IIRC, a .100" cut on the head only will give you about 9-1.
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| Compression Ratio = HP?? https://slantsix.org/forum/viewtopic.php?t=13502 |
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| Author: | Slanted Opinion [ Tue Jun 28, 2005 7:16 pm ] |
| Post subject: | Compression Ratio = HP?? |
Hi everyone, Virtually all the performance engines from the 60s ran CRs of 10:1 or better. Is shaving the head on the slant six a good way to increase horsepower? Do I remember correctly that a .100 cut will bring the 225 up to 10:1? How high can I go on 92 octane pump gas? Finally, what about knock control systems, like water injection, etc? Thanks for the input! -Mac |
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| Author: | emsvitil [ Tue Jun 28, 2005 7:40 pm ] |
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With iron heads, and no quench to speak of (even with a .100 cut), I think 10:1 is too high even with premium................. I personally wouldn't go past 9.3:1. |
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| Author: | slantzilla [ Tue Jun 28, 2005 7:59 pm ] |
| Post subject: | |
IIRC, a .100" cut on the head only will give you about 9-1.
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| Author: | Dart270 [ Tue Jun 28, 2005 9:23 pm ] |
| Post subject: | |
0.100 cut will give you somewhere between 9 and 9.5 depending on gasket thickness and actual piston-deck ht. Very cheap way to get high compression. I have run up to 11:1 on 92 octane, BUT with a long duration cam (240 @0.050" min) and careful timing curve. This is on the edge, but is doable. Lou |
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| Author: | 70valiant [ Wed Jun 29, 2005 2:54 pm ] |
| Post subject: | |
I took 0.100" off and got a calculated 9.8:1 ratio. I run 93 octane at 3* inital timing. With the 3.9 gears and only a BBD I was turning 16.35sec at 82mph in the 1/4 |
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| Author: | Russell Cable [ Wed Jun 29, 2005 3:17 pm ] |
| Post subject: | |
I got .115 cut off the head + .030 off the block with a cam that is 230 @ 50 . And I use 91 gas with no truble on the street and at the track. |
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| Author: | ceej [ Wed Jun 29, 2005 11:14 pm ] |
| Post subject: | |
Depending on the cam used, starting with a static compression ratio of, say, 9.5:1 on a stock 225 bore, stroke and rod length, I end up with a dynamic CR of 7.88:1 with an intake valve closure at 60 deg ABDC. (Comp Cams 264S) If the valve closes, just for yucks, around 45 deg ABDC, it raises dynamic CR to 8.58:1 More delay in valve closure results in less dynamic compression. If we take it down to 20 degrees ABDC we get around 9.32:1 With the cam I chose the Inlet Valve Mach index is 0.534 with the stock valve size, turning faster than I should. At an Inlet Valve Mach index greater than 0.6, volumetric efficiency falls off rapidly. This means my engine breaths much less than it's displacement. To fix this, we open the valve further, longer, install a bigger intake valve, or just spin the engine slower. I won't get it all back though. And the cam isn't designed to peak at the RPM I want. With larger Lobe center angles we get less overlap, tightening them creates more overlap. The 364S has 48 deg of overlap, which is rather on the extreme side. The Lobe center angle is 110 Degrees. More overlap moves the HP up to higher RPM's. 20-30 deg is more street-able. In the 50-60 degree range, we end up running with DD down the drag strip.  If you run at high altitudes, you can run your compression up higher than those at low altitudes. The dynamic compression is inversely effected by altitude and air temperature. Darn. I've got too much cam... And with that overlap, I won't be making HP where I want it. Lower RPM. Hmmm. Good thing I researched all this after spending money. CJ |
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| Author: | emsvitil [ Thu Jun 30, 2005 1:15 am ] |
| Post subject: | |
Quote:
With the cam I chose the Inlet Valve Mach index is 0.534 with the stock valve size, turning faster than I should. At an Inlet Valve Mach index greater than 0.6, volumetric efficiency falls off rapidly.
CJ What exactly is inlet valve mach index and how do you compute it???? Sounds like it deals with port velocity in some way....... |
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| Author: | ceej [ Thu Jun 30, 2005 8:32 am ] |
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You are correct. I used a piece of software online. If you search google for cams and lobe seperation, etc.. there are some online calculators out there. Arm yourself with all your specs on the engine and your cam. CJ |
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