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Old 01-23-2011, 08:48 PM   #16
supershaft
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Quote:
Originally Posted by chammyman View Post
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.
Shorter pistons with tighter packed rings have been proven very reliable in airheads for decades now. Longer RLR 's do have the advantage of reduced piston side loading but the reduced angle of the rod at the crank reduces torque. Increased angularity at the crank and thusly increased torque through leverage is obtained by a longer stroke OR a SHORTER RLR. Longer RLR's don't have a mechanical leverage advantage over shorter RLR's. Longer RLR's advantage is transferring more chemical energy into mechanical energy at higher RPM's (most say above about 4000rpm) via more dwell at TDC. At higher RPM's, the flame front literally gets to push harder and longer on the piston if it has a longer RLR. Then some think that longer RLR's also have the advantage of better cylinder filling to start with.
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Old 01-23-2011, 09:01 PM   #17
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Originally Posted by bgoodsoil View Post
Man, you guys know your stuff


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?



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?
As far as I know, Dr. Curve was the first one to draw up shortened pistons for Venolia to make specifically for an airhead. Same story with those Carrilo rods that Siebenrock is using. But ALL newer, more modern pistons are running tighter ring packs with higher wrist pins than days gone by. I think I would still run three rings in my street bike airhead. A lot of MX four strokes are now running two ring pistons.
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Old 01-23-2011, 09:06 PM   #18
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Quote:
Originally Posted by supershaft View Post
Shorter pistons with tighter packed rings have been proven very reliable in airheads for decades now. Longer RLR 's do have the advantage of reduced piston side loading but the reduced angle of the rod at the crank reduces torque. Increased angularity at the crank and thusly increased torque through leverage is obtained by a longer stroke OR a SHORTER RLR. Longer RLR's don't have a mechanical leverage advantage over shorter RLR's. Longer RLR's advantage is transferring more chemical energy into mechanical energy at higher RPM's (most say above about 4000rpm) via more dwell at TDC. At higher RPM's, the flame front literally gets to push harder and longer on the piston if it has a longer RLR. Then some think that longer RLR's also have the advantage of better cylinder filling to start with.
In all honesty the angles are very slight anyway I can appreciate what your saying a greater angle will in theory allow better torque output to the crank at lower rpm's but conversely you also have plenty of time for the piston to rock due to side loading. But again when discussing these things we tend to use very exaggerated examples.

regardless of R/S ratios peak combustion pressure is usually attained around 10 - 15 degrees after TDC with maximum effort able to be transferred to the crank journal when the rod is 90 degrees to it. With a longer rod this occurs further up the bore than with a shorter rod so this is why torque output is greater with longer rod - stroke ratios.

The views vary greatly on r/s ratios. Personally I find the engine decides what the score is. You can't tell it what to do, maybe predict it now and then but sometimes a crappy head design that shouldn't work makes a monster output at a compression ratio that is thought to be to high to use. Or a rod ratio that should have spat the piston through the block long ago allows it to rev to the moon etc.
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Old 01-23-2011, 09:10 PM   #19
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Originally Posted by pommie john View Post
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!!
I am curious how your push rod tube seals have been doing with 16mm shorter cylinders?
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Old 01-23-2011, 09:29 PM   #20
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Originally Posted by bgoodsoil View Post
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?

.

No secrets here :)

I run 98 octane unleaded pump fuel. It's available in every petrol station here ( Australia)


The compression ratio is 10.5:1
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Old 01-23-2011, 09:32 PM   #21
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Originally Posted by supershaft View Post
I am curious how your push rod tube seals have been doing with 16mm shorter cylinders?

The pushrod tubes (obviously) are shortened too and have to be bent a little to make them line up.
They don't leak, but I use a bit of silicone on them just to be sure.

The real problem is that the pushrods are not perfectly in line with the cam followers ( lifters) and they are prone to a bit of flexing.
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Old 01-23-2011, 09:32 PM   #22
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Originally Posted by chammyman View Post
In all honesty the angles are very slight anyway I can appreciate what your saying a greater angle will in theory allow better torque output to the crank at lower rpm's but conversely you also have plenty of time for the piston to rock due to side loading. But again when discussing these things we tend to use very exaggerated examples.

regardless of R/S ratios peak combustion pressure is usually attained around 10 - 15 degrees after TDC with maximum effort able to be transferred to the crank journal when the rod is 90 degrees to it. With a longer rod this occurs further up the bore than with a shorter rod so this is why torque output is greater with longer rod - stroke ratios.

