[Brad_bb]

Well I'm trying to flush out what will get me there. I've had a local Pontiac guru advise me to shave the tops of my pistons down by about .060 to get to increase cc's. On the other Hand(so to speak), I read a Jim Hand tech article on Pontiac Street Performance website, which advised against this and starting with a zero deck height.

64 389, .040 overbored = 4.1025, Stroke 3.75
Flat top forged TRW pistons with 4 small valve reliefs
Deckheight = Not yet measured, but let's assume .020
Head gasket = Not yet measured, but let's assume .042
Heads, 716's, Chamber volume, not yet measured, but have been smoothed and polished. I've seen 2 CC numbers for these heads, 68cc and 72cc.

Intake and carbs = stock tripower
Cam = the local Pontiac guru advised me to use Crane H-272-2
216/228 duration, 454/480 lift

Seems like different schools of thought. This is an engine largely for the street. I may even want to do a cross country trip. Car currently has 3.90 rear gears but may change to 3.23, and I haven't built the Muncie yet.

Anyone been through this and have some enlightenment.

[Dick Boneske]

If the heads have not been milled since new, they will likely be about 77 cc. You will end up with compression ratio above 10:1, but probably not the advertised 10.75:1 for a '64 GTO. In any case, this is much too high for a cast iron Pontiac head--closed chamber--for today's pump gas.

Until you measure the chamber size after polishing and valve seat grinding, you can't be sure of what actual compression will be. Anything over 9.5:1 with those heads will be a problem with any kind of pump gas now available.

The only answer I know of is dishing the pistons an amount needed to give about 9.25:1 comression ratio--unless you don't care about originality and switch to aluminum heads which will tolerate a higher compression ratio without detonation.

[Andre]

I had my pistons dished and dropped the compression down to 9.5:1. But with that I had to go to cam with different specs. Don't ask me about the particulars, as I put this in the hands of Pontiac Dude- Ken Keefer.

[Brad_bb]

I take it you guys are talking about static compression ratio. Have you read what Jim Hand wrote about dynamic compression ratio? He talks about cylinder pressure or rather dynamic compression, which is defined in large measure by the can and timing of the closing of the intake valve. Static CR assumes that the intake valve is closed at BDC so that you get the full measure of compression of the total volume. But in reality, it's almost never the case because the intake valve is not closed until sometime after BDC, allowing less than the full volume possible. The timing of the closing of the valve affects the dynamic CR. I suggest reading it on the Pontiac Street Performance website. It shows that there are more variables than many think.

If we do use the static compression ratio, and assume that the optimum would be 9.25-9.5:1, we can easily calculate backwards to determine how much volume we need(cc's) to achieve this, and measure and modify accordingly. This is likely much easier to do than trying to find the dynamic CR with a particular cam and working to achieve that.

In my situation, I want to remain with the heads I have and therefore I am locked in to a specific chamber volume(heads have not been milled to my knowledge, but I'll be checking their volume). I'm also locking myself into the stock tripower setup, which I'm sure is somewhat limiting for performance. Therefore it seems that there are only 2 variables left, cam selection, and removing material from the pistons. What are the advantages or disadvantages of dishing the piston as in an actual concave shape vs. milling the top flat off the piston? It would seem to be more difficult to machine and be certain of how many cc's you're removing.

[AJ GTO]

I just got back to GTO's later in my life. Drove a 64 in 64 - bought 65 upon return from NAM first time. Later converted to Vettes which I built & raced. Anyway my oldest has me back to Pontiac's - haven't forgotten Vettes - have 03 Z06 - but just picked up 64 GTO from St. Louis without engine. Picked up 64 389 and am asking same questions.

Plan on going .30 over, not decking block but maybe .001 clean cut, maybe .001 clean cut on heads. Std 64 heads are supposedly 68cc according to Pontiac Power.com.

Usually larger than propaganda as rule. Composite gasket decreases compression. As I remember it is easy to lose compression but harder to gain.

