Why does a tighter lsa need more compression?

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

Post by swampbuggy »

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
Wouldn't one need a whole lot more information on both engines to answer your question, Randy :?: Mark H.
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Re: Why does a tighter lsa need more compression?

Post by David Redszus »

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
What do you mean by "dynamic compression"? Do you mean ratio or pressure?

It is possible to calculate useful information if the following data is provided for both engines.

Bore = xx in
Stroke = xx in
chamber volume = xx cc
IVC= xxx BTDC
Rod = xx in
Fuel specs...
Inlet press = xx psi
Inlet temp = xx F
SCR =
DCR= to be determined
TDC press = to be determined
TDC temp = to be determined
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Re: Why does a tighter lsa need more compression?

Post by RevTheory »

David Redszus wrote: Sat Feb 15, 2020 1:25 pm
It is possible to calculate useful information if the following data is provided for both engines.

Bore = xx in
Stroke = xx in
chamber volume = xx cc
IVC= xxx BTDC
Rod = xx in
Fuel specs...
Inlet press = xx psi
Inlet temp = xx F
SCR =
DCR= to be determined
TDC press = to be determined
TDC temp = to be determined
Is it possible to make some reasonable assumptions on fuel, inlet pressure and temp or is it a complete crapshoot without equipment?

Good ol' pump swill, 91 octane, 10% corn juice
100*F ambient temp
180* thermostat + cold air induction
3,295 ft. elevation
30.05 in/HG
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Re: Why does a tighter lsa need more compression?

Post by David Redszus »

* Since static compression ratio is a ratio, it's meaningless and serves no useful purpose.
True
* Since dynamic compression ratio is poorly named, it's meaningless and serves no useful purpose.
Regardless of its name, it does serve a useful purpose in that it allows further calculations to be performed.
* Since cranking pressure is based on a slow starter speed, it's meaningless and serves no useful purpose.
Cranking pressure is a crude measure of cylinder balance or leakage. It does not determine performance.
* 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.
While that is the most accurate method to evaluate a running engine, there are other methods that can be used. A direct measure of compression pressure in a running engine is very useful. It would permit a calculation of compression temperature to determine fuel properties required.
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
That would be a very unusual IVC. But given a reasonable IVC and chamber volume, a reasonable DCR can be calculated. From which compression pressure and compression temperature can be determined for each crank angle. Now it is possible to evaluate the appropriateness of a given fuel.
; again, because everything is meaningless and serves no useful purpose.
No, everything is not meaningless; just some misused parameters, probably due to a lack of understanding and compounded by folklore.
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?
A very useful insight can be gained if the correct parameters are considered. But terms such as SCR, DCR, cranking compression, and the like are not really useful by themselves.
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Re: Why does a tighter lsa need more compression?

Post by David Redszus »

GRTfast wrote: 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.
Mike is absolutely correct: 0.050" numbers serve very little purpose since they do not reflect ramp height and duration below 0.050".
Seat to seat values should be the standard for valve events. But a cam mfg does not know the cam follower that might be used and might be reluctant to publish valve events. There is a difference between cam lift values and valve lift values. The engine does not care about the camshaft; it cares a lot about valve lift.
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Re: Why does a tighter lsa need more compression?

Post by randy331 »

swampbuggy wrote: Sat Feb 15, 2020 11:04 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
Wouldn't one need a whole lot more information on both engines to answer your question, Randy :?: Mark H.
Well, I was just pointing out how different "dynamic comp" can be with the same static comp.
Both had a cam chosen for the application with no thought to "dynamic comp".

I think MT engines posted how important "dynamic comp" is.

Randy
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