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I’m planning a rebuild on my non numbers ‘66 Tripower 389 using Butler’s 389 4.25” stroker kit. I’m planning to use all forged components as much for insurance as necessity. Even though I’m improving the engine I want to hang on to it’s 389 heritage.

That’s gonna stretch me a little so I’m planning to use the 093 heads on it for now and use dished pistons to get CR down for pump gas. Also planning to clean the chambers up and port match for better flow. They have new rockers, studs and springs from an SPC-7 cam upgrade this past year. Eventually I’ll buy 65cc KRE heads when I have the $$

Questions are, how much will it be possible to lower the CR using dished pistons, head gaskets, etc using the cleaned up 093 small valve 65cc heads? What numbers do you think it’ll make? The Tripower going back on along with the RA exhaust manifolds. I’m debating on reusing the recently installed SPC-7 cam or going a little bigger.

Challenge on the cam is I have power brakes and factory AC that I recently restored. Anyone ever fab an idle-stop solenoid bracket for a Tripower? I’d of left it all alone except it started knocking on me at the end of the season.

Thanks for your input
 

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If you go on the Butler Performance site and look under "rotating assemblies" they have a compression calculator. I just bought a stroker kit and rotating assembly from them and with 72cc chambers, 0 deck height, 22cc dished pistons, 4.25 stroke, 4.155 bore and .045 head gasket, they say my compression will be right around 10:1. Can't tell you how accurate it is yet, but you can change any variable you want....Scroll to near the bottom.

 

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If you go on the Butler Performance site and look under "rotating assemblies" they have a compression calculator. I just bought a stroker kit and rotating assembly from them and with 72cc chambers, 0 deck height, 22cc dished pistons, 4.25 stroke, 4.155 bore and .045 head gasket, they say my compression will be right around 10:1. Can't tell you how accurate it is yet, but you can change any variable you want....Scroll to near the bottom.

Thanks. Sounds like a nice build there. I’ve played with the calculators and seen the numbers. Wanted to see if anybody with similar build or experience had real world numbers. It’s my first experience building a Pontiac so I’m obsessing a little trying to learn.
 

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Thanks. Sounds like a nice build there. I’ve played with the calculators and seen the numbers. Wanted to see if anybody with similar build or experience had real world numbers. It’s my first experience building a Pontiac so I’m obsessing a little trying to learn.
I don't blame you. Lot of money to spend at the click of a mouse! Pontiac Jim on here has steered me right and kept me out of some trouble so far.
 

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The 093 heads are said to be 68 CC's and probably closer to 70-72 CC's. But, you did not say if you plan on replacing valves or are using what you have?

Are you just planning on refreshing a recent head rebuild and re-use all the parts or do you plan on rebuilding the heads using all new parts?

Gasket matching the intake runners and deburring/cleaning up the runners would be a basic. I am not sure what you plan to do in the chambers or bowl. But do all your work before having your valve job if you plan on doing any bowl work. Get a 3-angle valve job to help air flow if you have not already had this done.

Here is why I asked about your valves. Before ordering your stroker/pistons, you want to CC the heads to know exactly what they are. Original valves and a valve job may have sunk the valves a bit which can effect the CC's. Installing new valves can raise the valves slightly and effect CC's. And, the type/size of the valve used can effect CC's as some are flat, some have a slight "tulip" depression, and others have an extreme "tulip" shape - all of which can alter final chamber CC's. Very easy to do this yourself at home: CCing Heads - How To

Once you know how many CC's your combustion chamber is, you can then factor in the remaining parts of the formula needed to determine how much dish you will need in the top of the pistons. You will need to know how far down in the bore your pistons are "deck height." Typical Pontiac is around .015" from the top of the block's deck to the top of the piston.

