What is the power/reliability pro's and con's of the two? Setup would be drag racing.

690=102mm 11:1, Stage 2 cam, 1mm oversize valves, Porting

730=105mm, 12:1 or higher, con. rod, craycraft cam, 1mm oversize valves, porting

Is the 740 kit also new sleeve and cut cases or is it a stroker as well??

 

Given those two choices, the 730(technically it's 727cc not 730cc) would be both faster and more reliable.

The only difference in the 740, from a 727, is a 106mm piston.

 

730 for reliability... and faster...

 

Thats a basic 690, of course it will be faster, but you can build them to run just as fast....Every build can be done reliabley, it just takes a good builder of the engine to do it...JMO

 

730 will produce more power but set-up will play the biggest role. If you plan to run 300 feet both will run about the same et. If hill shooting the bigger 730 will take you to the top faster. I have seen a lot of 10-1 730's from Bob run and i am starting to think that may be the ticket. Watch the videos of rapfam vs howard stern a few times. Low vs high comp dragger and with a few changes to rapfams(cam, cdi) the results would be different. The 10-1 will take to nos better if you chose that route later on. Rapfam just fills it with super unleaded and rides/races. No issues that the others have.

I would take the gamble on low comp and run for a while. If you don't like it change to a hi-comp piston. The cost savings in fuel will add up fast. The CR cam is the way to go IMO at this point. I have ridden many many different built raptors and run all the cams and at this time the CR cam is it.

 

The addition of a new rod makes the 730 a more reliable choice in your examples, not the larger bore size. The 730 will be faster, I've had both Craycraft builds and I've drag raced other 686s. You will need a few more upgrades to take advantage of that particular 730 build - carbs, intake, dyna ignition, and a better clutch at a minimum.

I don't agree with 14mins about the high comp vs. low comp thing just yet. I think it's harder to tune a high comp motor and he just hasn't seen one completely dialed in yet. Different race fuels act very different in the motor. Now that I have an EGT, I'm learning that it's more than just changing main jets. I have a dyno baseline on my high comp CR730, we'll see where it ends up as I play with fuel, jets, and timing. I do agree with him on the Craycraft cam though, way better than a Stage 2.

You certainly wouldn't be disappointed with a CR730 LCC, Bob's bike was putting the spank on everyone at the Aledo mud bogs. Like 14mins said, it's not a big deal to change the piston if you decide later you want more compression. A higher compression will set you up nicely for alky, something I've been thinking about more and more as I try to noodle through octane/specific gravity/blends of VP race fuel.

 

ubetrn- To me you have the ultimate CR build. No expense spared! Yours is going to shine on the long hill shoots. I meant to say that group all the choices together and in 300 feet the times will real close. It will be set-up and reaction times.

All said and done the higher comp 730's should produce 5 to 7 hp more than the low comp 730's and after 2/3rds the way up the hill ubtrun will be smiling.

THE ROD CHANGE IS MANDANTORY on these higher hp quads now. I run the higher comp in my stock bore for the smell of the fuel[img]i/expressions/face-icon-small-smile.gif[/img] I have run against many of the CR 730's and once they get traction i am ducking for cover

 

I was thinking about taking out a loan and building a LCC 730, but man do those numbers add up FAST

 

how much is the build for the CR730 LC???