It's pretty well universally believed that the stock Fiero Intake, while it looks good, is a bottleneck to the little motor's performance. I've been givning this some thought for the last few months and I'm going to air some of those thoughts here.

One of the things that brought this on was when we had the Finale on the dyno the same day Rare87GT's car was on. They both had 3.4L engines. Amir's had a mildly warmer cam than our 3.4 crate engine cam. Both had headers, his more equal length type than ours. His also had a ported TB. The two engines made within a couple of hp of each other at nearly identical rpm. I then went back down when Amir and FieroX had the dynotuner on his car and we ran it on the dyno again. It gained a couple of hp, but not much and was running much richer. We talked that night about the intakes and what I thought the problem was and how to fix it and I've been thinking about this problem ever since.

Working backwards, let's start at the valves. The 2.8L in the Fiero has intake valves with a diameter of 1.72" or about 43.5mm. The cam has a lift on the intake side of about .394" or 10mm. That means that the area that the intake charge has to get out of there is the circumference of the valve head times the lift or approximately 1355 sq mm. Since at that point, some of the charge is fuel as well as air, we can reduce that a little to allow for just the amount of air. Let's use 90% by volume so we're looking at an area of about 1200 sq mm. Now that's bone stock. A high lift cam or other changes can vary that, but it's a place to start. don't forget that we have 6 of these little guys, so we are looking to feed about 7200 sq mm of area.

Now, let's start with some pictures and numbers. The intake ports as I've measured them are the same size or larger from the top of the middle intake on down to the head. To get the area here, I looked at a spare upper intake I have.

Not a particularly sharp picture, but you get the idea. These ports measure 43mm X 30mm with a slight are that's curved taken out for the area. I calculated this area, allowing for the curved portion, as being 1,235 sq mm. Since there are 6 of them, that makes a total port area of about 7,400 sq mm.

So, we need an upper manifold that will feed a port area of 7,400 sq mm. Remember what we came up with on the valve areas, roughly the same thing. So the lower and middle intakes appear to be sized, if not ideally, at least appropriately.

Next, I looked at the Throttle Body itself. This is a stock one, no porting.

As you can see, it measures just under 53mm in diameter. That gives us an area of 2,206 sq mm. IOW, less than 1/3 of the port area. Now you don't need to match TB area 1:1 to port area, but generally for a high performance engine you need about 60% of the area or better. Even at the 60% rule, we have half the TB area we really would like to have.

Some people (like Rare87GT) have ported a Throttle Body from Darrell Morse or others. And yet, as shown on the dyno when Amir and I ran our cars, there just really wasn't much difference. How can this be? Let's look farther into the manifold and see if we can find out.

Although it's a little hard to tell from this picture, but this is the snout cut off the upper throttle body at about the point the vacuum fitting is underneath it. This section was very informative.

The snout necks down to an area that is only 34mm X 51mm, and that's a generous measurement because of all the obstructions inside it. This gives us an are in the snout of about 1734 sq mm.

This explains why the results of the ported throttle bodies are minimal, at best. I'm not saying they won't help, but the factory throttle body is not the biggest bottleneck in the intake tract, the snout is.

In addition, I did some measurements of the runner length and dusted off some old textbooks. The lengths that I came up with when I measured indicated that the peak HP should come in at about 4000-4500 rpm. My dyno runs, as well as Amir's, both peaked HP within that range (4300 rpm in my case).

In order to up that power range, the runners need to be shortened quite a bit. Ideally (theoretically) about 100mm. Theory isn't always practice though, so this is somewhat of a cut and try, but what it does tell us is the runners are too long.

This is a picture of the top of the intake. It looks like there could be about 70 or 75mm removed without much trouble.

So, how much TB area do we need to feed these little monsters? Well, we can work backwards. We have about 7,400 sq mm of area we need to feed. We'd like to stay at least at a 60% level through the rest of the intake. This means we need about 4,500 sq mm or intake area. Doing some number crunching, that comes up to about a single 75 mm throttle body, ideally. It can be smaller, but that's what we should shoot for. Alternatively, we could look for a twin throttle body with openings of about 53mm each.

What can we use to get in this ballpark? Well, the '85-'88 twin throttle bodies as used on the Camaros have about a 49mm area. They also use the same P/N IAC the Fiero uses so that would make that part plug and play, electrically. The TPS is different, but if memory serves that's because of the pins and not the electrical outputs.

