489 R&P, good choice?

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Rapid Robert

Rapid Robert

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circle track with welded spiders. I have a 489 pumpkin & a line on some 5:13's. is this tapered pin (iirc) unit an adequate choice? thank you for your time. RR
 
I have no issue with the 489 part, but welded and 5.13 what are you going to do with the car.
 
i would look at a gearing calculator and compare that to what your motor set up is before committing the $$ to something that is as critical as that is to overall performance of the car.

depending on your tire size, with the 5.13's your 360 2bbl might run out of breath half way to the next corner.
 
Rapid Robert said:
OK I will dig in further
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You have to know, you can not guess at what your lowest corner rpm is and what your highest rpm is before going into the corner.

I haven’t tuned anything that turns corners for 15 years but only one guy in 25 knows what those rpm numbers are.

If your lowest rpm is 3500 and your highest is 6500 you can figure the gear ratio.

Really, the lowest rpm is for you to tune from. It doesn’t affect rear axle ratio but that number is critical

Any thing else is just a guess.

I’d say ask around where you race what gear ratio they are using but I doubt they know what the rpm is either.
 
Rapid Robert said:
circle track 1/3 mile high banked dirt track. 360 2bbl 904
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Open wheel?

I only ask as if the 5:13 is only a little off then you could “adjust” final drive desire through wheel height….pending rules and clearance.
 
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I know what I need to do now & I very much appreciate the high level tech and yes it is open wheel P205/70 or 75/15 required. iirc one of the chebby boys said they were running in the fives. back to my original Q, the reason I asked about the 5:13's is I have some 5.57 in there now but I cannot get the axles in all the way. I've never ran em just what I had in my stash. I looked in the sides & I see a round area with a small hole in the middle (that the axle end would butt up against I imagine) which a guy said is normal. Any thoughts? RR
 
If your lowest rpm is 3500 and your highest is 6500 you can figure the gear ratio.

Really, the lowest rpm is for you to tune from. It doesn’t affect rear axle ratio but that number is critical

Any thing else is just a guess.
Click to expand...
If you don't mind what do I do next with this info?
 
Newbomb Turk said:
You have to know, you can not guess at what your lowest corner rpm is and what your highest rpm is before going into the corner.

I haven’t tuned anything that turns corners for 15 years but only one guy in 25 knows what those rpm numbers are.

If your lowest rpm is 3500 and your highest is 6500 you can figure the gear ratio.

Really, the lowest rpm is for you to tune from. It doesn’t affect rear axle ratio but that number is critical

Any thing else is just a guess.

I’d say ask around where you race what gear ratio they are using but I doubt they know what the rpm is either.
Click to expand...
It's not really as simple as the lowest rpm. Much of the time is spent pedaling the car through the middle of the corner and exit depending on how slick the track gets. Your looking more for the rpm when you can get the pedal all the way down. You need to be in the power range of the motor when you get to full throttle.

First you need to figure out what the safe mechanical rpm limit is for your motor. How hard can you safely turn it which out breaking it? Is this number lower than the rule mandated 7000 rpm? If so then set the rev limiter accordingly.
Next find a cam that optimizes that rpm limit.
Last choose a gear that will optimize the power that the motor makes where you need the power.

You may hit the rev limiter before you lift for the corner. Through experimentation you will have to decide if the power off the corner offsets what you lose hitting the chip. We see this scenario with the crate motors all the time. They almost always are faster trading hitting the chip for more power off the corner.

What track is it? Eagle? What is your safe rpm limit? Does your cam match your rpm limit? are you on 70's or 75's? Does your driver have experience? Is he or she a top five or top 10 finisher or rookie? How heavy is the car with driver?

I have customers at the tracks near you. If you answer these questions I think we can get you a good starting place gear ratio.

The top 4 finishers each night are likely going to be within one ratio of each other when comparing crate to crate and open to open.

You should be on a floater. :)
 
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