Ryan-87GTA

iTrader: (2)

To put things simply, what were the differences between the L98 in an '87 GTA and the L98 in the '91 / '92 GTAs that helped them produce the extra ~30HP and ~25Ft/lbs of torque? I'm going to be rebuilding the L98 in my '87 GTA this summer and want to make my engine closer to the ones that were used in the last two years.

Drew

iTrader: (58)

Dual cats.

That's an over simplification, but if you run the single cat exhaust on a 90-92 L98 it'd make less power than 89. Some details were made milder to cooperate with speed density, the dual cats gave a serious performance boost that made up for the difference and then some.

bluegrassz

iTrader: (3)

When I pulled the heads on my 92 camaro 5.7 it had lighter flat top pistons with valve reliefs . The 89 motors had a small dish with valve reliefs.
The best I can find on info is the flat tops yielded 9.8-1 or 9.5-1 compression with a 64cc head and the dish pistons yielded 9.3-1 compression.
I am not saying all 91-92 motors got the flat tops. This would give around a half point of compression resulting in a few hp. That along with the duel cat exhaust, would probably get you those extra hp. Im not sure on the cam, but it also may be different.

dmccain

iTrader: (2)

I think the cams were basic the same except for 89 I think it was a bit hotter but not much. Ive always heard the dual cat system and the much better tuning with the later map system gave the L98 a few more hp.

Ryan-87GTA

iTrader: (2)

That would make sense. I was already planning on installing the dual cat exhuast system (alongside the SLP Tri-Ys if I can pull off a miracle and find them) so I am covered there.

Thanks for the tip on the possible piston variation, I'll look into that.

Definitely seems like the theme here is "Dual catted exhuast with a sprinkle of mild changes" which does make sense when I think about it.

Ryan-87GTA

iTrader: (2)

THE AIR INTAKE! Looking at pictures online, the '87 had this stupid intake that looked like it was designed by Rube Goldberg, and in '88 they did away with it for one long tube with two 90 degree bends and refined it's design in '91. I'm sure that is responsible for a fair share of the HP gain.

blacksunshine'91

iTrader: (10)

No, the intake tube does not free up that much power. Especially since the major restriction is the intake manifold. Any gains made by the tube would be negated by the choke points found in the manifold.

Drew

iTrader: (58)

The 85-87 TPI air cleaner on Firebirds is truly awful. 88-92 are all enough better that the SLP single basically used the ducting unchanged, they just added the box to draw air from the fender area, and some inserts to help direct the air. If a person looks at the numbers from year to year, the 87-88 break will show you the effect of the air cleaner change. 88-89 or 89 early-89 late shows you the dual cat impact. Late 89-90 shows you the SD and related changes impact. Late cars generally got some later hardware since GM was gearing up to go LT1, so you get some slightly different parts in later cars. Starting in later 91 you start to see the red trans dipstick handle, coarse texture Corvette runners, black throttle body plates, the brown terminal keeper on the TPS connector, the later style oil pan, and different pistons. The later the car, the more of those details you'll see as they used whatever was leftover. You won't see any published difference in power rating for any of those later changes.

Fast355

iTrader: (2)

I have pulled apart a later model engine that had the same pistons and PM rods that you would find in the ZZ3 engine as well.

Fast355

iTrader: (2)

The intake manifold is hardly to blame. I mean its not the best but it certainly is not the biggest choke point. That goes to the heads, factory cam and factory exhaust manifolds. I had a vortec head 350 that made almost 300 hp to the tires using the stock runners, stock plenum and stock throttle body with an untouched 3817 base that some claim flows less than the OEM base.

TransamGTA350

iTrader: (2)

Also 3 different camshafts for L98's. Not a whole lot of difference between them, but it plays an overall role in horsepower difference along with changes to the air intake, dual cats, compression and elimination of the MAF sensor restriction in the SD systems.
87' Camshaft-most mild
88-89' Camshaft-Most aggressive
90-92' Camshaft-In between (but closer to the 88-89' than the 87')

You have the exhaust covered. Keep the MAF system and change out the air intake ducting to the 88-89' design.