help with detonation problems
Thread Starter
Pro
Joined: Sep 2002
Posts: 541
Likes: 5
From: Epping NH
help with detonation problems
We just got my car up and running again last week and are having problems with detonation. I am running a totally stock set up (69 L-46) with "186" iron heads, Q-jet, etc... Delco distributor with points and compression estimated at about 10.6:1 (originally 11:1). The only deviation from stock is a Comp Cams XE-268 cam.
We have played a bit with the distributor and the idle to reduce the pinging. Unfortunately, we can't seem to find a sweet spot where she runs well. We had anticipated needing an octane booster, but the only thing that really makes her run well is 110 octane mixed with Sunoco Ultra 93 at about 8 to 2 ratio. Then, she screams down the road.
Since the 110 is very expensive, and unavailable if I leave the area, I am wondering what others are doing with high compression engines. Thanks.
We have played a bit with the distributor and the idle to reduce the pinging. Unfortunately, we can't seem to find a sweet spot where she runs well. We had anticipated needing an octane booster, but the only thing that really makes her run well is 110 octane mixed with Sunoco Ultra 93 at about 8 to 2 ratio. Then, she screams down the road.
Since the 110 is very expensive, and unavailable if I leave the area, I am wondering what others are doing with high compression engines. Thanks.
Drifting
Joined: Sep 2003
Posts: 1,325
Likes: 6
From: FRANKLIN GA
Back off the timing a little to around 32 total, run 2 steps colder plugs, and richen the carb a little. Do it in this order untill the pinging stops. The iron heads don't help, they hold a lot of heat. You might want to try a high volume water pump also. Your main objective is to cool the cylinder down some.
Race Director
Joined: Jan 2000
Posts: 13,011
Likes: 2,250
From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
C2 of Year Winner (performance mods) 2019
2017 C2 of Year Finalist
It's going to take some work. The 268H cam is a pretty short cam duration wise and builds a lot of cylinder pressure. I believe it's a lot shorter than the stock one was. it works very well with 8.5-9.5 compression motors.
But it should be awful responsive with some compression!
As noted, heat is your enemy. Do all you can to cool it down.
What is the timing curve in there now? Usually a fairly agressive initial and a limited total works well. Then you can control the rate it comes in with the springs.
I think you can re-curve dist to have 16-20* initial and then the rest in to a total of 36* maybe. That will take a larger advance bushing and then you play with springs to get it to handle it. You will also likely need to limit vacuum advance some. Put it on manifold vacuum..it will help cool it down, but as I said may need to be limited.
Sounds like it's time for more cam...aluminum heads and /or both!!
JIM
But it should be awful responsive with some compression!
As noted, heat is your enemy. Do all you can to cool it down.
What is the timing curve in there now? Usually a fairly agressive initial and a limited total works well. Then you can control the rate it comes in with the springs.
I think you can re-curve dist to have 16-20* initial and then the rest in to a total of 36* maybe. That will take a larger advance bushing and then you play with springs to get it to handle it. You will also likely need to limit vacuum advance some. Put it on manifold vacuum..it will help cool it down, but as I said may need to be limited.
Sounds like it's time for more cam...aluminum heads and /or both!!
JIM
Race Director
Joined: Apr 2000
Posts: 17,654
Likes: 190
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Back off the timing a couple of degrees and try with no vacuum advance connected first. I agree with what was stated above. You have a mismatched combination with a short duration cam in a high compression motor. Do a compression check, I bet you are 230psi or more, that and iron heads is your problem in a nutshell 
Why would you build a motor for the street that needed octane boosters ? You select the cam and pistons ( for the compression ratio you want ) at the same time.
If you wanted to run that specific cam then about 9.5 :1 would be the limit with iron heads. IF built that way you would probably only lose 2 hp or so over the combination you now have running 110 octane gas
I have 11:1 CR but I have a big duration cam. Still my cranking compression is in the 230psi range. But with aluminum heads, no vacuum advance, cool running motor etc. I can run 91 octane gas. I do run 94 though most of the time.
Why would you build a motor for the street that needed octane boosters ? You select the cam and pistons ( for the compression ratio you want ) at the same time.
If you wanted to run that specific cam then about 9.5 :1 would be the limit with iron heads. IF built that way you would probably only lose 2 hp or so over the combination you now have running 110 octane gas
I have 11:1 CR but I have a big duration cam. Still my cranking compression is in the 230psi range. But with aluminum heads, no vacuum advance, cool running motor etc. I can run 91 octane gas. I do run 94 though most of the time.
Last edited by MotorHead; Jul 8, 2006 at 01:24 AM.
