[etd66ss]

I recently bought a 72 GTO with an original numbers matching 400 that was rebuilt just about 1 year ago.

The car was lacking power, and idled poor. So I set off on tuning the 400. First thing I did was to check the timing. The timing was set to 6* initial and 26 total at about 2700 RPM. The car was a dog off the line, very underwhelming. I was advised to try to reach 34* total @3200 RPM. I readjusted the timing, the car ran much better but there was significant detonation on 93 octane. The plugs always looked good, it did not seem to be running lean.

The prev owner told me that the engine builder recommended #62 heads, the ones on the car are from 69. I also was given the original 7k3 heads when I bought the car. I have been advised that even with the #62 heads on the car, I should not have detonation problems with 93 octane.

So I dug into the distributor to see what the mechanical advance was doing. The builder installed a '75 HEI (400 2 bbl) with an MSD super HEI module in the 400. The mech advance actually was starting to come in at only ~650 rpm, I think maybe that's why the idle was so erratic. The vac advance was also hooked up to ported vacuum and wasn't advancing at all at idle, the car hesitated off the line.

I removed the weights and center plate to see what the stampings were. I had a 397 center plate and 139 weights, the springs that were installed were very weak. I tore apart some other HEI's I had and found a 370 center plate and a bunch of stiffer springs.

I am now on manifold vacuum for the vac advance and it idles much better, no fear of stalling at low rpm at stop lights anymore. I put in the 370 center plate and 139 weights, that's supposed to increase the mech advance from 20 to 25, I saw 23.

I now have timing at 11 deg initial and 34 total, running 100 octane and the detonation has stopped. If I increase the initial to 13 and total 36 @3200 rpm, I can make the car ping under heavy load on 100 octane. My static compression with all plugs removed and WOT on a warm engine was between 190-200.

I have been advised that I have a "small" cam with high compression heads and maybe a quench that is too large which is causing the pinging and doggish perf on 93 octane.

My goal is to figure out what to do so I don't have to spend $8.49/gallon for 100 octane, this is a driver nothing more.

Is the simplest/cheapest fix just to get the 7k3 heads machined and use those? Or replace the cam?

[Chris65LeMans]

What's your compression? Carb size? Cam specs?

When you say "detonate," do you mean "ping?"

You said that you don't have problems with 34 dry total timing - how does it run then?

[72LuxuryLeMansLa.]

I'm no expert but what I would do is pull the heads and cc them to find out exactly what the chambers measure. Next, while the heads were off I'd measure the deck height with the pistons at TDC. If the pistons are within .005"-.010" I would buy an appropriate head gasket and then choose a cam that would allow the compression I am making. I would guess that you do in fact have too small of a cam and that's why you are pinging...... But of course I'm half a moron!

I'm saying this because of the 190-200psi compression!

[lust4speed]

#62 heads when they started life came right in about 75cc's, and after a normal cleanup mill, they are probably down to 72cc's. Even with the pistons down in the hole .012" your compression ratio is going to be slightly over 10:1 (and if zero decked, you are at 10.3:1). No amount of game playing with gaskets or cams is going to be a solution for your pinging. Increasing combustion volume is the only tried and true solution - so either larger iron chamber heads or dished pistons.

You mentioned a desire for more performance, and the simplest and easiest way to get there is with a set of 72cc aluminum heads. While it might not appear to be the cheapest way at first, after you compare the performance gains and all the labor cost in getting your current combo and a set of iron heads to where you want it be, it definitely makes the most sense. It also won't really be that much more costly.

[screamingchief]

Need to know what pistons were used in the build before making any suggestions.

If they used the 8 valve relief cast pistons,then yeah there are gonna be problems,and any "fixes" mostly just amount to band-aids on a bullet wound.

I've seen those pistons ruin more 400 builds than I care to mention,that's why this is the very first thing that comes to my mind when I hear a fella is having problems with a "new" 400 cid pontiac build.

FWIW

Bret P.

[etd66ss]

I don't know what cam or pistons were put in it, all I have is this work order from the build:

http://i.imgur.com/aTIe4Li.png

You said that you don't have problems with 34 dry total timing - how does it run then?

The car runs fine at 34* total, still not impressive in terms of perf. However, I don't want to run 100 octane, I'd like to run 93. If I run 93 I have to retard to 26 total for the detonation to go away, this is where the timing was set when I bought the car. It almost seems to me someone retarded the heck out of the timing to mask a build issue.

