Thread: Comp Ratio check

Tweet

Just been reading some recent stuff on compression ratios, heads etc, and thought I would like to double-check cr on my combo.
I have a 454 .030" over (460 cu in), with Edelbrock Performer RPM-0 aluminum heads. These are quoted as having 110cc chambers.
We fitted TRW L2399-30 pistons which apparently are for open-chamber heads and have a .095 (height in inches?) dome. CR with 118cc heads is estimated at 9.52. Composition gaskets were used.
What cc's about would these domes displace??
As the pistons were designed for 118cc heads, would they have had to do any milling on the piston tops to fit the Edelbrocks?
Shop estimated about 9.8:1 - does that sound about right?
Sorry for so many questions, but if the cr works out less I may shelve my plans to fit a slightly bigger cam.

Thanks guys,
Chris

The effective cr calculator that techinspector mentioned is great! Worked for the Lunati cam figures that I have in the car at the moment. They are quoted at .050" - IO: 1 BTDC, IC: 37 ABDC, EO: 49 BBDC, EC: -3 ATDC.
BUT... the Comp Cams one (CB XM 284HR) I plan to install is quoted at .006" only. Figures are - IO: 32 BTDC, IC: 72 ABTD, EO: 79 BBDC, EC: 31 ATDC. These specs are for cam installed at 110 deg centreline. Duration at .050" is 230/236, advertised 284/290, LC is 112 deg.
Is there an approximate way to convert these figs to .050" ones, or do I have to contact Comp Cams themselves?

Originally Posted by grifo7

Just been reading some recent stuff on compression ratios, heads etc, and thought I would like to double-check cr on my combo.
I have a 454 .030" over (460 cu in), with Edelbrock Performer RPM-0 aluminum heads. These are quoted as having 110cc chambers.
We fitted TRW L2399-30 pistons which apparently are for open-chamber heads and have a .095 (height in inches?) dome. CR with 118cc heads is estimated at 9.52. Composition gaskets were used.
What cc's about would these domes displace??
As the pistons were designed for 118cc heads, would they have had to do any milling on the piston tops to fit the Edelbrocks?
Shop estimated about 9.8:1 - does that sound about right?
Sorry for so many questions, but if the cr works out less I may shelve my plans to fit a slightly bigger cam.

Thanks guys,
Chris

I found a chart on Flatlander's that gives the following c.r.'s ......
with 100.9 chamber 10.23
with 106.9 chamber 9.71
with 109.4 chamber 9.52
with 116.9 chamber 8.98
with 119.0 chamber 8.84

So it looks like 110 chambers should yield about 9.50:1

The problem with guessing like this is that you have no idea what deck height or gasket thickness Flatlanders used to attain these figures and you have no idea of the deck height and gasket thickness used in your motor. But for all practical purposes, I'd think 9.5 would be real close. I don't think you'd have to do any surgery to the pistons or chambers, the dome is only 0.095" tall.

Originally Posted by grifo7

The effective cr calculator that techinspector mentioned is great! Worked for the Lunati cam figures that I have in the car at the moment. They are quoted at .050" - IO: 1 BTDC, IC: 37 ABDC, EO: 49 BBDC, EC: -3 ATDC.
BUT... the Comp Cams one (CB XM 284HR) I plan to install is quoted at .006" only. Figures are - IO: 32 BTDC, IC: 72 ABTD, EO: 79 BBDC, EC: 31 ATDC. These specs are for cam installed at 110 deg centreline. Duration at .050" is 230/236, advertised 284/290, LC is 112 deg.
Is there an approximate way to convert these figs to .050" ones, or do I have to contact Comp Cams themselves?

Since we have no way of knowing how quick or how slow the closing ramp is, you'll have to get the info from Comp. Most cams today are ground asymetrically (opening ramp is different from closing ramp). Assuming a closing point of around 40 degrees ABDC, the DCR would be 8.161:1 with that cam.

Have you done a compression check with the current cam in place? Remove all the spark plugs and wire the throttle primaries open. Disconnect the coil. Run each cylinder through at least 4 to 5 cycles so you get a good reading. Then post your findings on here please. I'm curious to see the correlation between DCR and cranking compression. Thanks.

Thanks for your replies, Richard!
I will do a compression test, as soon as I get a comp gauge, and let you know the results. These will be with the current Lunati 50247 hydraulic roller cam.

Wonder what cc's those .095" domes would displace?

I will contact Comp Cams for the .050" cam specs.

Thanks again, Chris

You could figure out the dome cc's by placing the piston down the bore lets say one inch, then cc it, then compare it to a flat top piston cced one inch down in the same size bore, then you have your dome cc, not to sure on this, hope it helps

Originally Posted by chevy427tall

You could figure out the dome cc's by placing the piston down the bore lets say one inch, then cc it, then compare it to a flat top piston cced one inch down in the same size bore, then you have your dome cc, not to sure on this, hope it helps

Exactly right. That's the way I explain it in this paper I wrote last year....
http://www.streetmachinesoftablerock...opic.php?t=124

Thanks, guys.
No way of measuring it, as that engine is not coming out again for a long while, having spent a fortune on the rebuild!
Should have clearance for installing a new cam, once the rad is removed, though.
Chris