Ive been thinking about making something similar to this. But theres one thing about this one in particular that confuses me a little. Arent both of those throttle bodies stock fiero ones? Because everything ive ever heard says that two 52mm's would be way too much throttle body for the engine and theyd outflow it by a mile. I did some calculations and the 40mm throttle bodies from a late nineties GSXR750 crotchrocket would be about the right size. Or would it even make a noticeable difference? From my understanding its better to have throttle bodies that match the max cfm of your engine when theyre open all the way, because the butterfly is flat which causes less turbulence than one that outflows your engine when its only 1/2 or 3/4 open.
Am I overthinking this, or no?
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Originally posted by Frank2:
here's the goal: Build a mid engine car 1/2 as good as a Lotus Elise at 1/10 the cost.
If it were two throttle bodies feeding a common plenum, then yes it would be grossly oversize. But those are two separate plenums. So you only have one throttle body feeding each cylinder. It may seem counterintuitive, but that's how the math works out.
If it were two throttle bodies feeding a common plenum, then yes it would be grossly oversize. But those are two separate plenums. So you only have one throttle body feeding each cylinder. It may seem counterintuitive, but that's how the math works out.
You mean each cylinder bank (of 3) right? But each still doubled it's input correct?
If it were two throttle bodies feeding a common plenum, then yes it would be grossly oversize. But those are two separate plenums. So you only have one throttle body feeding each cylinder. It may seem counterintuitive, but that's how the math works out.
2.5 is right... When I did the math I calculated it as a 3 cylinder 1.4L engine with a common plenum and it came out to between 38-42mm depending on the application. Since this engine could be thought of as two 1.4L 3's stuck together wouldnt that work?
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quote
Originally posted by Frank2:
here's the goal: Build a mid engine car 1/2 as good as a Lotus Elise at 1/10 the cost.
As long as you can modulate the opening rate to get smooth throttle tip in performance, I don't think there is such a thing as too large of a throttle body.
As long as you can modulate the opening rate to get smooth throttle tip in performance, I don't think there is such a thing as too large of a throttle body.
'Cause its only gonna suck as much air as it can suck?
The computer/injectors just need to know how much fuel to match it with I suppose.
I don't think there is such a thing as too large of a throttle body.
I think where some of us get tripped up on this is that in the good ol' days, there was such as thing as too large of a carburetor. It's my understanding that this doesn't apply to a throttle body as (in the case of the 2.8) it's only supplying air and not an air/fuel mixture. Out of curiosity though, can throttle body injection have an issue with being "too large"?
[This message has been edited by Patrick (edited 05-30-2017).]
With dual throttle bodies, throttle input will be more aggressive than the stock setup*. So the fuel response to throttle input needs to be more aggressive, as well. Otherwise, the engine will go lean on acceleration.
I realize the math says that a 1.4 liter engine needs a smaller throttle body. But if you look at cars with small engines, most of them have larger throttle bodies. I spent quite some time combing through salvage yards looking for cars with small engines, so I could grab a couple small throttle bodies to use on a dual plenum project. Almost everything had a throttle bore of 50mm or larger, even the Hondas with 1.5 liter engines. As a matter of fact, the only car I could find with a smaller throttle body was a Geo Metro with a 3-cylinder. And it was a TBI unit, so I couldn't use it anyway. So if Honda can get away with a 50mm throttle body on a 1.5 liter engine, then we should be able to make a 52mm throttle body work on a 1.4 liter engine (i.e. half of a 2.8 V6).
* To make the car more driveable, you would use a progressive throttle mechanism that opens the throttles slowly at first, then ramps up.
So if Honda can get away with a 50mm throttle body on a 1.5 liter engine, then we should be able to make a 52mm throttle body work on a 1.4 liter engine (i.e. half of a 2.8 V6).
Are you sure you're not comparing apples with oranges? Comparing a DOHC engine that flows and revs quite a bit higher than a much more archaic pushrod engine?
You're right about the small engines using larger throttle bodies. But often those small engines rev a lot higher than our pushrod v6's... A honda might have its throttle body sized for peak power at 6k rpm while ours makes its peak at 4.
