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Why does a tighter lsa need more compression?

General engine tech -- Drag Racing to Circle Track

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GARY C
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Re: Why does a tighter lsa need more compression?

Post by GARY C » Wed Feb 12, 2020 10:06 pm

gmrocket wrote:
Wed Feb 12, 2020 2:31 pm
GARY C wrote:
Tue Feb 11, 2020 8:27 pm
LSP wrote:
Tue Feb 11, 2020 5:53 pm


What Super Stock engine builders focus on only cylinder pressure?
None!
Which super stocker is making more cylinder pressure?

10,000 RPM 750hp

10,000 RPM 740hp

Both same cubes and legal static comp ratio

I'm not saying this has anything to do with cranking psi. Maybe the one making 10 more has lower crank psi..
But your sure that it couldn't have more?

If it is lower could it be made better with more?
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Re: Why does a tighter lsa need more compression?

Post by GARY C » Wed Feb 12, 2020 10:10 pm

David Redszus wrote:
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.
True but that doesn't really explain the other example in my post of the EMC engine with the cam degree optimized and dynoed tested.

Not to mention my examples were based on the effect of IVC which would be changed due to lash.
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Re: Why does a tighter lsa need more compression?

Post by David Redszus » Wed Feb 12, 2020 10:25 pm

The difference between SCR and DCR is only the intake valve closing angle. If the IVC angle is 180 deg BTDC, then SCR and DCR will be identical. This is fine for compressors or slow running engines but will not flow enough air for a performance engine.

A later IVC, as with a long duration camshaft, will lower DCR, and depending on inlet pressure, and lower compression pressure as well.

While Dynamic Compression Ratio can be a misleading descriptor; so is cranking compression ratio and pressure.
Perhaps a better term would be motored cylinder pressure; but only if the engine is turned at speeds representative of actual operation. A pressure sensor in the spark plug hole of a running engine provides the real numbers we need.

Compression pressure, not DCR, does have an effect on horsepower for several reasons.

Increased compression pressure comes with increased compression temperature which will impact the ignition delay period, increase the flame speed, advance the combustion peak pressure location, and the onset of detonation. If the ignition advance angle is not adequately retarded, we could see some pre-ignition, as well as a premature cylinder pressure rise producing negative work.

SCR tells us nothing of value at all, except chamber volume.
IVC will determine DCR.
DCR along with inlet air pressure will determine compression pressure and compression temperature.
Compression temperature will affect the combustion process substantially, either positively or negatively.

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Re: Why does a tighter lsa need more compression?

Post by GARY C » Wed Feb 12, 2020 11:26 pm

My point was that seat to seat numbers affect DCR/Cranking compression or what ever someone thinks it should be called and the 2 real would examples I gave shows that a small change can be between 12 and 20 average hp and ft lbs.

What I say means nothing and is just what I have observed.
People should continue to follow whoever they believe if it fits their preconceived notions...after all thats how you learn new things, Right?

Interestingly many think the IVC is the most important valve event, until some one says DCR and then IVC means nothing.
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Re: Why does a tighter lsa need more compression?

Post by CamKing » Thu Feb 13, 2020 9:30 am

GARY C wrote:
Wed Feb 12, 2020 11:26 pm
Interestingly many think the IVC is the most important valve event, until some one says DCR and then IVC means nothing.
You're missing the point. IVC is important, but so are about 30 other parameters, and they all work together.
DCR is useless, because it doesn't take into account most of the parameters that control the engine's power curve.
You can't base iny formula off of 5% of the information.
What the optimum IVC point is, can depend on many other parameters. Change one other parameter, and the optimum IVC changes.
Change the size of the port, and the optimum IVC changes.
Change the altitude you're running at, and the optimum IVC changes.
Change the IVO, and the optimum IVC changes.
Change the rod length, and the optimum IVC changes.
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Re: Why does a tighter lsa need more compression?

Post by CamKing » Thu Feb 13, 2020 9:35 am

GARY C wrote:
Wed Feb 12, 2020 11:26 pm
My point was that seat to seat numbers affect DCR/Cranking compression or what ever someone thinks it should be called and the 2 real would examples I gave shows that a small change can be between 12 and 20 average hp and ft lbs.
On this, we agree 100%. That's why I'm one of the few cam companies to list the actual seat duration, not some arbitrary "Advertised" duration.
All my calculations for designing a cam for a given application, are based off of actual seat duration.
Basing calculations off of .050" durations is nowhere near as accurate, because there's too many variables that effect .050" duration.
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Re: Why does a tighter lsa need more compression?