The next thing needed is the thickness of the head gasket you plan on using. These can vary depending on what is used/sourced. Typical Felpro head gaskets have a .039" compressed thickness. I am using Cometic head gaskets at .027" for my build. The diameter of the gasket comes in a couple sizes, I see 4.20" and 4.30". You want a gasket that is large enough to stay outside the cylinder wall, ie not hang over. So you want to select one that matches your bore size and just a little more so it doesn't hang over. Typically, the 4.30" works, but again, you can also get custom sizes from Cometic. BUT, I would not order your head gaskets until you know what the deck height of your new dished pistons actually are. So for calculation purposes, Let's use .039" x 4.30" head gaskets.

Using the Wallace Compression Calculator, Compression Ratio Calculator - Wallace Racing , You plug in the engine, assume 389CI .030" over 4.25" stroke, Combustion Chamber CC - using 70cc, Deck Height - .015", Head Gasket thickness & diameter - .039"x4.3", and inserting different Valve Relief/Dome cc's into the formula to get a compression of 9.3 (an average of a compression of 9.0-9.5), I come up with a piston needing a dish of 27 CC's gives you 9.37 compression, and 30 CC's gives you 9.14 Compression.

OK, but there is more to all this. You want to try to get the quench/squish area in the range of .040" - .045." What is quench/squish? That is the measurement between the flat top of the piston and flat bottom of the head adjacent to the combustion chamber. This area helps keep the chamber cool (quench) and forces the air/fuel mixture (squish) into the combustion chamber for a complete burn and helps in preventing detonation issues.

To get this number above, you will need to know exactly what your deck height is with the new pistons. You will then use your head gasket to "adjust" the needed distance between the piston top and head. For example, let's say your new pistons are now .010" from the deck. If you used a Felpro gasket of .039", you add this to the deck height of .010" and get .049". Not really the ideal quench as you would want closer to .045". If your new pistons were the same as stock and .015", you would add that to .039" and have a quench of .054". Even worse.

Did a web search for 389CI head gaskets trying to find a custom thickness, but none exist. All I could find were head gaskets from Butler having a compressed thickness of .045" https://butlerperformance.com/i-24453585-butler-performance-pontiac326-389-421-and-early-1967-400head-gasketset-2spm-19375-2.html?ref=category:1234803 . That said, to get a quench of .045", which is what the Butler gaskets give you, you want to now have a "zero deck" piston. This is still doable.

Your options are to have the block deck milled down to get the top of the pistons to a "zero deck" height. I don't care too much for deck milling if not needed. The dished pistons from Butler will be Ross pistons and custom made. You can have the piston pin position changed being these are custom and will be swinging off of the 6.8" rods. If Pontiac uses a .015" deck height, you can move the piston pin holes lower and thus moving the piston higher up into the cylinder and getting the piston closer to "zero deck." So you could have the pin height moved down .010" moving the piston height up and giving you a deck height of .005". From that point, you can then have your machine shop check the deck height and if need be, mill the deck .005" to bring the piston top to "zero deck." Now with the piston at "zero deck," and the Butler .045" head gasket, you get the desired quench distance between piston & head.

By "zero decking" the pistons, you have now altered the compression ratio formula. So now what dish do you need for your piston if you use "zero deck" in the equation? 27 CC's gives you 9.5 compression, 30 CC's gives you 9.28 compression.

If you go the stroker route, I would email Butler with your known combustion CC's, and then give them the deck height of your present pistons, and let them know you would like to shoot for a .045" quench distance using their .045" head gaskets and want to know if the Piston Pins on the Ross pistons can be moved to put the piston higher up in the bore to get closer to a "zero deck" height and use a 30 CC dished piston to get you near a 9.3 compression for a street engine?

They should be able to give you an answer and/or provide you will their recommendation. I am no engine building expert, but know a little bit about engines. So see what they and your machine shop advise and go from there.

On the cam, your heads have press-in studs. Anything more than the SPC-7 (068 cam), you will need to have screw-in studs fitted to your heads as you will need additional spring pressures for the bigger cam and could experience pulling out the studs. You stated you have new studs, but did not say if they are screw-in? If screw-in studs were installed, then I might go more cam for the stroker engine.

