Right now I'm using 3.42 gears in a 6-speed car and 275 40 17 BFG Dr's that measure out at 24.5in tall.

The car feels like it stops pulling about 100ft before the traps, and is going through at about 6,300 rpm's. Horsepower looks like this-

5,350rpm's 711rwhp

5,500rpm's 702rwhp

6,000rpm's 660rwhp

6,300rpm's 600rwhp

Any opinions on what RPM I would need to go through the traps to get the highest MPH?

I'm thinking 3.08's would be ideal, but might be a total biotch to get out of the hole.

 

yea, thats a nasty drop off.. 100rw down!

you may just have to deal with it tho. 3.23 would help, but like you said.. getting out of the hole might be tough. you would be turning around 6k with 3.23s..

thats why i run a 28" tire at the track.

 

I would try running alittle more tire first. You are running such a small tire, you can experiment with 26's and 28's too see how it effects it. I bet with a 28 tire, it will increase the load and help ya MPH down the track!!!

Chris

 

Shouldn't you be looking at your Torque curve, not HP? I would think you would want to go through the traps in 4th at the exact point in your RPM range where the twisting force at the wheels in 4th is just about to drop below the twisting force you would get if you shifted into 5th.

 

With a turbo, though, the torque curve is very, very wide so you don't really need to look at it when figuring this. I'm with Chris.......a taller tire would do two things......bring your trap rpm down a few hundred and also give you more sidewall to absorb the shock of the launch. The question is if there is a tire available like that.

 

It may be wide, but for maximum acceleration and trap speed I do not see why you would shift until such a point where the TQ twisting the wheels is about to drop below what it would be in the next higher gear. This is ignoring other issues such as the bottom end grenading due to over-reving.

This link explains what I am talking about http://www.datsuns.com/Tech/whentoshift.htm

As far as I can see, you would want to use as MUCH of 4th gear as possible. If you cross the traps before your shift point, then you are being wasteful since you could have put a shorter gear in your rear end, gained twisting force at the wheels in all 4 gears (traction nonwithstanding), and thus increased acceleration.

 

draco-
you are right, you want to run out 4th gear as long as possible(INSTEAD of going into 5th)

thats why he wants to add some more rear gear, so he doesn't have to go to 5th because he is way past his power peak.

not sure what that link says, but if it says something about "peak tq" its wrong. Peak torque doesn't really matter when it comes to shifting. you want to maxamize the area under the HP curve over a run.

this is generally achieved by running slightly past your peak HP through the traps. sometimes its not that easy, as you also have to gear the car to get out of the hole.

basically, peak TQ doesn' matter when it comes to racing. HP is what makes you go fast, and for that matter.. HP is calculated off TQ.. so TQ does matter, just not PEAK TQ


even if you peak TQ is 100ftlbs, if you can rev to 20K(aka, F1 motors) you will haul ***

 

Yeah the link isn't talking about peak torque at all, but instead maximizing the area under the TQ curve.

Personally, to trap at maximum possible speed in 4th I would choose a gear that leaves me crossing the line at a TQ number 33% below my peak TQ (since 4th gear at 1.00 is ~33% higher then 5th gear at 0.74 OD).

Since I make ~460 peak rwtq, I would pick a gear/tire combo that lets me cross the traps at about 310 rwtq (not sure what RPM that would be tho since I havent dyno'd that high). This way I can know that I have used ALL 4 of my gears to the maximum.

 

He made peak torque at 4800rpm. He's not going to want to cross the finish line there. The thing would be a slug. For the quickest et and most mph, it's usually best to gear it to cross the traps 500rpm above the peak hp number. In this case, he makes peak power at 5350 so he would want to cross the traps at about 5800-5900rpm.

 

Are you in the top of fourth @ 6300 RPM? Well I would guess so with > 700rwhp!!! You aren't having traction problems off the line with just a DR? But I guess that's off the topic. I know you guys don't like trapping in 5th, but what about throwing like 3.73s in the rear end so that the car can still get out of the hole and on the top end you can end up just at or over the power in 5th gear. However, the added time of shifting one more time in the quarter may not quite make up for the extra steam, plus 5th is overdriven and it just may not work.

What if you went with a wrinkle wall style tire? I've been told that it will make you come off the line better because it is collapsed on take off and then expands and gives a little more gear on top. However, not exactly street car stuff.

 

racr4jc: A wot upshift into 5th would more than likely end up with a broken T56. Those things will take some abuse in 1-4, but 5th has much less torque capacity. It may not do it on the DR's, but I'd put money it'd let go on the first pass with slicks. Also, a wrinkle wall tire will grow about .3 to .5" which ends up giving you LESS effective gearing not more(unless you actually meant it would give you more usuable rpm range and then yes in this case you're absolutely correct).

 

ditto what shane said

 

I started looking into tires about a month ago and spoke to the guys at Hoosier and M/T and one of them (can't remember which) told me that for every inch taller you go in a tire, your rpm will decrease by ~ 300 rpm.

Marc