Need some opinions from those in the know.

Currently I’ve got 3.73 rear gears in Hemi Hunter. Did it hoping to keep rpm down and the engine live a long happy life. I’ve had a couple people suggest I might be better off with a bit more gear. 4.11 or so? Slicks are 30 inches tall. Any help would be appreciated.

 

I went the other direction. 3.89 to 3.50 to get rpm down thru the traps. Didn't like seeing 6600 thru traps.....motor has a boatload of runs on it....

 

I run 29s. With 4.10 and my converter slip i saw 6400-6500 in the quarter, with 4.56 I see just about 6800 This isn't digitally logged just what I see the tach at

 

What are you seeing now

 

3.73 to 4.10 is about 400 RPM at the 1/4 mile stripe.

This will vary some depending the characteristics of your converter.

Bruce, What was the decrease in RPM with the 3.50 ?

 

Only bout 300rpm Gary.....lost .03 in 60ft and .10 in quarter. Have to slow it down for 10.00 anyway.....

 

Well that math adds up seeing I gained about the same rpm from 4.1 to 4.56.

I think a 4.30 for me would be a nice middle

 

Car is 3405 with me in it. Some people have said I might be driving through the converter, that rpm in the traps might actually drop with a bit more gear. I’m about 63/6400 in the traps now. When I did my converter swap I went from 10.60’s to 10.20’s, picked up 6 mph. I wasn’t worried about going faster, just wanted to get the rpm down. It dropped rpm about 100, but picked up a bunch of mph. Fun going faster but didn’t really accomplish my goal of losing some rpm. Guess I shouldn’t fuss and just enjoy the damn thing.

 

I would leave it alone personally

 

There is a roughly accurate formula for converter slip out there base on mph and rpm gear and tire size. I say roughly accurate because as we all should know your slicks will grow in diameter with mph.......so the exact size of your tire isn't truly known at that exact moment.

But normally 6-8% is pretty good

http://www.wallaceracing.com/converter-slip.php

 

With Buick engines being relatively low rpm I always go for a tighter converter. Mine is only 4200 stall. Can't go more than 2000 at starting line. (footbrake)
Sure a 5K conv would launch better but you will drive thru it and not really get much rpm drop on shifts.
I like my motors, transmissions and converters to last many runs.....

 

Keep in mind. A converter's characteristics will change with Density Altitiude.

That's why you'll see guys running new best ETs in the Fall but the MPH doesn't come up accordingly.

 

Gary are you saying the air density affects the convertor directly? Why would that be?

Or maybe you are saying the change the engine performance due to air Density affects the convertor performance (that makes sense to me) ?

 

In my experience the 60ft and 330ft are the most effected numbers on the timeslip in good air.

Closer to, or below sea level like the guys at Cecil get. The engine will make more torque which is why everyone goes quicker in the spring and fall.

Torque creates ET unless you have a ton of available RPM and offset lack of torque with gearing.

I know guys that run heads up, NA classes that have different (looser) converters for the summer vs. the fall to minimize the lost ET when the DA rises.

 

OK, Thanks for that. So in the spring and fall when cool air is denser, using a convertor that grabs at lower RPM is a win. Versus hotter thinner air, where you need the higher RPM.

 

I always pick up mph with good air....but I run fat....

 

I try not to alter my tune too much, hate wrenching at the track......I slow down about .2 in the summer from my previous fall

 

Where does it make peak power at? Get that at the stripe.

 

I run a 31" tire and a 3.90 gear. Car actually picked up going from a 4.10. Last time out went 1.35 60' and 9.98@ 131.It crosses at6350.

 

What gains is this from the 4.10