Trying to clarify conflicting reports on this from both media outlets as well as dealers/individuals so wondering if any past or present GM peeps may know the answer: what is the factory hp rating for convertible Trans ams that left the factory with the 5mt? I'm sure by now we are all well aware that the LB9 5-spd was rated @ 230 hp from the factory from 1990-1991 and only 205hp with the auto. Was the 5spd Lb9 in the ragtop also only rated at 205?

 

It's the G92 RPO code that gets you the higher HP, along with this would come the N10 RPO for the dual Cat exhaust. IIRC you could get this option in a vert.
The link below is to the chart lists this info and as you see the 5 speed could be the lower HP without the G92 option. The G92 option was only available with the 5 speed MT.

thirdgen.org/tech/techdb.php

 

I think you meant "could not get this option". None of the 1991/1992 Trans Ams Convertibles had any performance gears, all 5-speeds had the 3.08 gears, and none had the N10 exhaust.

 

I believe the N10 was only good for extra 5-10HP on LB9. So you would get 225-220HP. With 3.08 gear and 5spd, you are most likely at 0-60 in 7s and 1/4 mile in 15s. You would not feel the effect of the N10 at WOT.

 

In the 90-92 years, there were two exhaust systems for the TPI motors. The lower output 305's with the tall rear gears got the single 2.25" cat exhaust system and were 205hp. This would be the 305/auto with 2.73 gears or the 305/5-speed with 3.08 gears. All convertibles with TPI came this way. The cars with the performance rear axle ratios 305/5-speed with 3.42 rear or 350/auto with 3.23 rear got the dual 2.25" cat with 2.75" cat back exhaust and were 230hp for the 305 and 235hp for the 350. Add 5hp to all of those for Z28's due to the lower restriction air intake setup. The performance rear gear with the associated horsepower increase in the 305 were only available on coupes without T-Tops (except for some Formulas).

So, to answer the question, if it's a convertible, it does not have the 230hp 305 and will have the taller rear gears.

 

I have an all original LB9 '89 5 speed, 3.08 (None N10/G92) with single 2.5" inlet cat + 2.75" cat back exhaust. It's not a single 2.25" all the way. Interesting to see that in '90-'91 GM would have reduced the exhaust to single 2.25" for the "305/5-speed with 3.08 gears." Only way to know is for the OP to measure the exhaust diameter, if it's still original.

 

G92 doesn't apply to 91/92 Firebirds unless the car is a 1LE. R6P on the Firebird in 91 at least includes the same components as G92 on the Camaro.

People like to say the N10 exhaust didn't do much, but if you drive a car with the performance package and one without, it's pretty hard to miss the difference. GM didn't invest in the dual cat system because it did nothing.

Convertibles got the weak LB9, at least for 91/92 Firebirds.

 

For sure. N10 was a game changer in the Camaro/Firebird vs Fox 5.0 rivalry.

 

89' and earlier are different. All of the low output LB9's from 86'-89' had the 2.75" single cat exhaust, but GM used the peanut camshaft from the LG4/L03 to reduce the horsepower. The high output LB9's got the same 2.75" single cat exhaust and the bigger cam from the L98. In the later part of 89' they also got the dual cat exhaust for an additional 10hp over the 88' and early 89' version.

When GM changed from Mass Air to Speed Density in 1990, all LB9's got the L98 camshaft and they used the single 2.25" exhaust from the L03 TBI motors to choke it down and reduce horsepower for the low output version of the LB9 (autos, convertibles and non performance rear gear 5 speeds). All high output LB9's (5-speed with performance rear gear ratio) in 90'-92' got the dual cat exhaust.

 

From 90-92, according to the Parts catalog, all F-body TPI engines got the same cam. Part number 10111773. So in reality the biggest determining factors for performance was 1) Displacement of the engine (350 v 305), the gear ratio, 2.73, 3.08, 3.23 or 3.42, the N10 exhaust was a small factor, as was the JG1 aluminum driveshaft and the transmission. Unlike 87-89 where the LB9/A4 cars got the same cam as the 1987 LG4 & 88-89 L03, this was not the case for 90-92 (Note the L03 got the same cam from 88-92).
As for the TPI+M5 vs TPI+A4 the engines would have been rated the same, but the Automatic transmission at the time sucks HP, so there was a reduction in performance there. The 3.08 gear used with the M5 would have had marginal improvements over the 2.73 used in the A4 cars.
ALL 91-92 Firebird Trans Am Convertibles with the LB9/MM5 Combo got the GU4 3.08 gear. There was no Performance gear ratio for the convertibles those years.
120 for 1991
122 for 1992
Keep starting in the early 70's HP ratings went from Gross (at the flywheel) to Net (at the rear wheels) this remained the same until more recently.

