[*67firebird*]

Situation: 670 heads on my 400, high compression, high compression and pump gas dont mix well at all.

My concern is what does the pinging sound like? I know the engine has been out of the car because the engine has some orange paint on it??? so I am wondering if there was any work done to the heads, because i really cant hear anything but my flowmasters. or is it something i cant hear, but i know it is happening because of high compression.

anyways any help is great thanks.

[GREATGTO]

listen for a slight pinging/ knocking sound under fairly hard acceleration, you should be able to hear it even with loud exhaust if its really bad, but its also possible to have inaudible detonation. Iv run several 400's with cast iron 72 cc heads without any problems, but my cam did have a lot of overlap which tends to bleed off cylinder pressure giving you a lower static compression ratio.

[GREATGTO]

listen for a slight pinging/ knocking sound under fairly hard acceleration, you should be able to hear it even with loud exhaust if its really bad, but its also possible to have inaudible detonation. Iv run several 400's with cast iron 72 cc heads without any problems, but my cam did have a lot of overlap which tends to bleed off cylinder pressure giving you a lower static compression ratio.

[GREATGTO]

sorry for the double post, hit submit twice

[*67firebird*]

ok, thanks, ill give it a shot this weekend. So are you saying i would be fine with the summit 2802, which has a 114 lobe sep or is it on the contrary a 110 lobe sep?

[chiphead]

Detonation often manifests itself as high engine temps at cruising speeds with no good explanation. It can also show up as blown head gaskets, or a "valve rattle" sound under accelleration. In severe cases it beats out rod bearings and breaks pistons.

Intake closing event is the most accurate way of telling how much compression a cam bleeds off. The later the closing, the more pressure bled off. I still say its best to use a lower static ratio and a cam like a XE grind, that builds cylinder pressure on pump gas street engines.

Ive been seeing a few posts about engines running high compression on pump gas and living. But they're running a cam that most would consider NOT street friendly.

Pay special attention to your timing curve, fuel mixture and sparkplug heat range.

[GREATGTO]

the cams i used always had a 108 lobe seperation, very choppy but thats what i like, you could probably use a 110 without any problems, you dont want a 114. Also dont let anyone sell you a ram air 4 cam either, its junk! I know Butler performance has some nice street cams that arent too radical and work good with pump gas, check them out.

[Doug Christie]

osvaldo, If you want to here what detonation sounds like here's what you can do. Advance your initial timing about 15-20*. With your foot on the brake, put your car in gear (if it's an automatic) and bring the rpm up till the car reaches it's stall. You will hear a rattling sound much like loose rockers. Do a before and after test to give your self a base line and don't let the rattling go on for long.

As a side note, it's not only the audible detonation you can hear that is a killer, there is inaudable detonation that can do just as much damage. I've knocked out two pistons with detonation that I never heard.

[*67firebird*]

yea i was looking at the 280H from compcams, it has a 110, and its lift is .480" on both sides. that narrows it all down. I Know the best way to drop compression is getting some heads with a 85cc or so, but thats out of the budget for now ill look at getting some heads areound december.

[GREATGTO]

well you could always run a thicker head gaskets, its not the best way to lower the compression but when on the cheap its the way to go, but I think you can run what you have just fine with the right cam like I said. DO you know what cam you have now? what else is done to the engine? headers, converter, intake, carb?

[GREATGTO]

Hey, I'm in a giving mood, I have a set of 6x heads you can have for free, they are totally new, springs, valve job etc.. If I recall correctly they are even set up for a cam in the 480-500 lift range, I had them on a 455 i used to run a couple years ago, just been sitting around, youre welcome to them if you want them. These heads on a 400 will give you a very drivable compression ratio, the ports have even been cleaned up on the intake and exhaust side, the car ran low 13's with only 9.6 compression ratio , let me know.

[Will]

I gotta disagree with Greatgto on a couple things here.

First, you do NOT want a moderate or smallish duration cam on a tight lobe separation if you are concerned about detonation with high compression in a street car.

Reason - these cams have early intake closing points and are designed to build high cylinder pressures in low compression engines. The Comp 280 magnum would be a bad idea. The XE274 wouldn't be much better. The Summit 2802 is actually a better cam for a high compression engine. There is nothing wrong with a wide lobe separation if it's used in the right application. The wider lobe separation generally means a later intake centerline and thus later intake closing point for the same duration. I know a guy running the 2802 in a 10+:1 400 into the mid 12s in a '68 'bird with a very basic setup and it works great for him.

I don't know whether it's a good choice for you because other factors come into play besides just the heads and displacement and I don't have those details about your combo.

Dynamic compression ratio is determined by the static compression ratio combined with the intake closing point. This is the actual compression ratio the engine sees while running. Dynamic compression ratio is why some cars can run high static compression ratios and big cams on the street. The intake closing point is late enough with the big cam that cylinder pressures at normal cruising RPMs where detonation is more likely to occur are much lower, or at least not any more than a lower compression engine with a smaller cam.

I would also disagree that the RAIV/041 cam is junk. It all depends on the combination. It's not the best cam for a lot of combos, especially 400s, but it can work well in the right situation. For a 400 that would be a 10.5:1 static CR, good flowing heads, free breathing exhaust and steep gears with a high stall converter or 4-speed. Cam technology has come a long way since the 041 was designed so it wouldn't be my choice for just about any combo, but I'm still going to defend it from the "junk" label. At least it was specifically designed for Pontiacs, unlike the Comp 280 "generi-grind" single pattern cam.

