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| Help with a LS4/F23 swap (Page 3/5) |
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Will
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DEC 14, 09:38 AM
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| quote | Originally posted by cam-a-lot:
Something to consider is the final drive ratio on the F23. It is not really well suited for high torque motors- it was originally paired with a gutless 4 cyl motor that is almost as depressing as the 2.5 that came in your Fiero. Maybe consider doing the gear swap to the FY1 F23 of the 2010 Cobalt SS? There are some threads documenting this process here on PFF. |
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The Sunfire FD is 3.94. That'll make the car a terror in the stoplight grand prix, for sure!
Changing the FD requires getting another transmission and building two out of one... maybe get the car running first and see how you like it.
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pmbrunelle
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DEC 14, 12:04 PM
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I don't really get the knee-jerk reaction that says higher-torque engine = taller ratios.
In the Porsche 992 PDK, I doubt they implemented an (econobox style) overall 1st gear reduction of 14.77 (4.89x3.02) because that's what they had on hand... no, it's about starting-line FUN.
Ratios taken from here:
https://www.caranddriver.co...-s-coupe_2021/413069
(sure, it has taller tires than the Fiero, but you can understand the sentiment)
I think that Justinbart used stock F23 ratios, and his car was pretty quick...
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Rotmilky
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DEC 14, 12:11 PM
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| quote | Originally posted by Will:
If that's actually the first day, there's a telltale in the background that you never intended to let the 4 cyl continue living in that engine bay  |
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I had already stuck a wrench on the crankshaft and knew it wouldn't budge. If it turned out minor and fixable, my plan was to get it running and then do some LeMons races until I needed that motor in the background. At some point, we were going to need a replacement...so better safe than sorry. 
I do wish we could have gotten the 4 banger going, though. Would have been fun to run it in Lemons stock.[This message has been edited by Rotmilky (edited 12-14-2020).]
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Rotmilky
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DEC 14, 12:48 PM
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| quote | Originally posted by cam-a-lot:
Something to consider is the final drive ratio on the F23. It is not really well suited for high torque motors- it was originally paired with a gutless 4 cyl motor that is almost as depressing as the 2.5 that came in your Fiero. Maybe consider doing the gear swap to the FY1 F23 of the 2010 Cobalt SS? |
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| quote | Originally posted by Will:
The Sunfire FD is 3.94. That'll make the car a terror in the stoplight grand prix, for sure!
Changing the FD requires getting another transmission and building two out of one... maybe get the car running first and see how you like it. |
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Thanks for the comments and warning. I'm keenly aware of the 'while I'm there' syndrome where mods and upgrades take so much time that nothing ever gets finished. So, we're concentrating on just getting the LS4 stuffed in there with the F23 first and foremost. If it ends up being poorly geared, I'll yank it all back out and make modifications at a later date.
If we do get it running, we're going to use it only for autocross and Lemons style races. It won't be a daily driver. For autocross, I really only run in 2nd gear and ideally I want it geared such that 2nd tops out around 60-70 mph. If I assume a rear tire of 235/65/R15, then that's a 27" tall tire. If the LS4 redlines at 6000 RPM, then my top speed is ~ 60 MPH. Looks like at least 2nd in the F23 is geared about where I need it for my particular driving style.[This message has been edited by Rotmilky (edited 12-14-2020).]
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cam-a-lot
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DEC 14, 02:55 PM
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I had an F23 in a Fiero with 3800 SC motor. Tranny is great , but first gear was useless. I can’t imagine you will get to 80-100 km/h in second gear unless the LS4 revs really high and pulls to 6-7k RPM
Comparing with PDK transmissions is really an apples to oranges comparison , since you are not pressing a clutch and shifting 3 times just to get up to 70 km/h
I never drove an LS4, so if it can rev and breathe at much higher RPM than my 3800 SC motor , then the gearing may not be an issue
Love the build, and hats off to you for getting the young ladies involved !
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Rotmilky
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DEC 14, 04:52 PM
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| quote | Originally posted by Dennis LaGrua:
All of these have been overcome by some talented swap guys but the key comes down to your fabrication skills. Not trying to dissuade you or be negative, Just trying to explain what you will face.
