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Old 01-20-2011, 07:10 AM   #1
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Airhead shorter piston longer rod experiences?

Just curious if anyone has any experience with running longer rods and shorter pistons in an airhead. What are the possible advantages of running these more modern style pistons? I'm not planning on doing it, just curious.



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Old 01-20-2011, 07:35 AM   #2
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In 1987 Jim Roche ran a R100 with a long rod engine in Mexico to over 150mph. I write about him and the motorcycle he called White Dog on my blog here

Mr. Roche has some of his writing on BMW's here: http://mailer.fsu.edu/~jroche/
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Old 01-20-2011, 08:16 AM   #3
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The kit on the picture is a 1070-kit and on this kit you don't have the thin sleeve on the cylinder that goes into the engine-case. This means that the usable area of the cylinder gets shorter and you have to move the piston out from the engine.

With a modern piston you can keep the total length of the (longer) rod and (shorter) piston standard.
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Old 01-20-2011, 09:35 AM   #4
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What years are these kits available for?

My dream is to build a fast airhead.
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Old 01-20-2011, 09:42 AM   #5
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Originally Posted by crazydrummerdude View Post
What years are these kits available for?

My dream is to build a fast airhead.

Thats Siebenrocks' kit. With the rods and camshaft and the 1070's its about $2400.

Check out their website... its for all years past 1980 I think.

Love it.
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Old 01-20-2011, 10:12 AM   #6
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It seems to me that Siebenrock went to longer rods for the same reasons the Harley guys did: Because they had to for running out of room hogging 'em out! A rare few of the HD guys figured out that they were getting performance gains beyond what the added displacement should have got them and narrowed it down to the longer rod length ratio. It seems that Seibenrock doesn't touch on that aspect of their big bore kits much.

The problem with those big bore kits is that the cylinder studs are in the same place as ever. The cylinder wall along the length of the cylinder's stud hole is paper thin as is the gasket area between the stud and pushrod tube holes and the bore. Plus the entire cylinder wall is way too thin to boot. I have seen big bore cylinders distort like a corrugated pipe 100% in unison with the cylinder's cooling fins! Talk about rings that can't seat!! I have also seen them leak oil like a mofo for having almost no gasket sealing surface in places.

Smokey Yunick is the guy to really read up on RLR. That is who Dr. curve got the idea from I think. I have helped put longer RLR rods in airheads and oilhead race engines that already had all the early and big performance gains done to them. The engines were at the point where small gains were hard to come by. The substantially longer RLR's net instant and substantial gains on the dyno! They give airheads more torque from low mid to high RPM and that translates into HP anyway you twist the throttle on!

I would love to run a long rod engine if I had the chance BUT I am leaving my bore at 94mm and my displacement at 980cc. There are GOOD reasons why BMW stopped hogging out their 750 at 1000cc's!!!!

supershaft screwed with this post 01-20-2011 at 10:18 AM
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Old 01-23-2011, 06:56 PM   #7
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longer rods..

How about just using longer rods and short piston with
stock barrels.

Any advantages?
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Old 01-23-2011, 07:33 PM   #8
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Quote:
Originally Posted by walkingbear View Post
How about just using longer rods and short piston with
stock barrels.

Any advantages?
Read supershaft's post again. He describes the advantage to doing so, and says he'd do it that way rather than going with a bigger bore.
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Old 01-23-2011, 07:34 PM   #9
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Originally Posted by walkingbear View Post
How about just using longer rods and short piston with
stock barrels.

Any advantages?
Just the usual, reduce cylinder wall loadings due to smaller thrust angles. Torque will go up.

The biggest issue will be pistons though, keeping the same stroke with the same heads and jugs will mean pushing the gudgeon pin further towards the crown which will generally mean moving the ring lands closer to the crown as well. Obviously you can go only so far.

Therefore you increase the rate of failure on the piston as the higher the ring lands the weaker they will be 9they will be made closer together) and the top ring land will be a lot more susceptible to detonation damage.

But on the other hand if your comparing 30 year old pistons (and their technology) with newly developed and perhaps coated slipper type pistons then tbh they will be about the same or the newer one (like for like) will have a longer life.

of course this is assuming AFR's etc are ok and detonation is avoided.
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Old 01-23-2011, 07:56 PM   #10
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Man, you guys know your stuff
Quote:
t seems to me that Siebenrock went to longer rods for the same reasons the Harley guys did: Because they had to for running out of room hogging 'em out! A rare few of the HD guys figured out that they were getting performance gains beyond what the added displacement should have got them and narrowed it down to the longer rod length ratio. It seems that Seibenrock doesn't touch on that aspect of their big bore kits much.
I've heard Harley did an experiment and got incredible gas mileage from a single piston engine by having a very long stroke. I wonder if this is related?

