How much difference can more compression make?

[grander]

Hi All,

Just a question on how much difference or power more compression can make?

I have recently stripped down the top end of a 360 LA where all measurements calculate out at a comp value of 8.73.

This made 280hp with a Crane f248, 248/[email protected]",110 lobe sep, .540/.560" lift with a 1.6 rocker and old torker manifold and 750 DP holley.

Other than that it is completely stock.

SO, my question is what sort of increase in power potential could I see if the CR was raised to something like 10:1 or similar?

Thanks.

Graeme.

 

[PurpleBeeper]

Total guess?......maybe 30-40 horsepower

 

[91r/t]

Not sure exact numbers wise, but look at the mid seventies 440's. They made alot less power compared to a 70-71 and they had 452 heads which were 906 heads comparably with hardened seats and flow pretty dang good. Sure they had a few differences and minor changes, but in 70 it had 375 hp. and in 78 it had 255 hp in the police interceptor package cars. Thats quite a loss that can be contributed to a decent compression drop. And yes before people freak out, other **** was taken away, but mostly compression. **** even in 73-74, the engine had 280ish hp. which was way lower than the 375 hp a couple years prior. Compression is a decent power builder. I think the motor home engines were like 8.1:1 or something like that. Man they were slugs compared to the good old days but still made decent torque compared to other engines of their time. Compression isn't the only power builder at all, but it sure can make some ponies appear for sure when added.

 

[wyrmrider]

Get real
they went from gross to net horsepower
however
all late engines have the same low compression as motor home engines but really below 8:1
real slugs compared to earlier engines and they run hot
and they had to run the high stall converter even with the standard cam
and run hotter with the magnum cam- real dogs but stock and magnum cams act like extra EGR for smog with really long closing ramps
late TQ manifold is better than early square 4 holler but heavy TQ is still the best single carb (with very few exceptions)
different cam, quench pistons like the KB's can lower EGT by 800 degrees
no more glowing cherry red exhaust manifolds
note that the more you drop the piston compression height the more throttle you have to give it to make the same torque which equals more heat and less gas mileage

 

[Lefty71]

Google compression ratio calculator by Pat Kelley. I don't know who he is, but he has free online tools to help you calculate dynamic compression which is far more important than the static compression you are asking about. Hopefully you enjoy some math. Put your inputs in very. very carefully, and in the end you'll have a clearer understanding of the relationship between cam timing and duration, and their effects on your motors compression. There are also horsepower and drag strip tool calculators out there, which can help you determine what your build can be, and therefore will cost, before you start turning a wrench. HTH, Lefty71

 

[YY1]

Post #3

You can't compare gross HP to net HP that way.

 

[IQ52]

Horsepower? I think it is fun to play with this calculator.

http://www.wallaceracing.com/hp-cr-chg.php

If correct, a whole 4.78 HP gain. Whoopee!

Spend the money on cylinder heads.

 

[slepr1]

You'll be happy with higher compression.

 

[727]

Higher Compression will open up a whole new list of cam choices.

 

[rumblefish360]

More compression is good to a degree. It helps make for efficient engine operation when the cam gets bigger. At your measured 8.73-1, I’d say you could use another 3/4 in ratio more (Max!) of compression to help the engine make better torque with that cam.

Roughly, 1 pt. Of compression is 3% in power. But it’s more about efficiency. How everything works together as a package. Balancing out the current package would be expensive on the lower end front.

IQ52’s mention of cylinder heads is a better bang for the buck in the long run. You’ll make more power from a well done cylinder head than upping the compression ratio by far. You could most likely gain a little bit of compression through a cylinder head milling.

FWIW, I would NOT! exceed a 9.5-1 ratio with (that cam &) iron heads and pump gas. Your actually better off slightly low on ratio than anything else on the whole engine.

Long ago I built a 9.8-1 iron headed 360. The cam was a 248@050 MP 292/.509. If the fuel I got that day was old, not up to par in its rating (USA 93) it would ping and run poorly. The car was also a 4spd w/4.10’s FWIW.

IMO, having done this a good bit for a long while with 360 engines, mill your heads flat to be sure they are and for a 1/4 pt. raise in the ratio and run the top grade fuel. Switch to a Edlebrock RPM intake to gain a noticeable improvement in torque with a zero HP loss. Use improved valves @ 2.02-1.60 and have your heads ported. Or move to an aluminum head like the new Trick Flow. Use a 1.6 roller rocker with it all to take advantage of the heads high flow capabilities.

Your gonna love what it does to the cars performance.

 

[biomedtechguy]

Horsepower? I think it is fun to play with this calculator.

http://www.wallaceracing.com/hp-cr-chg.php

If correct, a whole 4.78 HP gain. Whoopee!

Spend the money on cylinder heads.

I was going to invite your comments, but I see you beat me to it.
I thought of how much success you have had making BIG power with "low" compression.
To add my opinion, if an engine is riding the edge of detonation, that could mean damage and less reliable performance if the octane and fuel quality varies. Not worth it for a street/strip car.

 

[grander]

Thanks all for the thoughts. Some interesting info. I am actually going to build the motor "correctly" with matching quality parts , good heads, manifold etc. And of course more compression. Still undecided whether to keep the same sort of cam, currently the motor easily revs to 7000. And maybe a stroker too. Parts in Australia aren't cheap but will be in the in the ball park of about $11-12k...

 

[Triple Black 73]

Cylinder pressure makes power, not compression ratio. You're not making power because your valves are staying open way too long. By they time the valves closes, you have very little compression to increase the cylinder pressure. This is called dynamic compression ratio.

A cam that closes the valves sooner will make more power, or you can force more air into the cylinder, or you can increase the compression ratio, but anyways you cut it you'll need 12.5:1 and aluminum heads for that cam to make big power at high rpms.

What you really want is the max PSI for your giving application based on the compression ratio. You want to be as close to 165 psi for iron heads and 220 psi for aluminum heads. The different is that iron will retain more heat which leads to detonation at lower PSI. If you swap heads to aluminum, you need a cam to match the cam. In a prefect world you match the cam, pistons, and heads to build the max cylinder PSI (without blowing it up) at an target RPM range, then worry about the intake and exhaust flow based on that target RPM range

 

[rumblefish360]

Good job. Well said and written.

The mistake most people make with this is the close relationship of the static compression ratio and the working, in motion dynamic ratio coupled to the actual pressure within the cylinder with the lack of understanding of how a cam effects and changes the ratio. This also can change the octane requirements very quickly.

It is a little tricky. It’s something I don’t get into great detail about because it gets messy and confusing really quick. There is a balance in there that has a certain amount of give and grey area that can be covered by the fuel octane and band-aided with timing, which is mostly a loss of power.

As a basic rule, bigger the cam, the higher the compression ratio to bring up the dynamic ratio to properly run.