# Butler Compression Calculator



## extinctmake (Feb 8, 2011)

Been checking out the compression calculator on the Butler web site. It's caused some new concern regarding compression of the numbers-matching '65 GTO 389 I am trying to get built.

It's the Effective Dome Value (EDV) that has me wondering how am I going to get the compression reduced to 9.0:1 using my correct #77 65 cc cylinder heads. I assume what is meant by EDV is the cc of the dish in the piston? I have a formula right now of a bore of 4.0925 after an assumed .030 bore, a stroke of 3.75. I state a compressed gasket thickness at .045 and an EDV of 21cc and guessed the deck at .005. 

9.35:1

That is still higher than I anticipated. I plan on using 91 octane BP Ultimate. Are these figures I entered about par for the course? A deck clearance of zero bumps compression to 9.44:1. 

What is the most ccs for a dished piston? How deep or how far can the dish be cut into the piston crown? 

I am wondering if it is even possible to lower the compression to 9.0:1 without making the engine less responsive or ill performing. Perhaps I'm entering inaccurate figures in the calculator.


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## john23 (Mar 6, 2016)

where do you live and drive the care? what is your altitude?


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## Pinion head (Jan 3, 2015)

Have you had the 77's vatted & a few chambers propperly cc'ed, or are you going by some published chamber volume? many times original never machined Pontiac cyl heads have chambers that are bigger than published specs. An example... since the mid 90's, have always got a good chuckle when reading 72cc "13" casting heads. CC those virgin 13's &, they'll come in @ 78-80cc chambers.

Forged pistons are extruded & when you look up above the pin boss area it will appear cupped. The thickness of the deck of the piston can vary between different manufacturer's, that's where the machinist can inquire with the custom piston manufacturer. With the closed chamber heads a D shaped dish is preferred to a large diam dish. Hope this helps.


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## extinctmake (Feb 8, 2011)

John, I live in Nebraska, so altitude is not an issue. 

I have yet to have anything done to the heads just yet. The engine block, heads, crank is in for a cleaning and magnafluxing. 

Maybe the chambers are bigger than the advertised 65ccs. That would be a big help. I guess we'll find out once the machinist goes through them. I'm thinking I will have to have Ross pistons make some dished pistons. The machinist has said he likes Diamond pistons and pitched the idea of boring my 389 .060 and use a dished piston that is for a 400. I don't want to do that if I can avoid it. First, I don't want to bore the cylinders that far and second, I am not sure if a piston for a 400 that uses different chambers and valve sizes than my 389, would work very well.

I just was on that Butler calculator entering numbers and it made me wonder how much of a dish I will need to lower my compression to 9.0:1. I guess once more is known about the heads, then the people at Ross pistons can figure something out. 

I also saw a 389 Tri Power on You tube rebuilt by an outfit called DCI Motors. They said they lowered the compression by making a heart-shaped cut to the combustion chambers to make the heads 80ccs. I'm not sure how that works out, but I'm sure it probably costs the same as using a dished piston without hacking up some cylinder heads. 

Has anybody done or heard of anything like that?


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## Pinion head (Jan 3, 2015)

What Don @ DCI did on the tripower build is use a program in his CNC mill to open the chambers. Its not a butcher job, its actually a well thought out method to lower static C/R. SD Performance has been performing a similar mod for over a decade to '670's. Have a friend who had SD do the CNC open chamber mod, as well as port work to some killer 670's that's on his 433. I have a set of custom Ross pistons in my 448, & am looking into AutoTecs or Diamonds in my next engine build (9.8-1 455 HO). For a street type build, you could look at the KB 389 forgings Butler sells, then have your machinist work the tops over either in the lathe or with a CNC program.


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## extinctmake (Feb 8, 2011)

Thanks for the insight pinion head. I might have to see about lowering my compression through increasing the chamber cc opposed to getting a dished piston. 

I wonder if I could use a flat piston if the chambers in the heads were cut to increase the ccs?


