ROBERTDMERRILL

iTrader: (1)

Why does the 305 auto produce 205 hp and the 305 5sp produce 230hp?

91zeder

Same engine, 305 auto has one cat, 305 5-speed has 2 cats.

MrJ

the auto also has a smaller Y-pipe, manifolds, and catback as well.

91zeder

Are you sure the manifolds are smaller too?
Why did GM penalize 305 Auto cars with 25 less hp and 15 tq by giving the strong LB9 an inadequate exhaust?

ForzaAzzurri

The lb9 305 also got the l98 cam which is why it makes more power

PONTIMANIAC 90

Both the stick 305 and the auto have the same cam. I believe the 90-92 305TPI autos are 215hp and not 205. The 15hp difference is due to the twin cats on the 230hp stick car.

pete86iroc

um no?.., STOCK cats wont make a diference that big, its mostly inpart due to diferent drivetrain losses, a manual loses less % of the HP and torque than an automatic does.

PONTIMANIAC 90

Close but but completely wrong!Net HP ratings are given at the crank so drive-train loss doesn't play into the equation.

Ed Maher

Ding Ding Ding. The only difference between the G92 and non-G92 305s in 90-92 (BTW, not all 5spd LB9s are G92, the ones that rean't got small exhaust) is the non-G92 cars got the TBI sized exhaust from manifolds to the road.

In my own car's case, i went from 15.1 @ ~90.5 to 14.8 @ 93.x JUST by switching to L98 exhaust manifolds and dual cats (evne used the same catback cut to fit the dual cats. Conditions were near identical, and my ets are always repeatable and reliable. Thats a demonstrated 25hp difference in my book...

The better question is, Why did GM ever bother putting ANY low-output V8s in these cars. Offering most of these cars with pathetic excuses for V8s has forever tarnished the thirdgen reputation since most people experiences with them are from TBI or peanut cam MAF cars They don't even realize that what they got a taste of was not what these cars really had to offer.

And yes, i'm sure the manifolds were smaller. I've held both in my hands and the difference is painfully obvious.

ROBERTDMERRILL

iTrader: (1)

Thanks for the answers, but please "Can't we just get along?"

Kevin91Z

iTrader: (1)

Ed is totally correct. I've got pictures of the two manifolds if you dont believe him.

TPIterror

ALL Formula and GTA 305 TPI 5spd cars from 90-92 were G92. It was optional on the TA.

The G92 performance pkg or "performance enhancement group" (I believe the RPO was RP6) had a 3.42 posi rear, dual cats with the larger piping, rear disc, and an oil cooler.

Horsepower ratings are as follows:

1990- 200 hp/295 lbs tq auto.....225 hp/300lbs tq 5spd
1991-1992- 205 hp/295 lbs tq auto.....230 hp/300lbs tq 5spd

I read somewhere that the exhaust is only responsible for a
10 hp increase and that the rest was found in the cam..but i cant confirm this.

Breazlan

iTrader: (1)

I think you meant to say they were RP6 not G92. G92 (performance axle ratio package) was a race package similar to 1LE. Not positive, but RP6 (performance enhancement package) was included in G92 and 1LE.

90Iroc-Zee

The G92 LB9 5spds in 90 were given the L98 cams only the 305 5spd G92 tho. That is why, those 2 cats are a sham and once i purchase the edelbrock y pipe im converting back to 1. its an extra 2 feet of better flow which im sure the pipe (like an x pipe on a mustang) doesnt flow to great when both the exhaust gases meet.

90 IROC-Z
Formula
Trans Am M5 LB9 V8 9.3:1 5.0 (305) 230@4400 300@3200 TPI ---- 3.42 ---- ----

"In 1990, the base engine for the IROC was finally the LB9 TPI 305 rather than the base LO3 V-8. However, G92 had to be specified now to get the better cam/230hp LB9 motor in the 5 speed LB9 IROC-Z. Previously, all 5 speed LB9 IROC-Z's got the higher output LB9. One other difference between a 1989 5.7 IROC and a 1990 5.7 IROC are the pistons. The 350 (only) received lighter weight pistons thus a slight horsepower increase of 240 (1989) to 245 (1990). In 1990, TPI Camaros switched from MAF air metering systems to MAP systems. This eliminated the mass airflow (MAF) sensor."

enjoy, thats from tech data.

90Iroc-Zee

Cam specs :

305 G92 5spd / 350 AT 10111773* .413 .428 202 207 114.5 (116/-116)

Taken from tech data.

This is the dreaded peanut cam :

305 AT (LB9 10088155* .350 .384 179 194 109.0 (108/-112)

BIG difference in lift and duration, thats where the 15hp comes in. I doubt the exhaust gave the car 5hp.

TPIterror

Its actually a 25 hp increase (200-225)...I'd say 10 for the exhaust(like the Pontiac literature states) and 15 for the cam.

amandagregory

how can you tell which cam you have? I have a 305 TPI A/T single cat exhaust. A lousy 2.73 posi rear which i hope to change soon. If i have the 'peanut' cam, can I switch to the good cam and will it raise my rpms redline? And would i have to change to the 8k tach?

Kevin91Z

iTrader: (1)

It depends on the year. All the 90-92 305 TPI cars have the good cam, regardless of transmission. Its the 87-89 305 TPI automatic cars we dont know about, yet.

amandagregory

my car is a 90. i thought i read somewhere that if your tach went up to 8k for the 305 (with A/T) then you had the good cam. where did you confirm that 1990 has the good cam, regardless of tranny?

SMOKEN

well my 86 305 tpi ta came with the 350 exhaust but i believe there was no cam but the brochure said 210 hp and 270 tourque .i thought that when the hp rates at 4000 rpm that ment you had the peanut cam anyways my hp was 210 at 4000 rpm and tourqe was 270 at 3200 .so i figure that when gm says that 86 camaro was rated at 190 hp was due to the cam and exhaust

Breazlan

iTrader: (1)

You can't go by info you see in a sales brochure. Brochures are printed and released before the vehicles are released. You usually can find a disclaimer in a brochure that points out that the manufacturer reserves the right to make any changes at any time that is contrary to the info released in the brochure. As far as the rating for '86 Pontiac TPIs, just know that it is less than what it was in '85 even though the brochures say they are the same.