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Why does a tighter lsa need more compression?

General engine tech -- Drag Racing to Circle Track

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PRH
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Re: Why does a tighter lsa need more compression?

Post by PRH » Tue Feb 11, 2020 2:42 pm

MadBill wrote:
Tue Feb 11, 2020 2:10 pm
PRH wrote:
Tue Feb 11, 2020 1:15 pm
..
No one can think of a scenario where knowing the cranking compression might be helpful?
Well, if an engine of unknown specs cranked at say 220 psi, I probably wouldn't tank up the vehicle with 87 octane...
Similarly, if you were modeling an engine for heavy towing to be run on 87 octane fuel, and some of your inputs resulted in calculated cranking pressures too high for that octane, in that application....... you could make some adjustments to the plan to bring the cranking number within a more suitable range.
Somewhat handy with a die grinder.

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Re: Why does a tighter lsa need more compression?

Post by David Redszus » Tue Feb 11, 2020 2:47 pm

mt-engines wrote:
Tue Feb 11, 2020 1:34 pm
PRH wrote:
Tue Feb 11, 2020 1:15 pm
randy331 wrote:
Mon Feb 10, 2020 10:14 am

I agree.
Randy
No one can think of a scenario where knowing the cranking compression might be helpful?
Example
10:1bbc 496 dart 320 heads
274/285@.050 .740 .720 109 107icl
235psi cranking compression
740hp
Same engine :
268/276 @.050 .770 .730 112 108icl
210psi cranking compression
765hp
What did we learn? Nothing.. What did it prove? Nothing
What will it help? Figuring if lash is changing without taking the valve covers off i guess.
Interesting. If the SCR is the same for both engines, then why is there a significant difference in cranking pressure?

Why does engine A, with longer duration, and later IVC, produce a higher cranking pressure?

Why does engine B, with lower compression, shorter duration, make more power than engine A?

Clearly, compression has absolutely no predictive value. Then why do we concern ourselves with SCR or DCR, or cranking compression?

Perhaps we do not understand the role that compression pressure plays in the combustion process.

Hint: its about the compression temperature produced, not the pressure.

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Re: Why does a tighter lsa need more compression?

Post by Orr89rocz » Tue Feb 11, 2020 3:26 pm

Then why do we concern ourselves with SCR or DCR, or cranking compression?
Fuel choice i guess

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Re: Why does a tighter lsa need more compression?

Post by GARY C » Tue Feb 11, 2020 3:26 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 and it hurt average power as much as 20 hp and ft lbs. across the pull.

A similar example on an EMC engine here.
To take full advantage of the large airflow potential when running this small-cube engine, we consulted with Chris Mays at Comp Cams, who ground us a couple of small-duration solid flat-tappet cams to try with the most aggressive opening rates possible for a Chevy lifter. We were initially pleased with the first cam, a 236/242-at-.050 grind; however, I spent some time with a degree wheel and a dial indicator and realized that I could get the same intake valve seat-to-seat time, but with more duration everywhere else if I ran a 242/242-at-.050 cam, but opened up the lash on the intake. Dyno testing confirmed my theory, and we were rewarded with a 7-12 ft-lb gain in torque across the board https://www.hotrod.com/articles/0903phr ... vy-engine/
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Re: Why does a tighter lsa need more compression?

Post by Stan Weiss » Tue Feb 11, 2020 3:37 pm

GARY C wrote:
Tue Feb 11, 2020 3:26 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 and it hurt average power as much as 20 hp and ft lbs. across the pull.

A similar example on an EMC engine here.
To take full advantage of the large airflow potential when running this small-cube engine, we consulted with Chris Mays at Comp Cams, who ground us a couple of small-duration solid flat-tappet cams to try with the most aggressive opening rates possible for a Chevy lifter. We were initially pleased with the first cam, a 236/242-at-.050 grind; however, I spent some time with a degree wheel and a dial indicator and realized that I could get the same intake valve seat-to-seat time, but with more duration everywhere else if I ran a 242/242-at-.050 cam, but opened up the lash on the intake. Dyno testing confirmed my theory, and we were rewarded with a 7-12 ft-lb gain in torque across the board https://www.hotrod.com/articles/0903phr ... vy-engine/
Gary,
You would not happen to know how much reduction in cranking compression psi there was with that change?

Stan
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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: Why does a tighter lsa need more compression?