Your heads will be limited in flow, so you don't want to go too big on lift. Using stock length valves will also limit lift due to clearance/interference with such things at the spring retainer to valve guide. My guess would be to keep lift below .470", but with the heads off and the valve springs removed, you should be able to figure out how far the valve will go before the spring retainer hits the valve guide. You want to make sure you have a good margin of space between the retainer and valve guide.

The bigger the cam duration, the higher up into the RPM range you push your power band. So the SPC-7 as is has a good duration for the street and might be more to your liking. I prefer the 112 LSA on a cam with over 9.0 compression. If you can go the lift, the Crower 60243 would be a good choice in my opinion: Pontiac Hydraulic Flat Tappet Camshaft - Pontiac - Camshafts But if limited to lift, then Crower 60242 could be a good choice: Pontiac Hydraulic Flat Tappet Camshaft - Camshafts Both these cams are aimed at more performance over the SPC-7.

Also, assuming a 4-speed? If an automatic, you may or may not need a slightly higher stall speed on the converter. If a 4-speed, the higher stall speed is your leg. LOL

Check your rocker arms to make sure no binding at max lift in the area between the rocker ram stud and the slot in the rocker arm cup. They do make aftermarket stamped 1.52 ratio rockers with longer slots if you don't already have these.

If you do a search on "best cams" for your engine, you will get many opinions. It is hard to pin any one cam down until you actually use it and and hit the street with it. Then you will see if you love it, or hate it, as each engine/car build and intended purpose of the car is different.

So these are just my opinions and a few things to consider. 👍

Also going more cam may require bigger jetting on the tri-power so as not to lean out the air/fuel ratio and have the engine run hot and get that detonation problem because the engine is running to lean. You may also want to dial in your distributor advance to match the changes in the engine.
 

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Discussion Starter #6
Thanks Jim. That’s an incredible amount of great information. The same shop that did my recent cam swap will also be doing my build. They suggest the 093 heads w/ the recent updates are adequate and with some cleaning up and port matching, that they can get me by until I have the $$ to upgrade to KRE or E-heads. The 093’s were supposedly completely rebuilt by the PO about 4,000 miles ago, but like I said the springs and roller tip rockers went on when the cam was changed this year. No idea about the valves other than the car doesn’t smoke and ran well before the engine developed a knock.

My expectations when the aluminum heads go on is a street friendly cruiser/thumper with around 450-500 hp and similar torque numbers.

The transmission is an M21 rebuilt with M20 gear set when I changed out the 3.90 gears that were in it when I bought it. It’s a 10 bolt rear now running Yukon 3.36 gears. It was rebuilt at the same time as the transmission. I’m aware I may have to deal with upgrading (or replacing if they grenade) those at some point, but the car will probably never see the track or drag radials for that matter, so we’ll see. Those are future me problems 😎. I’m planning to have to dyno’d to get it dialed in on timing and A/F mixture.

if I can save money using an Eagle cast crank I may be able to stretch the budget to get the heads now. I wouldn’t object to reusing the relatively new 068 cam unless that’s gonna be too mild for the build. Based on what I’ve read in these forums that cam might not be enough for stroker displacement.

I’m struggling a little to make a (the right) decision and running out of time if I want it back this Spring. Thanks for all the great info and advice.
 

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Before you order any parts have the shop do a sonic test on your block. Pontiac got carried away with trying to save a little block weight for the 1966 model year and this coupled with core shift can have the cylinder walls dangerously thin. Saw results for a 66 block that was at .030" over and did a sonic test before boring and several of the walls were already only .045" thick. Have seen two other blocks right at the same cylinder wall thickness. Then about the time I was ready to condemn all 66 blocks the next one was one of the thickest walled blocks we've seen.

We just finished up a build for a 66 GTO and the cylinder walls were too thin so we substituted out a 400 block but still used the 93 heads, valve covers, intake manifold and such to have it look stock. Heads cleaned up at 68cc and the Butler supplied stroker kit required 36cc dished pistons. We were aiming for 9.3:1 but the Ross pistons came out just a little larger at 37.2cc's which along with other final measurements had actual compression at 9.27:1. By the time the owner paid for all the labor on the heads and new valves, springs, retainers, screw in rocker studs, and such he was 2/3 the way to a nice set of aluminum heads, but he wanted the stock look.