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I've got two teenage daughters. How could this possibly go wrong? .
I'm currently looking at the fab work as part of the 'Grease Monkey 101' educational coursework. Gives me a reason to teach them to weld and rull the mill. However, we're rapidly moving from introductory coursework to graduate level.
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Will
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DEC 14, 05:21 PM
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| quote | Originally posted by pmbrunelle:
I don't really get the knee-jerk reaction that says higher-torque engine = taller ratios.
In the Porsche 992 PDK, I doubt they implemented an (econobox style) overall 1st gear reduction of 14.77 (4.89x3.02) because that's what they had on hand... no, it's about starting-line FUN.
Ratios taken from here:
https://www.caranddriver.co...-s-coupe_2021/413069
(sure, it has taller tires than the Fiero, but you can understand the sentiment)
I think that Justinbart used stock F23 ratios, and his car was pretty quick... |
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And 996 Turbos have 3.82 x 3.44 in first gear.
BUUUUT... Corvettes & muscle cars have always had tall first gears. The standard from the ZF S6-40 in the C4 through the TR6070 in the C7 was 2.66 first with 3.42 final. Old T10 Muncies weren't far from this. They also have very small RPM drops from 1st through 4th gears.
There's a ratio curve for best possible acceleration from a dead stop to top speed. A CVT would follow this curve. Germans have been using this curve to define their gear ratios for a long time. GM has started doing the same on their 5+ speed automatics and new manual transmissions (F40, F35, F23, AY6, etc.) over the last 15-20 years.
The F23 3.58 x 3.94 = 14.1... and an LS4 will run that out very quickly. A DCT geared like this can just grab the next gear instantly. Shifting the transmission manually, especially with a large RPM drop from first to second, is much slower.
With a 4.89 first and 3.17 second, the PDK has a 0.65 "split", meaning after the shift, the engine is turning 65% of shift RPM. The F23 has a 0.585 split, so it actually drops more RPM going into second than the steeper geared PDK does.[This message has been edited by Will (edited 12-14-2020).]
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Rotmilky
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DEC 15, 07:05 PM
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| quote | Originally posted by Will:
The F23 flywheel should be adaptable. The center bore may come close to the bolt holes, but the center bore is only for locating the flywheel on the crankshaft. Make sure the new bore is a VERY close fit.
The stresses are fed into the flywheel at the crankshaft bolts, not the center bore. As long as the LS4 bolt circle and material outside of it are there, you'll be ok.
Since the LS4 also uses a larger circle with the same number of bolts, you can locate the LS4 holes in between the original holes and you'll be fine.
Do check the back side of the flywheel to make sure that the machined pad that mates to the crankshaft is large enough that there is machined material outside the LS bolt holes.
IIRC the LS4 is rated at 330 ftlbs stock, so a clutch rated for 300 flbs is marginal, especially if you start "doing things" to the engine, like adding the LS2 intake I think you mentioned... |
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Thanks for the information...and most critically the machined pad on the backside of the flywheel. The Sunfire flywheel (which we were going to use) doesn't have enough on there. We pulled the transmission off the original Fiero to see if that was better -- and it has a larger pad. I think it's going to work. See photo below. Sunfire wheel is on top, original Fiero on bottom.
The black dots are where the new flywheel bolt holes will approximately go. Clearly the Sunfire flywheel is a bad choice to remachine. We're going to try machining the original Fiero flywheel to fit the LS4 in the next few days....[This message has been edited by Rotmilky (edited 12-15-2020).]
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Will
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DEC 16, 10:06 AM
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Is the photo showing the friction side of the Sunfire flywheel?
If the area on the friction side of the flywheel where the bolt heads will seat is not machined, the new bolt holes will need to be spot faced as well.
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Rotmilky
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DEC 17, 08:42 AM
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| quote | Originally posted by Will:
Is the photo showing the friction side of the Sunfire flywheel?
If the area on the friction side of the flywheel where the bolt heads will seat is not machined, the new bolt holes will need to be spot faced as well. |
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No, that's the surface that attaches to the crankshaft.
You are correct about needing to spot face the friction side. The bolt holes are going to intersect the as-cast surface. I was going to remove the cast surface on the lathe, but I think I like your thought of spot facing better.
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