Quote:
The biggest issue will be pistons though, keeping the same stroke with the same heads and jugs will mean pushing the gudgeon pin further towards the crown which will generally mean moving the ring lands closer to the crown as well. Obviously you can go only so far.

Therefore you increase the rate of failure on the piston as the higher the ring lands the weaker they will be 9they will be made closer together) and the top ring land will be a lot more susceptible to detonation damage.
I think what you're calling a gudgeon pin we call a wrist pin. If so, I get what you're saying there. What do you mean by ring lands? The grooves for the rings? Maybe the stock pistons had acres of useable space so they're not pushing the limit on that? I thought Siebenrock's kit was well-vetted. I wonder who designed that thing?

What are AFRs?
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Old 01-23-2011, 08:02 PM   #11
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I run a pair of these in my airhead racer.



They are 95mm bore and 16mm shorter than standard. I run them with standard rods and had to machine 16mm from the barrel base and you get better ground clearance for racing.

There may have been a torque advantage to running longer rods....but I didn't know that at the time!!
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Old 01-23-2011, 08:15 PM   #12
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What octane do you run in that racer? What displacement does that come out to be? Any idea what that compression ratio is? Are these secrets and I'm not supposed to ask?

edit: I've always been a Russell fan.

edit, edit: I'd hate to shorten my barrels for ground clearance. Dragging the cylinders on the pavement in parking lots is one of my favorite things about the G/S.
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Old 01-23-2011, 08:16 PM   #13
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Quote:
Originally Posted by bgoodsoil View Post
I think what you're calling a gudgeon pin we call a wrist pin. If so, I get what you're saying there. What do you mean by ring lands? The grooves for the rings? Maybe the stock pistons had acres of useable space so they're not pushing the limit on that? I thought Siebenrock's kit was well-vetted. I wonder who designed that thing?

What are AFRs?
Gudgeon pin is wrist pin but I refuse to Americanise myself. Therefore I see colours, I eat doughnuts, roll on tyres not tires, have silencers not mufflers, the hood is a bonnet, the boot is what you call a trunk and all the rest of the grammatical differences. Engines also have big ends here...

Ring lands are the areas between the piston rings. They are thinner the closer the rings are to each other, thus more likely to break. The land between the crown and the top ring is the most easily damaged as if the crown deforms under detonation etc the thinner it is the more likely it will either break away or deform and pinch the ring.

The bigger the ring lands the more stable the piston will be, the further the top ring is away from the combustion chamber the less nasty forces it will see and will run cooler but the piston hotter...

Small ring lands can distort easier and with that comes all sorts of issues, from ring flutter to pinching.

The other factor is of course ring numbers, a lot of racing engines will perhaps only run 1 compression ring and an oil scraper or 2 compression rings and no scraper. Race engines are throw away commodities generally so damn the emission compliance, economy and longevity.

AFR air - fuel ratios. The biggest force on whether an engine lives, dies, makes power or empties the wallet.
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Old 01-23-2011, 08:25 PM   #14
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Hey thanks for that! I'm learning a lot in this thread already.

I guess the question is this: where does that Siebenrock kit fall on this? Is it the 'expendable racer' '100 octane' piston or can we run US pump gas (93 octane is the good stuff) and still keep the longevity?
Quote:
But on the other hand if your comparing 30 year old pistons (and their technology) with newly developed and perhaps coated slipper type pistons then tbh they will be about the same or the newer one (like for like) will have a longer life.
Anybody know if Siebenrock's pistons are coated? The cylinders are Nicasil. I guess Stagehand'll have to test this one out for us.

edit: from their site:
Quote:
To reduce the thermal loading on the machine, we recommend the fitting of an additional oil pan distance ring and/or an oil cooler for all Power Kit conversions.
Here's your homework Stagehand: fill up on 87 and see if you hear a tink tink noise.
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Old 01-23-2011, 08:35 PM   #15
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Originally Posted by bgoodsoil View Post
Hey thanks for that! I'm learning a lot in this thread already.

I guess the question is this: where does that Siebenrock kit fall on this? Is it the 'expendable racer' '100 octane' piston or can we run US pump gas (93 octane is the good stuff) and still keep the longevity?


Anybody know if Siebenrock's pistons are coated? The cylinders are Nicasil. I guess Stagehand'll have to test this one out for us.

edit: from their site:


Here's your homework Stagehand: fill up on 87 and see if you hear a tink tink noise.
the pistons in the pictures at the top of the thread are coated, thats the black bits, just a friction reducer more than anything. but worthwhile. Friction is basically heat thats not making you faster.

If you hear the noise its to late tbh. Knock sensors on EFI engines are excellent at detecting knock which happens around 15Khz in most SI engines. They can hear it a lot better than any of us and react far quicker.

Knock can always usually be sorted out without the detrimental way of just pulling back timing. But to do so usually involves effort and money, thus its usually done by pulling back timing.
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