have 51 hp need 65+ hp

Nitrous Shots for straights?

Does that mean forced induction allowed? :applause:

Now THAT would be cool. Get a turbo'd 650R...:applause:

Now THAT would be cool. Get a turbo'd 650R...:applause:

If 65hp is "competitive", then how 'bout twice that? :applause: It wouldn't be durable, but it would be fun embarrassing CBR600s with a thumper. :p

I have the pro com box and want to go to the vortex or tokyo mods system. After all my mods I noticed a small difference in performance between stock motor and my modded motor, throw in a simple little rev box and hold on, much stronger performance. Now non of my friends will ride my bike.

I'm just getting into the xrr motors, but some things stick out to me. I also agree with the lost velocity and to high octane.

-head work with extremely large valves

Did you also put in stronger valve springs and ti retainers? Are these ti or SS valves? At sustained road race high rpm you may want to go ti.

Did you hog out the port or just blend the bowl into the larger valve seat diameter?

-mikuni hsr 45 mm carb

For the rpm target of this motor this is probably to large. You should target 5-8k rpm due to the valve train limitations (non-roller rocker arms). Personal preference, but I would go with a Keihin FCR41 (since you don't have the mounting limitations of the stock XRR).

-i forgot to say that the combustion chamber is re-shaped for the big valves. not much of a squish band left.

You probably killed some power and compression if you hogged to much out of the chamber. The low psi numbers also seem to dictate you lost some compression.

-power mist to-137 fuel (5% oxygen)

To me the dynol 77 looks the best for your target compression.

-do i need a velocity stack

yes. You may want to experiment with intake length to find the sweet spot for the motor.

- is it possible to remove the counterbalancer and re-balance the crank? how bad does if vibrate?

The target balance point of the motor without balancer should be where you spend the most time. At that rpm you won't vibrate, but I can't promise what it will be like at idle. Make sure the person doing the balancing knows your intended use.

For a big single road racer you would probably be better off and happier keeping the balancer and having the crank treated to knife edging, lightening, polishing, balancing, and a falicon rod. Maybe some flywheel lightening if the prior isn't enough of a weight loss.

- 680cc

What is your cc limit? I thought it was like 660cc.

P.S. Don't worry about peak HP. It only looks good on dyno sheets. Maximize the area under the torque curve. With that light TZ chassis take advantage of your corner speed and utilize the drive of the XRR to get you up to speed quicker out of the corners (this is where the crank lightening can help).

Amazing stuff...

I have a 680cc motor and where I see a big lose in how quick the bike rev's is in the heavier piston (I don't do motard but, have done some track work) weisco 102.4mm piston is heavy. I went steel sleeve and like the idea from another post I read on Nikasil allowing much tighter cylinder clearance. I have ridden XR680cc bike with 16oz off the ballancer and the bike was almost too responsive.

Here is the line up on the next project (Dirt 680cc Baja bike)

Piston; http://www.rosspistons.com/moto_custom_pistons_detail.php?cat=74&item=95

Cylinder; http://www.mt-llc.com/index_home.shtml

Flywheel; http://www.procycle.us:80/

Jeff Homolka


2101 W 10th Unit B

Eugene, OR 97402

(541) 688-9543

Cam: Barnum's Pro mid cam or stage two Hot Cam

Crank: Machine Balenced

Rod; I have a Corillo rod (and it's rebiuldable) but, falicon rod might work on a second engine.

Head work; No porting for the dirt but, I hear some like it for motard. Everyone that I have talked to that races the XR650R says, NO!

Rocker arms would be awesome if they will fit. I have a mill but, I am not that good.

If I went with the above I would go 1mm oversize intake valve to soften the low hit, giving more up top where it would be needed.

I run a 42mm carburetor and it gives more on the dyno then the 40mm did.

I know that Motard and dirt are two different fields but, I thought maybe some of the sources might help for others that are looking to go 680cc.

Rocker arms would be awesome if they will fit. I have a mill but, I am not that good.

Can you further explain this? Are you looking into some kind of roller rocker?

I would love to make roller rockers fit but, I would have to have the facts that they wouldn't come apart.

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