Here, I made a crude paint drawing to show what Im saying with the larger tb causing turbulence.
In that drawing, both are flowing as much air as the engine can breathe. The 52mm is open halfway and the 40mm is open fully, because the 52mm flows more overall than the 40mm.
[This message has been edited by Boot (edited 05-30-2017).]
The 2.8 V6 is a low power, low RPM engine because it has a restrictive intake and a very conservative camshaft profile. The dual plenum intake will take care of half the problem. A better camshaft will take care of the other half. Some head porting (and maybe a little more compression) would be the icing on the cake. With the right camshaft and a decent tune, you'll have an engine that pulls hard to 6000 RPM or more.
The big question is driveability. With dual 52mm throttle plates, it will be more difficult to modulate the throttle. A progressive throttle linkage, as mentioned above, will help with driveability.
[This message has been edited by Blacktree (edited 05-31-2017).]
As long as you can modulate the opening rate to get smooth throttle tip in performance, I don't think there is such a thing as too large of a throttle body.
Yeah, but that's the hard part. The driveability is the limiting factor.
Actually, that's probably one major reason why throttle-by-wire is standard these days, so they can have a different "pulley shape" depending on RPM...
But I agree that a tweaked pulley shape (even if it's fixed) may be part of the solution.
If you don't have throttle-by-wire, and you size your TB for max hp, then you probably have an on/off switch at low RPM. Or conversely, you have a setup that is controllable at low RPM, but you're choking the engine at high speed.
Anecdotally, I remember seeing 85 kPa absolute of manifold pressure in my 2.8 at high revs... I don't know if this was the TB or the neck restriction in the stock manifold.
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Originally posted by 2.5:
'Cause its only gonna suck as much air as it can suck?
The computer/injectors just need to know how much fuel to match it with I suppose.
On a MAP-based system, the throttle position isn't even the key factor. The amount of fuel necessary depends on how much air pressure is present in the plenum. How that air pressure got there, be it from a small throttle wide open, or a big throttle half-opened, doesn't matter.
On these old ECMs, the throttle position is used for acceleration enrichment; to prevent a lean spot on tip-in. This is pretty much the same thing (just a software version) as an accelerator pump on a carb.
As you might try different accelerator pump cams to make a car respond correctly on throttle application, you would modify the acceleration enrichment tables (which inject fuel according to the variation in throttle position) in the ECM.
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Originally posted by Patrick:
I think where some of us get tripped up on this is that in the good ol' days, there was such as thing as too large of a carburetor. It's my understanding that this doesn't apply to a throttle body as (in the case of the 2.8) it's only supplying air and not an air/fuel mixture. Out of curiosity though, can throttle body injection have an issue with being "too large"?
In a carburetor, sufficient air velocity through the venturi (and the corresponding pressure drop, see Bernoulli) draws fuel into the throat from the bowl. In the bowl, the pressure of the atmosphere is pushing on the fuel. So it's really quite a small pressure difference that pushes the fuel into the throat.
With an oversized carburetor, the air doesn't go fast enough through the venturi. Therefore, the pressure drop through the venturi is small, and fuel is not properly sucked into the venturi.
On a TBI, the fuel feed to the injector is pressurized by a pump, so I wouldn't think that the above applies.
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Originally posted by Patrick:
Are you sure you're not comparing apples with oranges? Comparing a DOHC engine that flows and revs quite a bit higher than a much more archaic pushrod engine?
I think that the best (rough) yardstick for comparing throttle bodies is horsepower. And comparing cable throttle bodies only, not the by-wire wizbang stuff.
If an XX mm TB was installed on an engine generating YY hp...
So suppose that you want a dual TB 2.8 to make 180 hp (if you don't shoot for the stars, you won't even wind up on the Moon), then you should get two TBs from engines producing 90 hp.
Originally posted by 2.5: 'Cause its only gonna suck as much air as it can suck?
True, but the larger throttle body allows the velocity of the air passing the throttle body to be slower and slower moving air does a better job of turning 90 degrees to enter the intake manifold runners (this is part of the reason you want a large plenum for higher RPM performance - the larger throttle body just allows the air to start slowing down before it enters the plenum).
