ratio

How do you determine what first gear ratio to start with? Is there some formula for this like weight x HP x weather conditions? Roots vs Screw or do you just simply ask a buddy in the same class as you?? Just trying to understand what to look for rather then just guessing and hope it works out. Thanks

 

Ok thanks basically what im trying to do is play with ratios as the year goes on now that it gets a little cooler you make a little more hp so am I better off to change a ratio up or down to slow wheel speed or keep ratio same and do it with timing just something about pulling a lot of timing out on the starting line is what I don't really like?? I can only imagine what ratio Frankie Taylor had to run that quick!! Lol

 

Ok Mike thanks for the help I think I am trying to pull to much gear but still working the bugs out and trying to find the right ratio for what we are doing. Thanks

 

Its a complicated relationship with ratio and a converter.
I have an aggressive ratio with a loose converter and it launches with a lot of timing in the engine. Generally, I would think it needs more ratio or a tighter stator but its pretty fast with controllable wheelspeed.
I used to think the line was about 10 degrees of timing dumped minimum so you would have a lot of tunability in reserve.
I would go less than that now but if you had a car that would leave with no timing dumped I think in theory you are probably giving up some of the converters advantages.

On the flipside I've seen a few cars that launch with huge timing dumped and they appear to be very powerful but inconsistent and difficult to manage.
Probably, the more aggressive the ratio/stator the more skill required by the tuner to manage it.

 

Screw blower wheel speed?

Again, most of us have close combinations, but have many different wheel speeds at .025, .050 of a second or etc., what have most found to be ultimate at .025. We have settled in at 2200 to 2400rpm for .94 to .96 60's. Most of our local Saturday night tracks can handle that, but no more. Does that seem out of range? 2550 #s with drive-that would help us on converter and rear gear choice-Thanks! Jim

 

Ill guess that 2200-2400 is driveshaft RPMs, Wheelspeed will be different with different rear ends ratios.
Oure car likes aprox 1900 at 0.25 and 2900 at .50
2425 lbs, converter, 5.00 rear gear ratio, procharged hemi 0,96-0,98 60fot, with a 4.57 in the rear we have to lower the drive shaft RPMs to aprox 1600 at 0.25 and 2600 at .50 to avoid tire shake.

 

Curious as to what first gear and rear gear guys run with screw and roots automatic 1/4 mile stuff I know a lot of this stuff is kept some what a secret but I'm sure a screw blower is much diff then a roots?? We run 1. 44, 1. 44 with 4. 30 rear with a roots?? Works ok hard to control wheel speed early that's all

 

Have ran a 1.25x1.25 with a 4.10 rear and been 5.70's.
Had a 1.44 and a 4.10 and went 5.70's.
Had a 1.37 and a 4.10 and went 5.70's.
Had a 1.31 and a 4.10 and went 5.70's.

With that said, there were numerous converter changes that went with this data, as expected.
In a perfect world, id run a 3 speed all the time as its quicker through the middle and gives a more mechanical advantage than a 2 speed. Problem for us is with a CS1, you cannot get a compound ratio tall enough to make the 3 speed work (or just I haven't ). We would have to change to a CS2 to get what we'd need, ratio wise. Or the CS1 "N" box, which will house the 19% planetary. Either way, all mounts would change so CS2 is the way to go.

This is all with a 2615lb car and a Screw Blown BAE (C-Rotor).

 

Is anyone paying attention to RPM drop after the shifts or does this take a back seat as long as the wheel speed is good within the 1st second? I would think that the 1.25 and the 1.25 would run better because of less RPM drop? Maybe the converter worked harder with the two speed and this is the reason for the same ET? Did you change the tuneup any also in addition to the converter changes that you mentioned?

 

RPM drop is exactly why I said id rather run a 3 speed. Absolutely take that into consideration. But to be fair, RPM drop with the 2 speed wasn't as terrible as I had imagined before the button was let go. With the 2 speed, it had to be revved up pretty good to run well. We run anywhere between 253 and 258 MPH too, so its no slouch.
Tune up varied very little within these changes. Pump, stator and shim arrangement changed mainly. Motor was pretty close to being the same.

2 speed works, but its not optimal IMO. Plenty of ways to skin the cat.

 

I would think 2 speed would work better because of only one drop in rpm which would mean boost only drops once?? Never tried it but would like to any thoughts would be great