Static Compression VS Dynamic compression

[RevTheory]

There has to be some calculation that gives you a safe-zone for a given fuel that doesn't require Top-Secret security clearance. Maybe?

[Tom Walker]

What can help predict the fuel octane requirements of an engine are actual pressure sensors in the cylinder of the engine operating under the conditions you are expecting the engine to experience. Very expensive, but very fascinating and informative. I would like to have some of those type of toys!

[johnef]

There has to be some calculation that gives you a safe-zone for a given fuel that doesn't require Top-Secret security clearance. Maybe?

This is precisely why I asked the question. To figure out how far I can push a particular fuel (VP Q16), considering mechanical compression will be 18:1. I can bump up compression a bit more if all other undetermined factors permit.

My understanding was that dynamic compression ratio would net you a lower number, so which number do Fuel manufacturers go by when advertising the limits of a particular fuel.

And what Mike has made mention of, makes sense in my logic. But that leaves me exactly where I started.

So in keeping with my OP example, how would you tackle this or reverse engineer the limits of the fuel?

Understanding that 2 of the same engines could have, same static/mechanical compression ratio, same IVC, but 2 different VE% due to factors like cam profile, intake port efficency. Volumetric Efficiency % directly impacts the capability of one fuel. The higher the VE% the higher the octane rating.

Keep in mind I don't have any pressure sensing logging capabilities.

If I'm not making sense please correCT me.

[GARY C]

There has to be some calculation that gives you a safe-zone for a given fuel that doesn't require Top-Secret security clearance. Maybe?

This is precisely why I asked the question. To figure out how far I can push a particular fuel (VP Q16), considering mechanical compression will be 18:1. I can bump up compression a bit more if all other undetermined factors permit.

My understanding was that dynamic compression ratio would net you a lower number, so which number do Fuel manufacturers go by when advertising the limits of a particular fuel.

And what Mike has made mention of, makes sense in my logic. But that leaves me exactly where I started.

So in keeping with my OP example, how would you tackle this or reverse engineer the limits of the fuel?

Understanding that 2 of the same engines could have, same static/mechanical compression ratio, same IVC, but 2 different VE% due to factors like cam profile, intake port efficency. Volumetric Efficiency % directly impacts the capability of one fuel. The higher the VE% the higher the octane rating.

Keep in mind I don't have any pressure sensing logging capabilities.

If I'm not making sense please correCT me.

This is something I have wondered for sometime but in all of my search I have never fond anyone that connects DC to fuel with the exception of pump gas limits, on race gas everyone seems to go by SC.

Another thing I would wonder at that compression limit and rpm is which Q16 would be better, the ethanol blend or the methanol blend or would it make a difference?

ETBE
Q16 Yellow 116 >120 >118 0.7200 Leaded 5.64 psi 9.41%
MTBE
Q16 REG Yellow 116 >120 >118 0.7155 Leaded 7.25 psi 9.38%

https://vpracingfuels.com/master-fuel-table/

[digger]

Are these things predictable and reasonably consistent?

No, and that's what I've been trying to point out.
Moving the IVC on one engine, can have the exact opposite result, as moving it on another engine.

I disagree that's totally unpredictable within normal bounds.
If you are trying to predict cranking compression pressure the results/trends are somewhat predictable . Or you are trying to predict torque at a low rpm the trends are pretty predictable.

[CamKing]

Are these things predictable and reasonably consistent?

No, and that's what I've been trying to point out.
Moving the IVC on one engine, can have the exact opposite result, as moving it on another engine.

I disagree that's totally unpredictable within normal bounds.
If you are trying to predict cranking compression pressure the results/trends are somewhat predictable . Or you are trying to predict torque at a low rpm the trends are pretty predictable.

Cranking Pressure is not an aspect of a running engine.
I’m talking about what happens when the engine is running.

[digger]

Mike low DCR causes the engine to be relatively lazy at low rpm. By low rpm I mean an rpm where little runner "tuning" is occurring, there is little air speed and the engines not up "on the cam". high DCR seems to have more propensity for knock at the same low rpms. These are just general trends all else being equal. Due to the observations being evident at low rpm this is not something that's rally relevant to a "race" engine mostly for hot street engines.

[gruntguru]

"dynamic" CR: volume above piston at IVC / vol at TDC

That's my point. It's completely useless, because it doesn't represent anything that has any effect on a running engine.

You could equally say that of CR. "The volume at BDC and the volume at TDC do not represent anything that has any effect on a running engine".

I agree with one thing - "dynamic" compression ratio is a poor choice of terms. Does that mean its completely useless - no.

There are a lot of physical measurements that may not mean much in a dynamic situation. That doesn't mean they are useless.

[GARY C]

DCR seems to be another one of those simple tools that many find useful while others will tell you there is no place for it.

[cjperformance]

SCR along with VE/charge mass gives a basic look at (remember CR and VE both effect cylinder pressure so with say 11:1 CR the cylinder pressure is way different if you have 110% 100 % 80% or 70% VE) octane requirement and power potential at WOT thru peak TQ and HP.

DCR(i think most of us agree its named incorrectly, but anyhow) along with overlap (read overlap as cylinder charge dilution at closed or light throttle) gives a basic look at idle quality and vacuum, light throttle response and TQ , general low end driveability so is more closely linked to streeter/daily driver situations.

[digger]

I personally don't believe DCR affects idle quality any meaningful amount

[cjperformance]

I personally don't believe DCR affects idle quality any meaningful amount

Not so much the "lumpieness" per say but it does affect timing requirements and convertor requirements.

[swampbuggy]

Question for Camking. So Mike, the way i am understanding your slant on this topic is---------you can have 2 racing engines basically the same engine only differences would be lets say engine #1 has an I.V.C. point at 70 deg. ABDC and has a cranking compression of 240.
Engine #2 has an I.V.C. point of 85 deg. ABDC and has a cranking compression of 220.
Engine #2 can possibly make more power due to the fact that it has a superior induction system (air filter to intake valve seat), and might have a superior exhaust system that does a better job of helping emptying the cylinders as well as helping to increase cylinder filling starting at overlap.

Note: When i said basically the same engine i was thinking same brand, bore&stroke, static compression, cylinder heads (valve angle/combustion chamber size) . Mark H.

[plovett]

Question for Camking. So Mike, the way i am understanding your slant on this topic is---------you can have 2 racing engines basically the same engine only differences would be lets say engine #1 has an I.V.C. point at 70 deg. ABDC and has a cranking compression of 240.
Engine #2 has an I.V.C. point of 85 deg. ABDC and has a cranking compression of 220.
Engine #2 can possibly make more power due to the fact that it has a superior induction system (air filter to intake valve seat), and might have a superior exhaust system that does a better job of helping emptying the cylinders as well as helping to increase cylinder filling starting at overlap.

Note: When i said basically the same engine i was thinking same brand, bore&stroke, static compression, cylinder heads (valve angle/combustion chamber size) . Mark H.

I think the original intention of DCR was to give an approximate indication of octane requirements. Not power. I know there are lots of other variables for octane requirement.

JMO,
paulie

[Orr89rocz]

So this is like the 128 thing