So, all we need is a new upper with greater volume, a way to attach the Camaro TB, and shorten the ports.

This is the approach we're going to take. I'm not going to be very quick about this, but as you can tell since I've already taken the snout off I'm heading in that direction. The next step will be to carefully mill most of the top of a stock upper intake.

Rare87GT is going to use his Formula as a test bed for this. A couple of things anyone else contemplating this should be aware of. First, the EGR will be gone. It feeds into the snout which will no longer be on the intake. This could probably be worked around, but not this time.

I'm not planning on doing this for a living so don't come beating on the door asking to make one. This is to satisfy my natural curiousity.

We WILL have this on the dyno and will report the results, for better of worse, to the group. Maybe I'm way off base with my thinking and if so, the dyno will be the judge.

OK, comments, flames, or ideas are welcome from anybody that cares to give some thoughtful consideration to the problem. Run my numbers for yourselves, I may have made a mistake somewhere and be all wet, but I don't think so.

John Stricker

John,

This is interesting.
The only question I have is whether it's really necessary to multiply the 1200 sq mm by six, since all six intake valves are not open simultaneously, but one at a time, in order. Of course, 1200 is too small to feed the entire engine, but I would guess that it's somewhere between the 1200 and 7200.
I'm sure there's a formula somewhere. I don't claim to know, even for a minute, that I know what I'm talking about. This is just a SWAG, if you will.

Having said all that...

A few years ago, I saw a hand made custom intake for a V6-60, that sounds very similar to what you have described.
It had short, straight runners. Seems like they were about the length of the Fiero lower and middle manifolds. On top of these runners was a large, (apparently) open plenum. On one end of the plenum was a Corvette dual port throttle body.
IIRC, it had been designed and built for a 3.4 that the guy also had.

I wish that I had realized what I was looking at. All of this stuff was for sale, as the guy was selling all of his Fiero stuff, and getting into autocrossing a Miata instead of his Fiero.

I'm looking forward to seeing what comes of your project.

------------------
Raydar

88 3.4 coupe. A work in progress.

Out of my mind. Back in 30 minutes.

[This message has been edited by Raydar (edited 01-23-2003).]

Raydar,

You picked up on a good point that I intentionally glossed over because now we're getting out of the realm of simple area rules and into an area of flow, which is volume.

Suffice it to say that the numbers I gave are on the VERY high end of what is really required from a theoretical standpoint. In part, that's where the 60% rule comes from, and that's strictly for a high performance engine. A more sedate engine can do with much less.

One of the things I was trying to show was that the TB isn't the only limiting factor here. The TB alone is sized larger than the restriction in the snout so gains by porting a TB will be minimal at best. The other thing is that from a tuning stanpoint, the runners are too long for the rpm ranges we think of as high performance. If it weren't for the runner length, grafting a largher TB onto the manifold and bypassing the snout would be much simpler, but I'd like to move the power band up another 1,000 rpm or so, and I think a shortening of the runner by about 60 or 70 mm should do that.

Good thinking on your part on the timing/flow issues.

John Stricker

quote Originally posted by Raydar: John, This is interesting. The only question I have is whether it's really necessary to multiply the 1200 sq mm by six, since all six intake valves are not open simultaneously, but one at a time, in order. Of course, 1200 is too small to feed the entire engine, but I would guess that it's somewhere between the 1200 and 7200. I'm sure there's a formula somewhere. I don't claim to know, even for a minute, that I know what I'm talking about. This is just a SWAG, if you will. Having said all that... A few years ago, I saw a hand made custom intake for a V6-60, that sounds very similar to what you have described. It had short, straight runners. Seems like they were about the length of the Fiero lower and middle manifolds. On top of these runners was a large, (apparently) open plenum. On one end of the plenum was a Corvette dual port throttle body. IIRC, it had been designed and built for a 3.4 that the guy also had. I wish that I had realized what I was looking at. All of this stuff was for sale, as the guy was selling all of his Fiero stuff, and getting into autocrossing a Miata instead of his Fiero. I'm looking forward to seeing what comes of your project.

Well I for one will be tracking this thread cause I am interested in finding a way to make our little engine breath much better and as I see it the main problem with that is out intake.

