Does increasing compression increase horsepower?

[Gregski]

I've been looking at Chevrolet crate motors, two in particular and am wondering why the one with lower compression has a higher horse power output? Doesn't increasing compression increase horsepower after all?

The two engines and their specs, the cams are different specs but both hydraulic flat tappet, I get it, but the heads seem very similar. Both have 76cc chambers and 1.94 intake / 1.50 exhaust valves. Not power related but FYI both have 2 piece rear main seals and 4 bolt mains.

10067353 Base 195/260 HP Engine
8.5:1 Compression
Heads - Part # unknown (similar to #882 castings)
Valve Springs - Part #3911068

12499529 290 HP Engine
8.0:1 Compression
Heads - Part #93438648/93438648
Valve Springs - Part #94666580

[rich weyand]

1) They both actually have the same compression ratio. 8.5:1 nominal, 8.0:1 more likely.

2) They are exactly the same engine with a cam change. The cam in the 350/290 is out of a high-compression 60s-era engine and is ill-suited to the low compression environment. The thing has no stones in the bottom at all, so it is a brick off the line without a pretty high rpm stall converter. It makes a great 50-90 shot, though.

(Yes, I've had both, in the same truck.)

3) Best performance in a truck is the 350/260 engine (cheaper) and change out the cam to a modern cam designed for the engine and its compression ratio. Comp Cams 12-300-4 (my choice -- love it), 12-230-2, 12-234-2, 12-235-2, something like that.

How you gonna use the truck? Towing? Mudding? Just general on-road driving? Mostly in-town, or a lot of highway? Answer those questions, and I can help you pick.

[Matt Man]

I got the 190 horse in my K10, great engine I get between 10-14 mpg with a new quadrajet carb and edelbrock performer EGR intake manifold and all emissions on it. Has a good amount of torque, and passing power on the freeway is good.

These goodwrench engines are really good value IMO.

[Gregski]

Originally Posted by Matt Man

I got the 190 horse in my K10, great engine I get between 10-14 mpg with a new quadrajet carb and edelbrock performer EGR intake manifold and all emissions on it. Has a good amount of torque, and passing power on the freeway is good.

These goodwrench engines are really good value IMO.

Thanks Matt, would you happen to know the part number for the heads that came with it?

[rich weyand]

They're 882s I think. Stock smogger heads.

[1970pelle]

You will get better MPG's with a engine with high compression. For what that's worth.


Ken

[MikeB]

Sometimes all higher compression will do is increase the chance of pre-ignition, especially in 60s-80s era iron heads with D-shaped combustion chambers.

[68c10owner]

I had a 290hp crate engine in a el camino and the heads were 993 castings. Those engines are the same with the exception of the camshaft. The 290hp crate engine actually ran pretty decent with my 4spd but needed 3.73 or 4.11 gears. It comes with the L82 camshaft that originally came in a 11:1 350. Agreed its not well suited for this engine but ran ok.

Increasing the compression will increase the HP and torque of the engine but with the right camshaft you can make a 8.5:1 compression engine run very strong. Its all about cam timing events. I would recommend a custom grand camshaft from Chris Straub or Mike Jones but some of the more modern grinds almost have to run better than the 40yr old L82 grand does. I know Mike Jones has a camshaft that has similar lift but less .050 duration that he sells to guys looking to upgrade their crate engines like these.

[TwoFiftyShifter]

If I wanted to get a crate motor on the cheap. I'd probably just get the 260HP motor and slap a late model Cam like the Lunati Voodoo line in.

Should give you a 300HP motor with strong torque.

[68c10owner]

Originally Posted by TwoFiftyShifter

If I wanted to get a crate motor on the cheap. I'd probably just get the 260HP motor and slap a late model Cam like the Lunati Voodoo line in.

Should give you a 300HP motor with strong torque.

Couple things to remember doing this,

First, changing cams will void the warranty but for $1500 its not a lot of money should something go bad.

Second, the 260hp rating is with headers, mild performance intake and carb. Still about the best deal your going to find. The smaller voodoo grinds would work well, possibly Comps 4X4 grinds as well though I`m not a huge fan of Comp.

[wilkin250r]

Lots of good advice in previous posts, so let's address your literal question. By that, I mean the title of the thread. Rather than specific engines and cam grinds, let's talk general rules.

Yes, increased compression generally mean increased horsepower, but there's more to horsepower than just compression (I'm sure you know that). While higher compression increases horsepower everywhere along the power curve, it's a bigger boost in the lower end. You can almost think of it as increased TORQUE rather than increasing peak horsepower, and will actually decrease horsepower for the entire RPM range past the peak of the HP curve.

Motorcycle racing back in the day, a popular (and effective) strategy was a really aggressive cam for high peak and high RPM horsepower, and then increase compression through the roof to gain back everything you lost on the low end.

[MikeB]

Lots of good info above. Cam event timing is indeed as important as static compression ratio, because making torque is all about cylinder pressure. And the sooner you close the intake valve, the more cylinder pressure you make.

As an example, many years ago when I lived at 9,600 ft elevation in the Colorado mountains, my 69 C10 with 3.08 axle had the base Goodwrench 350 with a 204/214 Summit cam that I had installed when I lived down at 600 feet. Even that entry-level performance cam was not a good choice for the engine's very low static compression (more like 7.8:1 if you do the math). At the lower elevation it worked OK, and let the engine breath better above 3,000 RPM compared to a stock cam. However, at 5,000-10,000 feet, it was WAY down on torque right where I needed it the most driving up twisty mountain roads.

On Crane's advice, I installed their #114112 cam with 194/204 duration and 104 LSA. Yes 104. That cam got the intake valve closed MUCH sooner than the Summit cam, and must have increased cranking pressure by 15-20 PSI. Torque from 1,500-3,500 RPM increased dramatically. Steep 30 mph mountain roads that had required first gear coming out of a 180 degree turn could now be taken easily in second gear. And I was able to pull a loaded 5x8 ft trailer over a 8,000 ft pass on I-25 at 70+ mph.

It was an amazing difference, and much more of an improvement than I had expected. The down side was the engine would "hit a wall" at 4,500 RPM, but in truth I rarely revved it that high before or after the cam swap.

On another note, when I moved back to Texas, Crane told me I might have to retard the cam (close intake valve 4 degrees later) to prevent detonation. But the only time I had a problem was on 100 degree days with the A/C running. So I started running higher octane gas in the summer.

Wow, this is a long-winded post! Hope it helps someone to understand the interplay between static compression ratio and cam event timing.