quote Originally posted by streetrod1: Gm is not gonna make block dimensions differ much on the outside for a series engine.

The issue is GM did exactly this with the LS4.
The LS4 block is actually longer than the RWD version by about 1/8".
The LS4 uses the GM Metric Bellhousing pattern, not the SBC derivative used on all other RWD versions.
The LS4 does not have a starter provision on the engine and does not have the typical starter pad area on the block like the RWD versions, it was removed from the block for clearance to the transverse automatic transmissions.
The LS4 is missing a couple of the RWD engine mount holes on the sides of the block.
The LS4 crankshaft is 13mm shorter than all the RWD versions as well, 3mm in the rear, 10mm on the front.
The LS4 also uses a 142 tooth flex plate vs. the 168 tooth used on every other LS engine.

quote Originally posted by streetrod1: If I wanna use My crate 6.0 and my E trans controller..and some tucked in accessories will it work ? and what axle options do I have for axles and trans..the 4t65e maybe? FWd trans stuff is new to me so which one bolts to 6.0 that might last awhile and would allow me to do this swap in another fiero.

A RWD LS engine with transverse auto in a Fiero has been done twice that I am aware of.
One used the 4T65e-hd: http://www.fiero.nl/forum/Forum3/HTML/000083.html
One used the 6T70/75: http://www.fiero.nl/forum/A...160323-2-119430.html

To go with a 4T65e-hd or the 4T80, the very first thing you will need to do is cut off the starter pad area of the the engine block. Once it is gone, there is no going back, and the applications where that engine can be used again are greatly diminished. After that, you will need an adapter plate and the LS4 flexplate to bolt the two together as the bellhousing patterns are completely different and the 168 flexplate on the RWD blocks will not fit inside the bellhousing. You will also need a starter solution. For the 4T65e-hd a $95 bracket that will reuse the LS4 starter is available: http://fieroguruperformance.com/?page_id=1558. The other option is a custom oil pan with starter mount that is over $1K. FOr the 4T80, you are stuck with using the custom oil pan as the transmission valve body won't allow the LS4 starter to clear.

The 6T70/75 is more compact and does not run down the side of the engine, so the starter pad does not need to be removed from the block. You still need an adapter plate, the LS4 flexplate, a starter solution, and a different trans controller.

No one is currently making the proper adapter plate for the RWD LS to GM Metric transmission. The closest one that could be "made to work" is this: https://shop.streetdreamsby...bell-housing-adaptor . The thickness would be close, but probably not perfect. You would need to cut off the starter side at the base of the block (this section is not needed at all), and then mill a recess for the LS4 starter nose.

The path most chosen is the LS4/4T65e-hd. This is a stock GM power combo designed to work together, then you are just focused on mounts, axles, alternator placement, plumbing fuel and coolant. If the 5.3 doesn't have enough low end torque, internally the LS4 is just like all the other RWD engines, so heads, cams, and other parts interchange. You could bore, sleeve and/or stroke the LS4 to make it a larger cube LS engine. While the LS4 crankshaft is shorter, it is pretty easy to make the flywheel end the 3mm shorter (or just space the engine and transmission 3mm apart) and there is no need to modify the front of the crankshaft to use the LS4 balancer (it is used quite often on the LS/Porcshe swaps).

[This message has been edited by fieroguru (edited 12-24-2020).]

[This message has been edited by Will (edited 12-29-2020).]