[Burningbird]

I have a similar combo, 400 bored .030 over, ported #16 heads (75cc), "TRW" pistons .003 down in the hole, 10.1:1 SCR. 068 cam with 1.65 rockers .The timing is 12°@ 650rpm, 32°@3000rpm (max) and additonal 15° vacuum advance. Runs great and never pings on Swedish 95 octane which is equivalent to US 91 octane. It would be interesting to know what cam the builder put in your engine

[Overkillphil]

Very generic paperwork from the shop. I'd bet those pistons are the dreaded "rebuilder" 8 eyebrow cars paperweights as was also mentioned in another post. The combo is most likely mismatched regarding the cam and heads which is greatly magnified by the possibility of those commonly used rebuilder Pistons.

Are you able to contact the shop to see if they still have a file on the job? Knowing what Pistons and cam would be extremely helpful before you start tearing into it. If they don't have the paperwork, they may remember since they probably do few Pontiac's and as a result use the same approach each time.

[etd66ss]

Originally Posted by Overkillphil

Very generic paperwork from the shop. I'd bet those pistons are the dreaded "rebuilder" 8 eyebrow cars paperweights as was also mentioned in another post. The combo is most likely mismatched regarding the cam and heads which is greatly magnified by the possibility of those commonly used rebuilder Pistons.

Are you able to contact the shop to see if they still have a file on the job? Knowing what Pistons and cam would be extremely helpful before you start tearing into it. If they don't have the paperwork, they may remember since they probably do few Pontiac's and as a result use the same approach each time.

I plan to contact them today to see if they can give me part numbers. Hopefully I'll have some more info soon.

[etd66ss]

Ok so, I just got off the phone with the engine builder.

There is more to the story.

Last year the prev owner brought in the GTO to his shop to get the "rebuilt" 400 running better. The more the shop investigated the engine, the more it became apparent the prev rebuild was not a rebuild, but thrown together with used parts, a quick hone job and what he called "crap cast pistons". So this guy rebuilt the engine. The owner wanted the high compression heads and a very mild cam. The builder told him this would require higher octane fuel, but that's what the owner wanted.

He could not give me any part numbers, but said they bored the engine and put in TRW flat top pistons and a Crane RV cam that had lesser specs than the 1972 067 cam. All he could remember about the cam is that is was about 212/215 @ .050.

The car came in to his shop with what he called a butchered Q-Jet that he fixed up the best he could, and it already had the HEI which he said still needed work when the car left but the owner didn't want anymore work done to it.

He told me the compression is definitely over 10:1, and even if I put a bigger cam in it, it still could have detonation issues on 93 octane.

[HWYSTR455]

Ok, so you say this is a 'driver, and nothing more'? Then yeah, prep the 7K3s and swap them out, but, you won't be seeing any performance gains. You will be able to safely drive the car on 91 octane though.

Still would have to get the distributor and carb setup right too.

But it is the quickest and least-expensive route to get it drivable.

If you want more power, your best bet would probably purchase a new long block, from one of the known builders. You could drive the car with the engine you have now, swap when you get the replacement, and stash the OE block and heads. Could do a quality rebuild on it and store it, and include it if you ever choose to sell the car.

.

[etd66ss]

Originally Posted by HWYSTR455

Ok, so you say this is a 'driver, and nothing more'? Then yeah, prep the 7K3s and swap them out, but, you won't be seeing any performance gains. You will be able to safely drive the car on 91 octane though.

Still would have to get the distributor and carb setup right too.

But it is the quickest and least-expensive route to get it drivable.

If you want more power, your best bet would probably purchase a new long block, from one of the known builders. You could drive the car with the engine you have now, swap when you get the replacement, and stash the OE block and heads. Could do a quality rebuild on it and store it, and include it if you ever choose to sell the car.

.

So just changing the cam to something like this is not an option?: http://www.competitionproducts.com/H.../#.VdM8bE2FNus

I've already got a 66 Chevelle project $$$ so won't be doing a long block for this car anytime soon. I'd think to get the original heads in order I'm looking at more $$$ than a cam change.

[Burningbird]

I'm certainly no expert but depending on the trans, converter stall and rear gear, I would try a Summit 2802 cam. However if the block has not been milled the TRW pistons is probably a bit down in the hole.

