# Compression help



## Jason6512 (Dec 29, 2020)

So if I have a 400 bored to 4.155 and my stroke is 4.250 and my deck is 0 decked and my heads are 90 cc what pistons do I need to achieve 9.25to1 ?


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## Jason6512 (Dec 29, 2020)

I'm also guessing at a .042 head gasket


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## O52 (Jan 27, 2019)

Here is a great tool to manipulate what you have to achieve a predetermined ratio.






Compression Calculator


Instructions: Enter a value in the un-shaded boxes below as they pertain to your engine. Note: For pistons BELOW the block @ TDC - value in box must be a POSITIVE number (.000) Note: For pistons ABOVE the block @ TDC - value in box must be a NEGATIVE




diamondracing.net





As you can see there's a lot of variables to figure out.


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## Jason6512 (Dec 29, 2020)

Thanks 052 but I dont know what 3 or 4 things on that calculator that why I came here


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## Jason6512 (Dec 29, 2020)

So can anyone tell me. If a 400 with 4.155 bore with 4.25 stroke and 0 decked block and 90cc heads .042 gasket with +8 pistons does it equal 9.55to1 compression rods will be 6.625


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## O52 (Jan 27, 2019)

I plugged in your numbers and came up with 9.74.


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## Jason6512 (Dec 29, 2020)

Ok thanks


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## Old Man Taylor (May 9, 2011)

I'll try to attach an Excel spreadsheet I made for doing CR. Hopefully it works. It has the numbers for my 505, but just replace them with your numbers.

OOPS, it wouldn't let me do it.


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## Jason6512 (Dec 29, 2020)

Ok I'm still a little high so maybe I'll just not take so much off the heads make it 92 cc instead


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## Old Man Taylor (May 9, 2011)

I'll send you the spreadsheet I made if you give me your email. You will have to have Excel to use it, but you don't have to know anything about Excel.


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## BearGFR (Aug 25, 2008)

I've got a spreadsheet I built too, and it's even out here on the site "somewhere" or at least it used to be.

But, plugging in your numbers and making some assumptions for the measurements you didn't have:
Bore: 4.155
Stroke: 4.250
Compressed gasket thickness: 0.042
Chamber volume: 90 cc's
Deck clearance volume:0
Head gasket bore (assumed): 4.130 
Piston diameter (assumed): 4.151
Distance from piston crown to top of compression ring (assumed): 0.255

A piston dish volume of 10 cc's (including valve pockets) puts you at 9.566:1
11 cc's puts you at 9.489:1

Bear

p.s. I'll also attach my sheet to this reply. In order to get the site to accept it, I have to play some games. I have to 'lie' and tell the system that it's a .txt file. To get your copy,click the link, download the file and save it somewhere, then rename it to CompressionRatio.xls, then open it in Excel. You may get a security warning about enabling edit mode - it's safe. I haven't embedded anything ugly in it.


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## Jason6512 (Dec 29, 2020)

Cool thanks alot


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## lust4speed (Jul 5, 2019)

Both Wallace and Butler have compression ratio calculators on their site. While the .042" head gasket thickness is available from Butler, I prefer to use the FelPro composite head gasket and it is stated to have .039" compressed thickness and a 4.30" bore diameter. I gather FelPro went to the trouble to compute the average diameter since their gasket has reliefs for valve chamfers and varies from 4.20 to 4.40 as you go around the circumference. Anyway using your above figures Wallace calculator says a 15cc piston will give 9.26:1 and Butler calculator is a little more conservative stating a 16cc dish is needed for 9.23:1 ratio.

Zero deck height is a lofty goal and tough to obtain. Take a look at off the shelf pistons and you will see piston pin heights (compression height or CH) all over the map. Not entirely the piston manufacturers fault since Pontiac changed deck height a little from year to year. Have your machine shop measure your actual deck height and compute the actual piston CH that should be ordered.

Math looks basically like the pistons I just ordered for a current build using a 4.155 bore piston. We haven't decked the block yet but rough measurement was 10.225". We wanted to allow a .010" cleanup cut since the block deck was pitted, so the finished deck height is going to be 4.215". Taking away the rod length of 6.80" and half the stroke (4.25/2=2.125) we ended up with a CH of 1.29". Now the good thing about waiting to cut the deck until after the pistons are received is you do the mockup and measure the actual cut needed since despite proper measuring, you have variances in rod length, actual stroke, and where the piston manufacturer actually set the CH. Very slight individual discrepancies up to only .001" are always present; and on a good day the tolerances balance each other out, but on a often seen bad day they all stack up going one way. Then all we do is to adjust the actual decking to get us precisely back on track. Then as long as the machine shop is having a good day, the final decking with be within plus or minus .002".


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## Jason6512 (Dec 29, 2020)

Thanks 4 all the info I'm still learning and this will be my first Pontiac build. I just want a good running engine that is reliable.


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## BearGFR (Aug 25, 2008)

Jason6512 said:


> Thanks 4 all the info I'm still learning and this will be my first Pontiac build. I just want a good running engine that is reliable.


That being the case, then I'd say it's not "worth it" to try to walk the razor's edge on compression. Consider this: with everything else being held the same, the power difference between a 9.5:1 engine and a 10.5:1 engine of the displacements we're talking about (400 - 461) will be less than 10hp, maybe less than 5. Is that small difference in power "worth" having an engine that you have to worry about constantly, stay "on top of" its tuning and and its cooling systems, fret about whether that octane rating at the pump is accurate? - vs. one you can just fire up and drive anywhere without having to worry?

Bear


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## Jason6512 (Dec 29, 2020)

Bear not sure what your talking about but I'm trying to get 9.25to1 for pump gas not 10.5to 1


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## BearGFR (Aug 25, 2008)

Jason6512 said:


> Bear not sure what your talking about but I'm trying to get 9.25to1 for pump gas not 10.5to 1


The point I was making was that for a 'driver' street engine, I don't think it's worth worth the hassle of trying to milk every last bit of compression you can out of it - trying to walk 'right on the line' of detonation.


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## Old Man Taylor (May 9, 2011)

I like to target 9.25:1 for the street. Remember that California only has 91 Octane, and that has 10% Ethanol in it. No matter where you are, you never know what state you might be driving in.


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