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predicting cranking compression

General engine tech -- Drag Racing to Circle Track

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David Redszus
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Re: predicting cranking compression

Post by David Redszus » Thu Jun 25, 2020 2:00 pm

Nut124 wrote:
Thu Jun 25, 2020 8:28 am
tcb3274 wrote:
Thu Jun 25, 2020 7:15 am
What psi is considered ok for 93 octane? 200 psi?
Cranking compression is not a predictor of whether detonation is going to happen. A hot build will likely crank a lower PSI than a mild build with less CR.
Cranking compression pressure does not determine detonation or pre-ignition. Those are determined by
temperature, not pressure.

Increased compression pressure will raise chamber temperature prior to TDC. But other factors such as inlet ambient air temperature, intercooler efficiency, fuel evaporation cooling will determine the onset of combustion.

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Re: predicting cranking compression

Post by 77cruiser » Thu Jun 25, 2020 3:01 pm

Stan Weiss wrote:
Thu Jun 25, 2020 12:57 pm
Jim,
I don't have anything for start RPM in my software and it use IVC at the seat. What I get working backward for you engine and 181.9 psi.
Is IVC 70.35 ABDC and with a 103 ICL that is 294.7 seat_to_seat duration.

Stan
First 2 is my cam seat & .05.
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Jim

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Re: predicting cranking compression

Post by Stan Weiss » Thu Jun 25, 2020 4:30 pm

77cruiser wrote:
Thu Jun 25, 2020 3:01 pm
Stan Weiss wrote:
Thu Jun 25, 2020 12:57 pm
Jim,
I don't have anything for start RPM in my software and it use IVC at the seat. What I get working backward for you engine and 181.9 psi.
Is IVC 70.35 ABDC and with a 103 ICL that is 294.7 seat_to_seat duration.

Stan
First 2 is my cam seat & .05.
Jim,
So what did you get when you did a cranking compression test? Based on your engine specs and that those cam specs I get 196 PSI.

Stan
Stan Weiss / World Wide Enterprises
Offering Performance Software Since 1987
Do you use engine simulation software that uses cylinder head flow files?
We have a package of more than 3025 DFW or FLW Files

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Re: predicting cranking compression

Post by swampbuggy » Thu Jun 25, 2020 5:38 pm

OK Mark G. now tell us how many Degrees After Bottom Dead Center the Intake Valve Seats ? with normal running lash of course. Mark H.

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Re: predicting cranking compression

Post by 77cruiser » Thu Jun 25, 2020 9:45 pm

Stan Weiss wrote:
Thu Jun 25, 2020 4:30 pm
77cruiser wrote:
Thu Jun 25, 2020 3:01 pm
Stan Weiss wrote:
Thu Jun 25, 2020 12:57 pm
Jim,
I don't have anything for start RPM in my software and it use IVC at the seat. What I get working backward for you engine and 181.9 psi.
Is IVC 70.35 ABDC and with a 103 ICL that is 294.7 seat_to_seat duration.

Stan
First 2 is my cam seat & .05.
Jim,
So what did you get when you did a cranking compression test? Based on your engine specs and that those cam specs I get 196 PSI.

Stan
I think it was about 185 cold.
Jim

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Re: predicting cranking compression

Post by mt-engines » Fri Jun 26, 2020 8:48 pm

Coloradoracer wrote:
Thu Jun 25, 2020 8:37 am
Nut124 wrote:
Thu Jun 25, 2020 8:28 am
tcb3274 wrote:
Thu Jun 25, 2020 7:15 am
What psi is considered ok for 93 octane? 200 psi?
Cranking compression is not a predictor of whether detonation is going to happen. A hot build will likely crank a lower PSI than a mild build with less CR.
Case in point, my stuff......static compression ratio is 16.5:1....cranking compression is about 180 psi......According to the dyno, I make 1510 hp @ 7100 rpm, no nitrous.....that was on gasoline. I've since switched to methanol, and it definitely made a difference...would love to dyno it again...
Wow. How many cubic inches?

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Re: predicting cranking compression

Post by Coloradoracer » Tue Jul 07, 2020 8:23 am

665", 12* Chevy....

intake opens 24, closes 86
exhaust opens 74, closes 36

These are .050" numbers....
Mark Goulette
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Re: predicting cranking compression

Post by David Redszus » Tue Jul 07, 2020 10:33 am

Stan Weiss wrote:
Thu Jun 25, 2020 4:30 pm
77cruiser wrote:
Thu Jun 25, 2020 3:01 pm
Stan Weiss wrote:
Thu Jun 25, 2020 12:57 pm
Jim,
I don't have anything for start RPM in my software and it use IVC at the seat. What I get working backward for you engine and 181.9 psi.
Is IVC 70.35 ABDC and with a 103 ICL that is 294.7 seat_to_seat duration.

