Compression Ratio Question.......

[Oldschool]

Hypothetically speaking, let's say you have a 500CI engine that makes 500 HP with 10.5-1 compression. Using that same engine and assuming everything else remains the same, how much would the HP change if you took the compression ratio up to 11.5-1? Also, would the torque change much? I have a buddy running a 496 in a Charger and he is looking at options over the winter. Fuel is not an issue as he will buy higher octane fuel if needed. Thanks fellas.....

[cudabob496]

 I'll guess 25 hp

[Oldschool]

I'll guess 25 hp

That's kind of what I was thinking. Torque guesstimate?

[cudabob496]

That's kind of what I was thinking. Torque guesstimate?

not sure about torque.  But with a longer duration cam, and higher CR, hp gain would be quite a bit more.
I know the old wedge engines were advertised to be 20 hp more when going from
11.5 to 13.5 CR.

found this:

http://bgsoflex.com/crchange.html

[cudabob496]

I've read that in the old days, before there were all these high tech cams, they
made more power by increasing CR.  But now, CR its not that big of a deal, when
they can match a performance cam to your existing CR.

[Strawdawg]

Peak hp will increase about 2.2%.....say 11 hp.  You can find a table without much searching.

Torque improvement will depend upon the cam duration so it could be a bit more percentage size,

There are a lot of low compression engines..say around 8-1 that make very high peak hp at high rpm with good heads and a big cam.  They usually need a much higher stall converter to get moving, tho.


Power increase is a sliding scale so that the gain from 8 to 9-1 is a larger percentage than 12-13-1 would be.  Also, the increases shown in the table are for iron heads.  Aluminum heads, being less thermally efficient with regard to combustion, will gain slightly more going from 10 to 11-1 than would iron heads.  We are talking about a fraction of a percentage point given that all other things are equal other than head material.

[HP_Cuda]

Way too many variables but in large part depends on the cam, cooling but jumping one point might get you 20hp.

Heck you could outdo that with a better exhaust system.

[moper]

I think it's more of a "when" as oposed to "how much".more compression will bring the torque rpm peak down, and flatten the curve. It will add a little, but not that much. You'd get more from changing mufflers. but it will feel more snappy.

[Racer57]

Compression comparisons don't mean squat unless both engines have the exact same camshaft.

[cudabob496]

The initial question assumed everything else was the same.

[Strawdawg]

Hypothetically speaking, let's say you have a 500CI engine that makes 500 HP with 10.5-1 compression. Using that same engine and assuming everything else remains the same, how much would the HP change if you took the compression ratio up to 11.5-1? Also, would the torque change much? I have a buddy running a 496 in a Charger and he is looking at options over the winter. Fuel is not an issue as he will buy higher octane fuel if needed. Thanks fellas.....

It always helps to read the original question.... 

Or read Bob's post above....

[Racer57]

I was only commenting for those that may not know about camshafts effecting compression.

[cudabob496]

no problemo