IIRC the reason for the long runners is for the low end torque that we all enjoy in the 60 degree V6s.

Didn't 1SLO2M6 and 1FST2M6 take a 3.4 Camaro intake and turn it around and made it fit past the distributor? It's the U-shaped thing, I think 1SLO2M6 has it in his signature.

------------------

I have found that the 3.1 intake for aluminum heads can be turn in either direction. It fits both ways to a tee, bolts holes and all. This way if you wanted to go DIS or not it wouldn't be to much of a problem.

Right now I'm having a hell of a time finding a good intake setup for my 3.4 buildup project. I have a 3.1 intake I was messing around with, and found this little bit of information strickly by mistake.

Earl R.

While you've made some interesting points, I think your area theory does have some flaws. I'm no expert when it comes to 3.4's, having never worked on one, but I have worked on SBC TPI's. Stating that the 3.4 needs a 53mm dual TB seems a little off when most 350SBC use a 48mm or 52mm. I dynoed at 272rwhp using a 48mm. (That's about 320 with losses). A larger TB would help, but people that go the the 58mm have had problems with driveability.
Now this snout is definitely a blockage, and should be opened up or removed.
You mention runner length and cross sectional area. On a TPI system, runner lenth will affect position of max torque, the longer, the higher the max torque, but at lower RPM. The cross sectional area affects the amount of air, but also the speed of the air charge. The smaller the port, the less air it flows, but the air charge will flow faster, increasing the ram effect of the tuned runners.
It's all very complicated and by all means I'm not an expert, just passing some info I've learned.
It might be worth noting that several people on the thirdgen boards received successfull results by siamesing the intake runners in the base on adjacent ports. The runners from the plenum to the base were left stock. The results showed no loss of torque, but a decent rise in power.
It's definitely worthwhile trying to modify the intake. It takes a few pioneers to make it better for the rest of us. Good luck...

Runner length shifts where the power (torque) is made. This can have an effect on how much HP is made because it's a function of torque and rpm.

Of course, it's a trade-off in that you can make an engine too peaky and almost undriveable, or even to torquey and it will run out of breath when you push it.

The area idea was kept simple not for the audience but to try to keep the post at some kind of a reasonable length. I knew I could count on you guys to fill in the blanks with responses.

The driveability problems by use of a large TB are really puzzling to me. In a carb'd application, you want velocities fairly high so that it can pull fuel and properly atomize it. In a TPI, you don't have to worry about that. You have a big box that is a reservoir of air, so to speak, and you use the TB to put more or less air in the Lake. If you let it out faster than you can let it in, you'll be flow limiting HP. If you let it in faster than you are letting it out, you're not. You don't need high velocities through the TB to aid in mixing fuel/air, that takes care of itself in the ports. You don't even sense your MAP at the TB, it's in the plenum.

Be that as it may, that IS one reason I'm planning to find an early TPI TB because it has two, smaller ports instead of one, larger one. (I do have a N* TB here on the bench, that's a single port, and probably about 85mm).

I'm not sure of the specs of the engine you're talking about, but what Rare87GT and I are trying to do is to be absolutely sure that the upper intake is NOT the limiting factor and see what happens from there. His car is a 3.4, has an aftermarket cam, headers, ignition, etc. It will make a good test bed. In addition, if things work out, I will have another stock 2.8 that I might try a before and after, just to see what will happen on it. That way we can eliminate the variables of the modifications on Amir's car.

The TB on the 5.7 is actually one of the intake side's biggest limiting factors there. And there are TB available to take that 48mm to 58mm from the aftermarket. That takes it from ~3600 sq mm to ~5300 sq mm.

I'm sure you're right, that the stock TPI TB is probably overkill, BUT they're cheap, available, and adaptable to this application, and I really don't think the TB will be responsible for driveability problems.

Something else I do wrong or may screw up will probably cause problems, but I don't think the TB being too big will.

Good input guys, keep it coming. It makes me think this through more thoroughly.

