[bjohnsontx]

I have a 1972 GTO with a base 400 motor and auto. The prior owner rebuilt the engine and put an 067 cam in it but everything else is stock. Did all GTO's in 1972 have 3:55 : 1 rear end ratios? The car is beautiful to me but slow. It is supposed to have 250 net hp but it feels like 170 hp.

So, a friend of mine got a set of #13 72 cc heads for me at a swap meet. I am going to have them redone and install them. By my calculations, this will take my compression ratio over 10.5:1. Is anyone running this combination on 92 octane? Will I have detonation problems? I just installed an HEI distributor.

I want about 300+ net horsepower. Will I need to move up to an 068 cam to get the full benefit of these heads?

[74 SD455 Formula]

You will not be able to run 92 octane with that set up.You should have had your friend find you a set of 1971 Cast # 96 heads.Had a little bit of port work done and you would be much happier.If the car was all original I would leave it alone or at least save all of the original parts.Good luck anyway.

[getmygoat]

I had a 72 GTO with the base engine back in 1985. It wasn't slow (4 speed with 3:08)

Perhaps your definition of slow is different than mine, or maybe your car just isn't tuned well, etc.

I wouldn't consider a steeper gear than 3:55 on the street.

[gto4door]

Originally Posted by bjohnsontx

Did all GTO's in 1972 have 3:55 : 1 rear end ratios?

For 400 automatic cars the standard rear end ratio is 3:55. With A/C the ratio was 3:23. I believe 3:08 was optional. With your current setup the car shouldn't be that slow. You may want to make your quadrajet is well tuned. If you want to run quicker, I would recommend you swap to a 455 block for more torque, use your existing 7K3 head off the 400, 068 cam and ram air exhaust manifolds. That should cure the sluggish feeling!

[RicksGTO]

as suggested above, if your car feels that slow, start with the simpler things first like carburetion and timing, points, vacuum leaks, wires, all can easily cause sluggish performance, check everything out you can before swapping parts. On the heads, your 7K3 heads were rated at 96cc's with 2.11/1.77 valves so they are not bad heads for a 400ci. If they are original to the car and you want to keep it that way, you might consider measuring the chambers to be sure and then having them milled .030-.040 to raise your compression and get a fresh 3 angle valve job done while they are off. If you brought them into the 85cc range with milling, your compression would likely be somewhere in the 9 to 1 range so you could still run pump gas easily. The 13's by the way are usually in the 76cc-78cc range stock unlike what the Pontiac charts say.

[bjohnsontx]

Thanks guys. I went through the prior owner's paper work and found that the car has an Edelbrock 2157 cam in it. This is rated at 204/214 at .050 vs 067 which is 197/212 at 0.050. I also put a new carb, HEI, plugs and wires on the GTO. One draw back is that it has a wimpy 2" Midas dual exhaust system on it which I am replacing with a Pypes crossover system.

Also, it shows that the block was bored and honed but it does not say how much. So maybe my compression ratio is lower than the stock 8.2 to 1.

My other car is a 73 Formula 455 which I put a HEI, 068 cam in it. I saved all the original parts but I am a torque junky and the 72 GTO is no where close.

Any advice or experience would be appreciated.

[bjohnsontx]

Oh, this may have a big impact. I went thru some paperwork and found a note from the prior owner who said that when the engine was rebuilt that he bored it .030 and put TRW forged pistons in it.

Would this have lowered my compression ratio from 8.2:1 to 7.5:1? If so, when I put the #13 heads, I should be about 10:1 or less which should be streetable. Does anyone know if that is correct?

[Baron Von Zeppelin]

Boring the cylinder will raise the compression ratio, not lower it. You're probably around 8.5 - you'll be around 10.25 - 10.5 with 13's at .030 over.

The 067 and 068 cams are weak in terms of potential performance in a basically stock built 400. The 744 grind is about the minimum preference - JMO.

No need to use the 13 heads if you already have 7K3. Compression ratio is less than a 10% factor in total power. But a big factor in predetonation. Gas quality will not get any better - but worse.

Invest in your ignition - buy the calibrated recurve/shim kit from H-O Racing, or send distributor out to a specialist.

Buy Cliff Ruggles book on tuning Qjets to the engine specs. Then follow the guidelines.

Last 400 I built was set at 9.25 by old factors - which is probably closer to 9.0 corrected. 420 or 425 lift on intake (aftermarket cam) split pattern profile (more lift on exhaust). Rhoads Lifters / Automatic / 3.23 rear / 2" dual exhaust with H-over equalizer / HEI with H-O calibrated curve&shim kit / standard rebuild on a virgin 7043262 carb - car screams from bottom to the top.

You "may" be able to benefit from 1.65 rocker arms versus a cam swap. But it's recommended to grind a relief at top of pushrod holes in most cases.

[bjohnsontx]

Baron,

Thanks, I will run a compression check and see. I thought by making the cylinders larger and keeping everything else the same, that the compression would drop due to the large combustion chamber size.

[johnsma22]

No, boring the cylinders will increase the compression because of the increased volume. Increasing the size of the combustion chambers in the heads will lower the compression, but you will not be able to open up the chambers on the #13's enough to achieve a "streetable" CR on a .030" over 400 block. IMHO.

[Baron Von Zeppelin]

The compression ratio number will increase with boring the cylinder.

The actual PSI per cylinder may or may not be affected - I've never conducted a controlled test to see for myself. But it probably does climb a little.

