Advice on Compression Ratio needed

[wyrmrider]

They should have lived in the Fordgedtrue days, way before all the multi axis skirt grinds

 

[66Satellite47]

They have three different combustion chamber configurations. 88cc, 84cc and 75cc. Only the 88 is an open chamber. 84 and 75 are closed. The 60919 is the 84cc closed chamber. Don't know how that corresponds to casting numbers. Would be interesting to know.

OK, got the 88 cc's then.

 

[wyrmrider]

let's start over
88 cc open chamber heads
edelbrock says "88cc chambers are machined .100" for clearance with quench dome style pistons"
so check out the Icon and Keith Black quench dome style pistons and compare with your pistons
how are you planning on getting some quench?

 

[66Satellite47]

let's start over
88 cc open chamber heads
edelbrock says "88cc chambers are machined .100" for clearance with quench dome style pistons"
so check out the Icon and Keith Black quench dome style pistons and compare with your pistons
how are you planning on getting some quench?

Yup, my EDE RPM's have .100" open chamber. Look like a 906 iron head. The casting number tricked me. Ran the numbers & my 500" motor has 10.5 CR and runs fine on 91-92 pump gas.

 

[wsutard]

Thanks for all the replies everyone. Now I have a question about calculating out these compressions.

Question about piston CC sizing. When a piston says it has 4.4 cc's does that mean it is "taking away" or "providing" that many cc's to the combustion chamber?

 

[wsutard]

Also, I will be going with molnar rods which state 6.76 for the length. The Icon pistons I want to go with state 6.768 for the rod length. Does the .008 difference in the "rod length" numbers mean anything?

 

[lewtot184]

the only way to truly calculate compression is cc the head, know the gasket cc's, and do a down fill in the cylinders.

 

[Garys1969RR]

At .039" piston to head clearance, you are fine. I'm running .030" and no issues, and the motor runs really nice! I've spun it to 6000 Rpm. I've known guys to run .020"! I would stick with what you've got. Mock up the engine with a piston or 2, no rings. Set it to TDC, and check it. Let us know what you have.

 

[66Satellite47]

Also, I will be going with molnar rods which state 6.76 for the length. The Icon pistons I want to go with state 6.768 for the rod length. Does the .008 difference in the "rod length" numbers mean anything?

The 6.768 is the true "stock" rod length. FSM states 6.766 - 6.770 is standard. Also the 4.4 cc's I presume is the valve notch volume for a flat top piston. Therefore it's an ADD to the chamber volume. BTW, I just measured the valve notch volume on my flat top Ross pistons this morning at 7 cc for the two nothes. Also lewto's method of the "fill down" volume is the only accurate way for a dome piston with valve notches that may have been flycut for a big cam or larger than stock valve head diameter.
I did run a zero deck, .020 gasket & 915 head, steel rod combo with no problem. .039 is just fine.

 

[wsutard]

OK forum members, debate this. The UEM (they make Icon and KB pistons) compression calculator asks for 15 degrees of timing to be added to the cam spec. So in my instance it goes from a timing of 44.5 to 59.5. This extra 15 degrees results in a 8.5 DR. Other compression calculators do not ask for an extra 15 degrees and thus output a DR of 9.28.

Which should I believe/use? Add 15 degrees or not?

 

[wyrmrider]

15 degrees is a crutch estimate for solid lifter cams
it actually depends on the lash and intensity of the ramps think of tight lash vs 30 30 Duntov
if you want to do it more right calculate duration at valve open .006
dial indicator on the retainer

 

[BSB67]

if you want to do it more right calculate duration at valve open .006

Why is that more right?

 

[HEMI-ITIS]

I don't do the numbers,I have smart guys do that for me.
I would like to add water/meth injection into the conversation.For you on the edge or over the edge,W/M with premium pump can support demands up to 112 octane.When I was reading into it,sounded too good to be true untill I spoke to a few guys at the track and the stuff works.If you buy the special sauce with more meth,it can add a few ponies.
I was looking into the W/M in case I needed to lower the temp in my intake manifold at WOT.