Originally posted by Patrick: I think where some of us get tripped up on this is that in the good ol' days, there was such as thing as too large of a carburetor. It's my understanding that this doesn't apply to a throttle body as (in the case of the 2.8) it's only supplying air and not an air/fuel mixture. Out of curiosity though, can throttle body injection have an issue with being "too large"?
Carbs uses the venturi effect to pull fuel into the air stream and there is an optimum velocity range for them to work well. To large or too small of a carb will have the air velocity going through the carb outside of the ideal ranges and performance, response, and fuel economy will suffer.
Fuel injection uses fuel pressure to spray fuel into the air path with a fine mist, so air velocity isn't as significant for atomization.
Originally posted by pmbrunelle: Yeah, but that's the hard part. The driveability is the limiting factor.
Actually, that's probably one major reason why throttle-by-wire is standard these days, so they can have a different "pulley shape" depending on RPM...
But I agree that a tweaked pulley shape (even if it's fixed) may be part of the solution.
If you don't have throttle-by-wire, and you size your TB for max hp, then you probably have an on/off switch at low RPM. Or conversely, you have a setup that is controllable at low RPM, but you're choking the engine at high speed.
Over the years, I have seen various iterations of a cam shaped throttle cable pull. The one below is from the 75mm tb on the Ramjet 350. Notice the cable cam is offset from the center of the throttle blade shaft. So at tip in it takes more linear distance of the cable to rotate the throttle blade than it does as you approach WOT.
DBW just makes it easier as you have a table that translates % pedal to % throttle body open. You can change the header values so instead of the chart being 10% pedal = 10% TB and 20% pedal = 20% TB, you can make it nearly anything you want like 5% pedal = 1% TB, 10% pedal = 3% TB, etc... This gives you great tip-in throttle control and WOT performance.
I said: But those are two separate plenums. So you only have one throttle body feeding each cylinder.
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Originally posted by 2.5: You mean each cylinder bank (of 3) right?
You have to look at it from a different perspective. Remember that only one cylinder at a time inhales air. Let's use cylinder #1 as an example. With the stock setup, cylinder #1 is fed by a single 52mm throttle body. With a dual plenum intake and two Fiero throttle bodies, cylinder #1 is still being fed by a single 52mm throttle body. The throttle body on the other plenum does not feed cylinder #1. However, if you take the stock intake and modify it to use a twin-plate throttle body, now cylinder #1 is being fed by two 52mm throttle bodies. That's what I was getting at.
So you may be wondering, if each cylinder is still being fed by one throttle body, what's the big deal? Well, two things. First of all, the dual plenum intake won't be as restrictive as the stock piece. Second of all, since each plenum only feeds 3 cylinders instead of 6, that means the plenum has twice as long to refill with air after each cylinder inhales. So there will be more air available to each cylinder. And more air (plus more fuel) makes more power. Plus the dual plenums will have faster throttle response, because they get more air into the cylinders.
Side note: I have an ITB (individual throttle body) intake for the 2.8 V6. Each cylinder has its own throttle body. Guess what size the throttle plates are? 52mm... all six of them. But still, each cylinder is being fed by a single 52mm throttle body.
[This message has been edited by Blacktree (edited 05-31-2017).]
You have to look at it from a different perspective. Remember that only one cylinder at a time inhales air. Let's use cylinder #1 as an example. With the stock setup, cylinder #1 is fed by a single 52mm throttle body. With a dual plenum intake and two Fiero throttle bodies, cylinder #1 is still being fed by a single 52mm throttle body. The throttle body on the other plenum does not feed cylinder #1. However, if you take the stock intake and modify it to use a twin-plate throttle body, now cylinder #1 is being fed by two 52mm throttle bodies. That's what I was getting at.
So you may be wondering, if each cylinder is still being fed by one throttle body, what's the big deal? Well, two things. First of all, the dual plenum intake won't be as restrictive as the stock piece. Second of all, since each plenum only feeds 3 cylinders instead of 6, that means the plenum has twice as long to refill with air after each cylinder inhales. So there will be more air available to each cylinder. And more air (plus more fuel) makes more power. Plus the dual plenums will have faster throttle response, because they get more air into the cylinders.
