compression ratio vs. octane, what does hi-test allow?
Thread Starter
Race Director
Joined: Oct 2000
Posts: 15,013
Likes: 79
From: central FL, near the beach.
compression ratio vs. octane, what does hi-test allow?
has anyone seen a chart like:
87 if it runs good on 87 and is X Cr
89 can raise it y
91 can raise it z
93 can go q
i'm trying to figure if i run 91, how much more cr can i run,
now it's 87 and 9.7:1
personal experience helpful, if u changed pistons to raise cr.
thanks!
87 if it runs good on 87 and is X Cr
89 can raise it y
91 can raise it z
93 can go q
i'm trying to figure if i run 91, how much more cr can i run,
now it's 87 and 9.7:1
personal experience helpful, if u changed pistons to raise cr.
thanks!
Drifting
Joined: Sep 2003
Posts: 1,325
Likes: 6
From: FRANKLIN GA
Matt it's hard to say exactly how much compression a certain octane will allow. It has a lot to do with the quench of the piston to head, the shape of the combustion chamber in the head, the maximum advance on the timing, and the timing/duration of the cam. All of these things work together in controling detonation.
If your not taking any of the above into consideration, I would say that the safe comperssion ratio on 93 octane would be 9.5:1 on iron heads or 10:1 on aluminum head.
That being said, if you do your research and take all of the above and build for high compression you can get away with a lot. I'm running 11.8:1 on the street with 93 octane. It runs great and gives me no problems. I do have a .031 quench, alum trick flow heads with a good combustion chamber, I limit the advance to 32*, and have a cam with a long duration. Some guys run higher compression than me. It's all in how you set it up
If your not taking any of the above into consideration, I would say that the safe comperssion ratio on 93 octane would be 9.5:1 on iron heads or 10:1 on aluminum head.
That being said, if you do your research and take all of the above and build for high compression you can get away with a lot. I'm running 11.8:1 on the street with 93 octane. It runs great and gives me no problems. I do have a .031 quench, alum trick flow heads with a good combustion chamber, I limit the advance to 32*, and have a cam with a long duration. Some guys run higher compression than me. It's all in how you set it up
Burning Brakes
Joined: Feb 2006
Posts: 1,024
Likes: 1
From: AL
if you could find some old PHR articles on the sbc engine masters challenge, they will have some info. i'd consider those combos too much for the street, but they did run on pump gas (some didn't make all the pulls)
Thread Starter
Race Director
Joined: Oct 2000
Posts: 15,013
Likes: 79
From: central FL, near the beach.
Eddie
i did NOT ask that.(how much compression for a given octane)
I asked about how much the CR can be INCREASED when going to hi-test from 87.
the only change is the pistons.
ie, a guy has a car and he changes just the pistons.
the answer would not be, say, 11.8:1 or 9.5:1.
it would be something like
89 0.4
91-0.8
93- 1.2
i did NOT ask that.(how much compression for a given octane)
I asked about how much the CR can be INCREASED when going to hi-test from 87.
the only change is the pistons.
ie, a guy has a car and he changes just the pistons.
the answer would not be, say, 11.8:1 or 9.5:1.
it would be something like89 0.4
91-0.8
93- 1.2
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
Eddie
i did NOT ask that.(how much compression for a given octane)
I asked about how much the CR can be INCREASED when going to hi-test from 87.
the only change is the pistons.
ie, a guy has a car and he changes just the pistons.
the answer would not be, say, 11.8:1 or 9.5:1.
it would be something like
89 0.4
91-0.8
93- 1.2
i did NOT ask that.(how much compression for a given octane)
I asked about how much the CR can be INCREASED when going to hi-test from 87.
the only change is the pistons.
ie, a guy has a car and he changes just the pistons.
the answer would not be, say, 11.8:1 or 9.5:1.
it would be something like89 0.4
91-0.8
93- 1.2
with Eddie. Such a chart would not work with static compression. it would not be lineal. Too many other factors to take into account as Eddie stated. Flat tops are more efficient and have better flame travel than dish or dome.
Last edited by 63mako; Dec 13, 2007 at 02:11 PM.
Instructor
Joined: Feb 2007
Posts: 108
Likes: 0
From: PA
Temp is also a factor... what is octane is ok at 70 degrees for 9.5/1 may not at 95 degrees...
i also saw a show that did some nice explaining... higher octane does nothing if the CR doesnt need it.... it does not make more power....
rich
i also saw a show that did some nice explaining... higher octane does nothing if the CR doesnt need it.... it does not make more power....
rich
Thread Starter
Race Director
Joined: Oct 2000
Posts: 15,013
Likes: 79
From: central FL, near the beach.
not expecting linear, but i am told over and over that hi-test would allow more cr, and i'd like input on how much. so my 1st guess is 0.8
anyone have actual experience or a chart from anywhere? this is not rocket science.
Pro
Joined: Apr 2006
Posts: 625
Likes: 49
From: Southern California
I can understand your question, but the problem is, its kind of like asking "how high is up?". There really is no good answer that would always be correct. You might go up .8, while your buddy can go up 1.1, and yet another buddy could only go up .5. The point is, if you come up with some sort of chart, it will only be proven wrong as often as its proven correct. So, where would that leave you? Right back where you are now. Everyone's engine/car combo is different. Yes, even the car itself plays a roll, weight and gearing can absolutely have an affect on pinging. A given engine may not ping in one car, yet ping itself to death in another. And of course, engine variations are virtually unlimited. Your best bet is to put together a list of what is working for other people who already run a setup similar to what you want to run. That way you can get pretty close right out of the gate. 
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Thread Starter
Race Director
Joined: Oct 2000
Posts: 15,013
Likes: 79
From: central FL, near the beach.