[geeteeohguy]

Brad, recently I helped a friend rebuild a '66 389 for his '65 Lemans....he went with custom dished pistons and stuck with the stock small chamber heads. The heads cc'd at 69. With a 3.23 rear, M20 trans, HO manifolds and a Holley carb, this thing runs like a scalded cat....on 87-89 octane. I would recommend looking into a set of dished pistons for a CR of 9-9.5 to 1. With my own '65, I'm in the same boat as you, and I have been running boosted gas for years. My 389 won't run on 91 octane without ping, even in cool weather. On my '67 GTO, I got sick of playing the detonation game and swapped to lower compression heads. If I were going into the motor, like you are, I would research and get the custom dished pistons. There are specific dish shapes to match your combustion chamber shape, too, and this helps with efficiency.

[3 Deuce 64]

classical Pontiac head section intake Exhaust Volume


63-64 421HO/389GTO 9770716 1.92 1.66 68 cc

in Wallace Racing

389 325/330/348HP 9770716 1.92 1.66 72 cc
421 350/370HP 9770716 1.92 1.66 68 cc

I always thought it was 68 so which one is correct ?

[Brad_bb]

Apparently there is a slight difference due to the size of two different intake valve diameters. Actually I know valves were purchased for the heads I have and something was funny as to the size, and this probably explains it.

Geeteeohguy, when originally looking for forged pistons for this engine(hate to say it but it was in the early 90's), the TRW's were the only ones found and they are flat top with 4 small reliefs. Who has these dished pistons available for the 389? Second, I am hoping I can modify these if need be and not have to mess with a new set. So they could either be machined flat or dished if I knew how many cc's I needed to remove. It's not much to remove go get in the ball park. In a rough calculation, I figured removing about .045 to .06 flat would get the cc's in the range of 89cc's total. I'm trying to figure out if 89cc's is the number to shoot for? If I knew the total cc's to shoot for I could get there. Then I'd just have to figure out the appropriate piston shape in removing the material. I'd also have to verify my deck height by mocking up the engine, and actually measure the cc's of the heads.
Lastly, you said the heads cc'd at 69, but what was the total cc's at TDC? This would include the dishing, the deck height, gasket volume, and head. And what cam did he run?

[3 Deuce 64]

something to think about:
In his book Alcohol can be a gas David Blumes says the following:
"you can convert your car more closely to an ideal alchohol engine by increasing its compression to take advantage of alcohols high octane rating. With its practical rating of 105, you should be able to run alcohol in an engine with a compression ration from 12:1 to 18:1, and sometimes a little higher, without experienceing pinging or preignition.

So as the % of ethanol (alcohol ) increases our higher compresion stock motors should actually run better and burn cleaner or am I missing something .

[Brad_bb]

I kind of understand your thinking. Our pump gas here is 10% ethanol, however the rating I'm sure is done after the addition of the ethanol. Straight Alcohol may have a higher rating, but is not practical for regular street. That does remind me that I need to make sure my carb accelerator pump is compatible with the ethanol.

[b-man]

Milling the top of the piston would be a mistake.

You lose the tight quench distance between the piston top and quench pad in the combustion chamber. A quench distance of .040" is pretty much ideal, the thickness of a head gasket. Makes the fuel air mixture turbulent for more complete combustion and an even rate of burn, helps the engine run cooler.

Definitely dish the pistons, using a D-shaped dish if possible. Running the pistons at zero deck is also a big help in preventing detonation.

I had a '69 400 Firebird engine that had the tops of the pistons shaved to lower the compression, about .100" was removed. That engine would ping and rattle no matter how much you retarded the timing, the previous builders attempt at dropping compression to enable it to run on modern premium fuel was a total failure.

Right now I'm planning a '64 421 HO engine build, keeping the '716' heads. To get the engine down to 9.2:1 will require a 25cc dish in the piston, with the pistons at zero deck. Mind you, I have not had the heads cc'd yet but in this preliminary planning stage I'm just assuming they'll be 72ccs (I hope they have more than that).

One would think that the '64 421 HO used a larger combustion chamber than the '64 GTO 389. The 421 had the same compression ratio as the 389, but did not use a dished piston like the 428 did as compared to the 400. The 421 would need about 6 more ccs in the chamber, I hope when I get a chance to cc the 421 heads and compare them to some 389 heads there will be a measurable difference.

