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Why does a tighter lsa need more compression?

General engine tech -- Drag Racing to Circle Track

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Stan Weiss
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Re: Why does a tighter lsa need more compression?

Post by Stan Weiss » Tue Feb 11, 2020 6:17 pm

Race engine 300 cranking compression

https://www.yellowbullet.com/threads/ra ... on.618754/

George Bryce
Discussion Starter • #1 • Nov 24, 2013
If I go real fast and my engine cranks over at 300 psi...what should it be?
If I loosen the lash .010 and it pumps over at 320 psi is this better?
What if I keep working and I get it up to 350 psi cranking...and I need 2, 800 CCA batteries and need to wire them to give me 24 volts to rotate the engine to test cranking compression? Will it go faster. Looking for opinions . Thanks
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Re: Why does a tighter lsa need more compression?

Post by David Redszus » Tue Feb 11, 2020 6:57 pm

Some food for thought, on this engine the only change made was tightening the valve lash from about .030 intake and exhaust to .026/.028 there by increasing only the seat numbers
Maybe not. A change in lash will produce a change in IVC. How much did the lash change affect IVC?

A change in IVC will change compression pressure and compression temperature.
How much did compression pressure and temperature actually change?

How does compression pressure and temperature change with engine speed?

Compression pressure alone, has no effect on required octane.

Ignorance screams loudly when cranking pressure is correlated to octane requirement.

Two fuels with the same octane numbers may not provide the same detonation protection.

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Re: Why does a tighter lsa need more compression?

Post by hoodeng » Tue Feb 11, 2020 7:17 pm

In reply to mt's post, I think the thing that people should/could/ would seek first is that their final target engine can be achieved in their current engine being developed / modified.
I have had customers with engines that have been modified in stages over years that want to go to the next big step, not understanding the current assembly has hit its wall as far as mods go, and any more investment will compromise their goals.

When you tell them they are in different crank case territory, some think they have wasted their money getting to where they are at the moment or worse think they have been misguided from the outset...Or,,, some can see the wall coming and accept that, this guy is always a lot easier to deal with.

In my field the only component i have seen that has lived through the mod stages from mild to wild has been a decent programmable electronic ignition.

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Re: Why does a tighter lsa need more compression?

Post by GARY C » Tue Feb 11, 2020 8:27 pm

LSP wrote:
Tue Feb 11, 2020 5:53 pm
GARY C wrote:
Sat Feb 08, 2020 6:39 am
Not sure but if any of the Credited Super Stock engine builders matter, they only focus on cylinder pressure and could care a less about compression ratio. The same could be said for boosted and nitrous applications.
What Super Stock engine builders focus on only cylinder pressure?
None!
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Re: Why does a tighter lsa need more compression?

Post by gmrocket » Tue Feb 11, 2020 9:00 pm

PRH wrote:
Tue Feb 11, 2020 1:36 pm
Well, I was thinking in terms of builds that weren’t racing engines, and where what it makes for power isn’t as high on the list of importance as other items.

Don’t some of the engine modeling sims show the cranking pressure as one of the “results”?
Like if the engine is being loaded from 2500 rpm at wot ?

I think it does give you an idea if it will make good tq down low. Two engines of the same cubic inch, one cranks 150, the other cranks 200...my money is on the 200 one making more tq at 2500

This was talked about years ago when the EMC was pulling from 2500. Someone said the cranking psi means absolutely nothing for an EMC build....he was a competitor , said his engine was cranking 150.

I know mine was over 200...

His engine was way down on TQ so it didn't score well at all. I'm not saying it was solely from low cranking psi.

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Re: Why does a tighter lsa need more compression?

Post by gmrocket » Tue Feb 11, 2020 9:14 pm

GARY C wrote:
Sat Feb 08, 2020 6:39 am
mt-engines wrote:
Sat Feb 08, 2020 12:45 am
Why. Is this thread still going.. And why do people care what your engine cranks for compression? Im not building air compressors..
Not sure but if any of the Credited Super Stock engine builders matter, they only focus on cylinder pressure and could care a less about compression ratio. The same could be said for boosted and nitrous applications.
Compression ratio is absolutely critical in SS.. Every single SS engine builder , builds with the max allowed static comp ratio allowed by the rules...

They do care..

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Re: Why does a tighter lsa need more compression?

Post by LSP » Tue Feb 11, 2020 10:11 pm

gmrocket wrote:
Tue Feb 11, 2020 9:14 pm
GARY C wrote:
Sat Feb 08, 2020 6:39 am
mt-engines wrote:
Sat Feb 08, 2020 12:45 am
Why. Is this thread still going.. And why do people care what your engine cranks for compression? Im not building air compressors..
Not sure but if any of the Credited Super Stock engine builders matter, they only focus on cylinder pressure and could care a less about compression ratio. The same could be said for boosted and nitrous applications.
Compression ratio is absolutely critical in SS.. Every single SS engine builder , builds with the max allowed static comp ratio allowed by the rules...

They do care..
Most of the time absolutely, but there are some cases where a smoother dome profile trumps hitting a dome CC volume number.

