Some use IVC @ .050 like

Go Fast

http://www.gofastmath.com/Compression_R ... Calculator

UMC Pistons

https://uempistons.com/p-27-compression ... lator.html

Some Use Seat to Seat IVC like

Pat Kelley

http://cochise.uia.net/pkelley2/DynamicCR.html

Wallace Racing

http://wallaceracing.com/dynamic-cr.php

asking, as for the engine that I am building, the various compression calculators agree on the static compression ratio
but are far apart on the calculated dynamic compression. One set calculates to the low side of ideal, the other set calculates to the high side of ideal.

The calculators that use IVC Seat to Seat calculate a dynamic compression of 6.81,, which I believe is on the low side of being practical.
The calculators that use IVC @ .050 ( with the 15 added when needed) calculate a dynamic compression of 8.5, which may be on the high end of being practical.

I have found this interesting as when I planned out the motor in the 83 D150, The same calculators did not give the exact same result, but they generally agreed with in a few points.

It appears that there is something about this combination of compression ratio and the intake closing ramp that is confusing one set of the calculators, but which one?

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

The real issue is that those calculators are not being confused by one particular aspect, but rather they are born confused and always will be.

I've talked to some very prominent engine guys and they've told me 'I don't know how to determine (not calculate) dynamic compression ratio'. I guess anyone can calculate anything as long as you understand going in that a calculation is only playing with numbers and can be made to say whatever you like, regardless of reality.

It's a very difficult thing to do determine dynamic comp ratio. I'll bet the best people at it - major car makers - have some very sophisticated programs that get them close. Most of all, those programs are developed and fine-tuned in conjunction with real-world measurements. There's no internet program that has only 7 or 8 variables that is gonna be on the money.

And....dynamic compression ratio changes with RPM. It has to. I looked at the Wallace calculator and it doesn't even list RPM as a variable.

you are much more pragmatic then I am, working through the theoretical has never bothered me.
As George Box said "All models are wrong, however some are useful."

Said a simpler way, "For a model there is no need to ask the question "Is the model true?". If "truth" is to be the "whole truth" the answer must be "No".
The only question of interest is "Is the model illuminating and useful?".

And that was and is my question. Doc had already referenced the UEM Piston calculator,,
I was really just trying to find out what others had used and found useful in the area of dynamic compression calculators.



Thanks

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

The main factors at play here are how fully did we fill the cylinder, when did compression truly start and how much more do we compress the mixture from that point. Most calculators appear to assume 100% filling, use the IVC (intake valve closing) point as when compression starts and use the piston position at IVC and connecting rod geometry to answer the last question. What we really want to know isn't entirely answered by the dynamic compression. The real question usually is what static compression ratio works well for the engine I have and the fuel I use?

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Joshua

Yep. In the end, for most of us, all that matters is how much CR can you run for a given fuel. And even that will change based on cam selection and a bunch of other variables. For the near-stock Slant Six, the answer seems to be 9:1 on pump gas (and I don't consider myself to be a contributor to that consensus).

What's more...even if you did some magic and were able to run 9:5 to 1, the resulting HP increase is actually pretty small.

A true 8.5:1 was borderline with a stock 1967 camshaft. It would have probably been okay with better oil control, but that would have meant a rebore to take out the cylinder taper. With a street performance camshaft 9:1 would have been fine in that car.

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Joshua

Agree, and with the limited breathing ability of the slant head, it leads me to think that the dynamic compression calculators error to the high side for our motors.




So the recommendation is to use static compression as the main factor when determining octane level. I cannot say that I disagree. The advantage of using static compression
is that the formula is locked down and universally understood. The disadvantage to using only static compression is that it does not provide for input from the selection of the cam shaft.
And the cam is possibly the biggest single element that defines an engine's personality.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

Even engines that can achieve 100% VE (or better) only do it at their optimum RPM range.

That's why engines with 'hot' cams run crummy at low RPM's. And it's why fancy engines use variable cam timing.

I always compute static CR, then pick a cam from my experience, along with plenty of numbers and interviews of (and rides with) other racers.

Unless I am miscounting, I have run 10 different cams over the years, and with different cam timing (degreed centerline) for some, and different CRs for some, and different head flows for some. All on 225 engines.

Lou

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Home of Slant6-powered fun machines

I'm a fan of compression ratio. An engine with 'high' CR has a good, crisp throttle feel and you just know it's giving you all it's got.

Unfortunately, I'm an even bigger fan of engines that aren't detonating.....

and this is not being critical, but if over the span of experience described above, if you had collected dynamic compression data, wonder if it would have pointed to a
common index, for motors that performed well. The comparative date would need to be generated using the same calculator for all.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

I could probably come up with all the parameters if you want to feed the calculators, but it would take some effort. I'll put that project on my long-ish list... I had some notable fails early on, and those were extremely instructive.

Lou

_________________
Home of Slant6-powered fun machines

My experience has been that a moderately low compression Slant (9.5:1) ran better and better the more cam I stuffed in it and more RPM it spun. I also found out that when I broke down and added more compression (11.2:1) it ran more better.

_________________
Official Cookie and Mater Tormentor.

inputting the data is the ez part, if you do pull it together it would be interesting to
see what corelates, especially since there are good ones and fails.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

Dynamic compression is actual running compression that varies with RPM. It is hard to truly measure DCR.
Effective compression is the calculated compression number most of these calculators give us.
As already stated... This calculated number is a good "point of reference" because it addresses the cam profile.

For me, I find (measure) the actual point where the valve hits the seat, at the lash (valve clearance) I will use. (UEM says to use @ .050 and add 15 degrees but I use the actual IVC I have measured)

My experience:
Up to 7.7 "result" engines run OK on any fuel you put in them and will take a lot of ignition timing but you left some performance behind.
7.8 to 8.0 'result' engines run well on pump gas. I look for this range for most pump gas engine builds.
8.0 to 8.3 'result' engine builds run butter... but needing better fuel... Pump premium or race gas "blends")
Any SL6 engine build over 8 to 1 on the UEM calculator are more pron to detonation on today's pump fuel.

I have also noticed slight output differences between difference calculators so it is important to stick with one that you have experience with.
DD