So next year I hope to start my ls4 project. Right now I have a f40, but? I feel it will be more cost effective to go with an f23. Which do you feel is best for my swap? Is there a big difference between the gear ratios? Is one stronger than the other Please help me decided. Let me know thank you.

I think it's all going to depend on what you want to do with the flywheel.

I'm sort of in the same boat, but I haven't got anything for the swap yet(overseas).

someone might be able to clarify, but I thought I read somewhere that there won't be clearance for the passenger side tripod and the block. Because the Y block design.

I won't be getting a motor and trans for another month or two.

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SSFiero@Aol.com 87 Gt-5spd-62k miles.

My vote would be for the F40 if you actually plan on driving the car much on the interstate.

Your cruise RPM at 70 MPH with 25" tall tires will be 2073 with the F40 and 2560 with the F23. This 23% higher cruise RPM will result in a 3-7 mpg difference in fuel efficiency at highway speeds. Large displacement engines do not like to spin high RPM's at cruise, you will pull high levels of manifold vacuum and significantly drop your fuel efficiency. This is less of an issue on smaller displacement engines.
The effective first gear (1st * final drive) is very close between the two transmissions with the F40 one being slightly taller (1 mph greater speed at 6000 RPM).
GM Factory torque rating on the transmission is higher on the F40 than the F23. Many will disregard this rating and say the F23 is stronger because it has seen more time at the 1/4 mile.
The F23 is much less involved with regards to an installation standpoint and would be a lower cost overall swap.
If you buy 2 F23 transmissions and piece them together to make one, you can improve the 1st gear and lower the cruise RPM to 2359 (still 14% higher than the F40), but this change adds costs and complexity that starts to lessen the cost/simplicity aspect of the F23.

It all boils down to what you want, your skill sets, and available time/$$$ for the project.

F23 all the way. I think I'm at 2200ish on the highway. Mpg shouldn't be a concern as it will take you forever to offset the cost of the f40 if it even gives you better mileage. Only reason to go f40 is to tell teenagers you have a 6 speed.

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Turbo 3800 E85 F23 5spd spec5
11.17@132.6

I have built and driven both up against series II and III 3800SC engines, they both drive and shift quite nicely compared to the old school Fiero trannies but the F23 is less then half the work to do as well as a lot cheaper in the end. The F23 is more readily available to buy used with low miles and parts are much easier to come by where the F40 is pretty rare and parts very hard to find. Try going into a dealer and asking for fluid for an 07 F40 and a filler hole plug. You will most likely get back orders on the plug and the fluid is about $25 a bottle and I was told because of how rare it is dealers do not stock it so I was going to have to buy a case of 6 minimum when only needing two bottles. Plus the cost of the jack shaft and making up all the extra brackets and playing with oil filter adapters to clear every thing it makes for a much more involved swap. On the one I did I had to move the engine 1.5 inches to the pass side for proper axle centering which on a V8 would be next to impossible to do with out hitting the frame so different length axles would definitely be needed over the cobalt SS axles I used.
Put aside the cost and work and parts needed for the F40 over the F23 and the F40 does come out with the nice lower revs on the highway but is also a very close shift from gear to gear with very little room for error on the quick shifts. Personally I would go with an F23 over the F40. Dan

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The MPG difference is very very small... driving style will show more MPG gains than a better gear would. Rpm means very little when it comes to calculating mpg in today's world.

If you're going LS4, and want to be cost effective, then either go with something other than an LS4, or stick with the 4t65e-HD auto it comes with. An LS4 manual is a very significant amount of work, and will be expensive no matter what you do.

LS4's not really an easy swap either, despite the similarity in bellhousing bolt pattern and intended transverse application.

quote Originally posted by Will: LS4's not really an easy swap either, despite the similarity in bellhousing bolt pattern and intended transverse application.

who wants easy? If it was easy, everyone would do it.

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SSFiero@Aol.com 87 Gt-5spd-62k miles.

Wow! Great info gentleman. I will be doing alot of highway runs and small trips to the drag strip. Mpg is not all the important because I won't be driving everyday. I will think about. Either way im going LS4 with a 5 or 6 speed. I paid 1000 for a LS4 with 15000 miles. I had to get it. LOL! The 3800sc got sold to get the LS4. Thanks again everyone for the input.

Pictures on https://www.fiero.nl/forum/Forum1/HTML/087296.html look like there may be axle clearance issues with the LS blocks, and there may be issues with modifying the case for a starter mount pad.

