How much compression is safe on pump gas?
Thread Starter
Safety Car
Joined: Jul 2001
Posts: 4,871
Likes: 15
From: Detroit's West Side MI
How much compression is safe on pump gas?
LT1 Bottom end is being rebuilt (forged rotating assy), we're planning on 10:1 compression ratio. *edit* Displacement will be 383ci.
The motor (before the rebuild) made about 450 CHP. It's a road race ONLY car, but I run 93 / 94 octane pump gas sometimes. Should I stick with 10:1 or can I go a little higher without detonation issues or the possibility of engine damage? A little more power never hurts...
The motor (before the rebuild) made about 450 CHP. It's a road race ONLY car, but I run 93 / 94 octane pump gas sometimes. Should I stick with 10:1 or can I go a little higher without detonation issues or the possibility of engine damage? A little more power never hurts...
Last edited by ScaryFast; May 30, 2006 at 04:56 PM.
Le Mans Master
Joined: Jul 1999
Posts: 8,683
Likes: 6
From: Phoenix AZ
I'm running roughly 11.5:1 on 91 octane gas with no issues at all. A lot of it really depends on how well the combustion chambers in the heads are done. If they are stock I wouldn't go much above 10.8:1
Thread Starter
Safety Car
Joined: Jul 2001
Posts: 4,871
Likes: 15
From: Detroit's West Side MI
Heads are ported but I don't know what was done to the combustion chamber. I'll have to look into it.
Of course now we're talking 383 because I changed my mind an hour ago. Do the Same rules apply?
Of course now we're talking 383 because I changed my mind an hour ago. Do the Same rules apply?
Le Mans Master
Joined: Jan 2002
Posts: 9,036
Likes: 13
From: Edmonton AB
Camshaft selection figures into this discussion.
To generalize, short durations will require lower compression, while
long durations with lots of overlap allow/require higher compression.
What kind of cam profile did you have in mind?
.
To generalize, short durations will require lower compression, while
long durations with lots of overlap allow/require higher compression.
What kind of cam profile did you have in mind?
.
Le Mans Master
Joined: Jul 2000
Posts: 5,012
Likes: 5
From: OKC OK
Originally Posted by Slalom4me
Camshaft selection figures into this discussion.
To generalize, short durations will require lower compression, while
long durations with lots of overlap allow/require higher compression.
What kind of cam profile did you have in mind?
To generalize, short durations will require lower compression, while
long durations with lots of overlap allow/require higher compression.
What kind of cam profile did you have in mind?
google "dynamic compression ratio"
Safety Car
Joined: May 2003
Posts: 3,789
Likes: 0
From: [Quote=WOEII] Is dried brown doodie powders man! [/Quote]
Originally Posted by LT4POWR
google "dynamic compression ratio" While you're there, also google DCR Calculator for a neat (and free) little program that will calculate your DCR, amongst other things.
If you can't find it send me a PM and I'll email it to you.
Team Owner
Joined: Sep 2002
Posts: 22,209
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From: OBAMA IS HITLER
i've got my engine rebuilt/modified, and the measurements have it calculated right 10.2:1. the factory setup was rated at 10.25:1. i run 89 octane with no problems. datamaster recording over 20 min. spans have showed about 6-8 knock counts total -- and this only ocurred while shifting which is perfectly normal. no timing retard.
if you're 10.1:1 c/r and your quench is in the .045-.055" area that's pretty decent so you shouldn't have anything to worry about.....in my humble, dumbassed opinion.
if you're 10.1:1 c/r and your quench is in the .045-.055" area that's pretty decent so you shouldn't have anything to worry about.....in my humble, dumbassed opinion.
Last edited by Red Tornado; May 30, 2006 at 07:59 PM.
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From: Good health is merely the slowest possible rate at which one can die
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Originally Posted by Hot Rod 90
if you're 10.1:1 c/r and your quench is in the .045-.055" area that's pretty decent so you shouldn't have anything to worry about.....in my humble, dumbassed opinion.
Funny, but that's hardly the case! And yes, quench will become your friend in this quest. In my newest combination, it looks like I'm ending up right around .040" quench with the zero deck. This is considered the practical maximum although some squeeze it even tighter.
Team Owner
Joined: Sep 2002
Posts: 22,209
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From: OBAMA IS HITLER
i should have posted .035" - .055" (factory crap can get up to .060"+). sorry for the misinformation. and yes i know quench is not only your best friend, but uber-important in the overall setup.
Safety Car
Joined: Sep 2002
Posts: 4,030
Likes: 6
From: Maryland
Originally Posted by ScaryFast
LT1 Bottom end is being rebuilt (forged rotating assy), we're planning on 10:1 compression ratio. *edit* Displacement will be 383ci.
The motor (before the rebuild) made about 450 CHP. It's a road race ONLY car, but I run 93 / 94 octane pump gas sometimes. Should I stick with 10:1 or can I go a little higher without detonation issues or the possibility of engine damage? A little more power never hurts...
The motor (before the rebuild) made about 450 CHP. It's a road race ONLY car, but I run 93 / 94 octane pump gas sometimes. Should I stick with 10:1 or can I go a little higher without detonation issues or the possibility of engine damage? A little more power never hurts...
I had puzzled over this as part of my re-build. There is a dynamic compression ratio that you should calculate, the calcuators are on line, I think KB / Silvolite had a really nice one.
The DCR takes into account your cam -- more duration helps you in that you can run more compression.
I had a mild cam (214/224) and thin head gaskets (.029) plus low quench since he decked the block (?0.050?) and with all this I was at a static CR of like 11.2 or so. The LT ran fine on '93 with that.
The LT is a little more forgiving than the gen I small block since you have reverse cooling--cooler heads.
I would be conservative if I were you since road racing makes the motor live in the hot range. If you were to build a motor with too much compression, it might be fine up until say 230 deg. oil temp and then start to knock.
So does this mean that you aren't running your 350 at all after all that work? What happened? I know it was a junkyard block but...
Melting Slicks
Joined: Oct 2003
Posts: 2,951
Likes: 3
From: Mount Vernon OH
Camshaft overlap can be a make or break issue depending on what you have going on. Also, rod length and the amount of time the piston spends at TDC. You can detonate a 10 to 1 engine or you can buzz an engine near 12 to 1 depending on your combination and get away with pump gas. Something many overlook is at what RPM you load the engine and your timing advance curve...I have seen engines ping down low in the RPM band under a heavy load yet wake up and make killer power on pump gas once the rpm's are mid range and higher. There are lots of factors to consider.
Thread Starter
Safety Car
Joined: Jul 2001
Posts: 4,871
Likes: 15
From: Detroit's West Side MI
The cam has 6 degrees of overlap, 230/236.
I read up on DCR, thanks for the tip.
However, there is too much info I don't know to use the calculater. I'll have to look up the crank, head, and rod specs to check it.
I read up on DCR, thanks for the tip.
However, there is too much info I don't know to use the calculater. I'll have to look up the crank, head, and rod specs to check it.
Le Mans Master
Joined: Jan 2002
Posts: 9,036
Likes: 13
From: Edmonton AB
Originally Posted by ScaryFast
The cam has 6 degrees of overlap, 230/236.
looking at Ex dur - Int duration.
Do you happen to have all the details from the cam card?
I'll see if I can find a link that can shed some more light on cam
events.
Edit:
Here are a couple.
Last edited by Slalom4me; Jun 1, 2006 at 03:10 PM.
