Drag mini bike-Predator RPM issue??

smwtnbndr

smwtnbndr

Member
#1
#1
Setting up a mini to run 125' drag. Here is the motor breakdown:
Predator Hemi, governor removed
ARC connecting rod, piston is flush with top of cylinder
ARC ultralight flywheel
Dyno Cam F275
stainless steel valves
ported and polished head, not a work of art, just smooth
Mikuni Carb 22mm, main jet about 140
Walbro fuel pump, fed in between carb and head
I am running a Comet series 30 torque converter
Yellow springs (3100 rpm) in driver
Yellow spring in driven.
I have extended the shaft on the driven unit to cross over as a jackshaft.
On the jackshaft 17 tooth sprocket
Rear sprocket 60 tooth
12" tire
The bike comes out of the hold nice, but at 125' I'm at 5100 rpm
Having trouble getting this figured out.
The carb is running great, checked different size mains and needle positions.
I'm sure I forgot something, any help is appreciated. I have attached a pic of the bike, older pic.
 
OND

OND

#4
#4
So you think that you need more gear ? Sounds about right, go to a lower gear.

So you want to go smaller on the jack shaft gear or bigger on the rear sprocket.
 
Last edited:
smwtnbndr

smwtnbndr

Member
#9
#9
OND my dilemma is I think it should be turning more RPM's Maybe I'm way off base but how can I get more RPM's out of it in 125'.
You think it could be the torque converter? What would have to change with it?
 
OND

OND

#10
#10
Hopefully DB can tell you about the converter...all I run is clutches...I think you should be turning more rpm's to at the 125 mark....your engine should turn closer to 8000 rpm's.
 
Last edited:
massacre

massacre

#11
#11
needs more gear i would imagine. For 100 foot drags i was using a 11 tooth driver with 72 tooth driven.

Your current ratio with 17/60 is about 3.5 to 1 which is not great for drag racing.
If you can't make the rear sprocket any bigger, another option is to go with a 10 tooth front sprocket, that should get you closer at around 6:1
 
smwtnbndr

smwtnbndr

Member
#12
#12
Good call on the torque converter. I did a little homework today and it seems the torque converter I'm running will max out at about 6,000 rpm. Time to step back and bunt at this point.
 
DBMonster85

DBMonster85

New Member
#13
#13
Yea that's what I'm sayin you can throw all the gear in the world at it but the converter holds the rpm back. All I can say is change the weights to lighter aluminum ones and drill those. Do a little research on that or p.m me and I'll give you some info. I'm not saying gearing doesn't play a factor but 100' drag that torque converter tuned right should do it. I'm no pro by any means but we have already gone through this.
PHP:
 
Last edited:
FirePowerMinis

FirePowerMinis

Active Member
#15
#15
TAV should not be limiting max revs. It may shift early through the middle but once the ratio is topped out, the engine has to climb revs. It's likely needing shorter gearing overall. Yours should go in to overdrive by 10% so add that factor in to the gearing. Is the MPH way too slow? If it's close, you should need a 1/3 smaller drive sprocket
 
massacre

massacre

#17
#17
So all you guys think a 3.5:1 ratio works well for 125 ft drag racing?
This has not been my experience.

Also not sure what kind of T/C you are using but i have not heard of them "maxing out" at a certain rpm. I have heard of them only handling a certain amount of horsepower though.
Could that be the issue?

Have you tried increasing the gear ratio to 6-7:1 to see if that helps?

It's not as simple as just using the tire size and max RPM to get top speed.
The track is 125 feet long, will never get to 80mph in 125 feet.
Actual testing and tuning will be necessary to get this bike dialed in.
 
Last edited:
FirePowerMinis

FirePowerMinis

Active Member
#18
#18
massacre said:
So all you guys think a 3.5:1 ratio works well for 125 ft drag racing?
This has not been my experience.

Also not sure what kind of T/C you are using but i have not heard of them "maxing out" at a certain rpm. I have heard of them only handling a certain amount of horsepower though.
Could that be the issue?

Have you tried increasing the gear ratio to 6-7:1 to see if that helps?

It's not as simple as just using the tire size and max RPM to get top speed.
The track is 125 feet long, will never get to 80mph in 125 feet.
Actual testing and tuning will be necessary to get this bike dialed in.
Click to expand...

I think the OP is the only one who thinks it's remotely viable ratio. But it's not.
The TAV has a nice low gear, the only thing that's making it rideable now. A regular clutch would barely move it at 17/60.

My 80mph mention was the current theoretical speed with the gearing in place. Of course it won't come near that, regardless of gear
 
H

Havasu Dave

Moderator
#19
#19
I know I don't have a racey user name here, but I do know that manufacturers of torque converter sets apply maximum RPM limits on their products. (I've seen them on line in various ads)

I also know that a few members here have posted problems with belt stick at higher RPM. Since "maximum" RPM is generally past the RPM/Torque maximum, (the power curve) it stands to reason that falling torque coupled with belt stick on a torque sensing driven "might" cause some problems.

In real life, these conversations wouldn't even occur without a dyno sheet sitting in front of us, but we know how often those dyno printer ribbons go bad. :thumbsup:

Havasu Dave- (Race Gangster in Cognito, and overall Internet Thug)
 
FirePowerMinis

FirePowerMinis

Active Member
#20
#20
Nice... Math for baseline, track for fine-tuning ;)
These engines have been done to death and back, so fairly accurate curve characteristics can be extrapolated from the specs. Again, for baseline.
Belt stick at high ratio sucks, but I doubt he's near that. A Sharpie mark on the sheaves would tell how much sheave he's using.
I haven't seen an RPM rating on the TAV, but it's on a lot of my big Comet 102/108 stuff. I'd love to see what the mfg says on the TAV
Racer types should likely get a better, more tuneable setup like a Polar or other Jr Dragster stuff, even a 340.
 
You must log in or register to reply here.
Top