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Compression heights


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10 replies to this topic

#1 tdangelo

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Posted 06 April 2004 - 08:20 PM

Well when all else fails go find a book. Here are the compression heights on the EA71, piston 92mm across, and the EA81, piston 92 mm across,. Both pistons have a compression height of 32mm. This means that if you use an EA81 crank shaft and connecting rod with either piston you will get the same compression. Both pistons are listed as flat top pistons. The EA71 is not domed and therefore can not possibly produce a different compression ration than than the EA81 piston.

Granted the measurements come out of a Sealed Power piston catalog and are for a replacement piston. But if they are the suppliers of after market, for engine builders, I would tend to believe that they are as close to OEM as is possible.

Now the next question is where we are getting a 9.5:1 ratio from a SPFI piston in an EA81 Block. It appears that the connecting rod on the EA81 and the EA82 are the same length. Unless there is a compression height difference between the EA81 piston and the EA82 SPFI piston I would tend to believe that the compression ratios are the same. This leads me to conclude that the the OHC EA82 SPFI head has a different combustion chamber measurement than the EA81 head or for that matter the lower compression EA82 carb engine.

Anyone with "hard numbers" and experience on this?? I have built a few "pipe poppers" in my time. Some by interchanging parts and some by using "racing competition stuff. I am trying to make a "pipe popper" using stock parts and a creative machinst.

#2 RenaissanceMan

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Posted 06 April 2004 - 11:14 PM

Ok, heres the scoop on it all I guess, I researched into this too not too long ago when I was looking into new pistons for my ea82T.

76-79 EA71 1590cc 8.5:1
80-82 EA71 1595cc 8.5:1
82-89 EA71 1595cc 9:1
80-89 EA81 1781cc 8.7:1
83-89 EA81T 1781cc 7.7:1
85-94 EA82 1781cc 9.5:1 (carb & spfi)
85-94 EA82 1781cc 9:1 (mpfi)
85-94 EA82T 1781cc 7.7:1

From what I can tell, after looking thru the ITM catalog, is the following:

EA71 & EA81 pistons are the same, combustion chamber & stroke are different
EA82 N/A pistons are all the same (carb, spfi, and mpfi), however the dual port heads have a different combustion chamber therefore lowering the compression slightly.
EA82T and EA81T pistons are different, EA82T having the clearances for the valves

so in the end we come up with 4 different types of pistons for engines with this 92mm bore.

Flat top
Turbo interference
Non interference
Turbo non interference

can anyone correct anything I might have messed up with all that?

#3 subynut

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Posted 07 April 2004 - 12:21 AM

hummmm how come the compression ratio is 9.0:1 for the carb and mpfi, 9.5:1 for spfi with auto, and 7.somthin' for the turbo? These numbers are from the specs page in my owners manual for my 86.

#4 RenaissanceMan

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Posted 07 April 2004 - 12:42 AM

I got my stuff out of my trusty chilton, and also from what I remember browsing thru the ITM catalog. but I specifically know the mpfi being 9.0:1, where as the carb & spfi being 9.5:1

but remember the mpfi and turbo have the same heads, just different pistons...

#5 tdangelo

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Posted 07 April 2004 - 10:10 AM

I would appear that the only way to derive any "concrete answers" is to get some heads from the different engines in question and determine their combustion chamber volumes using a burret and triple distilled water. Some purists use alcohol in place of the triple distilled water but you must have an absolutely airtight valve and seat in each hole in order to retain all of the liquid.

The other approach is to find some one who has already performed this or find a Field Services Manual with that information. We may even be lucky enough to find an "authentic Subaru parts counter man" who would know where to go for that information.

#6 bigjim5551212

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Posted 07 April 2004 - 10:21 AM

First of all, i have not taken a Subaru Engine apart. I have worked on Datsun Engines mostly.

Pin heights mean something when you are building a hybrid motor. This would be like if you could build a 2 or 2.2 liter from using a legacy 2.2 piston. If the pin heights are the same and the pins are the same or something can be machined. To determine what the difference is in compression, you have to look to see if the rod length is the same as well, and then if all that is the same, it is all to do with cylinder head volume.

