Author Topic: 28/31 pistons  (Read 5189 times)

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Offline rebelce

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28/31 pistons
« on: January 08, 2013, 07:07:57 pm »
Could one of you fine folks tell me the piston pin compression height for the stock oppy pistons and the 28/31 pistons needed to run ARC rods? Trying to work up a package for the compression/breathing conundrum.    Thanks Arr
Armond Orr

Offline Jeff McKelroy

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Re: 28/31 pistons
« Reply #1 on: January 08, 2013, 07:56:40 pm »
What do you mean by "package"?  Are you just wanting to know the difference in performance?  I am asking because as far as ARC's rods go, you're pretty limited on your choices.  Therefore, wanting to know a piston pin height comparison between a stock rod and an ARC rod is unnecessary.  As far as compression, the compression is the same, since you have the same stroke.  As far as breathing, well, that's another story.  I would suggest the search bar, as there are lots if ideas floating around.

Offline rebelce

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Re: 28/31 pistons
« Reply #2 on: January 08, 2013, 08:55:39 pm »
    Evening Jeff, I'm looking at non-stock pistons.  It looks like a stock oppy has a compression height of apx. 1.426.  ACR lists their rod lengths but I haven't measured the stock 42 and 46 rods. I guess the question is:  What is the 28/31 piston (that ARC recommends) compression height?  I have trolled extensively with the search bar. Most were just like your answer (no offense).  Lots of words and question marks, no numbers.  If you'd like details, I'd be happy to go into it, BC me. :)

    Before building an engine, I like to know the piston deck height and shoot for zero.  Too many manufactures have a lax attitude when it comes to "within specs".  Many times it's easier to match the shortest piston with the longest rod.   For me, "a package" is one where all the parts are where I want them.  Before all this comes together I'll be asking for the cam specs too, can't do an exhaust or intake system without knowing.  I'm a newby to building oppy's and an idea is no better than your ability to make it happen. I just want to know my ideas are possible.

        Arr
Armond Orr

Offline Jeff McKelroy

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Re: 28/31 pistons
« Reply #3 on: January 09, 2013, 12:41:41 am »
I don't know the compression height.  You have two choices of rods, what's it matter?

I understand where you are coming from, but what I am trying to tell you is it ultimately will only matter to your "package.  Oppy blocks, pans, alignment dowels, shoot: even head bolt alignment, deck height, inspection cover flatness, connecting rod reliefs, valve guide-to-seat alignment, intake ports have minor differences block to block, crank to crank, casting to casting.  We are all faced with it one way or another.  When I used to drag race, we fought lifter bore alignments, rocker shaft alignment, on and on.  If you want a blueprinted engine, you will have to find an intelligent machine shop, some patience, and most definitely some cash.

Just like you said, a compression height can vary piston to piston.  Most of the time they're never truly 0.00" flush, if ever.  I had one that was -0.008" on cylinder one, and -0.012" in cylinder two!  In engineering, it's called stack-up tolerance.  A thousandth here, a thousandth there, it all adds up.  You are wanting specifics that while they can and may be offered, probably won't be consistent with your engine.  There's often a variable.  Hence the "within specs" answer.

So when it seems like people are being secretive, or are obfuscating the truth, please understand we are not like that here. We never intend to come across as mean-spirited, send you out the door with a pat on the back and a smile and a box (or 20!) of parts.  We want to help. But some parts of this beloved sport are indeed trial and error, with limited information, and classic blood sweat and tears. 

Offline rebelce

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Re: 28/31 pistons
« Reply #4 on: January 09, 2013, 08:02:50 am »
I guess I'm used to people saying "You can't do that!".  You know the conundrum I mentioned.  Increase the compression on a flathead, kill the flow. Both would be nice.  I think it was Barney Navarro who first posed the idea of using a domed piston to ease the flow transition between the valves and cylinder.To that end, I started looking at aftermarket pistons. Several have pins and bores that would work. Some even have a dome. Compression heights vary from 1.188 to 1.5, hence my question. There's a partial machine shop in the basement and garage (I wasn't about to pack a Bridgeport to the basement) and a flow bench being installed. It would be a fun quest, to see how much hp. could be rung out of an oppy without breaking the bank. I wish we had a data bank to keep all this kind of info but I suppose there must not be many who would use it.
Armond Orr

Offline garrywarber

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Re: 28/31 pistons
« Reply #5 on: January 16, 2013, 07:55:44 am »
I guess I'm used to people saying "You can't do that!".  You know the conundrum I mentioned.  Increase the compression on a flathead, kill the flow. Both would be nice.  I think it was Barney Navarro who first posed the idea of using a domed piston to ease the flow transition between the valves and cylinder.To that end, I started looking at aftermarket pistons. Several have pins and bores that would work. Some even have a dome. Compression heights vary from 1.188 to 1.5, hence my question. There's a partial machine shop in the basement and garage (I wasn't about to pack a Bridgeport to the basement) and a flow bench being installed. It would be a fun quest, to see how much hp. could be rung out of an oppy without breaking the bank. I wish we had a data bank to keep all this kind of info but I suppose there must not be many who would use it.

Arr,
I'm a real neophyte at these projects, but from wiser guys here I have concluded you could put 42 rods in your 46, causing the piston to rise above the block and enabling turning a dome in its top.  That's my plan, so I hope my conclusions are correct.  I remember a post somewhere where a member recommended exactly that.  Rooster and Flatheadpuller, am I correct?  In my favorites I keep a link to topic "New ARC rods for opposed twin" that may answer what you are looking for...
Garry
Garry Warber, Crazed Oppy Cool-Bore Fan

Offline Rooster

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Re: 28/31 pistons
« Reply #6 on: January 16, 2013, 08:54:38 pm »
You won't ever get me to agree that raising compression in a flatty is a good idea.
Bert stole my cookies!!
 I think he used them to bribe Ed into something naughty?

