Okay, so let's say 8.0:1 DCR is the safe zone for 93 octane. Let's say a good SCR for that is about 10.5:1. Well, if DCR is pretty much what dictates octane, then why would it be bad to have as high as SCR as I want? But for the sake of not sounding totally stupid, why couldn't one have 12:1 SCR, for example. When the SCR gets too high, does that kind of 'override' DCR on deciding detonation on what octane?

Also, before I order pistons and cam and decide compression, wouldn't the right thing to do be have my machinists do my headwork first? I'm thinking he'll put bigger valves and then have to open the chamber up a bit (781s), which would affect compression a bit, right? So, wait?

Trying to get the compression calculator to work. But, how can I determine LCA? Like, I'm just picking cams from Comps website to guess and test, and the catalog has LSA, but not LCA.

Will an adapter plate be horribly detrimental to my engine? Spreadbore carb on Squarebore manifold.

Sorry lots of questions in one post! Thanks!

You could have 12:1 and a pump gas DCR, like 8.0-8.3 or so. The draw back is you would need a quite large cam and all the associated support parts to go with it (gears, strong bottom to support the rpm the cam would provide, appropriate converter stall, and heads that would support the cam). The wild card with very high CR is there are other factors that can cause detonation. At the high rpm high compression implies, the cylinders are being packed with mixture and the cylinder pressures can get very high. Under 7000 or so this is less of a problem.

Lobe Centerline Angle (LCA) is the same as Lobe Separation Angle (LSA), just a different name. I like LSA because it is less likely to be confused with Intake CenterLine (ICL). Sometimes it isn't given directly, add the ICL and the ECL and divide by 2.

If you are interested in pursuing more knowledge of cams, I recommend David Vizard's book on the subject.

Do the head work first.

Bruce Crower had a matched engine set that did just what you are talking about years ago..

I think it was like 14 to 1 CR. SCR with a cam that bled it down to 9.5 to 1 DCR.

The theory is that you want the high CR. to sweeze h-ll out what little the cam allows to still stay in the motor at TDC dueto it's extremely late intake valve closing event.

But I think the consensus was that at part throttle, it didn't work worth a hoot b/c of the throttling effect the carb. had on the overall suck, (choked the suck even more), which really messed up midrange power production..

I have the old mag. with the article in it some where in one of my boxes..

pdq67

Originally posted by Pat Kelley:
You could have 12:1 and a pump gas DCR, like 8.0-8.3 or so. The draw back is you would need a quite large cam and all the associated support parts to go with it (gears, strong bottom to support the rpm the cam would provide, appropriate converter stall, and heads that would support the cam). The wild card with very high CR is there are other factors that can cause detonation. At the high rpm high compression implies, the cylinders are being packed with mixture and the cylinder pressures can get very high. Under 7000 or so this is less of a problem.

Lobe Centerline Angle (LCA) is the same as Lobe Separation Angle (LSA), just a different name. I like LSA because it is less likely to be confused with Intake CenterLine (ICL). Sometimes it isn't given directly, add the ICL and the ECL and divide by 2.

If you are interested in pursuing more knowledge of cams, I recommend David Vizard's book on the subject.

Do the head work first. =
And to add a little more confussion from stuff I have posted before,,, :D

With a big cam you will see lower cranking compression but you also have to use a little common sense with the 180 cranking is oK for pump gas syndrome
It may have low cylinder pressure at low rpm's but somewhere the engine should start getting efficient & the engine is going to see whatever compression you have.
If the fuel isn't good enough when this happens ugly things can & eventually will happen
If your plan is to use it on pump gas most of the time I would put it at 10.-10.5 for 91 octane & 10.5-11 for 93 & change the cam accordingly.

Or,,,,,,
Put slightly differently,,,,
Here's another thought to screw up the works,,
You can build an engine with say 13.0 & overcam the crap out of it & on paper the dcr will tell you it will run on 91 octane.
The problem comes when that engine gets up in the rpm & becomes efficient & starts actually seeing that 13.0. Now that dcr becomes less of an issue as you are starting to keep pressure in the cylinder & it will detonate.

It also works the other way.
We do restricted engines you just could not put a load on at low rpm in most cases.
Small cam hi compresson deals like 13.5+ with a cam of say 240 or so @ .050 & at low speeds they will detonate themselves to death if you ain't careful but from say 3500-4000 up they will never see that 13.5 compression as the intake is too small to let enough air in to fill the cylinders :(
Is this enough confusion yet??

Let's add some more confusion and variables to the equation............My 11:1 motor shows no sign of detonation, runs cool and has 225 cranking psi. Now, if I were to swap out my 4 speed for an automatic and then replace the 4.10's with 2.73's,......it would probably ping itself to death. So there is more to just the motor for determining detonation. Gearing, how much load on the motor, weight of the vehicle...

Wolfplace... an EXCELLENT explanation of why DCR is not the final word. smile.gif

good info on this subject, so I guess detonation at higher engine speeds does occur, I've been told in the past that at high engine speed detonation can't happen due to less combustion time per revolution.