John Stricker

quote Originally posted by RCR: While you've made some interesting points, I think your area theory does have some flaws. I'm no expert when it comes to 3.4's, having never worked on one, but I have worked on SBC TPI's. Stating that the 3.4 needs a 53mm dual TB seems a little off when most 350SBC use a 48mm or 52mm. I dynoed at 272rwhp using a 48mm. (That's about 320 with losses). A larger TB would help, but people that go the the 58mm have had problems with driveability. Now this snout is definitely a blockage, and should be opened up or removed. You mention runner length and cross sectional area. On a TPI system, runner lenth will affect position of max torque, the longer, the higher the max torque, but at lower RPM. The cross sectional area affects the amount of air, but also the speed of the air charge. The smaller the port, the less air it flows, but the air charge will flow faster, increasing the ram effect of the tuned runners. It's all very complicated and by all means I'm not an expert, just passing some info I've learned. It might be worth noting that several people on the thirdgen boards received successfull results by siamesing the intake runners in the base on adjacent ports. The runners from the plenum to the base were left stock. The results showed no loss of torque, but a decent rise in power. It's definitely worthwhile trying to modify the intake. It takes a few pioneers to make it better for the rest of us. Good luck...

quote Originally posted by FieroGT87: I have found that the 3.1 intake for aluminum heads can be turn in either direction. It fits both ways to a tee, bolts holes and all. This way if you wanted to go DIS or not it wouldn't be to much of a problem.

I don't want to hijack John's thread, but this is of interest. Starting a new thread.

------------------
Raydar

88 3.4 coupe. A work in progress.

Out of my mind. Back in 30 minutes.

There have been quite a few threads on this. I guess I'll throw my thoughts into the pile...
The Fiero intake setup is perfect for the 2.8 to 5500rpm. So, this means its to small/tight/restrictive/whatever for the 3.4 to 6000rpm. Simple enough. But, being its only 0.6 Liters & 500rpm different (20oz pop bottle) how "to small" is it? There been lotsa math thrown around at this, and I think 61mm throttle body was roughly the number to 6000 rpm. of course, this is just the beginning of the intake. for the most part, as long the smallest point from the throttle body to the cylinder is the valve opening, its good. but the rough surface, bad alignment of the plenums, and the bumps from the injector holes create some restrictions/turbulance. So, I think the intake can be made to be fine for a 3.4 to 5500rpm with the 57mm (seems to be max) throttle body/plenum boring, the intake cleaned/sanded smooth all the way thru, the injector bumps smoothed down, and the ports matched between plenums (espcially between the upper & middle)

I'm not sure that it is "perfect" for the 2.8 at all.

I owned a Chevy Celebrity with the 2.8L engine. It was the standard 2.8 with MPFI. It had the identical intake the Fiero has. The only difference, externally, was that it had DIS (which failed often, BTW).

The standard 2.8 has a milder cam and smaller valves than the 2.8 HO that is standard in the Fiero. The intake as it was designed was tuned to move a car that was at least 1,000 pounds heavier than the Fiero and in no way, shape or form consedered "sporty". It was optimized for lower rpm operation as is evidenced by the long runners.

The internal differences were taken care of by the factory for the 2.8L in the Fiero, but the stayed with the same intake that is better suited to a grocery getter than a sports car.

I'm hoping we can do better, and that it will work as well on a 2.8L as it does on the 3.4. Only time will tell.

John Stricker

quote Originally posted by Pyrthian: There have been quite a few threads on this. I guess I'll throw my thoughts into the pile... The Fiero intake setup is perfect for the 2.8 to 5500rpm. So, this means its to small/tight/restrictive/whatever for the 3.4 to 6000rpm. Simple enough. But, being its only 0.6 Liters & 500rpm different (20oz pop bottle) how "to small" is it? There been lotsa math thrown around at this, and I think 61mm throttle body was roughly the number to 6000 rpm. of course, this is just the beginning of the intake. for the most part, as long the smallest point from the throttle body to the cylinder is the valve opening, its good. but the rough surface, bad alignment of the plenums, and the bumps from the injector holes create some restrictions/turbulance. So, I think the intake can be made to be fine for a 3.4 to 5500rpm with the 57mm (seems to be max) throttle body/plenum boring, the intake cleaned/sanded smooth all the way thru, the injector bumps smoothed down, and the ports matched between plenums (espcially between the upper & middle)

First, here's what you're talking about:
https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20020208-2-014612.html

2nd: the Northstar throttle body (at least through '95) is ~76 mm.

quote Originally posted by Raydar: John, <SNIP> Having said all that... A few years ago, I saw a hand made custom intake for a V6-60, that sounds very similar to what you have described. It had short, straight runners. Seems like they were about the length of the Fiero lower and middle manifolds. On top of these runners was a large, (apparently) open plenum. On one end of the plenum was a Corvette dual port throttle body. IIRC, it had been designed and built for a 3.4 that the guy also had. I wish that I had realized what I was looking at. All of this stuff was for sale, as the guy was selling all of his Fiero stuff, and getting into autocrossing a Miata instead of his Fiero. I'm looking forward to seeing what comes of your project.

