Thread: Help! Tuning Pontiac 400!!

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I have a Pontiac 400 American blue block and it is bored out .0060
TRW forged pistons -- I have #48 heads -- with 280H comp cam -- clevite bearings -- double roller timing chain--edlebrock intake manifold--edlebrock 750 carb--and headers.

Runs fine until I go to race it and then it detonates at the higher RPM...will not exceed 4000 RPM ...HELP!

68chick

Check your timing first.Possible dist. problems,good luck,shawn

If detonation is truly your problem, you should start by checking your timing as recommended above. Assuming you have the proper octane fuel for your compression, I would first verify top dead center if you haven't already. To check TDC, I remove all of my spark plugs and insert a home-made piston stop in #1. I made my piston stop with an anti-foul adapter that I threaded on the inside to accept a 3/8 bolt. Then, with a ratchet, I rotate the engine clockwise until the piston touches the stop. I use a ruler or a carburetor float measuring gauge to measure the distance from the mark on the balancer to the "0" mark on the timing tab. Then I rotate the engine counterclockwise until the piston touches the stop and take the same measurement again. Ideally you wouldn't want to be more the equivalent of a degree or two off on a street motor. If you are off more than this, the ring on your balancer may have slipped or the balancer and timing tab may not be compatable. I know this is basic stuff, but you would be surprised at how many people and engine builders take for granted that is always correct. Keep in mind that if this is wrong, setting the timing will ultimately be a guessing game.

After I verify that the TDC mark is correct, I reinstall the plugs and remove the distributor cap. I check to make sure that the mechanical advance is not "frozen", and I check to be sure that the vacuum advance is working. There is no need to proceed with setting the timing unless the advance systems are working correctly. If everything appears to be working, then I get out the timing light. You will either need a timing light capable of checking full advance or a balancer that is correctly marked with a timing tape. When setting the timing, I concern myself only with what the TOTAL timing is. If you have no way to check total timing, go borrow or buy a light that can or just forget the whole deal. Now, don't worry about what the timing is at an idle, I would just deal with the total timing and let the timing at an idle fall where it may. Once this is set, you are finished unless you want to recurve the distributor, which definitely can improve low-end torque.

If this doesn't cure your problem you may have to begin looking at the fuel system. But I would certainly start with the timing first if you're sure that your problem is detonation and not something like valve springs, faulty ignition wires, fuel delivery/jetting, etc. At least at this point you will have peace of mind that these timing issues are correct.

Good luck!

like the other two here said timing and general tune-up plug wires
cap and rotor and all that good stuff but first off check you timing that's the main thing

Sounds like a nice setup. Are you running stock valve springs? With a taller cam you can get into valve spring bind. If your springs are tired they can float, which will also limit your ability to rev the engine. This is different than detonation, but it'll wipe out your engine quick. I ran into it with my Trans Am.

Hey 68
Ditto on the valve springs and wires. Is your cam running straight up. What kind of ignition are you running and what is the total advance. If you running points check for point float. I'm running a 280h cam in my t , your just on the begining of the power band. What did you set your valve lash at. Rich

I bought a car with virtually the same engine and they said it was built for power. That weekend at the track it wouldn't rev past 5,000. I was shocked. Turns out it had a set of 64 cc heads. That translate to 12.5 : 1 compression. It was aweful. I have since rebuilt that engine with 76 cc heads and with the TRW pistons it's pushing 10:1. That's borderline with iron heads. 93 octane is a must. When it gets hot does it ever diesel when you kill the engine?

68......I am having the same problem, same motor in a 69 GTO. I checked the dwell last night, my pontiac manual said the dwell should be between 30-32 degrees. I got the dwell meter on it..it was set at 25 deg!!!

Keep in mind I just bought this car and this is the first time and I am kind going over things......so I adjusted the dwell, then adjusted the timing. I took it down the road and it detonated at about 4000 rpm! really bad!

So I backed off the timing....drove it again....the valves started floating at around 4500 rpm!

Then I advanced it in the middle from where I had it the first time...drove it...now it has just a slight detonatation at 5000 rpm!

So should I be happy with that? Or is that not good enough? Is that slight detonatation gonna hurt me?

thanks

Brent

i have a 69 ram air III gto motor in my firebird right now i have been trying to find out exactly what the timing is suppose to be but i hav it set at 8 degrees before tdc. and seems to be fine also you should think of changing it over to a hei distributor. because the person before me had points and had a detonation problem too. so far after i have rebuilt it and changed it over to a hei i have not had a detonation problem. good luck with it.

-joe