Post by mt-engines » Tue Feb 11, 2020 3:42 pm

Stan Weiss wrote:
Tue Feb 11, 2020 2:36 pm
mt-engines wrote:
Tue Feb 11, 2020 1:34 pm

Example
10:1bbc 496 dart 320 heads
274/285@.050 .740 .720 109 107icl
235psi cranking compression
740hp
Same engine :
268/276 @.050 .770 .730 112 108icl
210psi cranking compression
765hp
What did we learn? Nothing.. What did it prove? Nothing
What will it help? Figuring if lash is changing without taking the valve covers off i guess.
Were these measured or calculated. They make no sense to me if CR was 10.0:1 for both.

Stan
Pumped on the dyno. Who cares what some arbitrary calculation is

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Re: Why does a tighter lsa need more compression?

Post by GARY C » Tue Feb 11, 2020 3:44 pm

Stan Weiss wrote:
Tue Feb 11, 2020 3:37 pm
GARY C wrote:
Tue Feb 11, 2020 3:26 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 and it hurt average power as much as 20 hp and ft lbs. across the pull.

A similar example on an EMC engine here.
To take full advantage of the large airflow potential when running this small-cube engine, we consulted with Chris Mays at Comp Cams, who ground us a couple of small-duration solid flat-tappet cams to try with the most aggressive opening rates possible for a Chevy lifter. We were initially pleased with the first cam, a 236/242-at-.050 grind; however, I spent some time with a degree wheel and a dial indicator and realized that I could get the same intake valve seat-to-seat time, but with more duration everywhere else if I ran a 242/242-at-.050 cam, but opened up the lash on the intake. Dyno testing confirmed my theory, and we were rewarded with a 7-12 ft-lb gain in torque across the board https://www.hotrod.com/articles/0903phr ... vy-engine/
Gary,
You would not happen to know how much reduce in cranking compression psi there was with that change?

Stan
No I never measured it but using Patric's DCR calc and assuming it was only a 4 degree degree difference (292 vs 296) it would reduce DCR from a 8.74 to 8.58
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Re: Why does a tighter lsa need more compression?

Post by Stan Weiss » Tue Feb 11, 2020 3:56 pm

mt-engines wrote:
Tue Feb 11, 2020 3:42 pm
Stan Weiss wrote:
Tue Feb 11, 2020 2:36 pm
mt-engines wrote:
Tue Feb 11, 2020 1:34 pm

Example
10:1bbc 496 dart 320 heads
274/285@.050 .740 .720 109 107icl
235psi cranking compression
740hp
Same engine :
268/276 @.050 .770 .730 112 108icl
210psi cranking compression
765hp
What did we learn? Nothing.. What did it prove? Nothing
What will it help? Figuring if lash is changing without taking the valve covers off i guess.
Were these measured or calculated. They make no sense to me if CR was 10.0:1 for both.

Stan
Pumped on the dyno. Who cares what some arbitrary calculation is
What was the recommended lash for each cam and what was the lash set to. Also what was the adv seat-to-seat duration of each cam.

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: Why does a tighter lsa need more compression?

Post by BradH » Tue Feb 11, 2020 4:10 pm

Sorry, I'm a Joe Amateur here. But, if you're saying cranking pressure doesn't mean sh!t, you must not have dealt with -- or have forgotten about -- pump-gas builds of "old school" no-quench, iron-head, big-block street/strip engines.

As long as my own cranking pressure measurements were taken the same way, there were definitely some octane guidelines that correlated well to those #s. If the engine was 165-ish, I was good on straight 93 or 93 E10 with a "normal" timing curve. When the #s were hitting 180+/-, I needed to blend a higher octane fuel to keep the occasional ping factor under control.

I don't believe in living & dying buy some calculated "adjusted CR #" that doesn't factor the changes in the engine's VE, etc., across the RPM range, and I've p!ssed off some people when I've called bogus on that approach. However, I believe there is something of a sanity check for octane-limited applications when I have this reference point -- even if it's a # that only applies to my builds.

Your mileage may vary and your opinion may differ. Welcome to the Free World.

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Re: Why does a tighter lsa need more compression?

Post by mt-engines » Tue Feb 11, 2020 4:28 pm

Stan Weiss wrote:
Tue Feb 11, 2020 3:56 pm
mt-engines wrote:
Tue Feb 11, 2020 3:42 pm
Stan Weiss wrote:
Tue Feb 11, 2020 2:36 pm


Were these measured or calculated. They make no sense to me if CR was 10.0:1 for both.

Stan
Pumped on the dyno. Who cares what some arbitrary calculation is
What was the recommended lash for each cam and what was the lash set to. Also what was the adv seat-to-seat duration of each cam.