I'm guessing this is what you're talking about?


I don't remember where I stole these pics from, they're just floating around in my overflowing fiero folder...
Anyway, I don't know about the effectiveness of this mod, but it definately looks like it'll handle anything you throw at it.

Kris
Just another Millenium Yellow '87 GT

quote Originally posted by Kris Duck: I'm guessing this is what you're talking about?

Kris,

Very similar.
I'm thinking the one I saw was an entire manifold, and not just a reworked plenum.
Picture a box like the one in the pic, sitting on top of runners fabricated from rectangular tubing.
I could be completely mistaken, though. On second thought, I'm not sure how that configuration would have bolted on the engine.
Wish I could remember more clearly.
This was 4-5 years ago. Belonged to a guy in the northwest Atlanta 'burbs.
Maybe he's lurking, and will see this and post here.

Will,

You're right about the N* TB. I was working from memory of measuring the horn to get the connector size, the inside Dia. is 76mm.

I've seen the picture that is in the posts following yours, and that is similar to what we're going to do. The difference is that all of these that have been built of various types don't have dyno tests that have been reported.

Heck, it may not help. That's the thing, we don't really know. Hopefully, we can change that.

John Stricker

quote Originally posted by Will: First, here's what you're talking about: https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20020208-2-014612.html 2nd: the Northstar throttle body (at least through '95) is ~76 mm.

quote Originally posted by jstricker: The difference is that all of these that have been built of various types don't have dyno tests that have been reported. Heck, it may not help. That's the thing, we don't really know. Hopefully, we can change that.

You followed the link I posted, right? 36 HP on a G-tech is not to be taken lightly. I'd call that a dyno test, and I'd say that intake sure as heck helps!

Will,

Yeah, I followed the link. I know that G-Tech's are pretty close, but I'd really like to see a car that runs with the stock intake, change the intake, and run it again. Preferably withing a couple of hours of the first run.

I'm hoping that I have this sorted out soon well enough that we can do exactly that, possibly with Amir's 3.4 and also with a stock 2.8 as well. It doesn't take long to change the top plenum's if you don't have to deal with cable rigging, etc., and I'm hoping to get this to a bolt-on state.

We'll see, it may not be possible to do it that well, but we'll see.

John Stricker

quote Originally posted by Will: You followed the link I posted, right? 36 HP on a G-tech is not to be taken lightly. I'd call that a dyno test, and I'd say that intake sure as heck helps!

If you want something "simple" here you go, but it's not cheap.

This S/C uses the lower iron head plenum on the 2.8-3.4 engines. It can be set up to use carb or TB. I'll find the link and post it later.

Earl R.

[This message has been edited by FieroGT87 (edited 01-25-2003).]

quote Originally posted by FieroGT87: If you want something "simple" here you go, but it's not cheap.

That's the Fageol blower. Supposed to work great on an S-10 or F-body, but we don't have the room for the belts - you'd also have to completely rework the cooling hoses (not impossible) to work around the snout.

I've got one of those Road Dyno's showing up here in the next couple days - if Cooter wants to send me his intake, I'll give it a try on my car and report back!

Sorry, but our former resident troll Shaun bought the dual throttle body intake and has since been silenced so we will never know how it would perform on a modified engine. I hate it that he pissed so many people off because he really knew his stuff.

The other intake I made- the modified 3.4 intake- is on 1slo2m6's car. I sure do wish that he and Travis would pay me for it

I can't wait to get this going John. Hopefully the G-tech results of 36hp or something is somewhat in the range. It should be right but dyno is the ultimate way to see what kind of performance we are looking at. With my car and the chip, I can have Wester's Garage tune a chip for the LT1 throttle body and more air flow and the need for more fuel up high. He will be able to do it for us and the application so it should work out great. Keep me posted John. Can't wait!

