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Old 12-27-2012, 10:06 AM   #72181
Feelers
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Question

Quote:
Originally Posted by doug s. View Post
when at top-dead-center, the top of the piston is in the same location as stock, it's yust that the crank journal is closer to the top of the piston, and the rod is shorter; so when the piston is at bottom dead center, it is further away from the top of the cylinder, thus increasing displacement...

doug s.
Does stroking the engine increase the amount of compression? It seems to me that it will. If so, would it still run on normal gasoline?

Would the stroker kit be available as a standalone kit without the big bore? It seems to me that wouldn't be a problem. It might have a tougher time at idle?

Is there an advantage of doing one vs the other assuming very similar final displacement?

I should probably be asking Jeff, but anyone with engine knowledge should be able to help I think.
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Old 12-27-2012, 10:19 AM   #72182
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Originally Posted by procycle View Post
I'm confident the stock trans will be up to it (with a billet 3rd gear set).
There's no reason to think it will need any more cooling than the current 790. It will gain a lot of internal cylinder area to shed heat. But really, a bigger motor doesn't just automatically make more heat. It depends on how hard you run it. With the longer stroke and more torque it should have an easy time of of it. In equal conditions a smaller lower powered motor has to be run harder to give the same performance as a larger more powerful motor.
According to physics, more power has a direct correlation to more heat , but as you said, the larger area should help offset that. I know given my propensity to pin the throttle on pretty much everything I ride, I'd be running it hard.
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Old 12-27-2012, 10:32 AM   #72183
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Quote:
Originally Posted by Feelers View Post
Does stroking the engine increase the amount of compression? It seems to me that it will. If so, would it still run on normal gasoline?

Would the stroker kit be available as a standalone kit without the big bore? It seems to me that wouldn't be a problem. It might have a tougher time at idle?

Is there an advantage of doing one vs the other assuming very similar final displacement?

I should probably be asking Jeff, but anyone with engine knowledge should be able to help I think.
All things being equal, yes, a longer stroke will increase compression. I will modify the dish in the top of the piston to compensate.

This will never be a kit. In any case it would not work with the stock piston because the stocker is too long below the wrist pin. With the stock piston you could only increase the stroke a couple of millimeters before the bottom of the piston hits the crankshaft. The 790 piston is a much more modern design and has the necessary clearance under the piston.

As far as what is an advantage - It totally depends on what the goals are for the end result. Longer stroke makes more torque at the expense of rpms and vibration and depending on the motor can be more difficult or expensive than just a bigger bore.
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Old 12-27-2012, 11:58 AM   #72184
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Originally Posted by PPCLI-Jim View Post
So when is production gonna start on it?.... I am fairly certain there would be interest in this kit....
Sure, plenty of interest until we start talking about the cost. A bare minimal 900 kit - custom built crank & rod, modified 790 kit, gaskets, etc would run at least $3000. Add a cam, 3rd gear and whatever will be necessary to keep the clutch hooked up and it would easily go $4k-$5k or more.

Nope. Never going to be a kit but if someone else besides me is crazy enough to want to do this I'll be happy to help point them in the right direction.
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Old 12-27-2012, 12:57 PM   #72185
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Quote:
Originally Posted by procycle View Post
whatever will be necessary to keep the clutch hooked up

Nope. Never going to be a kit but if someone else besides me is crazy enough to want to do this I'll be happy to help point them in the right direction.
Reminds me of the 750cc 2 stroke Maico single that Rick Sieman and DirtBike magazine built back in the last century
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Old 12-27-2012, 02:37 PM   #72186
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Quote:
Originally Posted by doug s. View Post
when at top-dead-center, the top of the piston is in the same location as stock, it's yust that the crank journal is closer to the top of the piston, and the rod is shorter; so when the piston is at bottom dead center, it is further away from the top of the cylinder, thus increasing displacement...

doug s.
Your thinking is a little confusing. Using the same rod and piston would make the piston HIGHER in the bore.

Most stroked engines use a shorter compression height on the piston (distance from piston pin centerline to top of piston) to make up the diffrence so the piston doessn't hit the head. A 10mm stroker would have 5 mm more at TDC and 5 mm more at BDC. So- all things being equal you would make the compression height 5 mm shorter so the piston top ends up in the same location. By doing this you increase the stroke but the piston TDC height from crank centerline ends up being the same.

You can use shorter rods but that often leads to piston to crank counterweight clearance problems.

And yes, increasing the stroke increases the compression. More "swept volume" with the same compression volume = higher compression ratio. Compression ratio = Swept Volume + Compression Volume / Compression Volume.
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Old 12-27-2012, 03:15 PM   #72187
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Quote:
Originally Posted by Mongle View Post
Most stroked engines use a shorter compression height on the piston (distance from piston pin centerline to top of piston) to make up the diffrence so the piston doessn't hit the head.
You can't do it that way with most modern motorcycle engines because the wrist pin is already as high in the piston as is practical. There just enough room between the wrist pin and the cylinder head to fit rings with no extra distance available to make a shorter compression height.