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## BearGFR (Aug 25, 2008)

Highly recommend you CC the combustion chambers in your heads to find out where you're at now. Pontiac chambers have been known to vary from published factory specs. You can do this yourself with a little creativity and you don't have to spend a lot of money on special tools to do it. Those tools make it _easier_, but they're not absolutely required. When I measured mine, I used a flat piece of plexiglas sealed to the head using a thin layer of grease around the outside of the chamber. The head was tilted slightly "downhill" towards the spark plug side (also used grease to seal the valves, since the valve springs were removed at the time and of course the plugs were still in). I'd drilled a small hole in the plexiglas so that it was at the highest edge of the chamber with the head tilted. Then I just used one of those HUGE plastic medical syringes to fill the chamber with water, being very careful to use the markings on the syringe to measure _exactly_ how much water I was dispensing. I even measured each chamber multiple times and averaged those results to minimize the effects of any small errors I might have been making. I did that for every single chamber. Yeah, it took a long time to do, but I was very confident that I got good, accurate numbers.

Calculating actual static CR is something you can do yourself, the key is having accurate measurements for all the volumes. You need: chamber volume (of course), accurate bore size (remember to account for any overbore that may have been done during previous rebuilds), head gasket compressed thickness and bore size (won't be the same as cylinder bore size), piston deck clearance (at TDC, how far "down" the bore is the top of the piston? Pontiacs that haven't been "zero-decked" are frequently "down" 0.020" or so.), and last but not least, volume of any valve reliefs/dishes in the piston crown. Factory flattops with the single eyebrow valve reliefs are usually around 6 cc's., for example --- but you can measure those too.

I built an Excel spreadsheet embedded in a Word document that will make all the calculations for you, I've uploaded it out here several times so you can probably find it with a forum search. Static CR is simply the ratio of maximum cylinder volume (piston at BDC) to minimum cylinder volume (piston at TDC). 

In your original post, you "guessed" at a lot of the measurements. That will bite you, big time. In a 389 (using your same assumption of a previous 0.030 overbore), a difference of just .005 in piston deck clearance will change static CR by 0.2, and a difference of a measly 1 cc in clearance volume (anywhere - chamber, gasket, deck, or piston crown) will change it by 0.1. Don't guess. Measure.

When trying to hit a compression target you have to know where you're starting from and go from there to try to hit a target clearance volume (sum of all the components that contribute to the volume remaining at TDC). But static CR by itself isn't the whole story. If it were, all you'd have to do would be to spring for a set of super-thick Cometic head gaskets and you'd be done. However, you also have to pay attention to "quench". What's the minimum distance from the piston crown to the flat part of the chamber at TDC? You want that to be as close as you can get it without ever running the risk of actual contact, accounting for thermal expansion of parts, slight rod stretch at high rpm, etc. You'll find various opinions on the target static assembly measurement. This is a dynamic thing and materials (steel vs. aluminum) and upper rpm limits come into play. On a cast iron street engine, "usually" the target is around 0.038 to 0.042. Race engines that run aluminum rods, heads, and have to live north of 6000 or 7000 rpm will need more - 0.065 or more. If your quench area isn't tight enough it both hurts combustion efficiency (bye bye, power) and also promotes hot spots in the chamber which makes the engine more likely to encounter detonation problems (even with a lower static CR).

All this for static CR, and we haven't gotten off into the whole dynamic CR debate yet... As usual on the Inter-tubes, you'll find opinions all over the map on that one - everywhere from "it's an imaginary idea, all that matters is static CR" to the exact opposite. I'm not 100% convinced either way at present on that one yet myself. 

Bear


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## extinctmake (Feb 8, 2011)

Wow Bear, that is a trove of information. I will definitely consult with the machinist regarding the quench factor. He already has my cylinder heads, so I want to think he will measure the chambers so we will know what we’re up against.

One of the first things he asked me was what compression ratio I was trying to achieve. I will take the liberty of showing him your post to make sure he is aware of the combustion efficiency factor you bring up.

I have yet to hear back from the shop. It’s been a month, but he knows I am not in a rush. I would like to know if my cores are in decent shape before making any further plans. 

The goals I have for this engine are not as extreme as most of those who are on this forum. I told the machinist I’d like to get the engine to live on 91 octane BP (Amoco) Ultimate fuel. I asked for a 9.0:1 ratio. I also want this engine perform as well as possible, solely for street use, under the restrictions I have placed on it for the sake of keeping it correct for the car. I am using the correct small carburetor, stock ignition, and of course a reduced compression ratio. I am going to switch to the H.O. exhaust manifolds to aid in breathing and those won’t hurt much (if at all) in show points.

Many thanks.


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