The other determining factor in compression is head gasket thickness. Thicker gasket equals lower compression

#7 tdangelo

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Posted 07 April 2004 - 01:39 PM

Datsuns were my "cup of tea for years". Both L series as well as A series motors.
That is where my knowledge of "pipe poppers" comes from as well as my backround in compression heights, cylinder columes, sweep volumes, share fuel charges, dedicated fuel charges and the list goes on ad infinitum. What I am looking for is, as you put it, "hybrid parts".

#8 discopotato03

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Posted 25 February 2008 - 11:18 PM

The easy way to raise any std CR is to deck the block cases and adjust the cam timing with slotted cam pulleys .
This is how my not finished EA82T MPFI has risen to 8.6:1 with std EA82turbo pistons .
Pulleys , slot the three bolt holes and dowel pin hole , set up and time correctly . Mark pulley then weld dowel hole and redrill .

Fixed , cheers A .

#9 GeneralDisorder

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Posted 25 February 2008 - 11:34 PM

First, this post is 4 YEARS OLD.

Second, the original poster is WRONG about the EA71 pistons. They are a few thousandths taller pin to top.

Third, you can't slot the cam pulleys on an EA81 - it's a PUSHROD engine. It doesn't even have them.

Damn it's annoying when people do this :mad:

GD

#10 discopotato03

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Posted 04 May 2009 - 10:33 PM

The easy way to raise any std CR is to deck the block cases and adjust the cam timing with slotted cam pulleys .
This is how my not finished EA82T MPFI has risen to 8.6:1 with std EA82turbo pistons .
Pulleys , slot the three bolt holes and dowel pin hole , set up and time correctly . Mark pulley then weld dowel hole and redrill .

Fixed , cheers A .


We later found out that there was a mistake in the calcs and the static CR came out just over 8 to 1 .

I think most could work out for themselves that the pulleys are on an EA82 ...

I would like to know if there is a factory 71/81/82 piston that could raise the CR of an EA82T to around 8.5 to 1 .

Cheers A .

#11 Gloyale

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Posted 04 May 2009 - 10:53 PM

Ok, heres the scoop on it all I guess, I researched into this too not too long ago when I was looking into new pistons for my ea82T.

76-79 EA71 1590cc 8.5:1
80-82 EA71 1595cc 8.5:1
82-89 EA71 1595cc 9:1
80-89 EA81 1781cc 8.7:1
83-89 EA81T 1781cc 7.7:1
85-94 EA82 1781cc 9.5:1 (carb & spfi)
85-94 EA82 1781cc 9:1 (mpfi)

85-94 EA82T 1781cc 7.7:1

From what I can tell, after looking thru the ITM catalog, is the following:

EA71 & EA81 pistons are the same, combustion chamber & stroke are different
EA82 N/A pistons are all the same (carb, spfi, and mpfi), however the dual port heads have a different combustion chamber therefore lowering the compression slightly.
EA82T and EA81T pistons are different, EA82T having the clearances for the valves


The stuff in red is incorrect.

EA82 Carb engines are 9:1

85,86 NA MPFI are 9:1

87+ NA MPFI are 9.5:1

EA82 SPFI are 9.5:1

Heads and combustion chambers are the same for Carb and SPFI engines.

So.......From what I can tell, the only 9:1 ratio pistons out there were form the 85,86 years. Another thing about the pistons, particular to those years, is that they had a totally symetrical wristpin hole. They can be installed in any one of the 4 cylinders. IIRC, I have seen carb pistons that have 2 sets of valve notches, so that they can be flipped either way.

Later, 87+ pistons have a wrist pin hole with a 1mm offset to fight piston slap. Becasue of this, the later pistons can only be installed correctly one way, and feature both "front" arrows and "up" dots for locating them in the correct cylinder.

I can't say for sure, but I think this offset may correspond to the different ratios.




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