A '64 389 with the '716' heads at zero deck would need right around 18ccs in the piston to achieve 9.2:1.

Dishing the pistons is the only way to do it right. Running the dished pistons at zero deck is the best way to go even if the factory didn't always do this. As a side note, every 455 I've ever torn down had the pistons at zero or -.005 below the deck from the factory.

Pontiac dished the 428 pistons (12ccs 428 piston vs 6 cc valve reliefs only in 400 pistons) so the 428 would have the same compression as the 400 engines, sharing the same heads for both engines.

Pontiac also used dished pistons in the export engines, taking 8.6:1 389s down to 7.6:1.

[Dick Boneske]

Originally Posted by 3 Deuce 64

classical Pontiac head section intake Exhaust Volume


63-64 421HO/389GTO 9770716 1.92 1.66 68 cc

in Wallace Racing

389 325/330/348HP 9770716 1.92 1.66 72 cc
421 350/370HP 9770716 1.92 1.66 68 cc
-------------------------------------------------------------------------------------------
I always thought it was 68 so which one is correct ?

Neither one of those tables lists cc's correctly. The '64 GTO heads were specified to cc at 72. The actual chamber size on '64 GTO's from the factory was 74-76 cc. The 421 HO heads were identical in valve size and casting number, but were bigger chambers to achieve 10.75:1 compression. None of the '64 421HO or GTO heads were 68 cc stock.

[geeteeohguy]

Brad, just the chambers cc'd at 69, heads off the car. For a workable compression ratio, you want about an 82-87cc chamber. I installed 87cc chamber heads on my '67, but with deck height, head gasket, and piston reliefs factored in, I ended up with about 9:1 compression. It runs fine on regular fuel. If I were to do it over, I'd have milled my heads a little for something like an 84cc chamber at the head itself. There is a ton of good information here, and a lot of knowledgeable people. B-man's suggestion of getting the pistons dished is a good one, and I think that Mr. P-Body, on this forum, knows all about this aspect. Good luck.

[Old Man Taylor]

Originally Posted by Dick Boneske

Neither one of those tables lists cc's correctly. The '64 GTO heads were specified to cc at 72. The actual chamber size on '64 GTO's from the factory was 74-76 cc. The 421 HO heads were identical in valve size and casting number, but were bigger chambers to achieve 10.75:1 compression. None of the '64 421HO or GTO heads were 68 cc stock.

I think the spec'd size of the 716 chambers was 68 cc's, but they typically measured 72. The 400 heads were spec'd at 72 cc's and typically measured 75 cc's.

[ironhead68]

Back in the '60"s, Hot Rod Mag ran an article on opening up closed chamber Pontiac heads, grinding out the combution chambers similar to the '68+ heads.
As I recall they dropped the SCR a lot, but then ran pop-up pistons and had pretty good HP numbers.
Could something similar be a possibility? Maybe not so much cutting and still run flattop pistons?

[Ragtop Man]

Be sure to cc the heads you plan to use before you get out your wallet.

Pontiac published CR's that were higher than delivered, sometimes substantially to give sportsman NHRA teams a little more leeway at tech inspection. The chambers are typically a skosh bigger than listed.

The longer the exhaust valve cycle and the later the exhaust valve closing point, the more static cr an engine can tolerate on the same gas. But, that pushes the torque curve up higher in the rpm range, and can negate the best qualities of a Pontiac V-8.

A dished piston is definitely the best way to go, there are some different ways to achieve it, but deepening the relief under the valves is the best way to go; keep the deck clearance as tight as you can without banging the piston into the head.

Rule of thumb with a traditional Pontiac and bathtub chamber is to just move the decimal point on the static cr to find your ideal R+M/2 octane with typical ignition advance... 8.7 = 87 Octane, 10.0 = 100 octane, etc. As noted below, the lower heat retention of an aluminum head will raise the octane tolerance, too, by how much is open for discussion depending on cam timing.

Don't worry if you are a touch light on the static CR; it will let you put a little more spark lead into the engine. Also, be sure your carb tune is right on the money; if it is running too lean, or not calibrated for the transitions, it will rattle like a poltergeist.