The views vary greatly on r/s ratios. Personally I find the engine decides what the score is. You can't tell it what to do, maybe predict it now and then but sometimes a crappy head design that shouldn't work makes a monster output at a compression ratio that is thought to be to high to use. Or a rod ratio that should have spat the piston through the block long ago allows it to rev to the moon etc.
Displacement remaining constant, more torque through greater angularity via longer stroke OR shorter RLR is much more than "just theory".

I think you are missing the point that longer RLR's do not have across the board increased torque over shorter RLR's. Shorter RLR's create more torque through greater mechanical advantage until the RPM's get high enough that a shorter RLR's piston gets itself out of the way before the flame front has a chance to push on it. Longer RLR's have less mechanical advantage over shorter RLR's any way you look at it. You are spot on about where the piston is per degree of crank movement but at lower RPM that aspect is no advantage since the slower speed of the engine is enough to get the piston to hang around for the flame front to push on it. That combined with more leverage through angularity means that short RLR's have the torque advantage just off idle.
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Old 01-23-2011, 09:40 PM   #23
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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.
It is my understanding the regular 100 power kit, not the 1070, is dimensionally and materially the same as or pretty dam close to OEM, since it uses the same size cylinder size, same nycasil, and a 9.5 piston which is the same as an R100RS piston of the same year. Only the pistons are lighter. Still have three rings, so to keep the same rod and increase compression, the gudgeon to crown has to be slightly bigger, and that should keep the ring lands big. To reduce the piston weight by 110 grams and have a big gudgeon to crown, more stuff must have come off the waist?
So running the same 87 or 89octane really shouldnt be a problem. I realize teh 8.2's can run irish cream, but the 9.5's arent into the 90octanes to run right, I do believe.

This is my fervent hope and dream, anyway.
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Old 01-24-2011, 12:07 AM   #24
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Richie Moores website has some useful information, including dyno charts, show results from various engine mods including long rods and short skirt pistons.

I believe Richie has a 103BHP airhead engine

Link here http://www.moorespeed.co.uk/technical
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Old 01-24-2011, 12:37 AM   #25
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now that was an interesting read! Thanks, Rob. I need to figure out a way to make a few thousand dollars of expendable cash.
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Old 01-24-2011, 02:51 AM   #26
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Long rod, what?


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Old 01-24-2011, 04:52 AM   #27
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Long rod, what?


Long Con rod with a shorter piston. Send Richie an email and ask him about it.

Richie Posted this on UKGSer :-

the long rod kit has more torque and more power . the standard 100gs has about 47ft/lb at the wheel and about 51 bhp . the long rod kit gives 57ft/lb and 64bhp on standard heads it is all a lot more efficiant due to side thrust and rod angle. i normally just use these for racing because it gives you a higher base level to start tuning but the cost is high.

Rob Farmer screwed with this post 01-24-2011 at 04:58 AM
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Old 01-24-2011, 04:53 AM   #28
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Imho the rod-length is most of academic interest.
You might (or not) gain 4-5% power between 3000-7000 RPM but the cost is high.

When it comes to reliability of big-bore kits it's worth to remember that there are different solutions on the market. I run a big-bore kit myself and I'm not worried at all.
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Old 01-24-2011, 09:58 AM   #29
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Originally Posted by AliBaba View Post
Imho the rod-length is most of academic interest.
You might (or not) gain 4-5% power between 3000-7000 RPM but the cost is high.

When it comes to reliability of big-bore kits it's worth to remember that there are different solutions on the market. I run a big-bore kit myself and I'm not worried at all.
Academic interests??? I have SEEN it work! As have MANY others!

I do think some even advocates of longer RLR's mistakenly focus longer RLR gains on rod angularity. Some famous tuners I think rightly think the gains mostly have to do with TDC piston dwell and flame front speed.

Different solutions to big bore kits?? The one solution they need and no one has done is moving the cylinder studs out to make more room. They ALL have the same issues as long as the studs are where they are. Of course, moving the studs out would require completely re-casting the heads AND cylinders.
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Old 01-24-2011, 09:58 AM   #30
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Quote:
Originally Posted by Rob Farmer View Post
the long rod kit has more torque and more power . the standard 100gs has about 47ft/lb at the wheel and about 51 bhp . the long rod kit gives 57ft/lb and 64bhp on standard heads it is all a lot more efficiant due to side thrust and rod angle.
Did I get it right - this guy believes in a more than 20% gain in torque/performance just by increasing rod length?

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