Cam and compression question

[BigBlockDude]

My dad has a 69 GTX 4speed, a few years ago he bought a rebuilt 1970 440 6pac engine for it from my uncle.
I am not sure of all the specs on the engine, heres what I know.
906 heads, stock
292 509 MP cam with Rhoads lifters.
Stock six pac intake and carbs
I blocked the heat crossover in the intake
The problem is when it gets warm it diesels when I shut it off, and pings when I time it where it should be. I'm assuming it has the 10.5 to 1, like a 6pac is supposed to. I am lookign for some ideas on what I could possible do to make it more streetable on 92 octane. Maybe a different cam to bleed off some cylinder pressure (I've read Rhoads lifters are supposed to increase cylinder pressure at lower rpms) , or any other suggestions. Ideally I would replace the pistons and have it rebalanced, but I have enough projects at this time.

[68DCharger383]

I was having a similar issue with my 383. It too has an aftermarket cam. Where your timing needs to be is all relative. WHat the book states is for a "Stock" scenario. In your case, I am pretty sure you would want to run more initial advance with your cam, but then you need to adjust your vacuum adance, as i did, to stop the pingin under light throttle acceleration. I run 92 or 93 myself. Also, I switched my vacuum source to the ported port on the carb, and not the full advance port. By doing so, i gain a noticable increase in my low end response. Mechanical adavnce will most likely need to be tailored to your specfica cam, but in general, pingin under light load is normally caused by too much advance. Of coarse knowing your static compression ration would help.

[BigBlockDude]

I'm not running vacuum advance.

[dgc333]

Dieseling when you shut it off indicates some hot spots in the cylinder that are igniting mixture. Get some top end cleaner that you pour/spray into the engine while it's running to clean any carbon build up on the head or pistons (misting water into the engine while it's running will work too).

This may also take car of your pining problem.

[dave571]

That 509 should bleed off lots of compression.

cgd333 may be on the ruight track.

you couild also try a colder plug.

Timing may be a big factor here too.

What is the initial? What is total? At what RPM?
I agree that vacuum advance is not needed on an application like that.

[BigBlockDude]

Originally Posted by dgc333

Dieseling when you shut it off indicates some hot spots in the cylinder that are igniting mixture. Get some top end cleaner that you pour/spray into the engine while it's running to clean any carbon build up on the head or pistons (misting water into the engine while it's running will work too).

This may also take car of your pining problem.

It has only a few hundred miles.

Also I think the rhoads lifters are probably whats giving it more cylinder pressure at low rpms. I thought about putting in standard lifters.

I've adjusted the timing, does not help much. Might try to make the curve a little shorter. Honestly, I think it just has too much compression for pump gas.

[dave571]

Originally Posted by BigBlockDude

Honestly, I think it just has too much compression for pump gas.

Really hard to say, without measurements from when the motor was assembled.

Speed pro's "six pack" replacement piston is spec'd to be .019 below deck, with 6 cc reliefs.

At .030 over, with a .040 composite gasket, 88 cc 906's, that will give you about 9.4:1. This would be typical, but who knows what was done. a couple of cc's either way on the head, .005 here or there on the deck clearance,then 10.1to 9.1 becomes a realistic ballpark.

I ran these pistons with 9.6:1 and a similiar cam, without the problems you are having.

I then bumped the compression up to 10.6:1 with 78 cc closed chamber 915's and .043 quench zone. It still doesn't have the problems you have described(although I suspect it pings under hard load, but I just can't hear it).


I don't think the compression is the fault. At least not by itself. It may be a contibuting factor, but it is not all that is wrong. my .02

[JLM440]

I don't think its a compression issue either, you need to check the tune, make sure its not running lean or have a vacuum leak somewhere, also check the heat range on the plugs, hot spots on the pistons will cause this also, bad cooling, too high of idle, it really has nothing to do with cylinder pressure, i had 195 cranking pressure on my BB & it NEVER had run on, but i've had 8.5.1 350 chevys have run-on do to a bad tune (lean condition/vacuum leak) my .02

[BigBlockDude]

I found out it had domed pistons installed. Just what kind of piston, I don't know.

[sundrop_440]

now I think your in trouble with high compression, your probably over 11:1 and I'll bet you have speed pro 2295 domed pistons, though that's a just a guess, have you tried running some higher octane through it to see hows it handles it,

[BigBlockDude]

I plan on trying some good gas. 100+ octane to test my theory. If runs great, I'll know what going on.

[JLM440]

Yep, thats your problem, your compression is over 11.1 for sure, you need racing gas, don't run that aviation stuff, run at least the red 105 VP or Sunoco, you can find it at most motorcycle outlets, i would also experiment with a colder plug, i'd say the NGK 5670-7 would work good, maybe even the -8, now you can also try a 50/50 mix of 110 & 93, but your always going to need racing fuel in it.