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Why does a tighter lsa need more compression?

General engine tech -- Drag Racing to Circle Track

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

Post by MadBill » Sun Feb 09, 2020 6:30 pm

maxracesoftware wrote:
Sun Feb 09, 2020 4:50 pm
..
Most probably using an even larger than 0.050" would be a better correlation .
It doesn't take much curtain area to feed an engine at cranking speed, say 200 RPM, so I think the seat-to-seat numbers would be more accurate since hydraulic intensity variations among various X° @ 0.050" lobes would have a significant effect on cranking pressure.
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Re: Why does a tighter lsa need more compression?

Post by gmrocket » Sun Feb 09, 2020 6:57 pm

9 times out of 10 you'll get a different DCR number using difficult DCR calculators.

Some will use the .050 number and tell you. Others say add about 15, some say 20 of it's an old style cam..or add 25 if it's like a really old school 60's type cam because of the slow ramps..

I get the odd customers asking what I'm setting their DYNAMIC compression ratio at....which is kinda weird since they have no idea what it means...I'll them give them a few links to different online DCR calculators and ask them to get back to me on the question about what I'm setting it at...all give the DCR numbers

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Re: Why does a tighter lsa need more compression?

Post by gmrocket » Sun Feb 09, 2020 7:00 pm

MadBill wrote:
Sun Feb 09, 2020 6:30 pm
maxracesoftware wrote:
Sun Feb 09, 2020 4:50 pm
..
Most probably using an even larger than 0.050" would be a better correlation .
It doesn't take much curtain area to feed an engine at cranking speed, say 200 RPM, so I think the seat-to-seat numbers would be more accurate since hydraulic intensity variations among various X° @ 0.050" lobes would have a significant effect on cranking pressure.
Yup,

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Re: Why does a tighter lsa need more compression?

Post by GARY C » Sun Feb 09, 2020 7:36 pm

MadBill wrote:
Sun Feb 09, 2020 6:30 pm
maxracesoftware wrote:
Sun Feb 09, 2020 4:50 pm
..
Most probably using an even larger than 0.050" would be a better correlation .
It doesn't take much curtain area to feed an engine at cranking speed, say 200 RPM, so I think the seat-to-seat numbers would be more accurate since hydraulic intensity variations among various X° @ 0.050" lobes would have a significant effect on cranking pressure.
If you had 2 different cams, one with a 280 seat to seat and one with a 290 but both had 250 @.050 how would the calculator adjust for the actual time the valve is off of the seat?
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Re: Why does a tighter lsa need more compression?

Post by Stan Weiss » Mon Feb 10, 2020 12:43 am

DCR unlike Static CR is not written in stone. While it may not change while the engine is running it is very easily changed. Pull a valve cover and change the valve lash setting and you have changed DCR (also cranking compression PSI).

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Re: Why does a tighter lsa need more compression?

Post by randy331 » Mon Feb 10, 2020 10:14 am

mt-engines wrote:
Sat Feb 08, 2020 12:45 am
And why do people care what your engine cranks for compression? Im not building air compressors..
I agree.

Randy

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Re: Why does a tighter lsa need more compression?

Post by David Redszus » Mon Feb 10, 2020 11:56 am

GARY C wrote:
Sun Feb 09, 2020 7:36 pm
MadBill wrote:
Sun Feb 09, 2020 6:30 pm
maxracesoftware wrote:
Sun Feb 09, 2020 4:50 pm
..
Most probably using an even larger than 0.050" would be a better correlation .
It doesn't take much curtain area to feed an engine at cranking speed, say 200 RPM, so I think the seat-to-seat numbers would be more accurate since hydraulic intensity variations among various X° @ 0.050" lobes would have a significant effect on cranking pressure.
If you had 2 different cams, one with a 280 seat to seat and one with a 290 but both had 250 @.050 how would the calculator adjust for the actual time the valve is off of the seat?
The issue you raise concerns the effect of valve curtain area for 30 or 40 degrees. That cannot be answered without additional cam lift data in one deg increments. For sure, the values at 0.050" are meaningless and should never be used.

MadBill raises the issue of compression ratio vs compression pressure. While the ratio, either SCR or DCR, will not change with engine operation, compression pressure (and therefore compression temperature) most certainly will change.

In any event, DCR is always more useful than is SCR. SCR is completely useless and is predictive of almost nothing.

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Re: Why does a tighter lsa need more compression?

Post by 68corvette » Mon Feb 10, 2020 12:13 pm

example camshaft of BBC 280deg@.05" would be about 300deg@.05" valve lift with 1.7:1.
If using calculators, its also important to understand how camshafts are measured.

Same goes to cam predictor Softwares.
If SW or simulation spits out 280deg of duration, that is probably measured from valve rather than at camshaft.
For direct acting camshafts the duration seen at valve would of course be the same.
Or if using seat durations, those should be the same as measured from cam.

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Re: Why does a tighter lsa need more compression?

Post by David Vizard » Tue Feb 11, 2020 12:13 pm

As the CR increases so the optimal LCA gets wider and visa-versa. If you are dealing with a cam company that tells you different then deal with a different cam company.
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Re: Why does a tighter lsa need more compression?

Post by PRH » Tue Feb 11, 2020 1:15 pm

randy331 wrote:
Mon Feb 10, 2020 10:14 am
mt-engines wrote:
Sat Feb 08, 2020 12:45 am
And why do people care what your engine cranks for compression? Im not building air compressors..
I agree.

Randy
No one can think of a scenario where knowing the cranking compression might be helpful?
Somewhat handy with a die grinder.

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Re: Why does a tighter lsa need more compression?

Post by CamKing » Tue Feb 11, 2020 1:23 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?
Not really.

You can have 2 engines, where everything is exactly the same ,except for the port size.
Both will have the same cranking compression and DCR, but the actual running compression and cylinder pressure curves will be different.
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Re: Why does a tighter lsa need more compression?

Post by mt-engines » 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
mt-engines wrote:
Sat Feb 08, 2020 12:45 am
And why do people care what your engine cranks for compression? Im not building air compressors..
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.

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Re: Why does a tighter lsa need more compression?

Post by PRH » 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”?
Somewhat handy with a die grinder.

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Re: Why does a tighter lsa need more compression?

Post by MadBill » 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...
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Re: Why does a tighter lsa need more compression?

Post by Stan Weiss » 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.

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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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