Amir

------------------

Maroon 1987 GT 5 spd: 2.8L
Ferrari Red 1988 Formula 5 spd: 3.4L

My Website: http://www.members.cox.net/rare88/

quote Originally posted by Cooter: Sorry, but our former resident troll Shaun bought the dual throttle body intake and has since been silenced so we will never know how it would perform on a modified engine. I hate it that he pissed so many people off because he really knew his stuff. The other intake I made- the modified 3.4 intake- is on 1slo2m6's car. I sure do wish that he and Travis would pay me for it

there was another guy that did it and had a full documentatio on his website and expects to gain around 30hp on his worked 3.4 with it. Im sure someone has the website

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--Adam--
1987 Blue GT 5-speed
IM AOL: GTFiero
Fiero mods almost done...i know that because im running out of possesions to sell for money

If this sounds nuts... well... please excuse my pre-coffee insanity.

What would happen if you put the upper plenum on the engine, but didn't bolt it down. Fire up the dyno, get ready to do the pull. Now have one person snap the throttle full wide, while someone else lifts the upper plenum up!

I'd say that quickly simulates a REALLY big honking throttle body!

Oh yeah, have a third person holding the key the whole time.

It sounds nuts.. but why wouldn't it work?

^^^ as long as the ignition system has a rev limiter (like an MSD 6AL).

Also...

quote The 2.8L in the Fiero has intake valves with a diameter of 1.72" or about 43.5mm. The cam has a lift on the intake side of about .394" or 10mm. That means that the area that the intake charge has to get out of there is the circumference of the valve head times the lift or approximately 1355 sq mm. Since at that point, some of the charge is fuel as well as air, we can reduce that a little to allow for just the amount of air. Let's use 90% by volume so we're looking at an area of about 1200 sq mm. Now that's bone stock. A high lift cam or other changes can vary that, but it's a place to start. don't forget that we have 6 of these little guys, so we are looking to feed about 7200 sq mm of area.

Keep in mind that we're dealing with 4-stroke engines here. Only half of the cylinders will be on their intake cycle at the same time.

Also, notice how the vacuum fittings on the 2.8 plenum are strategically placed at the point where the neck is the smallest. At that point, the air velocity will be the highest, and hence the static pressure will be the lowest. I'm not sure what bearing this has on custom plenums, but it may be a good reason for you to think about vacuum fitting placement. Just a thought.

And by the way, I'd be very interested in seeing a high-volume replacement for the stock plenum. Assuming the cost is reasonable and the results are dyno-proven, I might just buy one.

[This message has been edited by Blacktree (edited 01-26-2003).]

Now that sounds like some fun...

Can I borrow your car for a couple of hours??

John Stricker

quote Originally posted by watts: If this sounds nuts... well... please excuse my pre-coffee insanity. What would happen if you put the upper plenum on the engine, but didn't bolt it down. Fire up the dyno, get ready to do the pull. Now have one person snap the throttle full wide, while someone else lifts the upper plenum up! I'd say that quickly simulates a REALLY big honking throttle body! Oh yeah, have a third person holding the key the whole time. It sounds nuts.. but why wouldn't it work?

Great thread John. I'll be following with interest.

My personal theory is that the stock exhaust is much more restrictive than the intake. You mention that both your and Amir's car have headers. Do you have any pictures of them that you can post?

As for the intake being restrictive, here's an easy test that should tell you if you're on the right track. Hook a vacuum guage up to the intake. Do a dyno pull and watch the vacuum guage as the RPMs rise. If you have a decent vacuum in the intake manifold at high load high RPM then the something upstream (throttle body or intake snout) is too restrictive. You could even do this on the street if you had a long enough hose on the vacuum guage.

Based on what Blacktree said about the placement of the vacuum line fittings, you might want to check vacuum from that fitting that's right off of the main reservoir in the plenum.

Good work and keep us posted.

