I'm not really disagreeing here, I just don't understand why the DRC is so meaningful to all you guys.

Let's say I've got a 10:1 engine and I put a big cam in it. The intake valve is still going to be open when the piston starts its compression stroke. At idle, the piston will push some charge back through the intake so I'll get a real compression ratio that is less than 10.

However, why is this meaningful? At higher RPM's, the intake charge is going to be entering the cylinder at such a high velocity that more charge will enter the cylinder even when the piston is in its compression stroke. (This is the whole point of leaving the intake valve open longer).

So, shouldn't we be talking strictly about cylinder pressure rather than dynamic compression ratio? Seems to me, DCR only applies to low RPM conditions. At higher RPM's a big cam should create more cylinder pressure than a small cam. So isn't it possible that if I put a big enough cam in to give me a safe DCR for pump gas, I can still get detonation problems higher in the powerband if my heads flow enough? (Or does this just not happen because you can never get that kind of flow at those RPM's?)

Scott, from what little I understand, you are correct. At high rpms, when the time event is so short, DCR is less meaningful , static moreso.

HOWEVER, having said that, I got familiar with DCR since I was swapping from a lazy 114 lsa cam to an aggressive 110 lsa and needed to understand how to keep the engine happy on pump fuel. Still think using DCR as one aspect of a cam choice is important.

So while it is important to understand the limitations of the DCR calculation, it still serves an important role in understanding the interplay of cam choice and compression, as well as helping "fit" a cam to the engine. IM unprofessional HO.

True you will start building more pressure in the higher rpm's.. But usually the engine isnt under the high load conditions at that rpm that it is at low rpms. So it may still run on pump gas.

Panic has some good theories. However, the rpm at which the overfilling takes place is usually really high.

Exactly, the DCR definition that is common on this board is essentially meaningless.

If you hear any professional racers, or engine builders refering to "Dynamic" compression, they are refering to the actual conditions, present in the cylinder during WOT throttle operation, and under racing conditions, NOT the intake lobe closing point as it relates to the compression ratio.

Thus the "dynamic" compression they're refering to accounts for everything, and what it really is dynamic cylinder pressure.

The disappointimg aspect of DCR is that each cam manuf. lists their seat times at different point of lift. This does make a difference in DCR and cylinder pressure. But what is interesting is that I have also seen DCR calculated using .050 numbers. This carries forward the practical experience that we all have from the high compression short .050 duration factory cam motors that would detonate on good old fashioned leaded high-test. Motors like a 350-300HP SBC would ping on anything less than 94 - 96 octane, yet based on the long seat durations, we would expect these to operate on very low octane fuel. I am inclined to use the DCR as a reference tool but have learned to depend more on experience and .050 numbers.

Originally posted by Motor Martyr:
Exactly, the DCR definition that is common on this board is essentially meaningless.

Thus the "dynamic" compression they're refering to accounts for everything, and what it really is dynamic cylinder pressure. It still helps us 'amatuers' get it close. And aside from bad advice from local "gurus", or self proclaimed 'gurus', it give us a calculation that at least enables us to compare the effect of various grinds, and aviod stuffing a large cam in a low compression motor and building a dud.

I think Pat's contribution is solid and has efficacy, BUT you and the original poster do point out its limitations, as did Mike Lewis a few times.

I'll still run a DCR calc everytime I design an engine or pick a cam.( very infrequently!) One of many tools.

Originally posted by 427L88:
</font><blockquote>quote:</font><hr />Originally posted by Motor Martyr:
Exactly, the DCR definition that is common on this board is essentially meaningless.

Thus the "dynamic" compression they're refering to accounts for everything, and what it really is dynamic cylinder pressure. It still helps us 'amatuers' get it close. And aside from bad advice from local "gurus", or self proclaimed 'gurus', it give us a calculation that at least enables us to compare the effect of various grinds, and aviod stuffing a large cam in a low compression motor and building a dud.

I think Pat's contribution is solid and has efficacy, BUT you and the original poster do point out its limitations, as did Mike Lewis a few times.

I'll still run a DCR calc everytime I design an engine or pick a cam.( very infrequently!) One of many tools. </font>[/QUOTE]=
Gene,
Just to be clear, the difference is I NEVER said it was meaningless.

I do not presume to know all there is to know in the universe regarding engines.
Fact is after about 40 years I find out daily how much I don't know :(

I have posted at different time my thoughts on DCR as it is used here & here is a couple I think you are referring too:

"DCR or more correctly "effective compression ratio as Dynamic implies something in motion or transition as used in Pat's DCR calculator is a very important part of the equation when it come to cylinder filing,, or lack of cylinder filling.
There is a bit more to cylinder pressure than just when the valve closed & Brian did an excellent job of explaining most of it.

Most of Brains explanation surrounds what happens around peak torque or the RPM the engines intake, cam & exhaust is tuned to.
It is why a Pro Stock engine stays in a very narrow RPM band. You cannot tune the engine over a wide band as effectively & compromises need to be made for most of the engines considered here.