[etd66ss]

Originally Posted by Burningbird

I'm certainly no expert but depending on the trans, converter stall and rear gear, I would try a Summit 2802 cam. However if the block has not been milled the TRW pistons is probably a bit down in the hole.

Well the work order doesn't say the block was decked. The trans is a TH400 without any kind of shift kit in it and the converter is an 1800 stall. The rear end was rebuilt by the same shop, the work order says 3.28 but I believe it should read 3.23 as far as the ring & pinion.

[crm318]

X2 on the Summit 2802. I have a .060 400 with 16 heads and 10:1 compression. The car runs great, granted, I have a 2500 converter and 3:73 gears. I think it's worth it for $115.

[HWYSTR455]

Originally Posted by etd66ss

So just changing the cam to something like this is not an option?: http://www.competitionproducts.com/H.../#.VdM8bE2FNus

I've already got a 66 Chevelle project $$$ so won't be doing a long block for this car anytime soon. I'd think to get the original heads in order I'm looking at more $$$ than a cam change.

Nope, won't do it, believe your static compression ratio is too high, and only ways to change that is dished pistons or bigger chamber heads.

Technically, you can't properly choose the correct cam without knowing specific engine parameters, like, static compression. You would be taking a stab in the dark, at best.

IF, and only IF you were wanting to take a stab in the dark, in order to drop almost a solid CR point, you certainly would have to go way bigger than that cam, or any other mentioned yet on this thread. And it would still be a shot in the dark. And would suggest as wide an LSA as you can find, at min, 114. You would be able to drop the compression (dynamic), but the cam will be a mis-match for the rest of the combo.

.

[etd66ss]

Originally Posted by HWYSTR455

Nope, won't do it, believe your static compression ratio is too high, and only ways to change that is dished pistons or bigger chamber heads.

Technically, you can't properly choose the correct cam without knowing specific engine parameters, like, static compression. You would be taking a stab in the dark, at best.

IF, and only IF you were wanting to take a stab in the dark, in order to drop almost a solid CR point, you certainly would have to go way bigger than that cam, or any other mentioned yet on this thread. And it would still be a shot in the dark. And would suggest as wide an LSA as you can find, at min, 114. You would be able to drop the compression (dynamic), but the cam will be a mis-match for the rest of the combo.

.

I guess let me ask another way. If you had a '72 400 block and 69 heads to start with, what combo would you build for a street only car wanting to run on 93 octane?

[Lee]

Originally Posted by etd66ss

I guess let me ask another way. If you had a '72 400 block and 69 heads to start with, what combo would you build for a street only car wanting to run on 93 octane?

I'd use readily available dished pistons to knock the compression ratio down to about 9.0:1

You can SELL the 62 heads for enough money to offset most of the cost of getting the 7K3's prepped. I agree with HWYSTR455. The best solution for your situation is another set of heads.

Maybe consider getting a set of CNC'ed heads from Dave @ SD Performance. They will flow better than either set you currently have (more power) and he can help you size the combustion chamber to get you the compression ratio you would like to have.

[etd66ss]

I guess maybe I need to be schooled on flat top vs. dished. Flat top pistons still have milled pockets for the valves right? I guess I am a bit confused. I was told flat tops are better for quench and detonation when compared to dished.

So what I am hearing is, with the combo of my heads and pistons, there is no good cam solution for 93 octane for this engine.

[HWYSTR455]

If I had a block and 72cc iron heads, would probably do a stroker rotating assembly, and keep the RPM down, or, a 400 and move the RPM range up, depending on the car and intended driving.

400 would really only need a freshen up, rods and pistons. I would probably give it more gear though.

Yes, pistons have reliefs for the valves, but a dish is more CCs. You can have pistons cut deeper or order different CC reliefs, to target your desired static CR.

I've run a number of 72cc headed .030 & .060 over 400s, all were in the 9.92 - 10.3 range, and no matter what you do, it's a struggle to keep them from detonation. Not fun.

Switching to the bigger heads will drop the CR considerably, IMO too much, but on a positive note, the cam that's in there will probably be a good match. So it looks to me that's the least amount of work/money.

You could always back the timing way off like it was and just drive it like that, live with it. Maybe after you get the other project done you can turn your attention to this one.

.