Post by Stan Weiss » Thu Feb 13, 2020 10:51 am

CamKing wrote:
Thu Feb 13, 2020 9:30 am

You're missing the point. IVC is important, but so are about 30 other parameters, and they all work together.
DCR is useless, because it doesn't take into account most of the parameters that control the engine's power curve.
You can't base iny formula off of 5% of the information.
What the optimum IVC point is, can depend on many other parameters. Change one other parameter, and the optimum IVC changes.
Change the size of the port, and the optimum IVC changes.
Change the altitude you're running at, and the optimum IVC changes.
Change the IVO, and the optimum IVC changes.
Change the rod length, and the optimum IVC changes.
Or in other words a BBC with a stock length rod and a Pontiac with a stock length rod, with the same stroke (ex 3.75") and same IVC have different DCR and cranking compression.

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Re: Why does a tighter lsa need more compression?

Post by David Redszus » Thu Feb 13, 2020 11:36 am

Change the rod length, and the optimum IVC changes.
The rod length would have to change by a very large amount to make any difference in IVC.
Or in other words a BBC with a stock length rod and a Pontiac with a stock length rod, with the same stroke (ex 3.75") and same IVC have different DCR and cranking compression.

Stan
DCR does not determine IVC. The optimized IVC (considering Mike's parameters) will determine the DCR.
But DCR is only a ratio not an actual pressure. DCR coupled with inlet conditions at IVC, will determine
compression pressure and temperature. That's what is useful to know and important for performance.

Cranking pressure is not very useful since it does not reflect actual conditions at operating engine speeds.
That's where in-cylinder pressure measurement becomes so very important.

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Re: Why does a tighter lsa need more compression?

Post by Stan Weiss » Thu Feb 13, 2020 12:15 pm

David Redszus wrote:
Thu Feb 13, 2020 11:36 am
Change the rod length, and the optimum IVC changes.
The rod length would have to change by a very large amount to make any difference in IVC.
Or in other words a BBC with a stock length rod and a Pontiac with a stock length rod, with the same stroke (ex 3.75") and same IVC have different DCR and cranking compression.

Stan
DCR does not determine IVC. The optimized IVC (considering Mike's parameters) will determine the DCR.
But DCR is only a ratio not an actual pressure. DCR coupled with inlet conditions at IVC, will determine
compression pressure and temperature. That's what is useful to know and important for performance.

Cranking pressure is not very useful since it does not reflect actual conditions at operating engine speeds.
That's where in-cylinder pressure measurement becomes so very important.
David,
That is an interesting conclusion to draw from what I posted. That DCR determines IVC.

"DCR coupled with inlet conditions at IVC, will determine compression pressure and temperature." So if inlet conditions and IVC are the same and DCR / Dynamic stroke is different due to rod length. Is there not a change in compression pressure and temperature?

Stan
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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 GARY C » Thu Feb 13, 2020 1:36 pm

CamKing wrote:
Thu Feb 13, 2020 9:30 am
GARY C wrote:
Wed Feb 12, 2020 11:26 pm
Interestingly many think the IVC is the most important valve event, until some one says DCR and then IVC means nothing.
You're missing the point. IVC is important, but so are about 30 other parameters, and they all work together.
DCR is useless, because it doesn't take into account most of the parameters that control the engine's power curve.
You can't base iny formula off of 5% of the information.
What the optimum IVC point is, can depend on many other parameters. Change one other parameter, and the optimum IVC changes.
Change the size of the port, and the optimum IVC changes.
Change the altitude you're running at, and the optimum IVC changes.
Change the IVO, and the optimum IVC changes.
Change the rod length, and the optimum IVC changes.
I understand that, it just seems like the more people can learn about the cause and effect of these things that the better choice they can make before even ordering parts.

You see a lot of threads here were a guy wants a cam recommendation only to find out he has the wrong heads and or compression for his intended application because he just started ordering parts without knowing what he really needed, so now he needs a bandaid cam.

I know it has helped me a good deal on my last cpl of street engines to get what I was looking for the first time.
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Re: Why does a tighter lsa need more compression?