Quote:
You can use shorter rods but that often leads to piston to crank counterweight clearance problems.
Which is how this one is done with a 5mm shorter custom made Carillo rod. The thing that made it possible is the 790 piston which has a lot of counterweight clearance with the stock crank.
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Old 12-27-2012, 03:18 PM   #72188
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This post makes me appreciate my simple DR all the more. http://www.advrider.com/forums/showp...3&postcount=33

Every time a new thumper comes out I get excited and think it might be something to replace the DR. Then I read posts like this. All that just to change the oil.
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Old 12-27-2012, 03:23 PM   #72189
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Quote:
Originally Posted by procycle View Post
You can't do it that way with most modern motorcycle engines because the wrist pin is already as high in the piston as is practical. There just enough room between the wrist pin and the cylinder head to fit rings with no extra distance available to make a shorter compression height.

.

Yes, when we stroke the pin ends up being into the oil rings. No biggie though, you run a shorter pin with a support rail. Like so:




Two diffrent ways of getting the same tamato juice! We often go this route to keep rod length longer for geometry and because custom rods are more expensive then the pistons. Of course, you only need one rod...we need 8
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Old 12-27-2012, 06:30 PM   #72190
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Originally Posted by trailrider383 View Post
This post makes me appreciate my simple DR all the more. http://www.advrider.com/forums/showp...3&postcount=33

Every time a new thumper comes out I get excited and think it might be something to replace the DR. Then I read posts like this. All that just to change the oil.
What's the big deal, I use Rotella in my DR.
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Old 12-27-2012, 07:17 PM   #72191
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Quote:
Originally Posted by Mongle View Post

Two diffrent ways of getting the same tamato juice! We often go this route to keep rod length longer for geometry and because custom rods are more expensive then the pistons. Of course, you only need one rod...we need 8
You could also run the stock length rods and use a cylinder spacer on the DR. Just another tomato juice.
We would do that on some bikes that could run stock rods on stroked cranks (two piece rods with plane bearings made it easier)Though somewhere i have a cylinder spacer for a stroked RZ350 laying around.
Not gonna work on your V-8's though
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Old 12-27-2012, 07:19 PM   #72192
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We got many rods from Spade Carillo in the mid 1960's, for small block Fords and Chevys. Never had a rod problem.
They are a thing of beauty. Needing only one is nice. But, you should buy two, just to have one to hold and look at.
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Old 12-27-2012, 10:40 PM   #72193
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Originally Posted by Albie View Post
What's the big deal, I use Rotella in my DR.
Say what?
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Old 12-27-2012, 10:55 PM   #72194
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Quote:
Originally Posted by RichBeBe View Post
You could also run the stock length rods and use a cylinder spacer on the DR. Just another tomato juice.
We would do that on some bikes that could run stock rods on stroked cranks (two piece rods with plane bearings made it easier)Though somewhere i have a cylinder spacer for a stroked RZ350 laying around.
Not gonna work on your V-8's though
Yeah, but if you're after 10 mm more stroke there are multiple problems to overcome after spacing up the cylinder. Cam chain length, upper motor mounts, fitting carburetor and exhaust pipes, oil lines, etc. It's certainly a lot easier with a 2-stroke.
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www.procycle.us - Everything for your DR650 and lots of other great stuff!
DR900 Big Bore Stroker buildup
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Old 12-28-2012, 04:07 AM   #72195
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So the ongoing struggle to get my Safari tank dialed in continues. When I last checked in you guys told me to fill it up and see how far I can get before it craps out. Seemed like the logical thing to do so I filled it with 33 liters (still had some in the tank when I rolled up to the gas station) and rode it for 275 miles. Then 5 miles from town in 108 degree heat, it stopped. It looked like I still had a lot of gas left so the only thing I could do was shake the bike a bit and luckily got some gas into the engine. Once at home I let everything cool down and I sprayed the carb inlet elbow with ALOT of WD40 and proceeded to empty the remaining gas into another container. When the Safari finally ran dry I found that it still had an additional 4 1/4 liters of gas remaining…and I was pissed. So I stripped the tank off grabbed a set of vice grips and cranked the elbow over to the 90 degree position. Was pretty happy that nothing snapped off and after I put the tank back on I re-routed the lines and removed the cheap inline filter I installed.



This is what it looks like now.


Does this look right to you? How bad is that bend in the hose?

Going out for a ride tomorrow and hopefully I can get another 50-60 miles before it goes dry. If not, I might be putting the Safari in the flea market and picking up a Acerbis. I really need the range and don't want to carry external containers if I don't have to.



Stay tuned and thanks again for your help.
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