OK, you know how when you run a much to small carb or a restrictor plate you need to run a higher static compression ratio in order to maintain the same dynamic compression ratio?

If flow capacity at carb goes up, compression must come down X amount and if flow capacity goes down then compression must come up X amount.

So I got to wondering...

I know that when you restrict the intake the air tries to flow faster in order to maintain the same volume and does so with limited success, which is why you bump the static compression ratio.

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NOTE: This all assumes I understand this correctly to begin with, which is not a certainty.

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Given that fact, if it is a fact, and the fact that velocity, compression and flow capcity are all good, up to a point, (about 300f/s ?), then it follows that there must be an optimal balance between compression ratio and restriction (flow)at the intake.

I do know that changes in compression ratios can sometimes have surprising effects on the dyno and sometimes the surprise is that it doesn't seem to have a signifigant effect on Torque or HP.

Has anyone managed to describe the mathmatical relationship here so that if I know certain factors I can calculate optimal compression ratio?

 

As I understand it... your theory is correct. A restricted engine, will always show a loss in volumetric efficiency when compared to one without a restriction. I also understand, based on a little 'dynamic compression' thread here a while back in which Mr. Riddeck made some of this a bit clearer, that dynamic compression ratio has to do with the volume of trapped gasses on the compression stroke. Of course this has to do with the time of the intake-valve-closing event but that's irrelevent in this part of the discussion. So, as I see it, a restricted engine is going to show even greater losses than an unrestricted one and will have a lower 'DCR'... all other things considered equal.
As for the effect of compression on a dyno, more compression generally makes for more torque. That's a real tough one though because there are sooo many variables. You'd have to really compare the same engine with the same combustion chambers and at the same quench height. Cause changing any of those would effect the burn rate and that'd have an effect on maximum timng for best torque.
As for the math... I dunno. Good question for one of the more engineerish guys.

-Mindgame

 

Sorta. IMO, choose CR based on detonation potential with the fuel you use. Restriction will just limit VE and power; that's the main reason sanctioning bodies use them.

Unless rules prevent it, work on minimizing the flow restriction, and if you are trying to design an engine for a restricted class of racing, go to a pro who's done it before.

 

Well, it's not that I want to provide a restriction, it's just that the whole restrictor plate thing got me to thinking.

There is a relationship there and one thing is a function of another.

That can't happen ONLY with a restrictor plate.

No matter what your carb/intake setup it's ONLY going to flow a certain amount, which means that in the broad sense it IS a restrictor.

It's like when you use a supercharger.

Because your volumetric efficiency goes up over 100% you have to look at dropping the compression ratio.

It just seems to me that I if I can calculate my engine's volumetric efficiency then I should be able to determine just exactly what static compression ratio to use to hit a certain dynamic or effective ratio..

(For that matter, I'm not sure what the difference is between those two.)

It just seems like compression often seams to be a seat of the pants thing. You pick a compression on what someone else said worked or didn't work in some other guy's engine. I mean, unless you're a racer.

Do those "desktop dyno" programs provide a useful answer to this question?

 

Some programs do, some don't. "Engine Analyzer Pro" is pretty good at calculating VE, tuning pressures, average inet and exhaust velocities, etc. IF you supply accurate data. It also gives a relatively good idea of your engine's tendency to knock.

As you said, everything is interrelated in a engine. When one thing changes, like intake restriction, you need to change other things to compensate, like cam timing, port and runner sizes, etc.

The carb or TB isn't necessarily much of a restriction if it's sized correctly. Remember that most 4-bbl carbs are flow rated at 1.5 in. hg. (about 20.4 in. H2O, or .74 psi) at wide open throttle (WOT), but an NA engine with a large enough carb or TB might have less than 1.0 in-hg vacuum in the intake manifold, which means the carb isn't even flowing it's rated CFM (because this engine doesn't need it) and isn't really a restriction.

Similarly, a too small carb or other restriction might cause 3-5 in. hg. (or more) vacuum causing the carb/TB to flow well in excess of it's rating. For example, a 390 CFM carb (rated @1.5 in hg) on an engine which is causing 5 in. hg. manifold vacuum at WOT is actually flowing over 700 CFM. This is common on a NASCAR Modified.

Dynamic CR is usually defined similarly to Static CR, except instead of using the total swept volume (bore x bore x stroke x .7854) you use swept volume at intake valve closing which depends entirely on the cam.

"Effective" compression ratio might be called what the engine sees if it has a restricted intake. If there isn't much restriction at WOT, Dynamic CR is probably about the same thing. Remember, at part throttle (cruise) there is a very high intake vacuum, and very little flow so the engine sees an "effective" CR of maybe 4-6:1, or perhaps lower.

I guess I'm saying design your engine's Static CR (and dynamic CR) for what it can handle in terms of detonation given it's combustion characteristics, head material, octane, etc. and VE. You can achieve slightly over 100% VE with an NA engine, even on the street, so that is a consideration. Practically, fuel octane will be the most important item, IMO.

As far as restrictors go, there has been a LOT of development work done to extract as much airflow as possible through a mandated restrictor. Some organizations (NASCAR) put the restrictor plate directly below the carb while others put them far upstream allowing the inlet and outlet to the restriction to be tailored for max flow. Bottom line is that restrictors limit the max power fairly well. Getting the last 1-2% is the challenge.

 

The limiting factor is going to be cylinder pressure. A restricted engine will not let as much air in, so you don't add as much fuel, and get a much smaller bang, w/ less energy, which will have a lower peak cylinder pressure. I suppose you could pull a number out, and define what your max acceptable cylinder pressure is based on fuel, temperature, etc (i.e. figure out what is the auto-ignition temperature for the fuel you're running, and stay below it), and work backwards from there. Like OldSStroker mentioned EA-Pro would be able to model it for you. As long as you stay below your max acceptable pressure, you should not have detonation issues, and can design your engine to get as close to the edge as you feel safe. Another thing to consider is the diminishing returns from a higher CR. You'd get the most power from an infinite CR, but that won't happen. An improvement from 9:1 to 12:1 would be significant (in 10's of HP), while a bump from 12:1 to 15:1 would not get you the same gain. The whole diminishing returns thing. I'm sure John Heywood's book has the graph.

 

I've been using the word restriction in it's broadest sense.

In this context zero restriction would be having the combustion chamber open to the atmosphere, through the intake valve opening, without the valve being in place.

I'm not looking to run a restrictor plate to tune my engine.

In fact, I am looking to use forced induction.

I'm simply trying to get my head around the relationship between how easilly the motor can breath and how much the mix should be squeezed, so as to get the optimal cylinder pressure at the moment of ignition.