Static CR too high?

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David Redszus
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Re: Static CR too high?

Post by David Redszus »

Is there a reference that you know of to find the autoignition temp of 92 AKI gasoline?
Call the fuel mfg.
I have scoured the web and found nada. From your initial post, it sounds like I should be safe in that department. Yes?
Yes, depending on the inlet air temp.
I may have misquoted Larry and his remarks regarding the use of DCR to determine the relative safety of using pump gas. I think he was referring to cylinder pressure as it relates to VE: better VE = more air/fuel trapped into the cylinder = higher cylinder pressure & possible preignition even though the DCR/TCR remains constant. Does that sound more accurate?
Again, VE refers to trapped air mass. DCR (worthless) and TCR (worthwhile) do remain constant. But the conditions at IVC do not. A better flowing head would result in a higher pressure at IVC thus resulting in a higher compression pressure. An increase in chamber air density will result in higher combustion temperatures which would have an effect on detonation but not pre-ignition.
Were your calculations (based on my stated engine metrics) made with an assumption that the engine is operating at 100% VE and air density at sea level?
As a starting point I use 14.7 psi and 100F. VE is not considered because of the measurement difficulties.
How would bolting-on a set of heads that are capable of flowing more air/fuel change the results of the calculations? Or would they?
Little or no change.
Overall, it sounds like you believe that my combination should be safe with 92 pump gas. Yes?
It might be safe for short bursts of full throttle, not for sustained high load applications. Pump gas should be avoided whenever serious driving is expected.
Do you think that a knock sensor & timing retard may be necessary or helpful in this application?
A properly selected and fitted knock sensor is always helpful, especially when using pump gas; not so much with race gas.
MidWestCoast442
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Re: Static CR too high?

Post by MidWestCoast442 »

Okay. So a better flowing head would result in higher compression pressure due to greater volume of air trapped in the cylinder? But that has no effect on preignition or detonation?

Greater air density (affected by inlet air temp, altitude, ram air effect, forced induction, etc) results in higher combustion temps which can affect detonation, but not preignition?

Hopefully I’m understanding that correctly.

But I don’t seem to be understanding the effects that the new and better-flowing cylinder heads may have on all of this. You said that it can cause the pressure at IVC to be greater and, thus, yield higher compression pressure. Then you stated that a better set of heads would have little or no effect on the calculations from your initial post.

I plan to use the car for street/strip driving, not sustained high load. Am I correct to assume that sustained high load applications would not be appropriate because of the effects of heat soak?

Do you consider a J&S SafeGuard device to be appropriate for my application? Seems like it may be (relatively) cheap insurance.
David Redszus
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Re: Static CR too high?

Post by David Redszus »

Okay. So a better flowing head would result in higher compression pressure due to greater volume of air trapped in the cylinder? But that has no effect on preignition or detonation?
The volume of air does not change with head flow. The improved air flow has no bearing on Trapped Compression Ratio. An higher inlet pressure at IVC will result in a higher compression pressure, but does not change ratio. Do not confused compression RATIO with compression PRESSURE and TEMP.
Greater air density (affected by inlet air temp, altitude, ram air effect, forced induction, etc) results in higher combustion temps which can affect detonation, but not pre-ignition?
None of those factors will affect compression pressure or temperature. They will impact combustion temperature and pressure. Pre-ignition occurs prior to TDC while detonation occurs (usually) after TDC.
I plan to use the car for street/strip driving, not sustained high load. Am I correct to assume that sustained high load applications would not be appropriate because of the effects of heat soak?
Yes, heat soak is a factor unless a properly sized cooling system can dissipate the heat to the air adequately.
Do you consider a J&S SafeGuard device to be appropriate for my application?
Sure. I worked with John at J & S for many years, many years ago. He's a good guy to work with.
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Re: Static CR too high?

Post by MadBill »

I believe he's a SpeedTalker; don't know his nom de plume though...
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.
MidWestCoast442
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Re: Static CR too high?

Post by MidWestCoast442 »

Thanks again, David. This is all making more sense now.

When building a motor to safely operate on pump gas, which parameters & values are important to consider and not exceed?

I have already contacted John by email and there is a SpeedTalker from whom I wish to purchase a SafeGuard. Just waiting on a PM reply.
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Re: Static CR too high?

Post by CharlieB53 »

David Redszus wrote: Tue Jun 19, 2018 10:49 am
How about the compression temp when accounting for timing spark BTDC?
The compression temperature (and pressure) will change with each change in piston position. So, at any ignition angle we know the piston position and therefore we know the temp and pressure.
How does that compression temp change if we change the spark to 34* BTDC? 35?
The later the timing, the higher the chamber temp and pressure
I'm not so sure I understand correctly.

Granted, there is temp rise per degree of piston movement on the compression stroke.

However, wouldn't that temp rise much quicker once ignition initiated?

Making any advance in timing creating more heat at TDC?

Isn't this the whole purpose of vacuum advance on street motors? Mileage (somewhat) VS Performance.

This has always been a grey area that is constantly muddying the waters.
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Re: Static CR too high?

Post by David Redszus »

Granted, there is temp rise per degree of piston movement on the compression stroke.
However, wouldn't that temp rise much quicker once ignition initiated?
Yes it will.
Assuming an ignition angle of 34 deg. there will be an ignition delay of about 8.5 -9.0 deg so that
combustion will begin at approx 25 deg BTC. The flame kernal will be about the size of the spark plug gap and will grow quite slowly until accelerated by turbulent squish velocity at about 10 deg BTC. The rise in combustion temperature is added to the rise in compression temperature.
Making any advance in timing creating more heat at TDC?
Yes, it will.
Isn't this the whole purpose of vacuum advance on street motors? Mileage (somewhat) VS Performance.
Normally, yes. A closed or partial throttle requires more advance in order to maintain the correct position of the combustion pressure curve centroid.
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