Help with pre ignition ping

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Cudapete

Cudapete

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A friend of mine has a 372 (340 with 360 crank) for his 69 barracuda. He bought the motor from a guy who was going to put it in a rail or something. Any way they changed the heads,cam and intake to get to a street friendly 10.5:1 comp when he decided to sell it to my friend.The motor has a air gap intake, pocket ported x heads,and a mopar electronic ignition.The trouble is the car has never run real strong and pings violently under 1/2 to 3/4 throttle. He knows the guy who built it but wasnt around for the process so we dont know much more than this. The mopar distributor seemed to advance too fast so we changed the advance springs no change, then put it back and reset the timing to 32 degrees advance @ 2800 rpm's still no good, then we put in a pro comp distributor right out of the box brand new and set it to 32 degrees and the same thing happened. So my question is we started to wonder if maybe theres more compression than what the guy told him and of course I dont have a working compression tester right now so tomorrow hes gonna take a ride out to the local airport or delta sonic and get some high octane fuel and see if the ping goes away, Is this a feasible idea? will it hurt the motor to try this @ 32 degrees advance.If this is a bad idea what do you think we should look at next? Thanks
 
Are you sure the damper is good and didn't slip?
Is that 32*'s with the vacuum hose disconected and vacuum source pluged?

The aAV gas won't hurt the short term use for the experiment. I'd first retard the timing until the ping goes away and work from there.

Other things like a poorly installed cam could produce results like this.
 
rumblefish360 said:
Are you sure the damper is good and didn't slip?
Is that 32*'s with the vacuum hose disconected and vacuum source pluged?

The aAV gas won't hurt the short term use for the experiment. I'd first retard the timing until the ping goes away and work from there.

Other things like a poorly installed cam could produce results like this.
Click to expand...

x2
Compression gauges are fairly cheap at the local parts store.

would be interesting what the crank pressure is!
 
10.5 to 1 comp isn't always that street friendly with iron heads. Especially open chambered heads. Do you know what cam is in it and if so what are the specs? With 10.5 to 1 compression and open chamber iron heads you'll need a fairly decent sized cam (240 @ .050 minimum is my guess) or it'll have so much low end cylinder pressure it won't run on pump gas. My 360 is 9.8 to 1 comp. with X heads and a Lunati 234 at .050 cam and it won't allow more than 35 degrees of timing on 93 octane fuel without spark knock.
 
........Sounds like u have lots of compression, unless he had a special piston made 4 that crank u will b closer to 12.5..............usually accomplished by milling down a forged piston .100 thou...................kim........
 
thanks guys, to answer a couple things the 32 advance is with the vacuum disconnected and plugged off. The car doesnt ping nearly as bad with the timing at like 25 degrees advance @ 2800 but the ping never fully went away.I beleive the cam is some where in the 545 lift/ 240 dur but I cant be too much more specific at this point I'll see if hes got something more specific on that.I am going to get a new compression tester soon, he just figured he'd try the av gas as a quick experiment. thanks again
 
If he wants to really "know", pull a head and profile the cam. Itsounds like it's got more static compression and you can either lower the dynamic compression via cam or head gasket, or run better fuel. It is not acceptable to "live with" pining or detonation. It will very quickly destroy the engine.
 
ok so if the engine doesn't run very strong and pings... the cam might be degreed wrong.
I would have to see how many key slots are availabel and which was used.
If the cam / crank relation isn't right you can screw around with the distributer forever and never cure it.
 
Good point Red. I was thinking that as well in my first post. I have made that mistake in the past. Installing the cam in the wrong key way and giving it a in general dot to dot line up. I was probably just barley scraping by without the valve hitting the pistons.

While it did run, it did run like crap. It's rather upsetting to screw togther an engine with the cam that far outta wack.
 
So a little update on my buddys issue. He talked with the engine builder and he told him that the pistons are custom made wiesco High compression pistons with valve reliefs. to get the compression "down" they used a .090 gasket and the X heads. to get it close to 10.5 :1 The motor ran better with less ping with 5 gal of 93 mixed with 5 gals of 100 octane av gas, however it still had some ping in it which makes me wonder if it needs more of a higher octane race gas to completely eliminate the issue, or maybe less 93 and more straight 100 octane. We know the ping isnt exceptable over a long period however what we're hoping to do is try to eliminate the issue with out a major tear down for the summer and then pull the motor in the fall and change to a actual 10:1 piston. thanks guys
 
Even short term spark knock is not acceptable. I found that out when I was young and inexperienced. Built a engine and ran good but with open headers I didn't hear it spark knocking. 10 runs is all it took to take out the bearings. Spark knock is essentially driving the pistons backwards. It will destroy pistons, rods, and especially bearings real fast. Do what ever it takes to eliminate it right now or I can almost guarantee you will pay dire consequences. Anybody disagree?

Did he have anymore info on the pistons? Did they stick way out of the deck? That's the only reason I can see for running a .090" head gasket.

I'd check out the cam also cause like Red said it could be degree'd wrong. May be way advanced. Even if you don't know what it is you can at least see where the intake centerline is. That'll give you some idea. Did you ask the engine builder what it was?
 
I totally agree with you guys that the ping needs to be eliminate immediatly, and I agree it will defineatly destroy the motor we just wanted to try everything we could without a tear down first.The only info I got yesterday was what I posted but I do have an update... He set the timing to 35 degrees at 2800 with the mix of av gas and 93 and there is no ping and runs strong. So if you guys agree that those are ok timing specs for a 372 then it'll be an expensive summer to go cruisin then we'll pull the motor in the fall and change a few things around.Thanks again for your input guys I always get great info here.
 
I can only assume that you live near sea level(less then 1000 feet)

10.5:1 (I bet is more, or the cam is smaller) is just to much for pump gas at sea level.

When you pull the head off, REMOVE all Sharp edged in the combustion chamber and piston.........This could be part of the problem.............


Get a part time job, for all the racing fuel, and have some fun this summer :-D
 
My 340, with 9.4:1 compression, at 4200 feet, required 2 gal's of 110 octane, to a half tank of 91 octane fuel. To keep from pinging, at the top rpm, in 3 gear in the heat of summer.(Drag Racing)
 
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