Side note: I have an ITB (individual throttle body) intake for the 2.8 V6. Each cylinder has its own throttle body. Guess what size the throttle plates are? 52mm... all six of them. But still, each cylinder is being fed by a single 52mm throttle body.
On a MAP-based system, the throttle position isn't even the key factor. The amount of fuel necessary depends on how much air pressure is present in the plenum. How that air pressure got there, be it from a small throttle wide open, or a big throttle half-opened, doesn't matter.
On these old ECMs, the throttle position is used for acceleration enrichment; to prevent a lean spot on tip-in. This is pretty much the same thing (just a software version) as an accelerator pump on a carb.
As you might try different accelerator pump cams to make a car respond correctly on throttle application, you would modify the acceleration enrichment tables (which inject fuel according to the variation in throttle position) in the ECM..
Another good explanation. Yep I was just meaning the computer/sensors need to know so the injectors do their job right. You detailed the "how" well I think. Beyond that is we got crazy with the air next would be capacity for more fuel, more pressure and larger flow. But Id bet the stock system can flow pretty well. Then on the other had, those exhaust manifold logs...
True, but the larger throttle body allows the velocity of the air passing the throttle body to be slower and slower moving air does a better job of turning 90 degrees to enter the intake manifold runners (this is part of the reason you want a large plenum for higher RPM performance - the larger throttle body just allows the air to start slowing down before it enters the plenum).
That is the idea behind my intake. But I did something more, I tapered the runner diameter down as it got close to the lower plenum to increase air velocity close to the valves.
I would say that on a 2.8K you would see an increase in throttle response with a dual TB manifold but IMO power will be limited by the size of the valves and the narrow passageways in the lower intake manifold.
------------------ " THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite. "THE COLUSSUS" 87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H " ON THE LOOSE WITHOUT THE JUICE "
The issue on breathing with the V6 isn't the TB, it is the intake manifold (and the fact that the flow in the heads suck). Not sure whether or not the dual TBs would have any negative effect, but I'd bet they weren't really necessary to feed the amount of air the engine can use.
I use a single 62 mm TB on my lightly modded 3.4 engine (52 mm is stock) with no issues - of course the intake manifold is much better than the Fiero item and my engine uses more cam etc.
The issue on breathing with the V6 isn't the TB, it is the intake manifold (and the fact that the flow in the heads suck). Not sure whether or not the dual TBs would have any negative effect, but I'd bet they weren't really necessary to feed the amount of air the engine can use.
I use a single 62 mm TB on my lightly modded 3.4 engine (52 mm is stock) with no issues - of course the intake manifold is much better than the Fiero item and my engine uses more cam etc.
A side effect of this dual tb manifold would be that it eliminates the restriction in the factory Fiero intake neck :P
The problem I still see is cylinder imbalance. We all have it. Traditionally, cylinders 5 and 6 get all the air they want and 1 and 2 suffer. If you could take 2 Firebird 3.4 intakes and merge them so that there was a TB at each end, you'd have a better pressure/vacuum balance across all 6 cylinders. This would maximize power out of all cylinders better.
The problem I still see is cylinder imbalance. We all have it. Traditionally, cylinders 5 and 6 get all the air they want and 1 and 2 suffer. If you could take 2 Firebird 3.4 intakes and merge them so that there was a TB at each end, you'd have a better pressure/vacuum balance across all 6 cylinders. This would maximize power out of all cylinders better.
The severity of this issue can easily be tested by the DIYer at a relatively low cost:
Build a prototype (necessary if OP wants to go ahead with this project anyway).
Shoot a leaf blower shooting into one of the throttle bodies.
Take simultaneous flow measurements of each runner (so three measurements).
Flow measurements can be done with 3x identical junkyard MAFs, or even using pitot tubes and clear PVC hoses to make makeshift water tube manometers.
Possible conclusions:
Flow differences are negligible; no special requirements.
Flow differences are significant, but workable; use sequential fuel injection to supply more fuel to the lean cylinders in particular.