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Offline Jim Knutson

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Re: 28/31 pistons
« Reply #7 on: January 16, 2013, 09:46:32 pm »
You won't ever get me to agree that raising compression in a flatty is a good idea.
i second the motion...
If you quit when you're behind, you will never win.

Offline rebelce

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Re: 28/31 pistons
« Reply #8 on: January 17, 2013, 07:12:50 am »
You won't ever get me to agree that raising compression in a flatty is a good idea.


I think we can agree compression and flow are at odds in a flatty.  I'm just trying to find the best compromise for power.
Armond Orr

Offline garrywarber

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Re: 28/31 pistons
« Reply #9 on: January 17, 2013, 07:18:59 am »
You won't ever get me to agree that raising compression in a flatty is a good idea.

Well, I misunderstood then...  I had thought you were against shaving the head causing even more flow restriction is all.    A domed piston shouldn't restrict much if at all I suspect.  What is the reason you would be against that approach?
Garry
Garry Warber, Crazed Oppy Cool-Bore Fan

Offline Rooster

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Re: 28/31 pistons
« Reply #10 on: January 17, 2013, 10:10:13 am »
Well, I misunderstood then...  I had thought you were against shaving the head causing even more flow restriction is all.    A domed piston shouldn't restrict much if at all I suspect.  What is the reason you would be against that approach?
Garry
In my opinion...Flatheads have been around forever, not alot has changed in the design. Experienced, professional engine builders have attempted literally every approach to increasing CR in them and no-one has been successful at it. Domes, shaving, quenching, port reduction....you name they have failed with it. Flatty's make compression with the Cam, not the piston! It has been tried in In-line engines, V Engines and yes, Briggs as well as other small engines. It always ends up the same, increasing compression reduces flow and cylinder filling thus hurting the power out put of the engine.
There is always the guy that comes along and says ;"What if we could increase CR without hurting flow?" - My answer to that is simply; "Then you will have done what no-one else has been able to achieve. If that is your goal, have it!"
While your out chasing that great white whale...I'll be fishing for HP in a pond full of bass!

There are many proven ways to make power from a Flatty....why waste your time messing around with the one way proven to hurt power?

Bert stole my cookies!!
 I think he used them to bribe Ed into something naughty?

http://www.cutlooseracing.com

Offline BIG AL 202

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Re: 28/31 pistons
« Reply #11 on: January 17, 2013, 11:00:24 am »
WHEN YOU PORT CORRECTLY YOU WILL LOWER THE COMPRESSION [SLIGHTLY].     MAKE IT FLOW AND IT WILL GO!
ALWAYS VOTE FOR ED!   FONGS BROTHER FROM ANOTHER MOTHER!

Offline rebelce

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Re: 28/31 pistons
« Reply #12 on: January 17, 2013, 12:37:30 pm »
The streetable X7 Hudson's did OK running 9.2:1 CR, putting out 220hp from 308 cubes. You may very well be right Rooster, but the day I quit trying, is the day I lay my wrenches down and sit in the spectators section.  I race the vintage class at Bonneville, using the same rules and iron that's been used since the SCTA started in 1948 and the records keep going up. It is the nature of competition.
Armond Orr

Offline BIG AL 202

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Re: 28/31 pistons
« Reply #13 on: January 17, 2013, 01:05:22 pm »
GO FOR IT! THE REST OF US STILL RUNNING FLATTYS ARE STILL TRYING TO GET THAT MAGIC COMBINATION! SOMETIMES IT WORKS SOMETIMES IT DOESN'T!  LOL
ALWAYS VOTE FOR ED!   FONGS BROTHER FROM ANOTHER MOTHER!

Offline garrywarber

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Re: 28/31 pistons
« Reply #14 on: January 17, 2013, 04:23:50 pm »
In my opinion...Flatheads have been around forever, not alot has changed in the design. Experienced, professional engine builders have attempted literally every approach to increasing CR in them and no-one has been successful at it. Domes, shaving, quenching, port reduction....you name they have failed with it. Flatty's make compression with the Cam, not the piston! It has been tried in In-line engines, V Engines and yes, Briggs as well as other small engines. It always ends up the same, increasing compression reduces flow and cylinder filling thus hurting the power out put of the engine.
There is always the guy that comes along and says ;"What if we could increase CR without hurting flow?" - My answer to that is simply; "Then you will have done what no-one else has been able to achieve. If that is your goal, have it!"
While your out chasing that great white whale...I'll be fishing for HP in a pond full of bass!

There are many proven ways to make power from a Flatty....why waste your time messing around with the one way proven to hurt power?



I'm in over my head here Ahab!  You boggle me with making compression with the cam.  Obviously I'd best stay with stock parts and not get exotic.  In truth, I was just using the parts I had, and instead of trimming the piston top flat I was going to round it...  Doing mine on the cheap, which to me means as many cubes as possible with stock manufacturers parts.  An ARC flywheel is as exotic as I'm getting, and that's only to get the weight down.  Low compression also means one can run lower octane fuel, correct?  I'm just a regular guy... :doh:  

My thing is a flyrod for bluegills, but pond bass I could live with! :rockwoot:
Garry
Garry Warber, Crazed Oppy Cool-Bore Fan

 

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