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Old 12-16-2005, 05:07 AM
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Can anybody explain this for me

I have a 355 Chevy that I did a cam and intake change on. Here's what's going on. I had an XE286 cam and a Victor Jr intake on the engine, I know, way to big for a truck. So I put in a XE268 cam and a performer rpm intake to solve my low power problems. Ever since the cam swap the engine pings under light load on the highway. I have the distribiutor advanced so far that that the starter almost will not turn it over and it still is doing it. The way I understand it is that I am having preignition, which is why I advanced the distrib so far. Octane booster is no help. It has 461 heads, flat top pistons, the block to head mating surface was cleaned up but not zero decked or anything. My question is why would the engine ping after the cam swap. I thought less of a duration cam would mean less air into the cylinder and thus less of a compression ratio. Anywho, if anybody can exlplain or help me out with some tips I would greatly appreciate it. Once the mechanical weights fling out it runs fine, about 3000 rpm, using stock springs. Thanks, Sam

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Old 12-16-2005, 05:36 AM
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Advancing the distributor will only make the preignition worse.

Chris B
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Old 12-16-2005, 06:30 AM
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Like Fl_rider stated adavancing will make it worse. I would not say that that old cam was too big for a truck, but it may have not matched your particular application. I happen to run a Comp 280h in my truck and it works great, for me. You have to match all the componets. Look at the RPM range of the cam amd match the intake to that. I can't see where the 268 would need a stall converter, but I needed one on my 280. Have you checked your timing? What is the iniatial and what is the total? It may be necessary to install a re-curve it in the dizzy to bring the ignition in at a sectain RPM. You have to remember that Comp cams are ground 4* for the factory. The pinging could also be contributed to a lean setting in the carb. Have to done a plug check? There are a few things that could cause a ping, and I don't think it would just be the cam. What gearing do you have?
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Old 12-16-2005, 07:49 AM
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Installing a cam of less duration raises the effective compression ratio because the intake valve is closing sooner. In your case it probably was raised about 0.7, enough to cause some nice pinging. You may want to re-curve your distributor to have less initial advance and a shallower curve.
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Old 12-16-2005, 12:11 PM
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When you re-cammed and got pinging, you should have retarded the timing to stop it. You should also re-check you carb settings and check the dizzy curve as mentioned above.
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Old 12-16-2005, 04:09 PM
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what grade gas are you running in it? you may need to go up to the next rating...
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Old 12-16-2005, 04:24 PM
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Do you have a timing light to check where your timing is at? As mentioned already, if you advanced the timing to reduce preignition you are going the wrong way. Bring the timing back.
My guess is that the cam isn't causing your problems at all. It may be worth it to play with the curve of the distributor, but not yet. Retard the timing and see what that does. If you want to do it right get a timing light. Start by putting the initial timing at 8-12* and the total timing at 34-36*. That should be a good starting point.
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Old 12-17-2005, 11:02 PM
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Thanks for the replies fellas. I did notice that it got a little worse when I advanced the distributor. I'll retard it back and see what happens. Thanks again, Sam
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