Compression Ratio vs CID

[TT350]

The is one that’s I’ve been thinking for some time
please feel free to correct me if I’m wrong.

Ok here goes if you have two cylinders
One being 1 gallon in total volume
and the other being 10 gallons to total volume.

Now both cylinders have a compression ratio of
10:1
It seems to me that the larger cylinder will produce
more PSI than the smaller one.

Now with all that said more psi = more heat,
more heat = more likely to detonate.

So where I’m going with this is you can run a
higher compression ratio on a small CID engine
vs a large one if your using the same fuel.

I hear of a lot of 9:1 and 9.5:1 turbo pump gas motors
but most of them are small CID, will the 9:1-9.5:1
work on large CID motor on pump gas.

 

[myclone]

Re: Compression Ratio vs CID

Im a dumb ass for even thinking about replying

 

[SeattleSy#1255]

Re: Compression Ratio vs CID

It seems to me that the larger cylinder will produce
more PSI than the smaller one.

Is this simply because their is more air (volume) being compressed?


Is this basically how these small 4cyl. or 6's like Supras get their power...they use small bore and small stroke = less crank stress but = less power....thus they throw 30-40psi at it to make up for the power loss (with their 6 bolt bottom ends)?

 

[Don W.]

Re: Compression Ratio vs CID

you're correct for the wrong reason. 10:1 compression is 10:1 compression regardless of displacement. You'd get the same pressure reading on both the "large" and "small" cyls. if you ran a compression check. (Assuming identical valve timing.) CR is nothing more than the ratio of the total cyl volume (including combustion chamber) at bottom dead center compared to the volume at TDC. Smaller cyls can run larger CR's because of other factors like flame front travel speed. Once the motor is running you have other factors of course which add up to an "effective compression ratio." Very low at idle for example and going up from there as throttle and eventually boost is added. Also valve timing and volumetric efficiency to consider as well.

According to one chart I found 8.5:1 static with 16 lbs of boost gives an effective CR of 17.8:1 No idea how accurate this is.

 

[tippmann243]

Re: Compression Ratio vs CID

most of the smaller hi comp motors being turbo'ed also have aluminum heads, that also plays a role in detonation on a given octane rating

 

[TT350]

Re: Compression Ratio vs CID

most of the smaller hi comp motors being turbo'ed also have aluminum heads, that also plays a role in detonation on a given octane rating

Hotrod magazine done a test with steel heads
and aluminum and found the to be and old wise
tail .

I realize that cam profile plays a major
roll in cyl psi, All other things being equal
but the cyl volume.

 

[0260IN3]

Re: Compression Ratio vs CID

you're correct for the wrong reason. 10:1 compression is 10:1 compression regardless of displacement. You'd get the same pressure reading on both the "large" and "small" cyls. if you ran a compression check. (Assuming identical valve timing.) CR is nothing more than the ratio of the total cyl volume (including combustion chamber) at bottom dead center compared to the volume at TDC. Smaller cyls can run larger CR's because of other factors like flame front travel speed. Once the motor is running you have other factors of course which add up to an "effective compression ratio." Very low at idle for example and going up from there as throttle and eventually boost is added. Also valve timing and volumetric efficiency to consider as well.

According to one chart I found 8.5:1 static with 16 lbs of boost gives an effective CR of 17.8:1 No idea how accurate this is.

Don W is 100% correct. We have found (as well as many others) that bore size plays a big role in how much compression you can have with a specific type of fuel. A 4.50 bore needs to have less compression than a 4.00 bore to run the same fuel because of the distance the flame must travel in the cylinder. Also piston shape (domed vs. flat top) also plays a role in these same situations. A dome piston is actually less efficient than a flat top becasue the dome actually gets in the way. That is why when ever possible, get the most compression you can with the least amount of dome (or a flat top / dish piston)

 

[tippmann243]

Re: Compression Ratio vs CID

Hotrod magazine done a test with steel heads
and aluminum and found the to be and old wise
tail .

I realize that cam profile plays a major
roll in cyl psi, All other things being equal
but the cyl volume.

im talking about iron vs aluminum

 

[Don W.]

Re: Compression Ratio vs CID

That is why when ever possible, get the most compression you can with the least amount of dome (or a flat top / dish piston)

I seem to remember, prolly still out there, domed pistons with a notch so the flame front could get across the chamber so-to-speak.

 

[0260IN3]

Re: Compression Ratio vs CID

I seem to remember, prolly still out there, domed pistons with a notch so the flame front could get across the chamber so-to-speak.

yes I think there are still versions of that available