 

The strangled exhaust hurt the auto cars. I changed out my exhaust to 3" and removed the cat on my 91 LB9 along with a aftermarket chip and it went 14.7s no other mods across the quarter mile with the crappy 2.73 gears. N10 exhaust and a 5spd made a HUGE difference in those cars from the factory like daylight and dark.

 

Yes, all of the TPI cams were the same for 90-92, but there was an actual difference in flywheel horsepower between the performance rear gear 305TPI/5-speed and the non performance gear 305TPI/auto or 5-speed. For 90-92, the performance gear TPI's all got the N10 dual 2.25" cat and single 2.75" intermediate pipe exhaust. The non-performance rear gear 305TPI's got the single 2.25" cat and single 2.25" intermediate pipe exhaust from the LG4/L03. The single 2.75" cat and intermediate pipe exhaust from 89' and earlier was no longer available in 90-92. GM was still playing the same game of high output and low output 305TPI motors, except that instead of doing it with different camshafts, they just choked it with L03/LG4 exhaust. For these years the N10 was more than the 10hp difference over the non-N10 305TPI since it's the difference between dual 2.25" compared to a single 2.25" when the year before it was dual 2.25" compared to a single 2.75". Big difference.

For 90-92, all 350TPI's got the performance rear gear and the N10 exhaust. 305TPI's were not all the same. The exhuast system was the difference.

As for gross vs. net horsepower, both are measured at the flywheel, not the rear wheels. The difference with net is that it is supposed to be a better representation of the engine flywheel output as it is installed in the car, with all accessories (but no A/C running), full exhaust, airbox and emissions equipment in place.

 

The exhaust manifolds are different also. I got the crappy L03/LG4 2.25" "T-Pipe" single cat choke um up exhaust in my 91 Z28 vert. When I was in the process of junking the whole system I found this out. The manifolds are a 2" (1/78" actually) outlet into the t-pipe. The N-10 and L69 (single 2.75") got different exhaust manifolds with a 2.25" opening to match the 2.25" downtubes. I don't remember the PN but there are 2 different ones for the exhaust manifolds. I actually think the 10 HP rating for the N-10 system is under rated. When I made the swap using the L69 manifolds, a real y-pipe and 3" tube it made the car just come alive, biggest seat of the pants mod you can do if you have the "crappy" exhaust.

 

Yes! And you can't just bolt the better 2.75" Y pipe or N10 to those 2" exhaust manifolds either.

 

That's correct. There are two different exhaust manifolds for V8's for 82-92. The manifolds for the single 2.25" y-pipe (LG4, LU5, L03 & 90-92 low output LB9) are the small outlet versions and will not work with the larger y-pipes. The 2.75" single cat y-pipe (L69, 85-89 LB9, 87-89 L98) and N10 dual 2.25" cat y-pipe (89.5-92 LB9 w/performance rear gear, 89.5 L98 w/performance rear gear or 90-92 L98) all used the larger outlet exhaust manifolds that will only bolt up to the single 2.75" cat or dual 2.25" cat y-pipes.

In your case, if you changed the manifolds, y-pipe, intermediate pipe and muffler from the stock 91' Z28 w/ the single 2.25" y-pipe exhaust system to the full N10 dual cat exhaust system, the gain would be much more than 10hp. It would be more like 25-30. The 10hp advertised gain only applies to the 89' model year when it was compared to the single 2.75" cat exhaust system.

 

"In your case, if you changed the manifolds, y-pipe, intermediate pipe and muffler from the stock 91' Z28 w/ the single 2.25" y-pipe exhaust system to the full N10 dual cat exhaust system, the gain would be much more than 10hp. It would be more like 25-30. The 10hp advertised gain only applies to the 89' model year when it was compared to the single 2.75" cat exhaust system."

This makes sense, I feel like it was at least a 20HP-25HP increase. I did change the complete system and went with a Magnaflow 16450 Y-pipe which is a 3" outlet and deleted the cat.

 

Noticeable horsepower increase and probably sounds great too.