New Engine preignites at 4500RPM

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wazoo64

wazoo64

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Shot it the dark here but maybe someone has a magic wrench. Brand new stroker build with Mallory electronic ignition. When the engine was being dynoed the builder was having preignition problems at 4500RPMs. When he bypassed the ballast resistor the problem went away and the engine pulled 560HP at 6000RPMs.
Here's my problem. I just installed the new engine with ballast resistor and it is preigniting at 4500RPM. It runs great until it revs 4500. This ballast resistor, distributor and coil were all working fine before we yanked the engine for build. Nothing has changed other than new engine. Any ideas? I know its a shot in the dark but just looking for piece of mind before my tuner tries to tackle the problem tomorrow.
 
You don't need a ballast resistor with the Mallory CD style ignition. What model number is the mallory box?
 
I just thought of something. The distributor is grounded to the eddy intake manifold. Is that a problem? Shouldn't it be grounded to the block??
 
Who did the heads and what are they? Can you post very specific details on teh engine build? Like piston design, valve sizes, quench, an cam specs? Detonation starts in a cylinder where liquid fuel exists. Sounds to me like the issue is the chamber shape or possibly throat porting and valve job comination. If the builder could not reduce timing or jet to cure, it's a mechanical issue like that. Your ignition wont fix it, and his only bandaided the problem on the dyno. If the intake was a single plane, you might be able to correct that issue thru careful porting and damming. 360/408s have more tendency for this as the bores are typically .040" narrower, leading to more shrouding and poorer wet flow around the intake valve. If you remove a head and look at the dirty dome and chamber surfaces you will see the marks left by wet fuel puddling on the edges of the chamber and piston.
 
fwaugh said:
Nothing has changed other than new engine.
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!!!! That's funny.

'Pre-ignition' means the mixture is igniting itself before the spark occurs. Sounds like excess heat to me! Could be lean? Perhaps you need more octane too, depends on your C/R. Hot spots can also cause pre-ignition, but that's usually from loose carbon deposits. If the engine was recently rebuilt, that's not likely.

Maybe check your plugs to see what they look like. Perhaps you need to adjust the timing too, advance it so it starts to burn a little earlier. Keep it simple!

The purpose of the ballast resistor is to limit voltage so as to not burn everything up within a few hundred miles. Designed for points distributors initially. As an example, you don't need one with an MSD.
 
rmchrgr said:
!!!! That's funny.

'Pre-ignition' means the mixture is igniting itself before the spark occurs. Sounds like excess heat to me! Could be lean? Perhaps you need more octane too, depends on your C/R. Hot spots can also cause pre-ignition, but that's usually from loose carbon deposits. If the engine was recently rebuilt, that's not likely.

Maybe check your plugs to see what they look like. Perhaps you need to adjust the timing too, advance it so it starts to burn a little earlier. Keep it simple!

The purpose of the ballast resistor is to limit voltage so as to not burn everything up within a few hundred miles. Designed for points distributors initially. As an example, you don't need one with an MSD.
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I think preignition maybe the wrong term. Its banging like a drumset. Misfiring is probably a better term.
 
fwaugh said:
I think preignition maybe the wrong term. Its banging like a drumset. Misfiring is probably a better term.
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Detonation? Still a similar situation with excess heat. Pinging and detonation are signs of a lean condition.

It dosen't cost anything to check you plugs to see what they look like.

reading_spark_plugs.jpg
 
Good info to know. I think the problem has to be related to distrubutor. Maybe not getting enough juice to it. Mallory makes a coil that has resistor built in. Would that help or be a good idea. I would like to get rid of the resistor if possible. One less point of failure.
 
Pinging is lean, and can be fixed with either less timing, or more fuel. Detonation is not the same thing, is not caused by the same things, and can be totally silent or extremely loud. If the dyno guy couldnt get it out with timing or fuel, it's not going to be fixed with ignition or fuel in the car. My guess is, it's detonating lower than 4500, you just cant ehar it easy. But keep playing with it. Who knows.
 
fwaugh said:
Good info to know. I think the problem has to be related to distrubutor. Maybe not getting enough juice to it. Mallory makes a coil that has resistor built in. Would that help or be a good idea. I would like to get rid of the resistor if possible. One less point of failure.
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The Mallory unilite dist. ballast resistor is a part # 610 if i remember right. Summit has them listed. The coil with the built in resistor could fail also just as easily as a external resistor then you'd have to pay $70 for a whole new coil where-as if just the ballast resistor fails you only pay $15 for the resistor. I will say is if it's running the ballast resistor isn't' bad. When they go bad it won't run at all cause it cuts power to the dist.