[Brad_bb]

I'll be running the factory tripower, so it's a standard 2bbl center carb.

I dropped off the block, crank, piston assemblies, and heads at the machine shop today. They need to clean the block exterior(rust) without hurting the alread machined bores etc. They may need to do a light hone after, but no big deal. Then they will assemble the crank and pistons to measure the deck to piston top height. Finally they will CC the heads. This will allow me to calculate how much will need to be dished from the piston. I was thinking I'd leave the deck to piston height stock instead of zeroing it. They will inspect the flatness of the decks. I've also asked them to measure an record all important dimensions so I have a record of there it's at. After they get that far, I'll have to get some pics to post.

[Thumpin455_GTO]

Originally Posted by Brad_bb

I kind of understand your thinking. Our pump gas here is 10% ethanol, however the rating I'm sure is done after the addition of the ethanol. Straight Alcohol may have a higher rating, but is not practical for regular street. That does remind me that I need to make sure my carb accelerator pump is compatible with the ethanol.

Apparently 3 Deuce 64 has been doing some studying. Good to know I am not the only one.

You have E85 pumps all over around you. Go here and search your area
http://e85prices.com/e85map.php

Gas City in Shorewood
Two Gas City in Joliet on Laraway Rd
Four of them along I80
and two on 143rd st

Seems like the more I look the more I find. I would say its practical. I have to drive 22 miles to get to the only E85 pump in my area, I wish I had your choices. Ive had my 70 GTO running E85 since 07, makes lots of power and its getting 670 heads this spring to bring the compression up to 11:1.

If you want to run a Qjet or Holley I could help you get it converted. I havent done any two barrels yet, but they cant be that difficult to convert. How about trying a Holley for a while on E85 and see if you like it? Ive been thinking of doing a Tri power for the 65 I am rebuilding and converting the end carbs with base plates and the other mods. Might actually be easier to do than a Qjet..

Ethanol is not like methanol, you dont have to run top end lube, nor do you have to run the system dry and oil it. It wont eat your carb, and it will handle well over 13:1 compression easily. The upside of it is the price, gas is going up fast, some say it will hit $5 a gallon this year. I doubt E85 will make it to $3, and with the higher compression you will make more power and get decent mileage with it compared to dropping the compression to 9:1 with dished pistons or opening up the chamber.

My 70 has a stock fuel system in it, the only thing I changed was the carb and an electric fuel pump. I put the pump on it back in 94 because it would run out of fuel part way through second gear on a stock pump. That was when it was running gas. I usually ran 10% in it since it ran better and didnt ping on it, but it ran fine on 87 to 91 ethanol or not.

Not saying you have to, but its an option. Its better than pump gas, makes more power, runs cooler, and costs less.

[Ragtop Man]

I keep waiting for someone to do an optimized E85 motor with a turbo, so you could take advantage of the octane, the burn rate, and the latent heat of vaporization all in one motor. Seems to me a small displacement motor like a 350-366 with 15 lbs would easily pump 750-800 hp and be reliable as an anvil with one of the simple, proven DFI's and a pair of hairdryers.

To this point, none of the OE's have used all the technology at their disposal to do an engine like this, which could apply all the cool technology like variable valve timing, variable intake, etc, to really make the dang thing stand on the bumper. (Cadillac, HELLO!)

As a part owner of a farm in Nebraska, and madder than !@#$!@#$! at the use of foriegn fuels, it just makes sense.

[GT182]

I'm running a stock bore 68 or maybe a 69 XX 400 2bbl 265hp engine in my GTO. Haven't looked at the date code, and that doesn't really bother me what year engine it is. Cam has been changed and it's running the Carter AFB Competition Series carb with an added 60lb Melling oil pump. No other changes and it's running on Regular pump gas. No knocking or pinging at all. I was told it's pushing over 320hp but actual hp isn't known. It does go like a raped ape. Builder knows what he's doing, but he surprised himself on how good it turned out and that it can run Regular gas like it does.

Now if I could do that with the matching numbers 389 engine, I'd be really happy. Sadly it will have to be bored to at least .40 or .60 over. Thank's to the previous owner.