Determining best compression ratio in supercharged motor?

F1Fever · Post by **F1Fever** » Tue Apr 10, 2007 8:17 pm

It depends who wrote it, mine is written to calculate from naturally aspirated engine efficiency.

Ed-vancedEngines · Post by **Ed-vancedEngines** » Tue Apr 10, 2007 8:23 pm

F1 Fever,

My problem with most of the Dynamic Compression Calculators is this;
Most serious custom cams I have ever seen do not include any information about what the advertisd duration is. Even the ones cut to my specifications do not ever show on the cam spec sheets or cam cards what the advertised duration is.

The only ones I see what the advertised durations are are those that are advertising their durations in a catalouge.

Along the lines of this question or question written into a statement form is this additional question.
What is Advertised Duration?
How would you even mearure for Advertised Duration, if the engine was still in house.

Advertised Duration = Crark degrees at .006 lobe lift & .006 toClose?

Advertised Duration = Crank degress at first movement of lifter on cam lobe to last movement?

Advertised duration = Crank Degrees at Lifter moement from .020 to last .020 open?

I am not wanting to sound ignorant or smart alec in this, but I find it impossible to use Dynamic Compresion calculators becuase of this equation that is always requested or is necessary.

Along this same line of thought. How can the program work right when not all cam companies that are showing an Advertised duration are each one possibly using a different point of measurement?

Again talking about anyone figuring a Dynamic compresion.
Any engine measurements measured while engine is not heated and rotating at rpm, like it is when sitting on an engine stand will change and will not be the same in actual use. Even different connecting rod materials or strength will change from cold and static measurements.

This will sound very ignorant of me,but to date all of anything I have ever done with any engine requiring a lower dynamic compression for streetability has always been based on my own gut feelings using only my experiences of what has worked for me in the past and what didn't.

Please do not think I am trash taking your and others efforts to give us all more tools of help in design. I do appreciate all of the efforts of all of you.

I just am too ignorant or not with it enough to be able to use this calculation process.

Is there any possibility of anyone trying to build into the equations the possibilitis of ans of the variables I have written of?

Ed Wilson

F1Fever · Post by **F1Fever** » Tue Apr 10, 2007 9:33 pm

Hi Ed, the calc is written to calculate from .006" before IVC as the "advertised duration" right or wrong that's how it's designed.
In my head the way I understand it is when calculating DCR is the moment the intake flow into or out of the cylinder stops the calculations begin.
At 5 RPM the intake valve may only be open .00001" and still allowing air to escape, which will actually be a point beyond .006" (.009" valve lift) in crank degrees yet at 6500 RPM with a properly tuned intake runner airflow may stop at nearly .050" tappet lift in effect the dynamic stroke increases and therefore the DCR. Or at least MY version of DCR, not the "static" version of it. We've gone over it extensively in this forum and on the fourms.hotrod.com.

Is there any possibility of anyone trying to build into the equations the possibilitis of ans of the variables I have written of?

It can be done, but it's probably going to be too complicated for a simple internet app. Probably the most influential measurement not currently included would be to integrate the intake runner length/area into the calculation & that's getting pretty complicated for your average user.

Post by **Stan Weiss** » Wed Apr 11, 2007 12:41 am

Ed-vancedEngines wrote:F1 Fever,

My problem with most of the Dynamic Compression Calculators is this;
Most serious custom cams I have ever seen do not include any information about what the advertisd duration is. Even the ones cut to my specifications do not ever show on the cam spec sheets or cam cards what the advertised duration is.

Ed Wilson

Ed,
Manufactures advertised duration and where it is measured does not matter. The IVC is when the valve actually closes. Lets take a BBC using 1.7:1 ratio rocker arms. That has a valve lash of .017" that means at .010 cam lift the valve opens and closes. If the cam had a different valve lash the close lift at the cam would be different. Valve close position is one of the things that changes as you change valve lash or intake centerline position. Based on the bore, stroke, rod length and if any wrist pin offset, the piston position at valve closing is calculated and you get what is called the dynamic stroke. From this dynamic compression ratio is calculated. based on 0 PSIG in the cylinder at valve close.
Stan

