Dumb Compression Ratio/Octane ?
Thread Starter
Le Mans Master
Joined: Dec 2002
Posts: 5,270
Likes: 3
From: Seattle Area WA
Dumb Compression Ratio/Octane ?
I was playing around with excel and I figured my compression ratio to be at worst 10.55:1
I was reading on the web tonight and they said 91 octane was good only to 10.1
Is this true?
List em off:
87 Octane:
89 Octane:
91: Octane:
I swear when I was planning parts and pieces I found I could run on pump, but now this is scaring me!
TIA
I was reading on the web tonight and they said 91 octane was good only to 10.1
Is this true?
List em off:
87 Octane:
89 Octane:
91: Octane:
I swear when I was planning parts and pieces I found I could run on pump, but now this is scaring me!
TIA
Advanced
Joined: Oct 2004
Posts: 85
Likes: 0
From: Tucson AZ
Really depends on your setup.
10.55:1 is quite mellow on an LT1, mostly becauset of the short intake runner lengths. For a TPI motor this is on the high side of streetable. Need to log the car to see if the car is pulling timing to see if it's going to need even higher octane.
10.55:1 is quite mellow on an LT1, mostly becauset of the short intake runner lengths. For a TPI motor this is on the high side of streetable. Need to log the car to see if the car is pulling timing to see if it's going to need even higher octane.
Drifting
Joined: Feb 2004
Posts: 1,277
Likes: 1
From: Santa Maria CA
if you have a large cam that can bleed off cylinder pressure then you can run 91octane with alum heads. its possible to do it with even 12.1 with the right components.you should be ok still sonsidering the LT-4 has more than that stock.
Thread Starter
Le Mans Master
Joined: Dec 2002
Posts: 5,270
Likes: 3
From: Seattle Area WA
Hmmm, wow I geuss I will be crossing my fingers, but hey, worst case worse, slightly bigger cam....ok 
Ported Plenum
AS&M Hi-flow runners
Accel Base
5-angle valve job 113 d-ports (w/ GM 0.051" head gasket)
.030 383
5.7 rods
hyperuetectic pistons
LPE LT headers
LPE 211 cam (224/234 .496/.520 w/1.6 112cl)
Hopefully I will be ok!
Ported Plenum
AS&M Hi-flow runners
Accel Base
5-angle valve job 113 d-ports (w/ GM 0.051" head gasket)
.030 383
5.7 rods
hyperuetectic pistons
LPE LT headers
LPE 211 cam (224/234 .496/.520 w/1.6 112cl)
Hopefully I will be ok!
Last edited by USAsOnlyWay; Nov 5, 2004 at 01:38 AM.
Le Mans Master
Joined: Mar 2002
Posts: 5,889
Likes: 2
From: Castro Valley Calif.
St. Jude Donor '03 thru '26
I wouldn't play with this issue, it's too important. We all want to push the envelope a bit without leaving HP on the table when we plan our build. It's really all about the dynamic CR, not the static, try this link for a great worksheet, the download is near the bottom. This will even let you see the effects of advancing or retarding your cam for bleedoff
http://members.uia.net/pkelley2/DynamicCR.html
http://members.uia.net/pkelley2/DynamicCR.html
Drifting
Joined: Mar 1999
Posts: 1,931
Likes: 1
From: Norcross GA
What you calc'd was the static comp ratio. What really matter's as ping is concerned is the dynamic ratio, and overall design. Quench is the distance from the top of the piston to the top of the flats' of the cyl head. This distance has a lot to do w/ elim cyl hot spots. The other issue is the valve timing. This is what will give you the dynamic comp ratio. (See the above post about cam's.) 10.5 is no big deal on an LT1,... reverse cooling, alu heads,.....
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by USAsOnlyWay
I was reading on the web tonight and they said 91 octane was good only to 10.1
Is this true?
Is this true?
Originally Posted by USAsOnlyWay
List em off:
87 Octane:
89 Octane:
91: Octane:
87 Octane:
89 Octane:
91: Octane:
Originally Posted by Juddidiah!
10.55:1 is quite mellow on an LT1, mostly becauset of the short intake runner lengths. For a TPI motor this is on the high side of streetable.
RACE ON!!!
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Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
10.55:1 is quite mellow on an LT1, mostly becauset of the short intake runner lengths. For a TPI motor this is on the high side of streetable.
Who doesn't have a clue.
Who doesn't have a clue.
If runner length has any affect on detonation it is minimal. The LT1 does have a superior compression tolerance but it comes from the reverse flow cooling setup, not the intake.
I've seen street driven LT1's with 12.5:1 compression run just fine on pump gas.
Don't be fooled by absolute octane numbers either. Here in BG I can easily go to almost any gas station and get 93 octane. Drive to Denver and you won't find 93, it might as well be race gas. The reason is that atmospheric pressure plays a big part in cylinder pressure. At high altitudes you can run a much lower octane fuel than you can at sea level and get away with it.
The short and simple answer to your question is that there is no short and simple answer to your question. It depends on your cylinder head design, pistons, camshaft AND geographic location. Blanket statements that XX octane is only good for xx.xx compression are completely useless.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by USAsOnlyWay
Ok so if I get a Dynamic Compression Ratio of 9.6-9.7:1 I should be good with 91 Octane correct?
No 93 octane around here, either. I use 85.
RACE ON!!!
Melting Slicks
Joined: Apr 2003
Posts: 2,150
Likes: 6
From: NS
Originally Posted by SHINOBI-X
if you have a large cam that can bleed off cylinder pressure then you can run 91octane with alum heads. its possible to do it with even 12.1 with the right components.you should be ok still sonsidering the LT-4 has more than that stock.
You can run more compression with a larger cam. The compression game is a fine line...cross the line and you're cursed with adding octane boost at every fill up.
