[ratlover]

Hey guys,
I have a 9:1 454 +.040 with 781 heads and 36 degrees total timing. Under low rpm and heavy load (ie mash throttle without dropping clutch) it tends to rattle. this is with 92 octane gas. I have a silver and a blue spring on the dizzy. also have a crane adjustable vac advance. Total timing at 3000 is 36.initial is 12. block has been decked to a couple thou within zero. 3600 pound chevelle. I would think with the 781 open chambers it should like more advance. any suggestions? Balancer checked for accuracy and is dead on.

[Bumpstick]

Which pistons do you have...? Sounds more like you have more compression than you think you have...?

[ratlover]

I am running forged flat tops. shaved the heads .030" They appeared to be uncut before that. I did not have the equipment to cc them, but estimate it at about 113cc.

[Bumpstick]

The heads start at around 115cc`s but I would figure .030 off of them should make around 110-111cc`s and basicly a zero decked block with a reg felpro head gasket would make maybe 9.5 to 1 But also what brand of 92 octane fuel...? I am running a little more compression and the only 2 fuels I can run are Exxon Supreme and Chevron Supreme (both are supposed to be 93 octane)

Also what temp thermostat...? (195...?)

[ratlover]

I try to use chevron exclusively, mostly because once i have dialed in the mix and timing there will be no other variables. We have the 89,92 and 94 available up here. I have been using the 92,(recent gas prices)thinking that would be ample for the comression ratio. I thought the 781's were more around 119-122cc stock.(?) I am running consistently around 180-195 with a 180 stat.

[xntrik]

Open chamber heads are prone to detonation regardless of brand, size, or engine family. This was the common problem of the 70s smog engines, and one of the main reasons that compression got so low.

More modern technology maintains quench areas for detonation resistence.

You might have to use tighter advance springs to reduce the spark advance in the low rpm range where you are having problems without limiting the total timing.

A good distributor man can alter the advance curve by modifying the centrifugal weights so that timing is reduced at lower rpm and then above a certain rpm (such as 2500) the rate will increase more rapidly.

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[dh79]

Is the adjustable vacuum advance adjusted correctly? Try dialing it back a bit. They can make quite a bit of difference. That's where I'd start experimenting first. good luck,
dh

[ratlover]

I dialed in the vac advance so that it never exceeds 36 degrees, even with manifold vacuum. As the vacuum drops off, the weights kick in. advance verified at 3000. I was thinking of going to blue springs on both sides of the dist. or trying a stock one with stock weights. It gets up and goes if you drop the clutch, but I don't want to fry the pistons.

[Guy Hiltz]

Just for the heck of it try un-hooking your vacuum advance and see if that cures your problem.I was having a similar problem with my 10.5:1 compression Big Block Chevy and my buddy told me to try this and it helped eliminate ping at off idle/mashing the pedal and also made it more responsive.I checked the plugs a couple of times afterwards because I had read that un-hooking the vacuum would foul the plugs but this doesent seem to be the case,although I replace the plugs every year.

[ratlover]

Interesting. It's been a while since I was experimenting with the manifold vs ported (not that I want to get inot that debate!) but the manifold definitely seemed to make it seem more responsive and dropp ed the temp a full ten degrees. What about the old closed chamber rec port heads. Would the gain in compression and tighter chambers be worth the hassle of a head swap and reward with improved performance and no death rattle?

[Bumpstick]

Compression would go up for sure and the death rattle as you saw would get alot louder (end up with around 10.5-11.0 to 1 or higher...?) I think that (xntrik) might be on the right track and also the vac advance has to be tuned to match and work together with the mechanical side... Also go with the idea on the springs and try some different combinations...

My first suggestion would be to try the 94 octane as its kinda strange to have 92 also...? We have 87, 89 & 93 but are your octane rates on the same formula as ours...? Also I run a 180 thermostat and if my temps get any over 190 I get pinging also even with 93 octane but my setup is at 10.8 to 1 but have finaly found a good octane booster that does the trick (not 104+ either)...

[dh79]

Yeah, in canada chevron has 87, 89, 92, 94 octane.

I still think you should play with the adjustable vacuum advance some more. I played with one several years ago. I don't remember exactly but I think the instructions said to adjust it so that it starts to rattle under load, and then back it off half a turn so there's no rattle. Something like that anyway. I remember relatively small adjustments making a difference in part throttle high load detonation. There's no point in running around with too much vacuum advance. It's not helping your wot power any.

[NXS]

Your initial needs to be up around 18*, no vac advance{or very little, I rarely if ever run vac advance unless I have a cam with 480 or less lift} and total is good at 36. This is a performance engine, not an 8:1 smog engine.

[xntrik]

Originally Posted by ratlover

Interesting. It's been a while since I was experimenting with the manifold vs ported (not that I want to get inot that debate!) but the manifold definitely seemed to make it seem more responsive and dropp ed the temp a full ten degrees. What about the old closed chamber rec port heads. Would the gain in compression and tighter chambers be worth the hassle of a head swap and reward with improved performance and no death rattle?

I ignored the vac adv because based on what he said initially I understood that he meant full throttle, which kills the vac adv to zero. Try disabling the vac adv to see if the ping stops. If it does, then I would go with NXS idea of more initial and no vac at low rpm. Adv cans are available that begin to advance at different vac ratings, etc. for tunability. A stock can might not be what you need.

The full vacuum at idle will give you vac adv at idle which often does make the engine run cooler because the fuel is burned more completely. Sort of like NXS said, more initial adv.

The point of vac adv is to allow more than the 36, maybe even 44 or more under light load conditions for better efficiency and temperature control.

Tight chambers would be great,,,,, IF your compression stayed at 9.0..... which it will not unless you change pistons. Quench heads withs reverse dome pistons are the best.

Rule one= make it run right without vac adv, then add it for efficiency at light loads.

[enjenjo]

FWIW, I have a 454+30, flat top pistons, and 69 model big valve closed chamber heads. It runs just fine under most conditions with 87 octane gas. I also have a 402, with .125 domed closed chamber pistons, and later open chamber heads, that also runs fine under most conditions with 87 octane gas.
Both have HEI ignitions, with vacuum advance, manifold vacuum, and a curve kit.