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Discussion Starter #8
Before you order any parts have the shop do a sonic test on your block. Pontiac got carried away with trying to save a little block weight for the 1966 model year and this coupled with core shift can have the cylinder walls dangerously thin. Saw results for a 66 block that was at .030" over and did a sonic test before boring and several of the walls were already only .045" thick. Have seen two other blocks right at the same cylinder wall thickness. Then about the time I was ready to condemn all 66 blocks the next one was one of the thickest walled blocks we've seen.

We just finished up a build for a 66 GTO and the cylinder walls were too thin so we substituted out a 400 block but still used the 93 heads, valve covers, intake manifold and such to have it look stock. Heads cleaned up at 68cc and the Butler supplied stroker kit required 36cc dished pistons. We were aiming for 9.3:1 but the Ross pistons came out just a little larger at 37.2cc's which along with other final measurements had actual compression at 9.27:1. By the time the owner paid for all the labor on the heads and new valves, springs, retainers, screw in rocker studs, and such he was 2/3 the way to a nice set of aluminum heads, but he wanted the stock look.

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That’s some very useful information and exactly what I’m hoping for in my build. Thanks for the pictures and the inspiration. The engine comes out next week and I’ll be sure to get the block checked. My heads should be good to slap on until I decide on aluminum heads. The valve train was rebuilt this past summer when I went to an SPC-7 cam. There were no signs the heads needed a valve job but we’ll see. That defiantly answered my question about how far it’s possible to go with dished pistons.

Thanks again!
 

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Didn't read all of the posts, as they were very long, but you need to know this: aluminum heads need to run a full compression point higher than cast iron heads to make the same power, all else being equal. So if you have your pistons dished to use with the 093 heads now, they will be TOO dished to benefit from the aluminum heads to their full potential. I would: leave it as-is and run the 093's as built, or, install pistons with less of a dish to accommodate the aluminum heads that will run better with higher compression.
 

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Thanks GeeTeeOh. If I can’t put the cash together for E-heads in this part of the build we’re going try to bring compression closer to 10 for the 093 heads rather than dish them al the way to 9.5 CR. It looks like it’s all stock now at 10.75:1. Once I know how much the short block is going to cost me for machining and parts. I’ll make the final decision on the E-heads.

This forum has been a great resource.
 

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I’m planning a rebuild on my non numbers ‘66 Tripower 389 using Butler’s 389 4.25” stroker kit. I’m planning to use all forged components as much for insurance as necessity. Even though I’m improving the engine I want to hang on to it’s 389 heritage.

That’s gonna stretch me a little so I’m planning to use the 093 heads on it for now and use dished pistons to get CR down for pump gas. Also planning to clean the chambers up and port match for better flow. They have new rockers, studs and springs from an SPC-7 cam upgrade this past year. Eventually I’ll buy 65cc KRE heads when I have the $$

Questions are, how much will it be possible to lower the CR using dished pistons, head gaskets, etc using the cleaned up 093 small valve 65cc heads? What numbers do you think it’ll make? The Tripower going back on along with the RA exhaust manifolds. I’m debating on reusing the recently installed SPC-7 cam or going a little bigger.

Challenge on the cam is I have power brakes and factory AC that I recently restored. Anyone ever fab an idle-stop solenoid bracket for a Tripower? I’d of left it all alone except it started knocking on me at the end of the season.

Thanks for your input
I'm late to the party on this one, but let me chime in on a point that PontiacJim raised. "Quench" --- this is BIG TIME important! When I first built my 400 into a 461, I -really- wanted to use the round port 722 heads I had. I cc'ed them myself at 72 cc's. Right on the factory spec which was a little surprising. In a 461, in order to get compression down to 9.5:1, I had to do some ugly ugly things to that engine. 1) I used pistons with ROUND (not D-shaped) dishes cut into them. That by itself killed the quench in that build, but still didn't get me enough clearance volume to lower compression to where I wanted it. So 2) I used a set of Cometic .072 compressed thickness gaskets -in addition- to those dishes. It ran, and amazingly made right at 500 HP and 550 ft. lb's on the engine dyno when I broke it in. With the piston crowns so far away from the heads that they needed binoculars to see each other, I know I was losing a ton in combustion efficiency and power. Plus not having qood quench properties can make it MORE likely to get into detonation even with lower compression.

