Simple question: Does overlap affect cranking pressure?

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BradH
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Simple question: Does overlap affect cranking pressure?

Post by BradH »

Simple question: Does camshaft overlap affect cranking pressure?

I say no, and that it's a function of the IVC event.

Someone with whom I had a discussion on cam 'stuff' says yes, although I'm not going to post their explanation.

Thanks - Brad
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Re: Simple question: Does overlap affect cranking pressure?

Post by user-23911 »

No , of course not.

It's IVC.


Again........it's common sense which isn't all that common.
It might affect it a bit if the throttle is closed but again, that depends on how many cylinders you've got and how many throttles.
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Re: Simple question: Does overlap affect cranking pressure?

Post by MadBill »

"Of course not" implies the application of reasoning... :(
A related fine example of its frequent absence is the advice I've seen in print a number of times re how to dial in your cam. Per these learned sources, you just keep advancing it until the maximum cranking pressure is obtained. For some reason, the greater the intake duration, the more cam advance is required... #-o
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Re: Simple question: Does overlap affect cranking pressure?

Post by BradH »

BradH wrote: Wed Nov 29, 2017 11:42 pm Simple question: Does camshaft overlap affect cranking pressure?
Just in case I used the incorrect terminology, I do mean the cylinder pressure of a running engine, not just the PSI that shows on a guage while spinning the engine with the starter.
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Re: Simple question: Does overlap affect cranking pressure?

Post by digger »

Yes but bugger all on cranking compression . Actual running engine yes anything that affects mass trapped in cylinder does
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Re: Simple question: Does overlap affect cranking pressure?

Post by user-23911 »

In the case of a DOHC or quad cam driven by a belt......
When you get the belt wrong for whatever reason and it jumps a tooth....and depending on the engine for specifics....then
Your cranking compression goes up. IVC happens sooner.

When it's a running engine with the same problem, you'll get an increase in low down torque but it'll run out of breath in the upper RPMs.
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Re: Simple question: Does overlap affect cranking pressure?

Post by digger »

Opening the inlet valve earlier allows the air flow to start sooner if the exhaust is working semi decently (ie exhaust port is negative pressure ) to pull the cylinder pressure sufficiently lower than Inlet pressure . It also means during valve opening at a given crank angle the valve is further off the seat which has potential to increase flow on the valve opening period ( piston works on a bigger valve curtain area nd also this might translate to better "ramming" near ivc
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Re: Simple question: Does overlap affect cranking pressure?

Post by BradH »

digger wrote: Thu Nov 30, 2017 1:07 am Yes but bugger all on cranking compression . Actual running engine yes anything that affects mass trapped in cylinder does
At high(er) RPM, there is the benefit of improving scavenging and drawing more air-fuel mix in. But below that point, is an engine bleeding off cylinder pressure during the overlap phase?
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Re: Simple question: Does overlap affect cranking pressure?

Post by Amilcar »

If your saying only about compression pressure( not only cranking,but lower rpm`s too) the IVC will rules most.
With the same IVC, but with more overlap, the exh valve will stay further open on the beginning of the intake stroke. The compression pressure may result the same as before but, now with more dilution on the fresh incoming from the exhaust port, lower rpms output goes down.
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Re: Simple question: Does overlap affect cranking pressure?

Post by rfoll »

Dynamic compression also varies with throttle plate position. Most would be amazed at how low it is at highway speeds.
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Re: Simple question: Does overlap affect cranking pressure?

Post by pastry_chef »

A few years back there was an article, in their test increasing rocker ratio increased cranking compression on a SBC.

I might be able to find a post from Harold B (maybe on another forum), he said different LSA with the same intake closing will have different cranking pressure.
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Re: Simple question: Does overlap affect cranking pressure?

Post by David Redszus »

Gas dynamics during overlap are a function of: throttle, pressure ratios, overlap duration, valve lifts, valve sizes, and flow coefficients.

Of these, changes in engine speed and throttle have substantial effects on pressure ratios.

Once again, in-cylinder pressure measurement is required to comprehend reality.
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Re: Simple question: Does overlap affect cranking pressure?

Post by digger »

BradH wrote: Thu Nov 30, 2017 9:04 am
digger wrote: Thu Nov 30, 2017 1:07 am Yes but bugger all on cranking compression . Actual running engine yes anything that affects mass trapped in cylinder does
At high(er) RPM, there is the benefit of improving scavenging and drawing more air-fuel mix in. But below that point, is an engine bleeding off cylinder pressure during the overlap phase?
The exhaust can work really well at low rpm to depending on the configuration e.g largely length
If there is reversion during overlap It delays the start of the induction It never bleeds off because that implies there was once higher pressure which was then lost aka inlet reversion around ivc
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Re: Simple question: Does overlap affect cranking pressure?

Post by pastry_chef »

http://www.superchevy.com/how-to/81679- ... mpression/
Quote
"We performed an interesting rocker ratio test and discovered that changing from a 1.50:1 rocker to a 1.62:1 rocker arm was worth 5 psi of cranking compression. This may not be valid for all engines, but it did work on ours."

UDHarold wrote: Mon Sep 03, 2007 11:47 am The intake valve closing point mainly tells where the intake valve closes.
Get your favorite cam grinder to grind 2 cams with the identical profiles, but with TWO different LSAs, say 106 and 108, or 110 and 112.
Install one cam in the engine, then dyno it.
Install the other cam on the IDENTICAL intake centerline, so that opening and CLOSING points are the same for both cams.
The cams will have different cranking compressions, different power curves. This is not to say one is better than the other, only different.
What effects the REAL dynamic compression is HOW MUCH air and gas is in the cylinder when the intake valve closes, not at what degree ABDC the valve closes.
The different exhaust opening points affect the amount of back pressure, ie, reversion, that is present when the intake valve opens BTDC. The amount of reversion affects the rate of cylinder filling on the intake stroke, and the amount of air and gas in the cylinder when the intake valve closes. This is what gives you the actual Dynamic Compression. The operative word is DYNAMIC.
Other cam designers may differ in opinion.

UDHarold
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Re: Simple question: Does overlap affect cranking pressure?

Post by pastry_chef »

joe 90 wrote: Wed Nov 29, 2017 11:52 pm No , of course not.

It's IVC.

Again........it's common sense which isn't all that common.
Ironic.
Mike R
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