Post by David Redszus » Fri Feb 14, 2020 2:48 pm

Stan Weiss wrote:
Thu Feb 13, 2020 12:15 pm
David Redszus wrote:
Thu Feb 13, 2020 11:36 am
Change the rod length, and the optimum IVC changes.
The rod length would have to change by a very large amount to make any difference in IVC.
Or in other words a BBC with a stock length rod and a Pontiac with a stock length rod, with the same stroke (ex 3.75") and same IVC have different DCR and cranking compression.

Stan
DCR does not determine IVC. The optimized IVC (considering Mike's parameters) will determine the DCR.
But DCR is only a ratio not an actual pressure. DCR coupled with inlet conditions at IVC, will determine
compression pressure and temperature. That's what is useful to know and important for performance.

Cranking pressure is not very useful since it does not reflect actual conditions at operating engine speeds.
That's where in-cylinder pressure measurement becomes so very important.
David,
That is an interesting conclusion to draw from what I posted. That DCR determines IVC.

"DCR coupled with inlet conditions at IVC, will determine compression pressure and temperature." So if inlet conditions and IVC are the same and DCR / Dynamic stroke is different due to rod length. Is there not a change in compression pressure and temperature?

Stan
Stan
IVC determines DCR. IVC is an actual number, DCR is merely a calculated ratio; it cannot change during operation.

The influence of rod length is very small.

B=4.
S=3.75
chamber=77cc
IVC=110 BTDC
Rod=5.75
Inlet press=14.7 psi
SCR=11.03
DCR=8.47
TDC press = 252 psi

B=4.
S=3.75
chamber=77cc
IVC=110 BTDC
Rod=6.3
Inlet press=14.7 psi
SCR=11.03
DCR=8.40
TDC press = 249 psi

The difference in compression pressure is within measurement error.

In an actual running engine, the inlet air pressure will change with rpm thereby causing a change in
compression pressure. DCR remains unchanged.

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Re: Why does a tighter lsa need more compression?

Post by randy331 » Fri Feb 14, 2020 11:32 pm

I have a 355 with 10-1 comp I run in my work truck, it has a 204* at .050" duration cam.

I did a 383 street engine with 10-1 comp with a 256* at .050" duration cam.

Does one of them have the right "dynamic compression" ?

Randy

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Re: Why does a tighter lsa need more compression?

Post by GARY C » Sat Feb 15, 2020 5:15 am

randy331 wrote:
Fri Feb 14, 2020 11:32 pm
I have a 355 with 10-1 comp I run in my work truck, it has a 204* at .050" duration cam.

I did a 383 street engine with 10-1 comp with a 256* at .050" duration cam.

Does one of them have the right "dynamic compression" ?

Randy
Right dynamic compression for what, IVC? Probably so.
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Re: Why does a tighter lsa need more compression?

Post by RevTheory » Sat Feb 15, 2020 7:56 am

So let me repeat all of this back just to make sure that I understand it.

* Since static compression ratio is a ratio, it's meaningless and serves no useful purpose.
* Since dynamic compression ratio is poorly named, it's meaningless and serves no useful purpose.
* Since cranking pressure is based on a slow starter speed, it's meaningless and serves no useful purpose.
* Since the only possible way to know if your cylinder pressure will be safe for a given fuel requires expensive sensors that very few people have, it serves no useful purpose simply because you don't have access to it.

So if your valve closes at 59* ATDC and you'd like to run a meaningless compression "ratio" of 10:1 with iron heads on pump gas, you basically have no way of knowing beforehand if you've pushed yourself beyond a safe limit; again, because everything is meaningless and serves no useful purpose.

Is that about the jist of it or is it possible that some of this stuff can be useful as long as you don't try to make a religion out of it?

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Re: Why does a tighter lsa need more compression?

Post by GRTfast » Sat Feb 15, 2020 9:50 am

CamKing wrote:
Thu Feb 13, 2020 9:35 am
GARY C wrote:
Wed Feb 12, 2020 11:26 pm
My point was that seat to seat numbers affect DCR/Cranking compression or what ever someone thinks it should be called and the 2 real would examples I gave shows that a small change can be between 12 and 20 average hp and ft lbs.
On this, we agree 100%. That's why I'm one of the few cam companies to list the actual seat duration, not some arbitrary "Advertised" duration.
All my calculations for designing a cam for a given application, are based off of actual seat duration.
Basing calculations off of .050" durations is nowhere near as accurate, because there's too many variables that effect .050" duration.
Interesting to hear you say that. I always felt like something was being overlooked or ignored in the @ 0.050 duration numbers.
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