So i'd say i'm looking for somebody that milled the heads. let's see what the range is for actual cases.
There is a company that confirms the octane ratings for the state, to check gas stations so they don't cheat people. They put the gas in a variable cr motor and can tell the octane from the cr.
So it not only can be done, it is done frequently in the industry.
There is a company that confirms the octane ratings for the state, to check gas stations so they don't cheat people. They put the gas in a variable cr motor and can tell the octane from the cr.
So it not only can be done, it is done frequently in the industry.
Thread Starter
Race Director
Joined: Oct 2000
Posts: 15,013
Likes: 79
From: central FL, near the beach.
262xe 218/224/110 1.6 rockers
355 flat tops, 9.7:1 87 octane
.025deck, .015HG= .040 quench
1 3/4 headers, 2.25 duals, turbo muffs
650 DP 4165 holley 13" PV, .054" primary jets
8004 weiand blocked heat riser
cold air induction
0.2" 4 hole wood plate w/bowl heat shield
2100stall 3.08
14.2 mpg around town before 4 holer
Last edited by Matt Gruber; Dec 13, 2007 at 05:55 PM.
Pro
Joined: Apr 2006
Posts: 625
Likes: 49
From: Southern California
67cc bowl ported SR Torquer iron, back cut valves, intake valve unshrouded
262xe 218/224/110 1.6 rockers
355 flat tops, 9.7:1 87 octane
.025deck, .015HG= .040 quench
1 3/4 headers, 2.25 duals, turbo muffs
650 DP 4165 holley 13" PV, .054" primary jets
8004 weiand blocked heat riser
cold air induction
0.2" 4 hole wood plate w/bowl heat shield
2100stall 3.08
14.2 mpg around town before 4 holer
262xe 218/224/110 1.6 rockers
355 flat tops, 9.7:1 87 octane
.025deck, .015HG= .040 quench
1 3/4 headers, 2.25 duals, turbo muffs
650 DP 4165 holley 13" PV, .054" primary jets
8004 weiand blocked heat riser
cold air induction
0.2" 4 hole wood plate w/bowl heat shield
2100stall 3.08
14.2 mpg around town before 4 holer
Last edited by 540 RAT; Dec 13, 2007 at 06:03 PM.
Drifting
Joined: Sep 2003
Posts: 1,325
Likes: 6
From: FRANKLIN GA
67cc bowl ported SR Torquer iron, back cut valves, intake valve unshrouded
262xe 218/224/110 1.6 rockers
355 flat tops, 9.7:1 87 octane
.025deck, .015HG= .040 quench
1 3/4 headers, 2.25 duals, turbo muffs
650 DP 4165 holley 13" PV, .054" primary jets
8004 weiand blocked heat riser
cold air induction
0.2" 4 hole wood plate w/bowl heat shield
2100stall 3.08
14.2 mpg around town before 4 holer
262xe 218/224/110 1.6 rockers
355 flat tops, 9.7:1 87 octane
.025deck, .015HG= .040 quench
1 3/4 headers, 2.25 duals, turbo muffs
650 DP 4165 holley 13" PV, .054" primary jets
8004 weiand blocked heat riser
cold air induction
0.2" 4 hole wood plate w/bowl heat shield
2100stall 3.08
14.2 mpg around town before 4 holer
Matt, I would say you are borderline detonation right now depending on what gears you are running in the rear. The cam seems small to me for that compression and 355 cubes
I know that you are very good a tuning and that is probably what is saving you now. Most guys running your compression and the small duration cam (for a 355 with over 9:1) with stock gears would be pinging on 87 octane. If you do up it to 10.5:1 you might want to think about a slightly larger cam.
Drifting
Joined: Sep 2003
Posts: 1,325
Likes: 6
From: FRANKLIN GA
true about domes, so let's say for milling the heads. easier and cheaper anyway.
not expecting linear, but i am told over and over that hi-test would allow more cr, and i'd like input on how much. so my 1st guess is 0.8
anyone have actual experience or a chart from anywhere? this is not rocket science.
not expecting linear, but i am told over and over that hi-test would allow more cr, and i'd like input on how much. so my 1st guess is 0.8
anyone have actual experience or a chart from anywhere? this is not rocket science.
Thread Starter
Race Director
Joined: Oct 2000
Posts: 15,013
Likes: 79
From: central FL, near the beach.
Eddie,
My experience is small cams don't like low CR. they are dogs. Thats why i scoff at current DCR theory; it gives pathetic low cr w/weenie cams and the guys figure if they raise the cr w/biggercam it will fix it, which it often does, not because they need a big cam, because the car is a turd w/low cr.
just drove it 3rd time with the .054" jets and 4 holer; got on it for the 1st time and i was really surprised, no ping and it seems quicker; even if it is an illusion, i love it. The only thing it is on the edge of, is tire spin.
3.08 rear
Thanks for your reply.
EDIT: as for a bigger cam, can u guarantee it will spin the tires as it does now, without power braking, just by giving it 1/4 throttle, with a 3.08 rear and 2100 stall(stock)
My experience is small cams don't like low CR. they are dogs. Thats why i scoff at current DCR theory; it gives pathetic low cr w/weenie cams and the guys figure if they raise the cr w/biggercam it will fix it, which it often does, not because they need a big cam, because the car is a turd w/low cr.
just drove it 3rd time with the .054" jets and 4 holer; got on it for the 1st time and i was really surprised, no ping and it seems quicker; even if it is an illusion, i love it. The only thing it is on the edge of, is tire spin.
3.08 rearThanks for your reply.
EDIT: as for a bigger cam, can u guarantee it will spin the tires as it does now, without power braking, just by giving it 1/4 throttle, with a 3.08 rear and 2100 stall(stock)
Last edited by Matt Gruber; Dec 14, 2007 at 01:03 PM.