Stan
Stan, you are missing my point... Who the F cares about cranking compression? Who cares about calculated compression? It proves nothing.

Why dont you ask about starter rpm, engine temp, throttle blade position etc.. Bottom line is nobody i know thst builds competitive racing engines uses these calculators, wastes their time with these camshaft programs or relates cranking compression to anything other than it being an observed number.

You can make graphs and spreadsheets all you want.. My customers want to win.. And as long as the HP number is high they could care less about anything else..

Lash was the same seat duration was similar

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Re: Why does a tighter lsa need more compression?

Post by Orr89rocz » Tue Feb 11, 2020 4:37 pm

^^ agreed when fuel octane isnt limited but if it is then maybe there is correlation or some importantance

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Re: Why does a tighter lsa need more compression?

Post by GARY C » Tue Feb 11, 2020 4:44 pm

mt-engines wrote:
Tue Feb 11, 2020 4:28 pm
Stan Weiss wrote:
Tue Feb 11, 2020 3:56 pm
mt-engines wrote:
Tue Feb 11, 2020 3:42 pm


Pumped on the dyno. Who cares what some arbitrary calculation is
What was the recommended lash for each cam and what was the lash set to. Also what was the adv seat-to-seat duration of each cam.

Stan
Stan, you are missing my point... Who the F cares about cranking compression? Who cares about calculated compression? It proves nothing.

Why dont you ask about starter rpm, engine temp, throttle blade position etc.. Bottom line is nobody i know thst builds competitive racing engines uses these calculators, wastes their time with these camshaft programs or relates cranking compression to anything other than it being an observed number.

You can make graphs and spreadsheets all you want.. My customers want to win.. And as long as the HP number is high they could care less about anything else..

Lash was the same seat duration was similar
As I posted before this engine builder says he doesn't care about SCR but only cylinder pressure and references cranking psi in his statement. And he is not the only race winning engine builder who feels this way. All data is usful if it's used correctly.
Bill has built engines at the highest levels of both stock car and drag racing. In '85, '87 and '93 Bill was the nhra world champ in competition eliminator - a series of racing that clearly shows wh makes the most power. Today he's focusing on building engines for the nascar west series and looks to be having the same level of success. viewtopic.php?f=47&t=57586
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Re: Why does a tighter lsa need more compression?

Post by Stan Weiss » Tue Feb 11, 2020 5:08 pm

mt-engines wrote:
Tue Feb 11, 2020 4:28 pm

Stan, you are missing my point... Who the F cares about cranking compression? Who cares about calculated compression? It proves nothing.

Why dont you ask about starter rpm, engine temp, throttle blade position etc.. Bottom line is nobody i know thst builds competitive racing engines uses these calculators, wastes their time with these camshaft programs or relates cranking compression to anything other than it being an observed number.

You can make graphs and spreadsheets all you want.. My customers want to win.. And as long as the HP number is high they could care less about anything else..

Lash was the same seat duration was similar
I am glad that your customers are happy with what you do for them.

But how does that help the non-professional engine build that comes here for help? You say ignore what I and others have said, but don't give any recommended methods of how to get the job done.

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: Why does a tighter lsa need more compression?

Post by mt-engines » Tue Feb 11, 2020 5:21 pm

Stan Weiss wrote:
Tue Feb 11, 2020 5:08 pm
mt-engines wrote:
Tue Feb 11, 2020 4:28 pm

Stan, you are missing my point... Who the F cares about cranking compression? Who cares about calculated compression? It proves nothing.

Why dont you ask about starter rpm, engine temp, throttle blade position etc.. Bottom line is nobody i know thst builds competitive racing engines uses these calculators, wastes their time with these camshaft programs or relates cranking compression to anything other than it being an observed number.

You can make graphs and spreadsheets all you want.. My customers want to win.. And as long as the HP number is high they could care less about anything else..

Lash was the same seat duration was similar
I am glad that your customers are happy with what you do for them.

But how does that help the non-professional engine build that comes here for help? You say ignore what I and others have said, but don't give any recommended methods of how to get the job done.

Stan
Even an amatuer engine builder seeks what first? A HP goal, rpm goal. Street or strip etc.. It takes x amount of cam, x amount of head and x amount of cubic inch to get those goals period. Happy dynos do exist and some people goals are not feasible. Throw in some arbitrary calculations and it csn put people many steps backwards.

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Re: Why does a tighter lsa need more compression?

Post by LSP » 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?

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