High RPM Detonation

[jt1]

Took the Z to a HDPE at CMP in Kershaw SC this past weekend, and had a problem I've never had before. On the straights I would start to hear some pinging starting at about 4k rpm. Backing off slightly from WOT would stop it. I checked the timing, apparently the distributor had moved a little, it was at 38 deg total. Backed it down to 35 deg. This helped but did not eliminate the problem. Combo is 355, 10:1, Trick flow heads, Comp solid roller 224/224 @ 50, 525/525, Ebrock air gap, 750 DP w/69/75 jets, GM HEI ignition, Amoco pump premium. It was very hot, high was 97 deg. There was no detonation at lower rpm. Too much timing for the temp? Too lean? Colder plugs? Any help would be great, this has me puzzled. Thanks.

[Scottie-GNZ]

IMO, not enough octane to hold WOT for extended periods with that c.r. When racing, try adding a little race gas to the mix. Adding just 1 gal of 110-octane to 7 gals of 93 raises the octane to 95.125. Cheap investment to safely maximize the engine or you will probably have to keep backing off the timing until it stops.

 

In almost every decent sized town there is a service station that sells race gas at the pump (fill into gas can only). Here it is $3.49/gal and if you buy 2 gals on the way to the track, that is $7 vs probably $3.20 for the Amoco premium. The one drawback is that it willcontaminate your O2 (if you use one) but with a mix like that it will take a while.

[Mike C]

Is that roller the 268SR? Your combo is very similar to mine except for your aluminum heads. I have 9.8:1 single trough valve relief flat tops and ported iron 492 straight plug heads. Your Trick Flows should be much more forgiving. I have never run race gas in my motor, just 93 octane pump pi$$. I run 40 degrees of total timing (inefficient GM chambers...) Your jetting seems a bit lean, however, but it's apples and oranges compared to mine. I have a BG Stage III 750 (1040 cfm) and it has something like 75 jets in the front and 85 in the rear. What do you have in the way of distributors? Are you running vac advance and if so, ported or manifold vac? It is difficult to get an accurate total advance reading with ported manifold vac as the vac reading at WOT may differ significantly from your tested reading with no load on the motor. My solution was an MSD distributor where I could tailor the mechanical curve precisely with their bushings and springs, and just eliminating the vac advance completely. I have had no troubles with detonation (fastest time ever for my car was 106 deg F that day!)

[jt1]

What really throws me is that about a month ago I went to another HPDE at the same track, with temps about 93 deg. I had no problems that time, timing at 36 deg,which is what I normally run. The only change I have made to the car is to install a griffin 24 x 19 alum radiator. the car ran a little hot, maybe 230, in the 93 deg heat. With the griffin, temps never exceeded 210 deg. No other changes. Scotty, this makes me think the gas isn't the problem, UNLESS I got a bad batch of gas at the station. I will try some different gas next time, but I really can't test that till I go back to the track (4000 rpm is about 90 mph). Mike, the vac advance is not hooked up. I have messed with the springs in the HEI so I get all the advance at about 25-2600 rpm. No detonation below 4000. I pulled the plugs out this afternoon, looking for a lean condition, but the plugs (Autolite 3924's) are dark tan, looking a little rich if anything. No bluing on the ground strap. Of course, I came off the track and into the pits, not a clean cut off. Maybe I'll try some diff gas and risk the NCHP to test her out.

 

Thanks

 

John

[Scottie-GNZ]

Hey John,

 

good luck in whatever you decide to do, but it leaves me scratching my head. I just cannot understand why folks will make a major investment in buidling up a high-boost forced-induction or hi-c.r. NA engine then take it to the track and try to run it all-out on pump gas. Not only are you not tapping the full potential of the engine, you are at risk of doing damage. Be careful as there is a big difference between a quick stab on the street and running WOT down a drag strip or a straightaway.

[Mike C]

If that is directed at me Scottie, my motor was built to run on pump gas. It was a STREET car. You of all people should know that! I never gutted it, removed anything from it that I kept in it when I drove it and ran on street tires. I always wanted to know what it did as I drove it, not what it would do if I changed a bunch of things. In '88 when the motor was put together there were VERY few cars runnin mid 12's on the street. In the subsequent 15 years mid 12's seems to be pretty pathetic, but the motor is as good as new. Actually, if you don't need the extra ocatane in an NA motor, the slower burning fuel can cause the car to slow down or require even MORE ignition timing for the same power. I still don't feel like 10:1 is very high compression with aluminum heads.

[Scottie-GNZ]

Mike/John,

 

my comment was not directed just to you, but a general obversation and one I probably ought to just keep to myself. I have a street car but since I do not street race I do not tune it for optimal pump gas performance. I do hope my comment was understood though and that the concern was extended WOT at the racetrack. I would just hate to see someone tune on the street with pump gas and then have a problem running extended WOT on the track. It is just not the same conditions.

[johnc]

> The one drawback is that it will contaminate

> your O2 (if you use one) but with a mix like

> that it will take a while.

 

FYI...

 

Per my engine builder, 99% of the Lamba sensor contamination occurs at idle. Leaning out the idle as much as possible will reduce sensor contamination. For my engine I'm running two 4 wire Lambda sensors and using VP Racing's leaded C12 fuel. I can expect about 50 hours of life out of the sensors before they have to be replaced.

[jt1]

Been thinking about this. Does anyone think putting the larger radiator in the car, while reducing the water temps, caused the temp of the underhood air to go up enough to cause the engine to detonate? This was the highest ambient temp I've ever run the car in at a track event, and the water temps were noticably cooler than before. The extra heat being removed from the engine has to go somewhere, into the air passing through the radiator. Maybe the combo of the higher ambient temp and the heat from the radiator was enough to cause the detonation. If so, i guess the answer may be less timing/more fuel/better gas to deal with the increased temp of the air. Or even better, a cowl inducton/air box setup to get cooler air to the carb. This may eliminate the detonation and provide some power increase too. Any thoughts?

 

Scottie, I appreciate your concerns about gas. Pump gas was one of my goals when I built this engine. I want to have a car that I can drive anywhere and be able to take to a road course and flog when I want to, without the hassle of mixing gas. Better gas would almost certainly solve the problem, but my preference would be to find another answer. Thanks for your help.

 

John