Air starvation of the end cylinders is so severe that the manifold needs to be redesigned.
Feeding a plenum from one end is fairly common, so I don't think it's impossible to correctly execute this concept.
Look at the Honda B18C5 (who can argue that this is a bad engine?):
Originally posted by lou_dias: If you could take 2 Firebird 3.4 intakes and merge them so that there was a TB at each end, you'd have a better pressure/vacuum balance across all 6 cylinders. This would maximize power out of all cylinders better.
Or you could just increase the plenum volume. The whole purpose of the intake plenum is to have a bunch of air ready and waiting to be sucked into the cylinders. If the cylinders on the opposite end from the throttle body are being starved for air, then chances are the plenum isn't large enough and/or the intake ports for those cylinders are more restrictive than the others.
And as mentioned previously, a dual plenum setup allows each plenum twice as much time to refill between intake events. Plus there are half as many cylinders competing for that air.
Not an expert by any means on this area but have read a bit on the intake and exhaust design considerations. I have always been intrigued by the dual plenum concept on the 2.8 motor as it lends itself so well to it with the lower intake plenum shape.
In all the exotic cars such as Ferrari, Lambo etc. that have the throttle body at one end of the intake, the intake will get smaller as it goes to the far end of the engine from the throttle body. This, I assume, to compensate for changing air pressure and velocity as it gets further into the intake. The cylinder up front gets lots of volume and air velocity ( must not forget velocity in these discussions) and as each cylinder takes off more air the volume, pressure and velocity decreases as you go further back in a single size intake shape. To compensate for this, the intake size reduces toward the end cylinder. This allows for pretty steady pressure, volume and velocity of the air hitting the last cylinder in line.
The intake size would need to be calculated to ensure the last cylinder still gets the volume of air required to match up front cylinders but this would reduce some of the issues introduced with a single size intake size. Next you start to figure in the pulsing attributed to the valves opening and closing and then the size and shape of the exhaust to pull the exhaust out ....... Quickly gets pretty darn complicated........
I am enjoying the read so please keep posting ideas folks Don
In the plenum concept, the plenum is supposed to be a tank of slow-moving air having low velocities, and high static pressure.
For a "log with a TB on the end" type of manifold, the section of the plenum immediately downstream of the throttle body needs to be sufficiently large such that the air is slow-moving, despite the airflow of the entire bank flowing though that section.
As we work our way to the end cylinder, the plenum doesn't need to maintain its original size in order for the air to remain slow-moving.
I believe that with a tapered plenum, you could have the same performance by maintaining the original large cross-section all the way to the end.
However, you'll have a plenum that's unnecessarily large at the end. Does this hurt steady-state horsepower? I can't see how it would.
The gotcha here is that your constant-large-diameter plenum has a lot of internal volume. So when you snap open the throttle, it takes a long time for the plenum to fill.
So I believe that the tapered plenum is an attempt at maintaining slow-moving air everywhere, while minimizing the volume for good throttle response.
Back to our 2.8s, as a non-expert, I would just design a plenum by following "that looks about right" principles and then measure it. Easier than trying to become some kind of aerodynamics/CFD expert...
Since the plenum is only 3 cylinders long, I suspect that the throttle response penalty of not having a tapered plenum would be minimal.
Lets back up a bit and look at the LIM ports on a Fiero. The 2.8L injector boss sits right in the middle of the port. Will a larger throttle body really produce any significant advatage is the question.
------------------ " THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite. "THE COLUSSUS" 87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H " ON THE LOOSE WITHOUT THE JUICE "
Ive been thinking about making something similar to this. But theres one thing about this one in particular that confuses me a little. Arent both of those throttle bodies stock fiero ones? Because everything ive ever heard says that two 52mm's would be way too much throttle body for the engine and theyd outflow it by a mile. I did some calculations and the 40mm throttle bodies from a late nineties GSXR750 crotchrocket would be about the right size. Or would it even make a noticeable difference? From my understanding its better to have throttle bodies that match the max cfm of your engine when theyre open all the way, because the butterfly is flat which causes less turbulence than one that outflows your engine when its only 1/2 or 3/4 open.