Greatgto - that's a very generous offer you're making. What version of the 6X heads are they? have they been milled at all? In other words, what's the actual chamber size on them? Reason I'm asking is that while they produced 9.6:1 on your 455, they will make considerably less than that on a 400 as I'm sure you know.

While a low compression ratio allows you to run camel pee for gas without too much worry of detonation, it will also hurt performance in a big way. Just depends on the chamber size of those heads and what the final compression ratio would turn out to be. If it's somewhere in the 8.8 - 9.3:1 range then you can still make good power on pump gas, depending on the rest of the combo. If the final compression will end up below 8.8:1, I wouldn't do it. I'd stick with the original heads and just be careful with my tuning. Run the carb a little on the rich side, keep the timing specs conservative, duct cool air to the carb, block the heat crossover in the manifold and cap running engine temps at 170-180. A higher than stock stall converter and/or steeper rear gears will also help by lessening the load on the engine.

As for hearing detonation - well, sometimes you don't hear it 'til it's too late. I pounded the rod bearings in an engine from detonation and never knew there was a problem 'til I heard that distinct bearing knock sound start.

[Doug Christie]

Excellent post Will. My 10.5:1 462 did well with a 2802 (744 clone) on a 112 LSA. It had good idle properties with good vacuum. My downfall was trying to put my foot into it with 87 octane.

[BILTIT]

I ran stock 670's on my 72 400 for many years with a RAIV cam, and 3.73 gears. I ran the best gas i could find and kept the carb rich and temps down. The thing ran like a scolded dog! Never had any problems with it.

[GREATGTO]

I never gave any cam recommendation, I dont know anything about his car, I simply said most cams that run with iron heads and high compression tend to have a 110 lobe seperation, you can look at butlers site,all their mild hydro cams have a 110 lobe seperation.
As far as me saying the ram 4 cam is "junk" (no offense to anyone using it) l it is compared to the modern cam technology and grinds available today with the octane fuel we have to work with, thats what i mean by "junk".
The 6x heads I have are the 6x- 4 heads, they came off a 400,I had them true decked just enough to clean the surface, had the same thing done to the exhaust flanges, along with 3 angle valve job, new valves and springs.
The heads are 95 cc's if i recall correctly, been 4 years since
Iv used these heads on anything but they have about id say 1000 miles on them. Yes on a 400 his compression would be lower than that of a 455 with the 6x 4 heads on it, he never said what his performance desires were, you can get a good running 400 using these heads, its like you stated before , its all on the total combination of his car and engine.
His 670 heads are closed chamber heads, he might even be better off getting an open chamber set of heads rather than the 670's if he still desires a higher level of performance than that of which the 6x heads could deliver.I have several sets of 13's, 12' and 16's too , they all just sit in my shop, I dont run cast iron heads on anything anymore.

[MIke L]

Guy's...
Do you think those 6x heads be a good choice for a 30 over '65 389?
I currently have the original #77's with stock intake w/ blocked heat cross over, Carter 750 carb, '67 R/A manifolds, 2 1/2" exhaust,
and the Nunzi 2041 cam 222/232 @50, 109C/L, 70 O/L, 113 L/S, .453 and .470 lift with 1.52 stamped rockers?

Thanks
Mike

[*67firebird*]

Yes! I would gladly take the heads. that would be great. as far as my setup im going to go with the 280H from compcams, which is a 110. .480" on both side, i have super hook headers, performer intake, going for a 2300--2500 stall and i have a buddy selling me a auburn posi with 3.55, which will be on by next weekend.

also if I get the heads milled about 12 cc's i could be running at 9.3 compression, what do you guys think about that.

[BILTIT]

If i remember correctly 0.010 is ~6cc's, so you would want to mill them 0.020 to reduce volume by 12cc's. Anybody confirm?

[GREATGTO]

Mike L the 6 x heads work good for any pontiac engine when you dont want to worry about detonation, so yes they would work fine on a 389, like we stated before, the 6x heads are not a great performance head, but they can be ported etc and milled to get decent power and still enable you to run lower octane fuel.
When I ran these 6x heads on my 455 in a 68 lemans the car ran consistant 13.20's @ 104mph, very mild build up, performer intake, hooker 2'' 4 tube headers, holley 830 carb, msd ignition, roller tip rockers, flowmaster mufflers,turbo 400- 2000 stall converter , with a 390 gear ford 9'' in the rear, it was actually way to much gear for the combination, but it still ran good so I left it at that. The car probably would have ran better if I had a 308 or 355 gear. The car pulled hard all the way to 5000 rpm which was about all I wanted with a stock bottom end 455 with heavy cast pistons in it.

[Will]

A general rule of thumb is 1cc for every .005" milled off the heads.

The 6X-4s would be a good choice for a 400 if lightly milled, especially if the block has been zero decked. They flow just as good on the intake side as any other large valve head. The only downside is the 1.66" exhaust valve.

67firebird - if you go with the 6X heads then I would highly recommend the XE274 cam over the 280H. The XE274 is not only a dual pattern which will help the exhaust side of your equation (you need some help there), but the lobes are a much more aggressive design than the old 280H lobes, which means more power for the same duration. Just a better all around cam that will work great in a 9:1 400, though I'd recommend more stall. Don't be afraid of a high stall converter if you get a good quality "tight" converter. Something like the Hughes GM30 or Continental "tight" 10" will feel much like a stock converter during normal driving but will really wake things up when you put the go pedal down.

Greatgto - you're a good guy to offer those heads and it sounds like you're sitting on a veritable gold mine of iron heads. Got any 061s laying around you'd like to part with?