I was reading some stuff the other day and noticed something which puzzled me perhaps someone could enlighten me.
When looking at the numbers on my old 88 IROC I saw a chart for the 1991-1992 Z28
Heres my question :
For the 1991-1992 Z28 it says 350cid L98 bhp 245 and torque lbft 345 compression 9.3.
but... I also have an old Vette a 1991 350cid l98 its numbers are:
bhp 245 and torque 345 lbft BUT the compression in the Vette L98 is 10:1

Same engine different compression why the same output numbers?

 

Vette has different (aluminum) cylinder heads. Not the same engine.

 

Both are labeled L98 both have the 5.7L 350.
Seems to be the same engine with different heads.
Are aluminum heads not generally agreed to be better than iron heads? Why then would the HP be the same at different compression ratios?

 

They are not the same engine. They use the same block and cam and intake, but heads are by far the most important part of an engine (along with the cam). Heads are huge. Different heads will have different characteristics. That's like comparing a Ferrari with a v8 to a Corvette with a v8, and saying "When talking about V8 cars, aren't Ferrari's generally agreed to be better than Chevrolets?" while the ZR1 is setting better lap times.

The air flow rates of the different heads are different. Maybe the tune is different.

Aluminum is better because it's lighter and it's easier to machine good port shapes into it. The heads in the 88-91 Corvettes were among the first GM production aluminum heads ever. They are very primitive and are generally considered about equal to the iron 083 heads in the big picture. The 083 heads have a better intake port, and the 113 heads have a much better exhaust port from what I recall, and in the end both were bottlenecked by the TPI system. Also, the extra compression is generally seen as an opportunity to run a larger camshaft. I think GM used the same cam in both cars, so there wasn't much real benefit in the higher compression level beyond efficiency. It might have netted slightly more power, but not anything statistically significant.



From the 70s when were stuck in the 7.5:1 range, there's a big improvement to be had in moving towards the 9-10 range. But as you can see beyond there, the returns diminish dramatically. You quickly hit the limit of modern fuel to resist detonation. The reason hot rodders run such high compression these days is to facilitate the use of very large cams that bleed off massive amounts of compression at lower RPMs. The extra added airflow of these cams at high RPMs makes power, but the engine runs very inefficiently. Running more compression addresses that, but it also requires race fuel. There are probably several other factors in that, but that's at least one.

 

What "chart" were you looking at? If its a factory literature they were often typo'd. Even if it's not typo'd its still just HP ratings from the 80's. Hardly accurate. Hardly significant. Take two fresh factory stock 92 L98s and dyno them. More than likely you'll have different results.

 

One rule to remember for aluminum heads is that besides design differences the aluminum usually required 1 point higher compression to equal the power of an iron head. The reason is the aluminum dissipates so much more heat it actually pulls heat/energy from the combustion process. The extra point in compression is required to get that energy back.

 

I was reading something similar the other day also. I believe it was on Wiki when I researched the L98. Said that the Vette had the same HP rating although it had more compression. I will say that compression don't always mean horsepower. If you have all the compression you want and aren't feeding that thing enough with a good bump stick, there will be a flatline in horsepower numbers somewhere. If its a TPI motor, those 1.94/1.50 valves in those heads could be starving it. The exhaust manis could be different due to the tight squeeze under that hood. Who knows, there are tons of variables.

 

The L98 V8 was optional on Jan. '87–'92 http://en.wikipedia.org/wiki/Chevrolet_Camaro & http://en.wikipedia.org/wiki/Pontiac_Firebird models (rated at 225 hp (168 kW)-245 hp (183 kW) and 330 lb·ft (447 N·m)-345 lb·ft (468 N·m)) 1987 versions had 10 hp (7 kW) and 15 lb·ft (20 N·m) more and a change to hydraulic roller camshaft. Compression was up again in 1990 to 9.5:1 Camaro/Firebird and 10:1 Corvettes, but rated output stayed the same.

From Wiki when searched for L98.