Plenum design on high performance engines can be a big deal. There are several classes of designs that range from plain, but carefully executed to completely exotic to the point of being difficult to fabricate (and highly effective!).
Plenum design is *NOT* a big deal on a 2.8 struggling to make 60 hp/liter.
An engine's throttle body should be sized to produce minimal pressure drop across itself for airflow volume *AT PEAK HORSEPOWER*.
That means that at low RPM, the engine sees high manifold pressure at relatively small throttle openings because the engine isn't drawing much air. You might see ambient manifold pressure (effectively WOT) at 30%-40% throttle. This is evidenced by looking at throttle % for power enrichment in GM factory tunes. They can be down in the 30% range at low RPM. That means that GM believes the engine needs a richer mixture to run for a long period of time at 40% throttle at 2000 RPM. That's because the manifold pressure at 40% throttle and 2000 RPM is going to be the same as at 100% throttle and 5000 RPM. That means that combustion pressures and temperatures are going to be roughly the same too.
With "too much" throttle, you just get into power enrichment that much sooner.
However, as FieroGuru and others noted, a "slow" throttle cam can move the throttle 20% for the first 40% of pedal movement. I've actually experienced this. My dad built a Tuned Port 400 Chevy and put it in a Jaguar XJ-6 S2. That car had 3.31 gears. The car had tremendous low and mid-range pull. With the original tuned port throttle cam, it was hard to leave a stoplight without chirping the tires. He swapped on the LT1 throttle cam, which was much more progressive ("slower" at small throttle openings) and it really transformed how easy the car was to drive, without any change to WOT power.
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Originally posted by Dennis LaGrua:
Lets back up a bit and look at the LIM ports on a Fiero. The 2.8L injector boss sits right in the middle of the port. Will a larger throttle body really produce any significant advatage is the question.
I think the iron head 3.4 crew has pretty much settled that the neck right behind the throttle is the most restrictive section of the stock intake tract, demonstrable by swapping from the stock Fiero middle & upper straight to the Camaro upper without changing the lower and gaining power. I don't have a thread link, though. I thought it was you who did that.
[This message has been edited by Will (edited 03-08-2019).]
I have built a dual throttle body intake for my race car. It mimics the intake linked in this thread and I am not in the mood to upload images but for all intents and purposes it is identical. It uses 2 Fiero throttle bodies and has a cross over tube we use for our vacuum ports. The engine is a 3400 roller cam block with DOHC pistons and around 10:1 compression. Its a WOT Tech cam (https://www.wot-tech.com/strip-performance-camshaft.html) with full roller rockers and stiff springs to match. Heavily ported iron heads built for endurance racing. It also has a short true duel exhaust on it although it is not equal length, it was built for packaging more than ideal performance.
This combo pulls hard to 6500 although the power is starting to fall off. For racing we have it rev limited to 6000 to help with fuel usage as well as to ensure we can run the car for 12+ hours at a time. I did the dyno tuning and it makes 175hp at the wheels and 210FT lbs with tons of room under the curve.
It is a very torquey little engine that is very eager to rev with very good throttle response. We have even driven it quite a bit on the street and although there are some tuning improvements that it would benefit from for that type of usage it is very drivable and well mannered at part throttle.
I would highly recommend this type of intake and find it to be a very good match for these little engines. I believe it is a big improvement over stock and mitigates some of the glaring issues, namely the constricted intake and sharp almost 180 degree turn the runners take.
Originally posted by Boot: Its not light speed, but 175 rwhp in a car that originally had 110 (30 years ago) is a pretty big upgrade...
Sorry, Boot! My comment was directed at Will who seems to think that anything over 135 rwhp is good for an iron head V6 engine... His comment seems more like an old snide remark to me. It puts me to sleep. I'm running a similar setup to theDrue w/10:1 compression but I'm currently using a Firebird 2.8 intake ported to 60mm. I converted to DIS via '7730 swap and tuned it myself.
A few people have driven my 'race' car and find the power kind of unbelievable because it's unexpected...I guess.
[This message has been edited by lou_dias (edited 03-13-2019).]
