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Jahns pistons comp ratio

Posted By cos Last Month
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cos
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Hello Members   Have a 292 that was put together in early 1960s.  Has .060 over Jahns ( 5/8" dome cast pistons ?) as I recall raised comp. ratio 2 points. Car was a 55 with a 6 volt system that would not spin fast enough to start, had to change to 12 volt with 6 volt starter. Retired it and then octane rating dropped. Also recall that I had  to use 103 octane fuel available at Standard stations. Had non posted G heads as i remember only once blew head gasket. My question, anyone here have any info about Jahns pistons different comp. ratio?   If pistons do actually  raise comp 2 points   to 11.7   would using 471 heads/ thick gaskets get it back down.  Really not interested putting much $ in it ( finding 3.81 or 3.82 pistons) ?  Thanks Cos
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Cos, I would say it's doubtful that you have 11.7:1 static compression with those pistons, unless those non-posted G heads and the block decks have been heavily milled (not a good thing). I would be more concerned about your dynamic compression ratio, which determines what grade of gas you should be using. You'll need to know your h, rod length, and intake opening at .020 lift (advertised) to calculate DCR. UEM has a good online static/dynamic compression calculator, but don't add 15 degrees to intake opening @ .050, use specs @ .020 (advertised). The calculation might not be right on the money, but it will be close. Ted can also calculate the DCR for you, if you can provide the required numbers. 

I would think, considering that the engine has domed pistons, that the camshaft is not stock. Obtaining the specs might be difficult, if not impossible, without Cam Doctor. 
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Late 1960’s I had a 312, can’t remember the overbore with Jahns pop ups. If my memory is correct they were advertised at either 11.25 or 11.50. With G heads and an rpm 300 Isky cam (still around #300300) it required the 103 octane. scrubron Custom Supreme in WA state. My current 320 stroker is a static of 10.75 with the Mummert heads, and we did a bunch of juggling on the cam (special order) to get the dynamic ratio down to about 8.7x. The EFI computer controls the timing advance, but it’s right there with today’s fuel for knocking. If you use those pistons, check your numbers carefully, or marry the daughter of the racing fuel guy.

Edit. Looks like it doesn’t like the ch— in Che. Vron

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It will run on E 85, you just need to work the carburetor (for more fuel) 
charliemccraney
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If the question is "would using 471 heads/ thick gaskets get it back down" then piston dome volume doesn't matter.  Everything else equal, increasing combustion chamber volume will drop the compression ratio.

A .060 over 292 won't be much over 8.8:1 with G heads, flat tops and zero deck.  A 2:1 increase would require about a 16cc dome.  11.7 requires almost 22ccs
Everything else equal, 471s should make it about 9.1:1 or 9.8:1 depending on where it actually is right now.



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Many of those early Jahns cast/domed pistons had a 10cc dome.  A std bore 312 with the stock flattop pistons 0.024” in the hole, composition head gaskets, and 69cc heads will have the static compression ratio (SCR) at 8.66:1.  Using a 10cc dome on the pistons in place of the flattops will raise the compression ratio to 9.70:1 SCR.

The advertised combustion chamber volumes for the G heads is 69cc.  The combustion chamber volumes for the ‘113’ heads is 72cc while the combustion chamber volumes for the ‘471’ heads is 82cc.  Back in the day it was a common practice to use the 471 heads with the 10cc domed pistons which essentially kept the compression ratio the same as factory delivered.

Lorena, Texas (South of Waco)


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Thanks for replies. everyone. Ted answer my question as usual.  Ted do you remember if Jahns made   more than one  dome height, these pistons  dome is about 1/2" highest  point   ( half moon shape) and tapers down to  noting  over pins. Thanks again.
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It might take a while to find a set of 471 heads that would be a good candidate for a rebuild. The cost of the heads + cost of machine work to put them into service won't be cheap. Personally, I prefer flat-top pistons and smaller cc heads.

Would it be possible/practical to cut down the domes of the Jahn's pistons to lower the compression? I have no idea how much meat can be removed without ruining them. 
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cos (6/5/2024)
..Ted, do you remember if Jahns made more than one dome height, these pistons dome is about 1/2" highest point (half-moon shape) and tapers down to nothing over pins. ..

I am only familiar with the Jahns cast domed pistons having a 10cc dome.  While I currently do not have any Jahns pistons sitting here to look at, I do have some older JE and Mickey Thompson cast pistons that also have the 10cc domes.  Even some of the older SBC cast pistons used a 10cc dome so that must have been an industry standard for a while.  Both the JE and M/T pistons are cast in a hollow dome configuration, so completely removing the domes on those would be an issue.  But if memory serves me correctly, the older Jahns pistons can be cut to a smaller dome and/or a flattop configuration as they were cast with a solid dome.



Lorena, Texas (South of Waco)


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If Cos doesn't have an issue with the bores, the cheapest route would be to cut down those domes. Just keep track of which piston belongs in which bore.  Would balancing become an issue (just thought of that)?

Ted, would you re-ring the pistons without touching the bores?


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