At low RPM, DCR as it is being used here will suffice when trying to run crap gas with high compression.

When you start getting up near peak torque everything changes & you better damn well have fuel with enough octane to support the cylinder pressures you are making.
This is not as simple as where the intake valve closes, it now becomes what is truly Dynamic compression ratio which includes where the intake valve closes & how much cylinder filling you have done taking into account all the other factors like how efficient the intake & exhaust system is at a particular RPM, which is essentially what volumetric efficiency or VE is about.

VE is a ratio of how much you filled the cylinder verses what you could have filled the cylinder (100%) at atomspheric pressure & in a well tuned system (or forced induction) can reach well over 100%.
IE: "overfilling"

Here is the rest of the post:

"First let me say I think that Pat's DCR calculator is an excellent tool when used with a bit of common sense.

Here's some of my thoughts about DCR & high compression that I have posted before.

With a big cam you will see lower cranking compression but you also have to use a little common sense with the slam a big cam & lower the DCR syndrome
It may have low cylinder pressure at cranking & low rpm's but somewhere it is going to 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 the plan is to use & lean on your engine with pump gas most of the time I would prefer to limit the static compression with aluminum heads at 10.-10.5 for 91 octane & 10.5-11 for 93 & change the cam accordingly.

You are right in that cranking compression which is part of the DCR is determined from when the intake valve closes.
The higher the static compression the higher the cranking compression will be the sooner you close the intake valve.
This I think we all agree on.

Now there is a lot of other things that will effect cylinder pressure to a certain extent, like RPM, Barometric pressure, cam intensity, engine temp, air temp, humidity & probably a lot of other little things I missed as well.

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 & with enough load without proper fuel, it will detonate.

The load part is why lighter low gear cars will usually tolerate more compression than a heavier or higher geared car.

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 compression 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 4000-4500 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??

Anyway, you are only compressing what is in the cylinder from the time the intake valve closes & this can be completely different depending on a number of things but most of them are listed above I think :confused:

Anyway,,, these are just a few of my uneducated thoughts on the merits of of just using when the intake valve closes to calculate DCR.

Someone else can pick up the VE, quench, swril, chamber efficiency & timing issues :D "

Good stuff Mike. I never said you said.... ;)

I reworded my last post since Brian and I had a foncon. IMHO he was being a bit too 'absolute' in his reply.

I consider myself very close to the top of my game, professionally speaking, and it PAYS to be humble at times, while at the same time being extremely confident in what you know, your opinions, and not letting yourself get swayed by "popular opinion" or "the crowd".

Fine line there!

And as I explained to Brian, that 'simple' DCR calc has kept many a TC viewer from building a slug ( overcamming a low compression mill), or overestimating what could be done on pump gas. Good tool, but it doesn't explain all.

Now if one of you pros could define a set of simultaneous optimization equations, using Pat's DCR, Gary's standing wave harmonics, simple gearing/rpm formulas etc., and have Pat program them all in visual basic, then our work would be done here......

tongue.gif

Ok back to being humble, er confident, er humble.....WORK graemlins/waving.gif

Not that this means anything, but there is an amount of credence to examining the DCR. One of the Japanese automakers (Toyota? I think) started exploring this years ago as a means to pull power packages together.

I had previously done a search on this, there is a lot of interesting info on the topic. At any rate, I do use Pat's calculator. I find the entire set of calculators to be very heplfull.

The DCR calculator came about because of questions about running pump gas in 10:1 engines. Most all the answers at that time were anecdotal. I recalled reading in David Vizard's book on SB cams (p.21, IIRC) his piece on using the compression ratio after the intake valve closed to determine the octane needed. In this piece he stated that 8.0-8.5 was about the highest CR that 92 octane would work at (and that this figure has been known since the mid 30's). He calculated the Dynamic Compression Ratio (his term) using percentages of piston travel. His recommendations (which I have adopted) are 8.5 max DCR with reservations and 8.3 for general street use. The thinking is that the static compression is not what the engine see. It sees the CR after the intake closes.

The DCR calculator isn't the end all in cam/CR matching but will point out a mismatch, say a 308º cam in a 9:1 engine. It probably isn't too useful for a Pro Stock engine or one with a huffer. But, I thnk anyway, for most amateur builders, it is a way to match up a cam with a CR and be in the ball park. No one is required to use it. All it really does is put a number where before there was mostly speculation. It does not tell you the Dynamic Cylinder Presssure or the VE. These require a much more sophisticated calculation. There was a Vizard web piece on this subject but the site is gone now.

As for very slow ramp cams, I now suggest using the .050" duration +15º. This seems to matchup better with reality.

As to putting a very big cam in a low (10:1) compression engine. Yes, it will make power at high rpm but will be very weak at lower rpm. I had an engine like this. An 8.5:1 with a 308º cam. No power below 3000 rpm. Ran pretty well above that. With 10:1 it would run better and with 11:1 even better,yet.