The F40 seems to be the only feasible manual transmission to use with a transverse LS4, without any adapter plates. However, it will be a quite difficult, and expensive swap to do. You probably should have kept the 3800.

EDIT: Oh, and the mileage doesn't really matter, because you'll really be needing to pull the motor apart anyway to get rid of all the DoD equipment.

[This message has been edited by dobey (edited 11-07-2012).]

quote Originally posted by FIEROFLYER: I have built and driven both up against series II and III 3800SC engines, they both drive and shift quite nicely compared to the old school Fiero trannies but the F23 is less then half the work to do as well as a lot cheaper in the end. The F23 is more readily available to buy used with low miles and parts are much easier to come by where the F40 is pretty rare and parts very hard to find. Try going into a dealer and asking for fluid for an 07 F40 and a filler hole plug. You will most likely get back orders on the plug and the fluid is about $25 a bottle and I was told because of how rare it is dealers do not stock it so I was going to have to buy a case of 6 minimum when only needing two bottles. Plus the cost of the jack shaft and making up all the extra brackets and playing with oil filter adapters to clear every thing it makes for a much more involved swap. On the one I did I had to move the engine 1.5 inches to the pass side for proper axle centering which on a V8 would be next to impossible to do with out hitting the frame so different length axles would definitely be needed over the cobalt SS axles I used. Put aside the cost and work and parts needed for the F40 over the F23 and the F40 does come out with the nice lower revs on the highway but is also a very close shift from gear to gear with very little room for error on the quick shifts. Personally I would go with an F23 over the F40. Dan

The F40 uses the same fluid as the Cobalt SS and Saturn Redline as they both use the Manual Saab 5spd and it's what GM sold me when I purchased it, cost ~$15 a qt. Don't loose the filler hole plug. I wouldn't use Cobalt axles earlier than 2008 found on the SC cars on an engine more potent than the engine they were designed for, as the Cobalt owners are breaking them and those found on the Turbo models 2008 and up are stronger. Put a good muffler on whatever you choose and cruise rpm will not matter as much. Fuel economy is just as much a matter of cam specs as it is gearing. If it's a swap it doesn't matter, the swap headache is still the same, it's how long it lasts that makes the difference.

quote Originally posted by dobey: Pictures on https://www.fiero.nl/forum/Forum1/HTML/087296.html look like there may be axle clearance issues with the LS blocks, and there may be issues with modifying the case for a starter mount pad. The F40 seems to be the only feasible manual transmission to use with a transverse LS4, without any adapter plates. However, it will be a quite difficult, and expensive swap to do. You probably should have kept the 3800. EDIT: Oh, and the mileage doesn't really matter, because you'll really be needing to pull the motor apart anyway to get rid of all the DoD equipment.

there's jack shafts out there that came stock with some getrag transaxles. I'm sure they can be made to work with the F23.

I don't see where there's an issue with the starter mount?
I really wish I had access to engines and transmissions here. 27 days.

quote Originally posted by KissMySSFiero: I don't see where there's an issue with the starter mount?

Must build one from scratch and cut/weld on the transmission case to install it.

quote Originally posted by KissMySSFiero: there's jack shafts out there that came stock with some getrag transaxles. I'm sure they can be made to work with the F23. I don't see where there's an issue with the starter mount? I really wish I had access to engines and transmissions here. 27 days.

The jack shafts don't help much if the block is interfering with the output of the transmission. It may or may not be OK, but it's a possibility.

As for the starter, the F23 hydraulic bits look like they might interfere with where the starter pad needs to be put in for the LS4 starter.

quote Originally posted by fieroguru: It doesn't look like I took an actual picture of the interference, so tonight I will take the hybrid tripod with the Fiero tripod cage and slide it in the end of the intermediate shaft to get an actual picture.

Here is the fiero tripod housing along side the LS4 block. The housing just barely clears in this picture, but it does not have the boot on it. With the boot installed there is interference.

quote Originally posted by Will: Must build one from scratch and cut/weld on the transmission case to install it.

That's going to be done on the F40 too.

quote Originally posted by dobey: The jack shafts don't help much if the block is interfering with the output of the transmission. It may or may not be OK, but it's a possibility. As for the starter, the F23 hydraulic bits look like they might interfere with where the starter pad needs to be put in for the LS4 starter.

This is where I want to have the parts in front of me.

quote Originally posted by KissMySSFiero: That's going to be done on the F40 too.

Part and parcel with an LS4 swap.