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Compression ratio help

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4K views 46 replies 6 participants last post by  Bierman7  
#1 ·
Some of you may know I have been working on rebuilding my 302 recently. With that being said the pistons I got from my machinist look to be low compression pistons which I do not want he assured me my compression will be around 9.5 but I am not sure how to go about checking this... I have trick flow 170 heads with 61 cc combustion chamber. It's a stock bottom end except its bored 30 over and I am using cast pistons...( I know I should get forged but due to time and money constraints these are more affordable for me, in a few years I plan on getting forged pistons when I have more time and money as well as space for my project) the pistons are sealed power part no 272ap. I'm not entirely sure how to go about checking the compression ratio. Any help is appreciated or if you need more numbers ill try to get them. Thanks


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#6 ·
Looks to me like 8.6 to 1. This is assuming 1cc for valve reliefs, the published dish size is correct(I calcated almost 11.5cc dish), your headgasket thickness is .035 compressed, your piston is .005 in the hole. If head gasket compressed thickness is larger or your piston is further in the hole, you will have even less compression.
 
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#10 ·
Yep, just did it again, 8.64. Use bore 4.030, stroke3, use cyl hd vol 61cc, use 12.5cc negative dome(dish), .005 down the hole, .035" head gasket thickness, youll get the same results.
 
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#12 ·
-.110 is how deep it is, in inches, the calculator is asking for cc's. I took the diameter of the dish, duvided by 2, multiplied by pi, and got the area in square inches, then to find the volume of a cylinder( the dish is just a shallow cylinder in mathmetics), you multiply by the .110 of the depth of the dish, which gives you cubic inches. I plugged that into my fancy conversion software and got nearly 11 cc's of dish and added 1 cc for the valve reliefs. There you have it.
 
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#14 ·
Basically, lol. You typically gain power by going to a 306 because you unshroud the valves AND you have a chance of increasing the compression. I highly suggest you choose another machine shop. They don't know their ass from their elbow.

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#15 ·
Those are truck pistons. You're just learning a hard lesson that I learned in a similar fashion. Do it by cutting costs never turns out how you want it to. Doesnt mean going broke, just means having to wait and save. I looked on summit and some forged pistons that would get you 9 to1 or more would cost in the high 300 range to low 400's. Look for a good used set. Get them mic'd and checked before you buy.
 
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#17 ·
Get different pistons.
 
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#19 ·
Pay to have them switched or proove to the guy he's full of ****, they are low comp, and get him to take them off and refund your money. I absolutely hate shops that do this underhanded ****. Bottom line, get a good set of pistins and do it once.
 
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#20 ·
Oh I plan on complaining about all this took him over a month to do the minor work I needed done and when both me and my dad asked about the pistons and compression he said they will be 9.5. I'm so sick of dealing with him. I wanted to have the car done two weeks ago.


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#21 ·
Dont rush your project, it takes the time it takes. Good luck, he,d lose any law suit, so he should be willing to oblige you.
 
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#26 ·
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#27 ·
I would NOT settle for those pistons. My two cents. I'd make him fix it on his dime or refund. Never been to small claims court thank goodness, but I hear it's easy and a simple threat of it can get results. If he continues to give you grief and not fix it, report him to the Better Business Bureau.
 
#28 ·
Well I don't know what ill do I called and asked him about I and he swears they are at least a 8.9 to 1 compression ratio which is still lower than what he told me originally. I am pretty sure stock compression is between 9 and 9.5 please correct me if I'm wrong.


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