Stan
First 2 is my cam seat & .05.
Jim,
So what did you get when you did a cranking compression test? Based on your engine specs and that those cam specs I get 196 PSI.

Stan
Stan,
You might want to remind folks that calculated pressures are often in units of psia while measured pressures are in
units of psig.

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Re: predicting cranking compression

Post by Stan Weiss » Tue Jul 07, 2020 11:23 am

David Redszus wrote:
Tue Jul 07, 2020 10:33 am
Stan Weiss wrote:
Thu Jun 25, 2020 4:30 pm
77cruiser wrote:
Thu Jun 25, 2020 3:01 pm


First 2 is my cam seat & .05.
Jim,
So what did you get when you did a cranking compression test? Based on your engine specs and that those cam specs I get 196 PSI.

Stan
Stan,
You might want to remind folks that calculated pressures are often in units of psia while measured pressures are in
units of psig.
David,
Way back my MD-DOS software calculated psig. When I rewrote my software for Windows I screwed up and calculated psia. Not long after that I realized my mistake and ever since then the user can calculate either psig or psia depended on which button they click.

Also I am not sure how many people understand that and how a change in BP or temperature effects cranking compression.

Stan
Stan Weiss / World Wide Enterprises
Offering Performance Software Since 1987
Do you use engine simulation software that uses cylinder head flow files?
We have a package of more than 3025 DFW or FLW Files

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Re: predicting cranking compression

Post by pdq67 » Tue Jul 07, 2020 10:39 pm

Very interesting thread, but I am more interested in dynamic compression ratio, (DCR), than static compression ratio, (SCR).

If not mistaken, it is a better predictor of fuel detonation.

And rod length does enter in here, but not to a great extent.

Please correct me if I am wrong.

And not meaning to hi-jack the tread.

pdq67
Last edited by pdq67 on Tue Jul 07, 2020 10:51 pm, edited 1 time in total.

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Re: predicting cranking compression

Post by af2 » Tue Jul 07, 2020 10:46 pm

pdq67 wrote:
Tue Jul 07, 2020 10:39 pm
Very interesting thread, but I am more interested in dynamic compression ratio, (DCR), than static compression ratio, (SCR).

If not mistaken, it is a better predictor of fuel detonation.

Please correct me if I am wrong.

And not meaning to hi-jack the tread.

pdq67
Nope!
And I ave to quote another.
GURU is only a name.
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Re: predicting cranking compression

Post by af2 » Tue Jul 07, 2020 10:47 pm

David Redszus wrote:
Tue Jul 07, 2020 10:33 am
Stan Weiss wrote:
Thu Jun 25, 2020 4:30 pm
77cruiser wrote:
Thu Jun 25, 2020 3:01 pm


First 2 is my cam seat & .05.
Jim,
So what did you get when you did a cranking compression test? Based on your engine specs and that those cam specs I get 196 PSI.

Stan
Stan,
You might want to remind folks that calculated pressures are often in units of psia while measured pressures are in
units of psig.
To Paul again
GURU is only a name.
Adam

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Re: predicting cranking compression

Post by Stan Weiss » Tue Jul 07, 2020 11:03 pm

I don't know how others do their calculation of cranking compression pressure. But for me one of steps along the way is the calculate DCR or as some call it trapped compression ratio.

Stan
Stan Weiss / World Wide Enterprises
Offering Performance Software Since 1987
Do you use engine simulation software that uses cylinder head flow files?
We have a package of more than 3025 DFW or FLW Files

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af2
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Re: predicting cranking compression

Post by af2 » Tue Jul 07, 2020 11:16 pm

Stan Weiss wrote:
Tue Jul 07, 2020 11:03 pm
I don't know how others do their calculation of cranking compression pressure. But for me one of steps along the way is the calculate DCR or as some call it trapped compression ratio.

Stan
When we figure it out then every strain of valve size and combustion chamber and intake will have a perfect #?????? Oh And Cam!
GURU is only a name.
Adam

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Re: predicting cranking compression

Post by af2 » Tue Jul 07, 2020 11:20 pm

Predicting ring flutter is way more ..... what can I say?
GURU is only a name.
Adam

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