predicting cranking compression

[tcb3274]

Is there a way to predict cranking compression before a actual build?

[Nut124]

In my limited experience, cranking compression is mostly determined by static CR and intake cam timing i.e. when does the intake valve close, deg after BDC.

If cranking pressure is way lower than expected, then cam timing may be retarded, or static CR way lower than planned. Valve clearance can affect this too. Inadequate clearance increases cam duration.

[rebelrouser]

A lot of engine programs will give you a pretty close estimate of cranking compression. If you put good accurate numbers into them. I use performance trends and Pipemax, but there are others. Also remember cranking speed, battery charge, starter condition, throttle open or closed, all affect compression gauge readings.

[gunt]

on a very basic math as i was thought , your compression ration x 14.7 + 14.7

basics of your compression ratio x atmospheric pressure plus one atmosphere , but i believe that's on 60-70's tech and tolerances

[David Redszus]

In my limited experience, cranking compression is mostly determined by static CR and intake cam timing i.e. when does the intake valve close, deg after BDC.

That's pretty close.

Cranking compression pressure is given by the following equation:

Pressure = (IAP)*(IV/FV)^k

where
IAP = initial air pressure (psi)
IV = cylinder volume at intake valve close (cc)
FV = final volume at TDC (cc)
k= ratio of specific heat for the gas (air = 1.4, air/fuel = 1.36)

so...for a SCR of 9.0-1, cranking pressure would be 201 psia or 186.3 psig.
The calculation is for absolute pressure; subtract one atmosphere to correct for gauge reding.

At cranking speed we can use atmospheric pressure for IAP. At higher engine speeds, inlet air gas
dynamics will alter the initial pressure.

The cylinder volume at IVC is dependent on bore, stroke, rod length and inlet close.

The final cylinder volume is at TDC and would include chamber volume, piston dish or
dome volume, gasket volume and crevice volume.

The ratio of specific heats will vary with dry air vs air/fuel at various fuel ratios.

If we know the chamber volume at every piston position, we can calculate the pressure at
any crank angle. As well as the chamber temperature due to compression.

Now lets add fuel evaporative cooling and perhaps a little bit of turbo boost.

[swampbuggy]

David, i have got to ask you this, do you have a Degree in Mechanical Engineering ? You always add deep complex ( but interesting no doubt ) information to our discussions. Thanks Mark H.

[Stan Weiss]

In my limited experience, cranking compression is mostly determined by static CR and intake cam timing i.e. when does the intake valve close, deg after BDC.

That's pretty close.

Cranking compression pressure is given by the following equation:

Pressure = (IAP)*(IV/FV)^k

where
IAP = initial air pressure (psi)
IV = cylinder volume at intake valve close (cc)
FV = final volume at TDC (cc)
k= ratio of specific heat for the gas (air = 1.4, air/fuel = 1.36)

so...for a SCR of 9.0-1, cranking pressure would be 201 psia or 186.3 psig.
The calculation is for absolute pressure; subtract one atmosphere to correct for gauge reding.

At cranking speed we can use atmospheric pressure for IAP. At higher engine speeds, inlet air gas
dynamics will alter the initial pressure.

The cylinder volume at IVC is dependent on bore, stroke, rod length and inlet close.

The final cylinder volume is at TDC and would include chamber volume, piston dish or
dome volume, gasket volume and crevice volume.

The ratio of specific heats will vary with dry air vs air/fuel at various fuel ratios.

If we know the chamber volume at every piston position, we can calculate the pressure at
any crank angle. As well as the chamber temperature due to compression.

Now lets add fuel evaporative cooling and perhaps a little bit of turbo boost.

David,
It has been so long ago that I wrote the code that I don't remember why now. But I used ^1.29

Stan

[David Redszus]

In my limited experience, cranking compression is mostly determined by static CR and intake cam timing i.e. when does the intake valve close, deg after BDC.

That's pretty close.

Cranking compression pressure is given by the following equation:

Pressure = (IAP)*(IV/FV)^k

where
IAP = initial air pressure (psi)
IV = cylinder volume at intake valve close (cc)
FV = final volume at TDC (cc)
k= ratio of specific heat for the gas (air = 1.4, air/fuel = 1.36)

so...for a SCR of 9.0-1, cranking pressure would be 201 psia or 186.3 psig.
The calculation is for absolute pressure; subtract one atmosphere to correct for gauge reding.

At cranking speed we can use atmospheric pressure for IAP. At higher engine speeds, inlet air gas
dynamics will alter the initial pressure.

The cylinder volume at IVC is dependent on bore, stroke, rod length and inlet close.

The final cylinder volume is at TDC and would include chamber volume, piston dish or
dome volume, gasket volume and crevice volume.

The ratio of specific heats will vary with dry air vs air/fuel at various fuel ratios.

If we know the chamber volume at every piston position, we can calculate the pressure at
any crank angle. As well as the chamber temperature due to compression.

Now lets add fuel evaporative cooling and perhaps a little bit of turbo boost.

David,
It has been so long ago that I wrote the code that I don't remember why now. But I used ^1.29

Stan

Stan
That number would be correct if you were predicting compression pressure for a methanol/air fuel mixture.
David

[mag2555]

The Cams spec a the quality of the build has a huge impact on cranking numbers!

[tcb3274]

What psi is considered ok for 93 octane? 200 psi?

[GRTfast]

The Cams spec a the quality of the build has a huge impact on cranking numbers!

Yup. My new cam lowered my cranking pressure by around 10% (Intake valve closes later) and the engine makes more power.

[Nut124]

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.

[Alaskaracer]

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...

[77cruiser]

_SnapShot.jpg

Pipe max gives this, mine cranks 185, I have no idea of cranking rpm, never checked.

[Stan Weiss]

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