So -- Talk to Butler. Make sure that you get pistons with D-shaped (not round) dishes. You might even have to go with a stepped dish design to get enough clearance volume without killing the quench properties of the engine. I have a spreadsheet here I built to run all the calculations when I was building (and rebuilding) mine. If I assume that your 389 right now has a +0.030 overbore and the Pontiac-usual 0.015 "down" deck clearance, using 65 cc chambers, and factory nominal head gasket thickness of .045 - You'll need at least 24 cc's of dish volume to get down to 9.98:1. Personally I'd feel safer with 9.5:1 and that would take a whopping 29 cc's of dish. (I guessed at the head gasket bore size and used 4.125) I don't know if that's possible, but considering the relationship that Butler has with Ross pistons it might be. But if it requires them to build custom ones as opposed to using a standard off-the-shelf variety, it's going to drive the cost up a lot. Unless there's some reason you really are set on using those heads, it might actually be less expensive to bite the bullet now and go with a set of aftermarket aluminum heads. With aluminum you can get away with (and actually need) more compression that you do with iron. 10.3-10.5:1 is plenty safe with aluminum heads.

Bear
 

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Discussion Starter #12
Thanks Bear. These are very convincing arguments for stretching my budget to get the aluminum heads and never look back. May have to look at alternate iron heads to keep it on budget. I’ll know soon.
 

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Thanks Bear. These are very convincing arguments for stretching my budget to get the aluminum heads and never look back. May have to look at alternate iron heads to keep it on budget. I’ll know soon.
I you're OK with alum heads, or any heads that are not period correct, but down on funds, why not consider 6x-8 heads. Can use flat top pistons, with zero deck height, & have good quench. Those heads are very common & probably the cheapest & most-used heads for Pontiac stroker builds. Probably still quite few sets out there for $200 or less. I have a couple of sets myself. But, because of weight, shipping heads ain't cheap.

As for the cam. IMO, an 068 cam is much too small for that many cubes. Many say you need close to 230° @ .050 lift on the intake. I'd consider 220° @ .050, an absolute minimum. For that size cam, a Lunati 10510312 might be a good choice.

If you wanna stay with low lift, for whatever reason, the Melling SPC-3 will give you 224° @ .050. The famous SD455 engines came with the 744 cam specs, & was under-cammed. The Summit 2802 is a sort of higher lift & cheaper version of that cam. Another cam in this range is a Crane 283951.


Next up would probably be the Crower 60243. But it's nearly $200. Too much, IMO. The Lunati 10510502 is 285/300 adv, 228/235 @ .050, with .465 lift. It is advertised with a 110° LSA. Many Pontiac guys like to have at least 112° LSA. But, CC lists most all their Pontiac cams with a 110° LSA. Some say 110° is just fine with big cube engines. But, I've read that some cam grinders will grind their shelf cams with most any LSA you specify.

Street/Strip Hydraulic Flat Tappet Cam - Pontiac V8 285/300

For the "at least 230°@ .050" cams, I like the looks of the Howards 410051-14 cam, if you wanna stay with conventional lifters.


My personal favorite has always been the Melling SPC-8, 041 clone cams, with original Rhoads lifters.


But, some can't stand the ticking sound the Rhoads cause. Next best lifters would be the Hylift Johnson "R" lifters. They're said to have a faster bleed down rate than regular lifters, but not as fast as Rhoads, therefore not making the loud ticking sound, which Rhoads cause. With either of these lifters, I think a big cube engine would make enuff vac to work vac brakes.
 

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Discussion Starter #14
Good point. I’m considering using different iron heads and have looked at the 6x-8’s and 6x-4’s. Thanks
 
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