Tire swap or gear swap??

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RockinRobin

RockinRobin

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My 70 Duster runs out of rpm about 2 seconds before the 1/4 mile traps. Currently have 4.56 gears and 12.0 x 29.0 x 15" slicks. Nobody makes a 12.0 x 30 or 31" tall slick. Next step up is 13.0 x 31.0, but only tube type are available, so I would be buying tires and tubes (currently running tubeless Hoosiers). I don't trust myself to get the pre load right, so I would spend some $$ to have someone change out the gears for me.
In the current setup It runs 6.60 1/8th mile consistently and 10.2/130mph 1/4 mile with me backing off the last 2 seconds.
I can't decide whether to do the tire swap or the gear swap. What would you do if it were your drag car??
 
You will have a hard time fitting 28" or taller tires in a stock A-body wheelwell. I think its time for 4.30's .
 
69MOPE said:
You will have a hard time fitting 28" or taller tires in a stock A-body wheelwell. I think its time for 4.30's .
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It has been narrowed and full tubbed. Used to have 31's on it when I bought it, should have stayed with that size.....
 
I would do the gears. Especially when finding the right slicks are an issue.
Also, less rubber to fling around and haul down the track should be worth some ET.
 
What rpm are you shifting at? 2 seconds seam like a long time. going from 4.56 to 4.30s going to be enough?
 
Cudafever said:
What rpm are you shifting at? 2 seconds seam like a long time. going from 4.56 to 4.30s going to be enough?
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The rev limiter is set at 7200rpm and the shift light is at 6800, with reaction time, I'm shifting right around 7000.
 
Cudafever said:
I like that calculator, the only thing that sucks is that you can't pick a lower gear set then 4:10
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You can if you edit the gears; just click the box that has the gear neumerical value with the bullet point to the left. Add what you want.
 
What rpm is it rolling over at? From what I see with the 7k shift point, you're close with the gearing now?

If you don't need more tire, I'd keep the 29's and go to a 4.30 if she's only pulling at 6500. If it's done around 6200/6300, I'd drop it to a 4.10.

As long as you're looking to be a consistent racer and not set any records :), I'd go conservative with the gearing to save the engine over a season. Might not be as fast as possible, but the win lights and less maintenance is good stuff.....



P.S. I knew guys, many moons ago, that ran big blocks capable of 7k and geared them to shift & stripe in the high 5's. They'd last 3, or even 4 seasons with nothing more then oil changes, plugs, and a valve spring change or two.....:). And they were very competitive running bracket points.....
 
If you can fit it, I would go with a taller tire....bigger contact patch.
 
Rule of thumb is to cross the line 200 rpm's before shift point. That way you are still pulling if the race is tight. You will then have the choice to tap the brake if you think you have the win to avoid break-out, or keep it pulling hard as you cross the line. Easier on the engine if it does not over rev too. If traction is not a problem, go down on the gearing. If traction is a concern, go up on the tire for more tire contact.
 
UPDATE:
Went with Hoosier 31.5 x 12.5 x 15 tubeless slicks.
mph is now up to 132-133 which is where I thought it should be, but I can't get the car to hook. It either spins or hops. 60' times are down. since these tires are 3.5" taller, that should have raised the axle off the ground by1.75". I'm thinking I need to raise the front to compensate. should be a simple torsion bar adjustment. Also getting some new adjustable rear shocks from Calvert.
 
drop to a 4.10 gear, I was running 4.56 in my car and 28.5 slicks and I changed to 4.30's and still ran out of motor right before the traps so ended up with 4.10's and worked great went a bit slower but saved ragging my motor out. You should save a bit changing gears because new slicks aren't cheap.
 
RockinRobin said:
UPDATE:
Went with Hoosier 31.5 x 12.5 x 15 tubeless slicks.
mph is now up to 132-133 which is where I thought it should be, but I can't get the car to hook. It either spins or hops. 60' times are down. since these tires are 3.5" taller, that should have raised the axle off the ground by1.75". I'm thinking I need to raise the front to compensate. should be a simple torsion bar adjustment. Also getting some new adjustable rear shocks from Calvert.
Click to expand...

If your other tires were hooking, these new ones should! they have more contact patch.

They may like different tire pressure then what you have been doing. probably less psi.
 
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