Shrinker · Post by **Shrinker** » Wed Apr 11, 2007 1:56 am

Hello all; when you comp gauge a motor the first pump read on the gauge is the dynamic pressure of the cylinder with its cams inlet closing point. The second and subsequent reading up to the max are the readings of the heat left behind in the cylinder adding to the compression pressure of the next stroke. Here is an example
pump 1=95psi
pump2 = 125 psi
pump 3 = 150 psi
pump 4 =160 psi
pump 5 = 165 psi
pump 7 = 165 psi

In this example the engine pumped to max in 5 pumps so it probably has a streeter setup in it. The first 95psi pump indicates 95/14.7 = 6.46:1 effective compression stroke after valve closure. I dont know exact formulas of how to calculate it but I find resonable powerful engines that go good, pump 175 on the third pump. If they dont do that then their lame ducks that should be streeters. I think engines need to get over 175 on the third pump in order to make real power. 900 hp engines pump 145, 200, 240, 260, 280. There does appear to be some commonality to pumping pressures. This 145 first pump indicates 10:1 effective compression stroke and the valve timing comfirms this on those engines.
I think any formula that doesn't take into account the overlap of the exhaust and the heat return that way is in sufficient. A very large influential factor in pressure generation is the temperature of the start of compression.
regards Shrinker

John Wallace · Post by **John Wallace** » Wed Apr 11, 2007 11:00 am

First off, I think the Dynamic Compression Ratio number should just be a tool to use, not gospel.

My equations are basically like F1Fever's except I use the closing point of the intake valve.

I think Ed's comments on the 'Advertised' durations are valid.
The main reason the cam companies started using .050" numbers was to get a standard and meaniful measurement.

What is the 'advertised' duration taken from? .006" , .020" , .002" , other?
This will have an effect on the calculations.
The main parameter of the compression calculation is the closing point of the intake.

allblowdup · Post by **allblowdup** » Wed Apr 11, 2007 11:52 am

I agree that dynamic compression should just be a tool. While trying to build a blown pump gas (87 octane) engine that needs to make max torque at 5000/5500 I ran into a situation. The dynamic compression ended up so low that combustion effeciancy suffered badly. The whole engine needed tons of advance and way rich mixtures until WOT and higher rpm. It would dirty the oil and even missfire unless the afr was richer thatn 13.5. Dynamic compression was 6.5. Now that I know what to look for I can see why it ran so poorly. Made decent power on 87 octane 1.7 hp per ci (5 psi).

Post by **Stan Weiss** » Thu Apr 12, 2007 11:39 am

In a street engine we are looking at the 2000-3500 RPM range and pump gas. If I have a combination that produces a DCR of 8.34 in an N/A engine. I reduce the SCR so the DCR is 7.29 and in this RPM range add 3 PSI boost. I will have almost the same cylinder pressure before ignition.

John Wallace · Post by **John Wallace** » Fri Apr 13, 2007 1:37 pm

Patrick Hale's book, Engine Pro - The Book, talks about 'Effictive Compression Ratio'.
Maybe that would be a better calculation?

Doesn't mention boosting though.

Post by **Stan Weiss** » Fri Apr 13, 2007 3:00 pm

I may have this all screwed up but I see the ECR for a N/A engine as almost a Horizontal line. Whereas the ECR for a engine with forced indication raises as the boost raises. So at low RPM with almost no boost it will be below the N/A engine and it the mid range will cross the N/A engine and at high RPM will be well above the N/A engine.

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plovett · Post by **plovett** » Sat Apr 14, 2007 9:01 am

Stan Weiss wrote:I may have this all screwed up but I see the ECR for a N/A engine as almost a Horizontal line. Whereas the ECR for a engine with forced indication raises as the boost raises. So at low RPM with almost no boost it will be below the N/A engine and it the mid range will cross the N/A engine and at high RPM will be well above the N/A engine.

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That makes sense to me. What we're ulimtately talking about is cylinder pressure isn't it? It seems like in the boosted situation you have to optimize static compression for the highest ECR the engine encounters and so it is not optimized at other places on your curve?

paulie

iadr · Post by **iadr** » Sun Apr 15, 2007 3:09 pm

This is off a British site. It's in easily modified Excel form, so if nothing else, it may work as a starting point for one of our members to evolve from, or as a talking point.

Though, guys over there seem to believe in it just as it stands... I didn't try it, just happened to run into it reading up on something else.

http://www.turbominis.co.uk/up/calculat ... rger-1.xls