Grounding to the intake is fine because it bolts to the heads which bolt to the block so it's all electrically interconnected. You should also have a ground wire running from the intake to the firewall to properly ground the engine to the body.

Have you put a timing light on it to make sure you got the timing right after installing the engine? Make sure it's what the dyno guy had it set at.
 
I hope I'm not over simplifying, but what are the plugs gaps set at? They may be too wide.

Like moper says, more info on the build is great too.
 
My tuner figured it out today but I didnt get the live report from him. When I speak with him next week I will post his findings.
By the way for any SoCal subscribers Riverside Carb and Electric is a great place to go for tough problems. Larry will fix anything and do it right.
 
fwaugh said:
My tuner figured it out today but I didnt get the live report from him. When I speak with him next week I will post his findings.
By the way for any SoCal subscribers Riverside Carb and Electric is a great place to go for tough problems. Larry will fix anything and do it right.
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Maybe ask him what method he used to figure out the issue, i.e. what steps he took to arrive at a conclusion. I'd like to hear what the issue was.
 
moper said:
I agree.. I'm curious how the dyno guy missed it...
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The dyno guy and engine builder is Joe Sherman a fairly well know Chevy, Ford builder but not Mopar. He had all kinds of issues getting the dyno up and running just because doesnt do small block Mopar. He had to get a lakewood bellhousing to fit my TTI's and a few other minor tweaks to get the small block mounted right.
Anyway, on his first couple pulls the engine was going strong to 4500 RPM and then started the bang, bang, pop, pop stuff. He couldnt figure it out but said he thought the ballast resistor was causing the issue. I gave him the greenlight to bypass it knowing we may fry the module. First pull without resistor was 582HP. Problem fixed. Final pul with my carb was 562HP with 492 torque. His carb was bigger and built up.
Once I dropped the engine back in my car I hooked it up exactly the way we had the engine in my car before build. It was working great before with Mallory Unilite and resistor. About 450HP.
When we fired up the car it ran fine until 4500RPM and then bang, bang, pop, pop. At that point I knew I would need help and my tuner guy fixed it easily. When I pick up the car and get the report I will post it.

Question???? Maybe a stupid one. Why do the Mopar engines need the ballast resistor and Ford, Chevy dont?
 
fwaugh said:
Question???? Maybe a stupid one. Why do the Mopar engines need the ballast resistor and Ford, Chevy dont?
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The other makes use them, just depends on the design of the ignition system. Go to an auto parts store, there's a bunch to chose from.

Again, a ballast resistor was originally used to limit voltage to the system. I'm not sure exactly why it was used on electronically-controlled ignitions, since those systems are controlled by a module. Theoretically you shouldn't need one.

Hard to believe that your ballast esistor was limiting your performance like that, seems like an unlikely culprit though anything's possible. I would still like to hear the outcome of the problem.
 
I agree with rmchrgr that it seems unlikely the ballast would cause the problem so when as you find out what was wrong please post it. I'd like to know what the problem was too. Sometimes a high speed miss can be an easy fix and sometimes it can make you beat your head against the wall cause you can't easily find it.

As far as ballasts go I know the old Chevy's had them built into the coil and assume Ford must have done the same although Ford's module was plenty big enough to put one inside it so that may be what they did. Or they could have even controlled the current/voltage electronically. Mopar's module was too small to put that big resistor inside with all the other parts.
 
Ah. That's why. It had ignition misfire... Not detonation. BIG difference there. The Ford IIRC doesnt use any resister. The GM uses a wire in the ignition system to step down system voltage so point sdont get fried and coils dont overheat. I fyou had a coil (Like the Mallory Hi Fire or some MSDs, you dont use the resistor. The ignition boxes (like MSDs and Crane, or JAcobs) control the coil's positive side voltage, so you take the resistor out totally.
 
Were the 5 and 7 wires touching at any point? What was the total timing at? Ford and chevys sometimes use resistance wires. Chevys have a cloth covering on theirs.
 

fishy68 said:
That'll do it. Glad to hear you got it.

I'm fighting a high speed miss coming in about 5800 on my 360 right now. Wish it was that easy to fix. I'm about out of ideas.
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are your 5 and 7 wires crossing anywhere? at the cap? msd box? my friends chip went south on him. a 6k chip started dropping spark off at 5500
 
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