my combo was originally built for 3.73 gears. I just swapped out my 3.90 gears for 3.73. I had problems with the 3.90's making a lot of whining noise. But anyways my 1/4mile times went from low 11.70's to mid to high 11.90's. Is that normal when swapping from 3.90 to 3.73. I didn't think it would be that much of a change

 

Did you take the car out of its power band? What RPM are you going thru the traps with the 3.90's, compared to the 3.73's?

 

go to 4.11s and you will probably pick up 2 tenths its all in the gearing

 

going through the traps at 5700-5800. I am shifting much higher than I was before. It was 6000 before and now its at 6500 I am shifting

 

I'd suspect that the 2nd quote explains more of the performance difference than the first.

Not alot of sense in making 2 changes at the same time, then blaming only one of them for the results. Un-confound the experiment by restoring one of the confounding variables to its former value. In this case, shift the same as before; then, do what LOGIC would dictate given the gearing change effect, which is, shift at a SLIGHTLY LOWER RPM, rather than a MUCH HIGHER one. That's backwards.

 

I tried shifting at several different rpms. It seemed to run best at 6500

 

Seeing that you have a Miniram, that explains the high shift point. I shift at 5700 and go thru the traps at 5800-6k on a 26" slick.

What size tires are you running? Did your 60ft times change?

 

running mt et streets 26X11.50X16. My 60ft are in the mid 1.6's. I use to have them down to 1.53 ish. Thats when I had an electric water pump. Anyone think going to a 28X11.5X16 would make any difference?

 

i am wondering if my combo is mismatched a bit. My goal was to get a 11.5x timeslip. Looks like I am getting farther from that. Anyone have any suggestions (thats not a power adder) to get down to my goal??

 

Well, a 3.90 gear will launch harder than a 3.70.....that explains the 1 tenth lost there. A taller tire will lower your rpm at the traps, but it will also allow you to shift back at the 6k range you were before.

 

I hear ya, my goal is 11.5's also. I'm actually changing to a 27" Hoosier QTP....6k is beyond the power band of my 383 HSR...I need to shift and hit the traps at 5700, and I need a little more MPH.

 

on my dyno sheet my max hp is at 5800. My ideal shift point would be around 600-6200. I am debating at getting 28 inch tall mt et streets. You think that would make much of a difference between my 26 inch et streets?

 

IMHO, I think you're shifting to high. For every inch taller in tire size you will lower your RPM by 250, and vise versa. So you would be knocking off around 400-500 rpm with the 28" tire.

If your max is 5800, why would you shift 400rpm past the peak?

 

Why is that? Your heads/cam setup and compression should allow for easily 6500 rpm range?

Shouldnt be shifting higher with the 3.73's now over the 3.90's. That isnt a big change in gear. taller tire is just gonna make it worse. taller tire reduces effective gear ratio. So now its gonna feel like you went from 3.90's to 3.42's. I'd keep the same tire and keep working on the launch. I think the combo is ok, maybe better with a TAD bit less stall, or keep the converter and go next size up on the cam. Something in the mid 230's deg duration. It should peak near 6000-6200 then and give you abit more room

how did the weather compare from before and after. Air quality can easily make a 2 tenth difference.

 

I agree the taller tire is probably heading in the wrong direction, and weather makes a big difference in ET. All that has to be accounted for.

Maybe you should have kept the 3.90's and just wore ear plugs...problem solved!

I thought so also.....but when I ran last time, my best ET came from a 5800 shifts. However, the earlier I shifted the more my MPH dropped. From what I've read the HSR is pretty much out of breath after 6K. But honestly, I'm still playing with my setup so my car is still being tuned/adjusted. That's just what Ive come up with so far.

Orr, What shift points, gears, tires size were you running with your stroker motor?

 

IS your HSR ported at all? I ported mine out alittle ways, gasket matched to the heads and with the cam I had in that thing, it peaked at 6250 rpm, held flat to about 6500-6600 or so. I shifted at 6600 sometimes, sometimes 6800. Didnt seem to make any difference in ET or MPH. Car had a broad top end.

Dyno I'm not sure why it only ran to 6500-6600 because I had the limiter set to 7K and we hit the limiter on the dyno run, so it may have held power to 7K but only showed 6600 on the chart?

MPH dropping off with different shifts is a sign that its losing power and needed to be shifted higher. ET changing tho may be gearing? I'm not sure.

 

Orr, its not ported at all. Seeing that I haven't dyno'd my car I really don't know the exact power band. I'm just guessing off the time slips and also from data logs. I'm waiting till some of this extreme heat passes then I'm back at the track.

Joeblue, maybe you should make another trip to the track and try to duplicate or better the results. That's what I need to do. That way you don't go buying tires, etc, when maybe.....that's not the fix. And I've learned the hard way, make one change at a time, come to a conclusion and then keep moving.

 

This is the norm for drag racing.
But ideally, you'd get a dyno sheet for your engine, find out the gear ratios in your manual transmission, then calculate your shift points.
Ideally, you want to shift high enough that your driveshaft torque after the upshift is the same as it was before the upshift.
And this will typically be at much higher RPM than your peak HP.
Shifting at HP peak, you'll have less driveshaft torque after the upshift.
All this and more can be found in John Lawlor's Auto Math Handbook.
However, you may discover the need to upgrade your valvesprings, possibly your pushrods, and maybe your rod bolts.
Many racers gear the car to be past the HP peak through the traps also. As your revs go up, your torque drops off, but if you gain more multiplication than you lose in engine output, then it's still a net gain.
The goal is the highest possible average torque through the driveshaft for the entire 1/4 mile.
If this is a mostly track car, then I'd do a 4.30:1 gear and the 28" tires.

 

Get a good datalog and send me the file. I'll take a look and see what its doing. Dyno is very helpful in determining powerband and your air fuel ratios with their wideband. Find a shop, do 3 pulls for 60-70 bucks.

 

I emailed them to you.....and yeah, I definitely need to hit up a dyno soon.

 

Thanks for the info, I always wondered about this.....learned something new today.

 

I have been having problems with my car starting. It actually would not start at the track last week. When I got it home I opened up the distributor, and found rust everywhere, and on everything. So hopefully that could of contributed to my 2 tenths loss last week at the track.

 

That definitely isn't helping ......

 

put in the new distributor. Took a couple pics of the old one. Anyone know if this would affect my 1/4 time?

 

might not be getting complete spark to the plug, so mixture may not be igniting completely. This would likely have shown up with how the motor was running at higher rpms... basically missing or breaking up. If it didnt do that, then it may not be hurting anything but it certainly doesnt look good.