390 with rattle under acceleration
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Joined: Jun 2004
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From: salmon idaho
390 with rattle under acceleration
hi i have a converted 360 to 390 with a fresh rebuild . it runs great but on heavy acceleration or a heavy load it has a light rattle . it almost sound's like a diesel under accleration . i dont know if it is preignition or someting else . from a dead stop with heavy acceleration it sounds normal and very healthy . i run 87 octane . it has c4ae heads and a c4 4 barrel intake, i don't know what it's from . the block was a 74 . im running an edelbrock 650 cfm carb , rv cam , headers and a dual exhaust . it was a low mile motor so i reused the 360 pistons . i think it has 10:1 compression but im not shure . the trany is a slushbox .
Post Fiend
Joined: Mar 2004
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From: Union, Washington
It's called pinging and will shortly knock holes in your pistons. Either more octane or less timing. 10:1 is to much compression for regular and sometimes even the next step up in octane, also todays gas gets more oxygenating additives in the winter causing your engine to run even leaner(more ping thank you EPA).
Post Fiend
Joined: Jan 2003
Posts: 8,600
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From: Iowa
What cam are you running in it? A bigger cam will allow you to run more compression. I have a 10 to 1 compression 390 with the crane 343941 cam, and I get no pinging at all on 89 octane 10% ethanol. I also run 10BTDC, with the light advance springs in the distributor, and I have the distributor setup for more total advance.
If its the stock cam, I'm not surprised its pinging.
For a temporary measure retard the timing until it stops pinging, and use higher octane gas. The permanant solution will be a bigger cam. Plus its more fun.
If its the stock cam, I'm not surprised its pinging.
For a temporary measure retard the timing until it stops pinging, and use higher octane gas. The permanant solution will be a bigger cam. Plus its more fun.
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Less compression is the best answer (the super thick head gaskets help alot). Anymore than 9:1 or 9.5:1 isn't gonna work unless you are willing to spend the money for premium fuel. All the bigger cam does is let air bleed back out the valve at lower rpms by holding the valves open longer, not really the best thing for fuel economy in this day and age.
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From: salmon idaho
thank's everyone . it sounds about like i figured . ill try retarding the timeing 1st . if that doesn't work i read a post with a recipy for octain boost a while back . something about laquer thinner etc,etc. for rusty70f100 the cam im running is a clevite rv part # 229-1776 S2026 . the speck's are (dur. 204 int. 214 exh. ) (lobe spread 112 )( valve lift .486 int.- .512 exh. )(cam lift .281 int.- .296 exh.) with this and the rest of my info does anyone have a guess about how many horses im putting out ? thank's .
