Dynamic Compression Ratio: Solving the Mystery with Excel

[911st]

My guess is, how the intake side of the cam is actually timed is what effects dynamic compression, not the lob spacing per say. One might widen the lobes but the timing of the intake valve on the compression stroke could end up the same, more, or even less.

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Why do we care about dynamic compression? Is it important or just a byproduct of a given cam design.

Dose it actually result in higher cylinder pressures?

Dose lowering DC make a motor less sensitive to detonation?

On a low compression 930 the stock cam has about a point higher dynamic compression than an SC cam so we would think it would make more pre-boost power. However, most seem to agree that going with the SC cam improves pre-boost power.

Why is that?

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Also, why or how dose increasing compression add power?

Is it because it accelerates the burn rate and with a faster burn rate, ignition can begin later during the end of the compression cycle and thus create less drag.

And or is it because with higher compression, more exhaust it pushed out of the cylinder so there is less potential dulluition.

And or, is it because with more compression comes more vacuum or suction during the intake cycle and more air & fuel is thus drawn into the cylinder.

I am guessing and do not know if this is true or if one is the prime advantage.

What is cam lift? From my crude calculations, by the time the valve is about half way open it is already open enough to match port size as to volume on most 911 motors. Even very a modest amount of lift and larger port.

Is lift more about making a steeper ramp to get the valves open quicker and close faster so they achieve the ports flow potential longer?

I do not know if any of this makes sense as I am guessing at this. Any help understanding would be appreciated.

Thx

[IXjamesXI]

I don't know what forum etiquette is for bringing back some of these old ones from the dead, but here goes. I suppose it will just be ignored if this is something people stopped being interested in..

Anyway, I am working on a 3.2 short stroke, all the normal specs with the addition of EFI, twin plugs, and 10.5:1 CR. Probably going with a lower overlap cam just to make EFI easier to tune, but I was told that I should watch out for high cylinder pressures because of that. I have been playing with this spreadsheet and I also added on mine a section to calculate an estimate of dynamic compression at different RPMs as suggested by petevb in a previous post in the thread. I guess it shows you the progression of compression as revs increase, and also how the intake closing time can affect ultimately how much compression you make and when in the rev range. But where the comp ratios peak in my chart isn't in the range where I would expect to find peak VE.. I would imagine peak VE is where torque peaks. The peak compression ratio numbers in my chart line up with the RPMs where I would imagine peak power would be observed.

The chart of effective compression ratios is also the same for a 993 supersport cam and a DC60.. I guess just due to the combination of intake valve timing and lobe centers which are different for each cam but which end up giving the same results on the chart. However isn't it a fact that a DC60 is going to make more power higher in the rev range compared to a supercup cam which is going to make power over a much wider range and less overall at its peak?

I was also under the impression that (assuming your induction system supports it) that a high overlap cam like a GE/DC60 is going to be "safer" with regards to cylinder pressures at low RPM because it is less efficient there. I guess what I really want to know is, what can I get away with. Running an SC cam (not that you would ever want to) would seem perhaps way more prone to detonation than say a GT2 Evo cam because of higher CR's early in the range. Do you want to optimize an engine so that maximum compression occurs as late as possible, at your max revs where detonation would be less likely to occur? I am sure there is a lot wrong with my thinking here and I look forward to people more knowledgeable than myself hopefully adding some of what they know. Of course the rule of thumb I think is that 10.5 and twin plugs is OK on street gas. Now I am learning that perhaps there is far more to it than that.. a cam could make a big difference from one engine to another..

[kenikh]

When I built this spreadsheet it was for ballpark reference. It was designed to compare apples to apples, using a common and rational (albeit limited) methodology, so that one could compare "known" engine configurations against unknown commodities. My approach was focused on "at rest" dynamic CR.

You are correct: VE is the key. If you can proxy intake volume and velocity against pumping force and resistance (functionally BMEP), then overlay it on a power curve, you can get the level of fidelity you're looking for. The wildcard is load, which can change the equation.

As far as reversion from high overlap cams mucking with your EFI - it isn't EFI's fault. EFI is inherently reversion tolerant. What ISN'T reversion tolerant is single throttle body/common plenum intakes. EFI on individual throttle bodies will happily accept DC100 cams, assuming you set up your system with the right probes for calculating your throttle curves.

One last note: knock control has gotten cheap. With knock control, using 993 knock bars and stock Bosch sensors, you can run 12.5:1 on premium pump gas and twin plugs on common plenum design cams. The 964s and 993s do it with massive jugs and a smaller motor like a 3.2SS has significantly better flame propagation, due to less distance required for flame travel.

Net-net, since you are going EFI, buy a good ECU with knock, twin plug your motor, get some JE FSR pistons at 11.5:1, go with the 993SS cams and you should be good to go.

[IXjamesXI]

Kenik, thank you again for putting this spreadsheet together, and also for your advice. I am going to take another look at knock control. Some threads say its just not reliable enough, but some of those threads are pretty old so maybe things have changed since then. Using your DCR calculations, a 964 engine stock, assuming that the static cr is around 10.5:1 (and not the published 11.3:1) gives a DCR of 8.05 ish. I guess that is my baseline for a known quantity that "works." Using a supercup cam on my 3.2SS dimensions gets a DCR of about 7.87 ish.. so knowing that I would be starting out "lower" than the known qty I guess I should interpret as a positive.

At any rate, I think I shall look around and see if I can find a set of knock sensor bridges. Thanks again !!!

[kenikh]

Pelican used to sell the bridges. 993 part number, $50/ea.

[Jesse911]

Very old thread but the link to excel file no longer works. Can anyone reupload this? Would love to check some data for my engine build.