An elementary type example. Lets say your combustion chamber volume is a 4 oz cup - and your cylinder volume is a 32 oz quart can. The ratio is 8:1.

If we trade the 32 oz can, for a 34 oz can, the ratio becomes 8.5:1

Forgot to mention earlier, your 72 looks real nice. Shame that drunk fell down beside your Car while taking the picture. Now go make that bicht run like it should - and quit blaming your cylinder heads !

[unruhjonny]

regaurdless of what everone here has said, I've run #12 heads in my '70 Formula with both 067 & 744 cams, and had no problem with detonation, or any other problems to speak of.

The 067 cam was run with the engine bored .030, and ran fine as long as I never put anything less than 91 rated premium gas (and stayed far away from ethonal blended gas!!) in the car.

When I rebuilt the engine I chose to bore it another .010 over, so at a total of .040 over I replaced the 067 cam with a 744 grind. Once again it ran fine as long as I stuck to my gas "rules".

IN both instances, my engine ran much much smoother, and idled lower when subjected to 100+ octane gas...

I should note - you said you were a "torque junky";
the 067 cam did make a stronger bottom end - the larger grind cam sacrifices some low end torque for top end power.

If tourque is your thing - don't go any grind "larger" than a 068 cam.

From what Ive been told (and read) the 744 cam proably won't make enough vacuum for your automatic transmission...

Were I in your shoes, I would opt to keep your stock heads in place, this means you're goign to have less "extra" car parts lying around... and truthfully you can probably get your money back for them fairly easily...

Just an FYI.

[getmygoat]

Originally Posted by unruhjonny

regaurdless of what everone here has said, I've run #12 heads in my '70 Formula with both 067 & 744 cams, and had no problem with detonation, or any other problems to speak of.

How far "in the hole" were you? - I think there's a conflict between small chambered heads, zero decking the block for best performance, etc, and detonation.

While you can reduce compression for pump gas using a small chambered head via piston shaving, this results in an "in the hole" condition, which contributes to detonation, and reduces combustion efficiency generally.....

I think....

i.e. those lower compression engines of the early 70's might run better today than the higher compression ones from 70 and earlier will today.

[Matt Meaney]

as a fellow torque junky, and a firm believer in better living through cubic inches, here's an idea. keep all the externals original and swap the crank out for a 455 unit. since it's recently been rebuilt no need to bore just get std bore 455 slugs. then add a 041 cam with rhoads lifters for torque-o-plenty with a nice idle and good vacuum. this is very pump gas friendly. keeping the log manifolds and small pipes it'll do high 13's at close to 100 on 9" slicks. at least my car did. then i went to headers and 3" exhaust and hello high 12's at 104 and change. just my .02 fwiw.

[bjohnsontx]

Matt,

I like that idea. I have not thought about changing the crank to pump it up to a 455. The engine is already bored .030 and has TRW pistons in it. So do I only need to changes out the crank?

[Matt Meaney]

Originally Posted by bjohnsontx

Matt,

I like that idea. I have not thought about changing the crank to pump it up to a 455. The engine is already bored .030 and has TRW pistons in it. So do I only need to changes out the crank?

if you change the stroke (crank) you will also have to change the pistons. the stroke, the length of the rods, and deck height of the block are the determining factors as to where the pin is located in the piston. you shouldn't have to bore out the engine any more, as a std. 455 bore is the same as a .030 400.

there are 3 ways i can think of right now to acomplish this:

#1 butler sells an aftermarket crank that will fit the 400 and has a stock sized rod journal so you can use your existing rods. the least expensive i think.
#2 have a factory crank machined to fit and get a p-dude main conversion kit. still can use your existing rods.
#3 the most expensive is an aftermarket crank with chevy rod journals, which require new rods.

imo it would be best to have a pro engine builder put this toghter as some light machine work will most likey be required to get the proper clearences.

[gto4door]

The easiest and most inexpensive way is to find a 455 out of a '71 -'75Grandville or Bonneville. I'm sure you can find one in the San Antonio/Austin area at a reasonable price. Buy an 068 cam and take the cam and the 455 block to a machine shop for a standard rebuild. Then bolt on your 7K3 heads, intake, carb, exhaust manifolds, distributor, oil pan, starter, etc. from your 400. Then you will have the torque and performance you originally asked for. The 7K3 heads are very good heads. These heads are 96cc and bolting these on to a 455 will give you a slightly higher compression than the 8.2:1 you currently have in your stock 400 setup. The 068 cam will also give you the smooth idle to run your A/C (I'm assuming you have A/C since you are in Texas).

Store your matching number 400 block in the corner of the garage. And if you ever blow up the 455, you still have the original 400 to fall back on. Or if you sell car, you have the matching number block to go along with car.

I did this 455 conversion to my '72 GTO years ago and was very pleased with the results. The 455 block I used came from a '73 Grandville. Externally, everything looked the same as the stock 400, but the car performed so much quicker. I later added the repo D-Port ram air exhaust manifolds and HEI distributor, the car really screamed after that.

[bjohnsontx]

GTO4Door,

Thanks. When I originally thought about the head swap, I was going to take the 7k3 heads and put them on my '73 Formula 455. I am running milled 4X heads and a 068 cam in it which works great. I want the same performance in the GTO so maybe I should just do the same thing. I really don't have any room for the 400 block anywhere but a friend of mine just mentioned that he came across a 455 block.

And yes, A/C is a must!