Technical 327 piston compression ratio question

Good morning everyone, I just bought some sealed pro pistons, other than the over-boar they are identical To stock. The listed CR is 8.7:1. I plan on using the the stock 1.94 heads with upgraded springs for a better than stock cam. Am I going to be limited on cam options and overall CR after the motor is assembled? This is the first motor I’ve built my self so I’m not to knowledgeable and any info is greatly appreciated.


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Year of your 327 and casting # of heads please.

 

Got a part #, so we can find out the cc's volume of the valve reliefs?? I'm assuming these are flat top pistons??

What head are you using?? Casting #'s??

Some 327 piston compression ratio specs are for a 62cc chamber head, and some are for the smaller 58cc head....so between what head you are using and what the piston volume is there can be a pretty wide swing in potential compression ratio.

Whether the block has had the deck cut or left stock, and what thickness head gasket you plan to use can also affect compression ratio, by more than 3/4 of a point.

Us the wrong combination of stuff and those "8.7:1 pistons" can become a real 7.2:1 disaster of a build.

 

65 small journal and 462 factory heads


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It’s the original 462s the part number for the pistons is 310AP-40


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I’m trying to find my pic with full casting #’s


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The time to work out compression details is before the engine is assembled. What's your intended outcome for the engine? With today's fuels you most likely will want to stay in the 9:1 to 9.5:1 range. You have your pistons and heads. Get piston compression height and cylinder cc numbers. Pick a head gasket. Crunch some numbers and you will know with those components if you need to deck the block to get the CR you want.

 

If wrist pin height is dead on as old pistons there should be nothing to worry about. I've given up believing spect out specs a long time ago.

 

My goal is a fun street motor that’s reliable. I took a pic of all the castings numbers before I dropped the block and heads off at the machine shop but am unable to find it. The heads are still at the shop unfortunately AO im gonna call my guy and see if he can get me the castings for them to narrow down what exactly I have.


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Once you get the short block together, check your deck clearance before just buying any head gasket that fits. You want to get a tight "quench area" to add performance, compression, and help with detonation. I try to keep this area between 0.038 and 0.045 it really helps.

 

I just checked that piston number. It shows a 1.654 deck height, when combined the the 327 crank and a 5.7 rod, you get 8.98". If you have not decked your block it will be approx. 9.025...this would put your new piston at 0.045 in the hole at TDC! Get your block decked to 9.00, otherwise no head gasket in the world will give you an optimum quench area.

 

All of the above♤♤♤
Also, if you plan on putting a lot of miles on this engine and the heads' valve seats are not machined yet consider having hardened valve seats installed due to the lack of lead in most of todays' gasolenes.

 

I will have to look at the paper from the shop if I’m correct I don’t think it was decked...


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I got the castings, block is 3858180 and the heads are 3890462


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It is your build, but I would not assemble it with the pistons that far down in the hole. Fel-Pro makes a coated shim type head gasket that is only 0.018 thick, but that would put your engine at a terrible 0.063 quench area. Don't get me wrong, it will run OK, but not optimum. This is where the old adage "sucker pistons" started getting thrown around. The mfg. started making pistons where they took the machine work (decking) into the equation. See they would rather you have too much clearance rather than an interference!

 

Thank you for the info, what piston spec’s would you suggest?


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Ted isn't saying to get other pistons. Slow down and listen and take it step by step. Go back and read reply #10 and #11.

That said, who's putting engine together?

 

Would these be more optimal?


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I've run it through a compression ratio calculator 3 times....62cc head, 5cc piston relief, .015" thick x 4.100" bore gasket, piston down .046" in undecked block due to "rebuilder" low height piston.....9.51:1 compression but quench a terrible .061"....it might need nothing short of premium in 75° + weather. The short stroke may be what saves you from that tho'.
9.3:1 if the head is 64cc.

Decking the block will raise compression ratio, but the improved tighter quench will improve detonation resistance despite the higher compression ratio. 1/10 of a point for every .005" cut
I don't like putting them together with wide quench either, they can get finicky on fuel grade with wide quench. I don't have enough feedback data to see if that is even worse with todays Ethanolized fuels.

 

https://www.summitracing.com/parts/slp-h660cp40
This is a 2 relief piston that show a height of 1.675" when combined with your 5.7 rod and 327 crank, it comes out to 9.00. IF, your deck has not been touched and is 9.025, then the Fel-pro head gasket that I mentioned would give you a very good quench of 0.043. I have to mention, this is all theoretical because the mfg. probably fudges a little, your deck may not be exactly 9.025" etc. etc. Once a short block is assembled, you check and make decisions. I still think you will be disappointed with the pistons you have. The piston above, with a correct head gasket will probably put you around 9.5 to 1 + this would make good power with a healthy cam and your 462 heads.

 

Basically the same piston spec but without the lost .020" reduced height, so it would be better from that standpoint. Also forged, it is a lot stronger piston.

If you deck your block .020" and keep the pistons you have you would get the same resulting tighter quench and 0.4:1 higher compression those L-2165 pistons would get you in the undecked block.

 

Run the numbers through a compression ratio calculator...undecked block, .015" thick gasket, 5cc relief, 64cc head says 9.7:1 compression.
The thing I don't like about the H660CP hyper is you are only $25 away from getting the much stronger L-2165 forged piston.

 

I will be, at this point with everything going on with the pandemic my machinist had to close and builds a lot of dirt track motors so now that he’s back he’s swamped and has guys spending way more money so they back logged. Buying a different piston would be easier than having the block sitting at the shop for a few months or more.


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That's cool. Now instead of all the math floating around to understand you can install a piston (as in same one) in each corner of block with out rings and see how far your piston is in the hole. Other surprises may surface.

 

Thanks for all the info!! I leaning towards a better slug too. At this point I much rather use the advice given and spend a few more dollars than have the block at the machinist for a few more months.


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Good luck! I beat the snot out of a lot of hyper pistons (naturally aspired) back when they were cheap and AMERICAN made! If I remember correct, the Speed Pro's are from India now. I do agree with Ericnova72 though. the forged pistons are considerably stronger.

 

I’m hoping it’s going to be a smooth process


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Now that I have better info I’m gonna see what other options are out there. Is there any pistons that are still produced here in the good ole USA?


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The process I mentioned cost you nothing as compared to having machine shop do it. Also you get to see and learn in front of your own hands and eye's.

 

I doubt that any "cheap" cast or hyper pistons are AMERICAN made anymore. I think that some good name forged pistons probably are. But a $700 - $800 piston set isn't in everyone budget!!