 

[wyrmrider]

Why is that more right?

Because it takes the valve lash variable out of the problem
say you spec a solid cam at 290 @ .020 but your lash is .016 or .030- in one case your duration is going to be more than advertised, in the other shorter
now try and compare with a different company with a completely different approach - trying to compare at .050 cam lift has the same problem
.006 at the valve is the SAE recognized way of measuring- also takes rocker ratio into consideration

 

[66Satellite47]

From my understanding, the use of the .050 duration was adopted to give a uniform point which would apply to most critical area of cylinder head flow and discount the wide variability of acceleration and deceleration ramp rates from the different cam manufacturers. For the compression calculators, by adding the lower lift duration in, a better approximation of the cylinder pressure is obtained. The .050 duration & valve timing is commonly given by most cam manufacturers rather than valve timing at some other tappet lift.

 

[BSB67]

.006 at the valve is the SAE recognized way of measuring- also takes rocker ratio into consideration

Really.

Anyways, this is what I think we do agree on, the "dynamic compression" calculations that are the rage these days are marginal at best. The calculation is fine, its the inputs. At what point does the cylinder actually start to build pressure, and like mentioned above, where exactly is the VALVE at that point. As pointed out, lobe profile, hydraulic verses mechanical, rocker ratio, and lash setting impact where the valve is and not some number for the cam lobe. And when some folks use these calculators but don't understand the finer points of what it is attempting to show, they will get a result but possibly draw an incorrect conclusion.

If you look for dynamic compression calculators online you will fine at least 4 different ways of generating a result, and the result they generate are all different. So it is funny when folks say stuff like 8.5 is the DCR limit. It could be, but would be more of a coincidence than exact science.

When you know something about the cam, near seat valve timing events, post engine assembly compression testing and success and failures regarding detonation, then you start to have a feel on how to use the results from the DCR calculators, and understand that it is not the ultimate final answer, but just one of several thing to consider when making compression ratio/cam choices.

 

[lewtot184]

I don't get into the dynamic compression thing much. to me it's a moving target. what's going on at 2000rpm isn't the same as what's going on at 5000rpm. what's going on at the valve is more relevant to me; the lobe is just the first step to the valve. I agree with BSB67 about these calculations being all over the map.

 

[IQ52]

OK forum members, debate this. The UEM (they make Icon and KB pistons) compression calculator asks for 15 degrees of timing to be added to the cam spec. So in my instance it goes from a timing of 44.5 to 59.5. This extra 15 degrees results in a 8.5 DR. Other compression calculators do not ask for an extra 15 degrees and thus output a DR of 9.28.

Which should I believe/use? Add 15 degrees or not?

When using the UEM calculator, and the .050" timing, add the 15 degrees. Other calculators like the Wallace, use the advertised cam duration and the +15 degrees is not used as it is already figured into the calculation.

If you are using the UEM calculator and the timing @ .006 lift, do not ad the 15 degrees.

Dynamic compression is figured from when the intake valve closes. At .050" lift the valve is open .050" x the rocker ration, be it 1.5, 1.6 or whatever. The UEM calculator wants you to add 15 degrees to the timing and that estimates when the valve physically closes and compression begins to build.

Hopefully this makes some sense.

 

[wyrmrider]

you're better off estimating than using a fixed 15 degrees for a solid lifter cam
if your lash was .015 the lifter would rise .035 at .050 and with a 1.5 rocker over .050
if it was .030 it would only rise .020 and with a 1.5 rocker the valve .030
comparing cams solid cams using .050 gives only a really rough estimate
.050 is useful for degreeing in cams, that's it

 

[IQ52]

you're better off estimating than using a fixed 15 degrees for a solid lifter cam
if your lash was .015 the lifter would rise .035 at .050 and with a 1.5 rocker over .050
if it was .030 it would only rise .020 and with a 1.5 rocker the valve .030
comparing cams solid cams using .050 gives only a really rough estimate
.050 is useful for degreeing in cams, that's it

Yep, solid lifter cams and hydraulic cams with the same duration at .050" lift are two different animals in my opinion.