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Re: Why does a tighter lsa need more compression?

Post by GARY C » Tue Feb 11, 2020 10:28 pm

gmrocket wrote:
Tue Feb 11, 2020 9:14 pm
GARY C wrote:
Sat Feb 08, 2020 6:39 am
mt-engines wrote:
Sat Feb 08, 2020 12:45 am
Why. Is this thread still going.. And why do people care what your engine cranks for compression? Im not building air compressors..
Not sure but if any of the Credited Super Stock engine builders matter, they only focus on cylinder pressure and could care a less about compression ratio. The same could be said for boosted and nitrous applications.
Compression ratio is absolutely critical in SS.. Every single SS engine builder , builds with the max allowed static comp ratio allowed by the rules...

They do care..
Every single one? Really? And I thought my statement was bold.
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Re: Why does a tighter lsa need more compression?

Post by gmrocket » Wed Feb 12, 2020 2:05 am

GARY C wrote:
Tue Feb 11, 2020 10:28 pm
gmrocket wrote:
Tue Feb 11, 2020 9:14 pm
GARY C wrote:
Sat Feb 08, 2020 6:39 am


Not sure but if any of the Credited Super Stock engine builders matter, they only focus on cylinder pressure and could care a less about compression ratio. The same could be said for boosted and nitrous applications.
Compression ratio is absolutely critical in SS.. Every single SS engine builder , builds with the max allowed static comp ratio allowed by the rules...

They do care..
Every single one? Really? And I thought my statement was bold.
Yup, 100%

Saying the could care less...no.
Last edited by gmrocket on Wed Feb 12, 2020 2:11 am, edited 1 time in total.

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Re: Why does a tighter lsa need more compression?

Post by gmrocket » Wed Feb 12, 2020 2:08 am

LSP wrote:
Tue Feb 11, 2020 10:11 pm
gmrocket wrote:
Tue Feb 11, 2020 9:14 pm
GARY C wrote:
Sat Feb 08, 2020 6:39 am
i


Not sure but if any of the Credited Super Stock engine builders matter, they only focus on cylinder pressure and could care a less about compression ratio. The same could be said for boosted and nitrous applications.
Compression ratio is absolutely critical in SS.. Every single SS engine builder , builds with the max allowed static comp ratio allowed by the rules...

They do care..
Most of the time absolutely, but there are some cases where a smoother dome profile trumps hitting a dome CC volume number.
Then that would your max...like I said.

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Re: Why does a tighter lsa need more compression?

Post by David Redszus » Wed Feb 12, 2020 10:56 am

As a design exercise, assume two engines with identical SCRs of 14-1.
Will they have the same cranking compression pressure? If not, why not?

Clearly, two engines with identical SCRs can have different DCRs as indicated by their compression pressures.

If compression pressures are different for the same SCRs, what effect does that have on performance?

Conversely, we could design two engines with identical DCRs but with different SCR numbers.

We can increase SCR in an engine by increasing piston dome volume, or reducing squish clearance.
The effect on flame speed and performance for each could be quite noticeable.

It is remarkable that over years of discussion and heated opinion, there has been no explanation of
how increased compression actually works to improve torque (not power). Or cause detonation.

What is missing from the loop of understanding is combustion pressure data via pressure measurement.
Clint, at TFX has attempted to show how engines actually work many times. Yet folklore persists.

Merely assembling an engine that was designed by someone else does not qualify one as an engine builder.

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Re: Why does a tighter lsa need more compression?

Post by CamKing » Wed Feb 12, 2020 12:42 pm

It's pretty simple. If cranking compression equated to HP, everyone would be running cams with intake centerlines around 40. That'll give you a ton of cranking compression.
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Re: Why does a tighter lsa need more compression?

Post by n2omike » Wed Feb 12, 2020 1:09 pm

People want to take something fairly basic... and read WAY too much from it.

Big cams need more compression ratio, as they close the intake valve sooner, trapping less air.
"Dynamic" compression ratio gets you in the ballpark, as to a workable compression ratio.

Most of us know tightening the LSA cranks up cylinder pressure, narrows and lowers the rpm range, and makes the engine less smooth and more 'punchy'. We also know narrowing it a bit for more overlap seems to help overly large intake ports.

99% of us are DONE at this point. We don't know enough to RELIABLY go past this without dyno testing on a given engine combination... and then our results are limited to that very combination.

Many of that 99% will attempt to speculate past the above, but it is just that... SPECULATION.

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Re: Why does a tighter lsa need more compression?

Post by Stan Weiss » Wed Feb 12, 2020 1:11 pm

GMRocket / Walter,
Weren't there some years based on the EMC rules that you had your ICL in the 90's?

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Re: Why does a tighter lsa need more compression?

Post by CamKing » Wed Feb 12, 2020 1:18 pm

Stan Weiss wrote:
Wed Feb 12, 2020 1:11 pm
GMRocket / Walter,
Weren't there some years based on the EMC rules that you had your ICL in the 90's?

Stan
a 90 ICL wouldn't have near the cranking compression as a 40 ICL.
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