ricekiller28

just trying to get an idea of what kind of torque and hp ratings i would be getting with 8.5 comp on a 383 forged LT1 with 847 cam, fully ported heads with 1.6 rr, ported intake, longtubes, cai, and other little stuff. I got a rear mount turbo, but wondering what kind of power is it making without it? Is that a good cmp ratio for a twin tt running bout 20 psi or more? Does low comp decrease hp n torque significantly?

rskrause

See

engineermike

You can figure about 5% lost for every full point of compression. If the engine should have been 450 hp at 11/1, then it's probably around 395 at 8.5/1.

rskrause

Way less than 5%, see the chart.

Rich

engineermike

I got that 5% from an old Vizard book. Obviously, it varied depending on the starting point. Going from 7.5 to 8.5 netted more than 5%, but going from 15 to 16 netted much less.

My own personal experience came from a short block swap I did several years ago. I had a 3rd gen that was 360 cid and 8.5/1 compression, running 12.10's at 113 mph. I kept everything else the same, but went to 383 cid and 11.5/1 compression. The car then went 11.30 at 120 mph. The math says I went from 380 hp to 455 hp with +23 cid and +3 points compression. From everything I've seen, the extra stroke only got me 10 hp or less, which leaves 65 hp from compression. Doing more math says that the compression increase gained right at 5% per full point compression increase, which lines up very well with Vizard.

Just playing devil's advocate, lets say the cid increase gave me a proportional hp increase. So, going from 360 to 383 cid would get me from 380 to 405 hp. That leaves a 50 hp increase from the 3 points of compression alone. This is still over 4% per full point.

That's why I just call it 5%/point.

Mike

rskrause

The chart is from Vizard's book and it is pretty accurate based on my experience. You did not just change the CR so I think your example does not enlighten the discussion.

Rich

engineermike

Yes, I made 2 changes at once, but the cid change was small and we have a good idea of the amount of power that's worth, so it's not too hard to segregate the 2.

Furthermore. . .

According to AI, "if we change the compression alone (independent of quench) then you'd see ~6-9hp difference for .4:1 lower compression." Assuming a 450 hp motor, doing the math tells us that 1 point is worth 4.2%.

MM&FF says, "every point increase in compression is a 4% increase in power".

This was all posted in the other thread, but the fact that my results agree with theirs tells me I did a good job of separating the effects of cid versus compression.

Edit: It has been proven that changing more than one variable at a time can get good experimental results. Do a search for "Dynamic Matrix Control", or DMC.

Mike

rskrause

Well, let's agree to disagree.

Rich