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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 »

CamKing wrote: 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.
Why would I want my intake valve closing for 30 + degrees before peak piston velocity?

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

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Stan Weiss wrote: Wed Feb 12, 2020 1:28 pm
Why would I want my intake valve closing for 30 + degrees before peak piston velocity?
???
a 312 degree cam on a 30 ICL would close at 6 ABC
It would also open at 126 BTDC
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Re: Why does a tighter lsa need more compression?

Post by Stan Weiss »

CamKing wrote: Wed Feb 12, 2020 1:34 pm
Stan Weiss wrote: Wed Feb 12, 2020 1:28 pm
Why would I want my intake valve closing for 30 + degrees before peak piston velocity?
???
a 312 degree cam on a 30 ICL would close at 6 ABC
It would also open at 126 BTDC
Maybe a picture will show what I could not with words.

Stan

ab-ICL-40.gif
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Re: Why does a tighter lsa need more compression?

Post by CamKing »

Stan Weiss wrote: Wed Feb 12, 2020 2:01 pm
CamKing wrote: Wed Feb 12, 2020 1:34 pm
Stan Weiss wrote: Wed Feb 12, 2020 1:28 pm
Why would I want my intake valve closing for 30 + degrees before peak piston velocity?
???
a 312 degree cam on a 30 ICL would close at 6 ABC
It would also open at 126 BTDC
Maybe a picture will show what I could not with words.

Stan


ab-ICL-40.gif
Oh, you were trying to be logical about this.
The answer to your question is, you don't.

I'm just pointing out that cranking compression has no connection to HP.
if more cranking compression equaled more HP, we would see much ICL's in the 30's You graph shows why that's not a good thing.
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Re: Why does a tighter lsa need more compression?

Post by gmrocket »

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

Post by n2omike »

gmrocket wrote: Wed Feb 12, 2020 2:31 pm 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..
There is a difference between cranking pressure and combustion pressure. It think some are getting these intermingled.
All else being equal, the engine trapping more air/fuel in the cylinder for the compression cycle will make more combustion pressure and more horsepower.
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Re: Why does a tighter lsa need more compression?

Post by Stan Weiss »

n2omike wrote: Wed Feb 12, 2020 2:47 pm
gmrocket wrote: Wed Feb 12, 2020 2:31 pm 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..
There is a difference between cranking pressure and combustion pressure. It think some are getting these intermingled.
All else being equal, the engine trapping more air/fuel in the cylinder for the compression cycle will make more combustion pressure and more horsepower.
Sometimes there in lays the problem. It still does comes down to how efficiently it is used. That is why I still look at BSFC and / or BSAC and not just fuel lbs/hr and SCFM.

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Re: Why does a tighter lsa need more compression?

Post by digger »

CamKing wrote: Wed Feb 12, 2020 2:15 pm
I'm just pointing out that cranking compression has no connection to HP.
who's saying it does?
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Re: Why does a tighter lsa need more compression?

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digger wrote: Wed Feb 12, 2020 3:59 pm
CamKing wrote: Wed Feb 12, 2020 2:15 pm
I'm just pointing out that cranking compression has no connection to HP.
who's saying it does?
Unfortunately, there's a lot of people that think that. I've seen multiple people say they advance their cams, to increase cranking compression, and that will increase HP.
I see lots of people say that the cam they were recommended would be down on HP, because the DCR wasn't high enough.
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Re: Why does a tighter lsa need more compression?

Post by n2omike »

CamKing wrote: Wed Feb 12, 2020 4:12 pm Unfortunately, there's a lot of people that think that. I've seen multiple people say they advance their cams, to increase cranking compression, and that will increase HP.
I see lots of people say that the cam they were recommended would be down on HP, because the DCR wasn't high enough.
These things CAN be true... especially if the engine has a fairly low compression ratio for the combination. It's a little sloppy to say cranking compression/DCR doesn't matter at all, as it gives us a good guideline to go by. It might not be the tool to get things within a razor's edge of being perfected, but it can get things squarely in the ballpark.

For the amount the vast majority of people/racers understand... there are some good 'guidelines' such as DCR out there. They at least allow us to see how bigger (longer duration) cams need a higher compression ratio, compared to an engine with a smaller cam.
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Re: Why does a tighter lsa need more compression?

Post by digger »

CamKing wrote: Wed Feb 12, 2020 4:12 pm
digger wrote: Wed Feb 12, 2020 3:59 pm
CamKing wrote: Wed Feb 12, 2020 2:15 pm
I'm just pointing out that cranking compression has no connection to HP.
who's saying it does?
Unfortunately, there's a lot of people that think that. I've seen multiple people say they advance their cams, to increase cranking compression, and that will increase HP.
I see lots of people say that the cam they were recommended would be down on HP, because the DCR wasn't high enough.
i guess you could probably argue it often increases power at 2500 rpm or something before the engine is on the cam and before the runner lengths, ramming are starting to help. i look at it that if i have a street engine where i care about bottom end i want 200psi rather than 150psi on pump fuel and provided i haven't screwed something up then i'm going to be better off in the case of the former. it may not make more peak or topend but its usually more fun on the street i.e. less soggy and more responsive
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Re: Why does a tighter lsa need more compression?

Post by 70GS455 »

n2omike wrote: Wed Feb 12, 2020 1:09 pm

Big cams need more compression ratio, as they close the intake valve sooner, trapping less air.
Closes it later

Bigger cam = later closing
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Re: Why does a tighter lsa need more compression?

Post by n2omike »

70GS455 wrote: Wed Feb 12, 2020 4:44 pm
n2omike wrote: Wed Feb 12, 2020 1:09 pm Big cams need more compression ratio, as they close the intake valve sooner, trapping less air.
Closes it later
Bigger cam = later closing
Oops. Said that wrong. Obviously, big cam closes the valve later. Edit that to say:
Big cams need more compression ratio, as they close the intake valve LATER, trapping less air.
:)
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Re: Why does a tighter lsa need more compression?

Post by digger »

n2omike wrote: Wed Feb 12, 2020 5:02 pm
70GS455 wrote: Wed Feb 12, 2020 4:44 pm
n2omike wrote: Wed Feb 12, 2020 1:09 pm Big cams need more compression ratio, as they close the intake valve sooner, trapping less air.
Closes it later
Bigger cam = later closing
Oops. Said that wrong. Obviously, big cam closes the valve later. Edit that to say:
Big cams need more compression ratio, as they close the intake valve LATER, trapping less air.
:)
they trap less air at low rpm due to reversion, but presumably one uses the bigger cam to trap more air at some "high" rpm.....so then it could actually make more peak torque and cylinder pressure which should make you revisit the theory
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Re: Why does a tighter lsa need more compression?

Post by GARY C »

CamKing wrote: Wed Feb 12, 2020 2:15 pm
Stan Weiss wrote: Wed Feb 12, 2020 2:01 pm
CamKing wrote: Wed Feb 12, 2020 1:34 pm
???
a 312 degree cam on a 30 ICL would close at 6 ABC
It would also open at 126 BTDC
Maybe a picture will show what I could not with words.

Stan


ab-ICL-40.gif
Oh, you were trying to be logical about this.
The answer to your question is, you don't.

I'm just pointing out that cranking compression has no connection to HP.
if more cranking compression equaled more HP, we would see much ICL's in the 30's You graph shows why that's not a good thing.
Isn't that like a straw man or Logical Fallacies kind of argument? I think most here are looking at a few degrees of fine tuning with in the confines of their combo and not a Summit budget flat tappet vs a Pro Stock solid comparison.
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