------------------
Doug Chase
'88 Fiero Formula 5-spd (autocrosser)
'88 Fiero GT 5-spd (daily driver)
'85 Fiero GT 5-spd (rally car)

I ran a boost/vac gauge on my 3.4 and at higer rpms it does build a vacuum in the intake, even with a bored throttle body and ported intake.

allow me to second the restrictive exhaust theory:
https://www.fiero.nl/forum/Forum2/HTML/031341.html

This may be a dumb idea/question but here goes. Why is the intake snout at one end of the manifold?
Now the dumb part!! why not just install two throttle bodies on top of the plemum and block off the snout? The top is flat. easy to mill drill and bore.
The air filters would be outside the hood for nice cold air. You could build an airbox to protect them.It would look like an old funny car????? HMMM think about it!!
sounds simple to me.
Any one want to donate a spare throttle body and plenum?

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RED 85 GT MY DEAL'S GAP DRAGON SLAYER
WHEELER'S PERFORMANCE MOTORCYCLE SHOP NEAR DEALS GAP
check out TAILOFTHEDRAGON.COM 318 curves in 11 miles
and now # 2 white 86 gt fastback

Maybe.

Typically, the intake side is much more critical than the exhaust side because you have the piston pushing the gases out.

I can tell you from many years of building hi-performance and race engines that the exhaust does not make as much difference as most people think.

I have a '73 Pontiac Grand Am that has a worked 455 in it. Back in the late '70's, I took it bracket racing a few times. It had a Walker replacement exhaust system (stock size pipes), factory cast iron manifolds, and turbo mufflers. The car ran, normally, around a 13.8 quarter. I took the manifolds off one summer and put a pair of Hooker headers on it. At the track I'd open them up so no exhaust back of the headers. It ran a 13.75.

I'm not saying the open Hookers were worth only .05 seconds. These runs were differenct days (and in fact, different years). What I'm saying is the difference wasn't very dramatic, except it made me feel bad-a$$ to be in the car when it went rump-rump at the line.

It just depends on what you're running BEFORE the change. In my case, it was already running the good, Pontiac, RA exhaust manifolds and a good, free flowing exhaust system. In that case, it didn't gain me much.

Now if I had been running the LeMans manifolds, and single exhaust, the results would have been more dramatic.

In the case of the Fiero, the factory manifolds aren't as good as headers, but they're better than most cast iron manifolds available. The system is just a bit undersized, but the killer is the stock cat and muffler. Remove the cat and put on a Spin Tec or other high flow muffler, and I doubt you're going to see much of a difference.

Regardless, on Rare87's car, he already has WCF headers, supposedly the good ones not the shorty's, and a good exhaust, so that should be pretty much out of the equation and we can see just what difference the intake makes.

I did have the WCF shorty's when we dyno'd, no cat, stock 125,000 mile muffler. Our HP numbers were almost identical.

Time will tell.

John Stricker

quote Originally posted by lou_dias: allow me to second the restrictive exhaust theory: https://www.fiero.nl/forum/Forum2/HTML/031341.html

Doug,

The Finale DID have headers. WCF shorty's. I can't recommend them or WCF, but I'm not getting into a pi$$ing match about that here. It now has factory manifolds back on it.

Amir's car had the better (?) WCF headers. The two cars made within about 2 hp or something of each other. We had the stock muffler, IIRC Amir had a low restriction. We had no cat, I don't know if Amir's has a cat or not.

The thing about this, and exhaust does play a part, that I want to examine is where the power is at as well as how much. By my measurements and calculations, the runners are simply too long for a performance engine. They're going to be shortened.

The dyno will tell us if this is a good thing or a bad thing.

John Stricker

quote Originally posted by Doug Chase: Great thread John. I'll be following with interest. My personal theory is that the stock exhaust is much more restrictive than the intake. You mention that both your and Amir's car have headers. Do you have any pictures of them that you can post? As for the intake being restrictive, here's an easy test that should tell you if you're on the right track. Hook a vacuum guage up to the intake. Do a dyno pull and watch the vacuum guage as the RPMs rise. If you have a decent vacuum in the intake manifold at high load high RPM then the something upstream (throttle body or intake snout) is too restrictive. You could even do this on the street if you had a long enough hose on the vacuum guage. Based on what Blacktree said about the placement of the vacuum line fittings, you might want to check vacuum from that fitting that's right off of the main reservoir in the plenum. Good work and keep us posted.

We must have different plenums, then. Mine has 2 vacuum fittings in the snout, right near the thinnest part. There's only one vacuum fitting by the thermostat housing. The engine is a 2.8 from a '86 SE, by the way.

quote Originally posted by Blacktree: ^^^ as long as the ignition system has a rev limiter (like an MSD 6AL).

