Info for those wanting to do a long rod motor

This article was sent to me by the person who wrote it, Mike. It was put on another site apparently and I wanted to share the info with you all. I have talked with Mike over the phone several times about this motor and have been looking into build one myself. I know when Hugh posted about a ling rod motor there was a lot of interest.

I left-out Mike's info and signature at the bottom, just in case he didn't want people trying to contact him, but I have been sitting on this info for a while, and apparently he is OK with sharing it now.

Enjoy.

Here ya go:

The quest for the ultimate XS 750 dirt track engine


This project started with the intent on giving our rider an advantage as compared to the competition (XR750) in vintage dirt track racing. As the reader may or may not know this is a real challenge. It is so much of a challenge that the owner (CD Martin) considered the purchase of a XR. CD spent several years in the 80's as an owner/mechanic attending grand nationals, so he had experience with the XR 750. However, he already had two Champion framed XS's one a 700 the other a 750. After much discussion CD decided to stay with the Yamaha's.

We began by purchasing the Engine Modification Guide from Craig Weeks (a must have in my opinion) and searching the web, we began to develop our build plan. The plan included the following.

The Lilly CNC ported head including guides and valves

Megacycle #30 cam with adjustable sprocket

Venolia pistons/perfect circle rings (gapless)

RD valve springs

Honda CR500 rods

Weighted crank, balanced

Barnett clutch and springs

ARD ignition

36mm round slide Mikuni carberators

JEMCO exhaust

So we started by sending the head to Craig's vendor for the porting and guide installation along with early XR 750 valves. Then came the custon Venolia pistons that have the Lilly domes (11.25-1 compression) but with the wrist pin moved upward 9mm compared to the early 256 Yamaha rods and using the late 20mm pins. This was done to accept the Honda CR 500 rods. Why the longer rod you ask, check out http://www.stahlheaders.com/Lit_Rod Length.htm, this will explain the why. Our intent was to smooth out the power as the XS's are known for it's 2 stroke type of power delivery on the dirt. That is the one of the advantages of the XR 750 a smooth power delivery and I believe the long rods are one of the contributing factors. The XR has a rod to stroke ratio of 2.115-1, I believe they, H-D, would have perferred a 2-1 ratio. The following is what I have researched from avaliable off the shelf rods that I am aware of, although a custom rod may also be purchased.

Yamaha early -256 135mm length rod to stroke ratio of 1.824-1

Yamaha -447 130mm length rod to stroke ratio of 1.756-1

Yamaha -533 Europe 140mm length rod to stroke ratio of 1.89-1

Honda CR-500 144mm length rod to stroke ratio of 1.945-1

We used Wiseco Hot Rods for our project, I believe you have to buy a complete crank from Honda as a rod kit is not offered. Also of note due to piston pin location it requires a teflon button because the pin encroches on the oil ring location, these are supplied from Venolia. For those who might want to use these pistons and long rods, order from Venolia job number 135554, these are 80mm bore which is standard 750. The rings I sourced thru Craig Weeks.

As for the crankshaft we installed .9 lb rings each on the two inboard flywheels, they are avaliable from Hoos Racing call Roger Johnson his number is on their web site. The assembly of the crank was done by Chet Wilson Engine service in Wichita Ks. Jerry Wilson runs the shop and is an avid motorcyclest, his dad Chet was the first one to put a V-8 Chevy in a sprint car. As far as the rod instl the small end was bushed to accomodate the 20mm pin and the crank pin was shortened 3mm (compare to the 650 pins to be sure) Jerry also balanced the crank to a factor of 64% (I think that's right, consult Craigs paper). As a final step weld the crank pins to the flywheels, this is a must as they will twist, even on a dirt tracker, I know of thru first hand experience.

Engine assembly is next. The upper and lower cases will need to be relieved to accomodate the flywheel rings, just set the crank in one case at a time to determine where to and how much to relieve, then join both cases with crank installed and check for proper rotation. The rest of the asembly is pretty straight forward just follow the tolerances described in Craig's paper. To get the proper squwish we had to remove .035 thou from the top of the cylinder with no base gasket and a .026 copper head gasket. So it is critical that this is checked. We degreed the cam per Mecacycle specs, only took 9 attempts but that's how it goes.
So, how did it work? As all projects we were behind the 8 ball for time as we wanted to go to the Kansas fair races and it was a last minute thrash. We finally got the whole thing assembled and went to the dyno. Now, we had a Maxxis dirt track tires and they will slip on the dyno, we were lucky enough that they had a rear wheel with a Dunlop CD 8 and they don't slip. This next part is really strange, we could not get the air fuel ratio right on the L-H carb, changed needels, needel jet, pilot jet and main jet to no avail. So we switched carbs left to right and the problem followed the carb. What was happening was a blubber up to about 6,000 rpm or so. It made 68.6 hp with the blubber. We never could find the problem with it, replaced it and no more blubber.
As I said we were short on time so we went to the first race untested, I will never do that again as we fought gremlins for the whole series of races but that is another story. I have to say the thing will really turn the RPM's and seems to be smooth, we geared it too low as we were turning it 9,000 or so. The good thing is our rider liked it as the power was smoother then the short rod he was used to riding, as he said it took the 2 stroke hit out of it. We still have things to do and I believe more power is there. Might try flat slide Mikuni's and a # 40 cam, I would think the the power would be somewhere in the 70's.