Ok, Done. Installed my Mallory 685 box last night, so I now have a fully adjustable rev limiter. What's your address?

Even if my psycho idea didn't work... it'd still be a hoot to try! If someone does it... please get video.

John my exhaust does not have a cat on my Borla system.

^ ^ ^

Raydar,

Haven't had too much time to work on the manifold this week, I'm going to try to fire up the mill and do some work on it this weekend. If I make any progress, I'll post some pictures on it.

I'm going to start by milling the top off and then taking some more accurate measurements on the runner lengths. I'm still looking for a throttle body. While right now I'm still leaning towards an early LT1 body, I'm keeping my options open up to and including making one out of block of aluminum. Just depends on what I come up with.

John Stricker

quote Originally posted by Raydar: ^ ^ ^

John---

AFAIK, the drivability problems with too large a throttle body are related to reduced throttle control & drastically increased airflow at lower throttle angles. Quadruple the size of the throttle body, and (I'm not up for the math involved to figure this out "correctly" right now, and this is a way-off base dramitization, but for illustration-) at 25% throttle opening you have as much air going in as WOT on the original throttle body. Harder to modulate, TPS gets "freaked" out a bit, etc...

Raydar---

The custom intake you're talking about was Michael W. Smith's, on a '96 DOHC. Wicked-evil-cool project in tune-ability...with pretty awesome results.
BTW---check your Email...

Take care all---
Jeremy B.

To a point, you're right. The stock older LT1 body is two, 48mm throttle bores. That makes it about 170% of the stock area. But what my thinking is going to is that the two bores will make it much more controllable.

That is, they're smaller, so smaller openings, in degrees of movement, will make less of an opening than a single, 68mm butterly in a bore would make. That's the main reason I'm planning on the LT1 body, if I can make it work.

The problems are many. Size, of course, allowing for distributor clearance. Also the throttle cable hook up is on the wrong side, IIRC. We can't move it around much without throttle cable modifications. All problems that will have to be looked at and, hopefully, overcome, remembering that I'd like this to be a true "bolt it on and go" swap so that if it doesn't work, 10 minutes will put us back to stock.

John Stricker

quote Originally posted by Fierotech: John--- AFAIK, the drivability problems with too large a throttle body are related to reduced throttle control & drastically increased airflow at lower throttle angles. Quadruple the size of the throttle body, and (I'm not up for the math involved to figure this out "correctly" right now, and this is a way-off base dramitization, but for illustration-) at 25% throttle opening you have as much air going in as WOT on the original throttle body. Harder to modulate, TPS gets "freaked" out a bit, etc... Raydar--- The custom intake you're talking about was Michael W. Smith's, on a '96 DOHC. Wicked-evil-cool project in tune-ability...with pretty awesome results. BTW---check your Email... Take care all--- Jeremy B.

I got some time this weekend to work on the intake. I started by cutting the ribbed area out as I wanted to save it for a potential other project.

I then took cuts across the top with the mill in the approximate area that I thought I needed to go to in order to get my runner lengths shortened by about 60mm. This allowed the rest of the top to come off in pieces.

I proceeded to mill the inside runner dividers out of the manifold. I took them down to within .030 (~1mm) of the bottom of the intake floor. I was going to mill them flush, but the floor isn't flat so I decided to leave them a little proud rather than try to mill them on a taper.

Finally, I milled the front end, the snout end, of the manifold off in a straight line.

The manifold now looks like this. I also milled the top of the ribbed area smooth, just because it was terribly pitted and only took about 10 minutes. Aluminum is so nice to work with in the mill.

Notice that the straight area on the snout end is NOT perpendicular to the sides. I just wanted a straight line taking as little material off as possible and with all the humps and dips there that's what it ended up as.

I also left the back area intact to add strength. I actually ended up shortening the runners about 52mm. That was as far as I dared go and maintain the structural integrity of the manifold in one piece.

Now we need to shape our aluminum pieces to fit the areas including the front, and mill the finished tops perfectly smooth. Maybe I can get that done next weekend.

Here's the millings that were left after I was done. I love turning big pieces of aluminum into little bitty pieces of aluminum. That pile is about 9" tall.

John Stricker