Is the installation of the long rods worth the effort? In my opinion, yes. I am so convinced I think I will put one in my TT-500 based dirt tracker.

http://www.xs650.com/forum/showthread.php?t=14124&highlight=CR500
Check out this thread it also got some good info in it

Patch....... I'm quite surprised Mike let loose of this info myself and he said anyone needed any assistance to contact him.
And remember that welded up bath tub chamber I was working on for my long rod engine? Well I got a call the other night,did two more pulls at 28" and on a 3" fixture,D-shaped intake port(port volume "SMALL") Still needs a little more work to get the #s up at 100&200 from 4th pull to get the E/I flow ratio down to around 85 to 90%. I'm extremely pumped up now and can't believe with all that welding shrouding the valves hasn't compromised the heads breathing abilities.
Third pull
100 91
200 132
300 170
400 185
480 198
LIFT CFM
Fourth Pull
100 80
200 119
300 181
400 201
480 205

Exhaust
100 79
200 106
300 126
400 140
480 142

Jack-

It sounds like he was waiting to finish this article before he released all of the info. I don't blame him, these guys had a bunch of work in this motor and I'd be a little pissed if someone published the info without consent and did none of the work....

Patches said:

Is the installation of the long rods worth the effort? In my opinion, yes. I am so convinced I think I will put one in my TT-500 based dirt tracker.

Click to expand...

I thought I had already de-bunked the myth of long rod engines.

Spare yourself the effort of building a long rod engine to see the difference in piston movement. There's this branch of math called "trigonometry". You can use it to see the effect of of a long rod vs a short rod without having to spend a dime.

You want to know the difference in piston movement between a stock and long rod at 10 deg ATDC?? .0004"

I'm just wondering. In the articles it says that the long rod helps reduce tire spin. "Our intent was to smooth out the power as the XS's are known for it's 2 stroke type of power delivery on the dirt."
I raced 80cc indoors on concrete and very little dirt, but I am familiar with coming out of a corner using throttle finese to keep from breaking loose or standing up as you jump back on the power.
So, what would be the advantage of doing this to a street bike on asphalt? I like the idea, but it seems to be mainly a dirt application.

littlebill31 said:

So, what would be the advantage of doing this to a street bike on asphalt? I like the idea, but it seems to be mainly a dirt application.

Click to expand...

Their are several advantages and that'll depend on how you look at'em from a performance stand point of view vs riding style. First of all if one chooses to use the standard 360 degree firing order and depending on the rod ratio chosen,you can install more modern short skirted piston of greatly reduced reciprocating mass weight which in turn when combined with the longer rod allowing less snapping of the piston from TDC can and will reduce engine vibrations,dependent of balance factor choice of course. For example the piston gram weight for my L/R combination will be roughly 220 grams vs 301 for a stock XS pistons,that's a significant amount of reciprocating mass lose that effectively reduces the inertia loading at the small end of the rod,less stress loading on the crank at higher RPMs and quicker revving that's more controllable at the throttle in my opinion. The engine torque curve across the board is flatter offering a broader smoother power band allowing the engine to really pull like a freight train at increased RPMs,you have to run the engine tighter which might be a disadvantage to some.What's really neat is the exhaust pulse sound changes to a more deeper throatier sound,that's alone is worth at least 10HP Longer rods are more forgiven on intakes ports that are lazy in the flow department and their are pumping that occur during low RPM operations but you can somewhat over come those loses if you have the know how but overall L/Rs pull harder on the intakes after mid stroke. You have less piston skirt load during thrust thus adding to friction reduction, more stabilized piston to enhance ring seal.
I hope this answers some of your question and one more thing,for asphalt racing,whether it be on the tract or the street,L/Rs will allow the engine to pull real hard down the straights to overcome those bigger engines using shorter rods,case in point check out www.jsmotorsports.com. Jim has a piston and rod combination that takes the Norton R/R of 1.63 to 1.8 plus and has taken the Norton engine performance to a new level with some of fasted Nortons to ever hit the tract in years. i'll post a vid of a 750 that kicks butt

Jack said:

Their are several advantages and that'll depend on how you look at'em from a performance stand point of view vs riding style. First of all if one chooses to use the standard 360 degree firing order and depending on the rod ratio chosen,you can install more modern short skirted piston of greatly reduced reciprocating mass weight which in turn when combined with the longer rod allowing less snapping of the piston from TDC can and will reduce engine vibrations,dependent of balance factor choice of course. For example the piston gram weight for my L/R combination will be roughly 220 grams vs 301 for a stock XS pistons,that's a significant amount of reciprocating mass lose that effectively reduces the inertia loading at the small end of the rod,less stress loading on the crank at higher RPMs and quicker revving that's more controllable at the throttle in my opinion. The engine torque curve across the board is flatter offering a broader smoother power band allowing the engine to really pull like a freight train at increased RPMs,you have to run the engine tighter which might be a disadvantage to some.What's really neat is the exhaust pulse sound changes to a more deeper throatier sound,that's alone is worth at least 10HP Longer rods are more forgiven on intakes ports that are lazy in the flow department and their are pumping that occur during low RPM operations but you can somewhat over come those loses if you have the know how but overall L/Rs pull harder on the intakes after mid stroke. You have less piston skirt load during thrust thus adding to friction reduction, more stabilized piston to enhance ring seal.
I hope this answers some of your question and one more thing,for asphalt racing,whether it be on the tract or the street,L/Rs will allow the engine to pull real hard down the straights to overcome those bigger engines using shorter rods,case in point check out www.jsmotorsports.com. Jim has a piston and rod combination that takes the Norton R/R of 1.63 to 1.8 plus and has taken the Norton engine performance to a new level with some of fasted Nortons to ever hit the tract in years. i'll post a vid of a 750 that kicks butt

Click to expand...

pfft!! more theory with no hard data to back it up.

Explain to me again how a .0004" difference in piston movement creates these massive power gains??