Razor89

I've been trying to figure out if I really would benefit from a different set of gears.

I currently have the 3.33 gears with the Dana 44 and ZF tran.
Please look at my public profile before you just post your opinions on what the best gear ratio should be for a TPI car. My profile is pretty close to my current setup.

This car is for the street with some time on the drag strip and a couple auto Xs. It's not a daily driver and I don't care about the fuel economy.

In the 1\4 mile I trap in at 108mph~ in 4th gear @~4650rpm, High 12 sec.

What do you guys think?

96 lt-4

with all the torque you have i don't think switching gears up or down would help you much if any.if i were to change it i would try a 3.07.a 3.45 may be a smidgen quicker in the 1/4(if you could hook it up) but not worth a swap.with the auto x thrown in you are probably right where you need to be.i am by no means a professional,just my opinion.

Razor89

For the last couple of years I have been thinking the same thing. 3.45 or 3.54 would help a little and might not be worth it.

A couple of knowledgeable sources have said that 3.73 would work great with my mods. I have the money, thats not the problem, I have it because I didn't waste it on something else. I don't want to waste it on gears.

If I decide to go LT1 / MiniRam later on I think it would be a sure deal.

88BlackZ-51

It certainly would be interesting to see you go to a miniram, and a gear and see what e.t.'s your car would achieve. But for now, I think the 3.33 is a good gear for your hp/tq curve's.

What rpm are you changing gears at now? Looking at the graph I am guessing 5000 rpm's.

Razor89

I've been trying to shift at 5,200 rpm but I am to slow. I have my first shift light at 5100 and all four led's start flashing at 5200. By the time I react I'm getting close to 5500 plus.

If I'm not careful I will hit my 6k rev limiter. My TPI will pull hard past 5500.



The graph above shows how bad I am at speed shifting.:o

_twisted_

If you are that slow on the shift , adjust the rpms down on the light until you find your sweet spot for nailing the target shift rpm

Razor89

Thanks, I've tried that but 1st and 2nd are hard for me to time just right. 3rd and 4th I have more time to react.

Razor89

Anybody think I would benefit from the 3.73 gears?

My calculations show that I would shift into 2nd @5.2k 40mph
3rd @60mph
4th @ 83mph and with my current trap speed at 108 I would be at 5.2k also.

Is my math good?

LT4BUD

Normally you would want to trap in 4th gear near the top of your RPM limit....if so, consider this:

(5500/4650)*3.33=3.93

So if you still trapped at the same speed 3.93 would put you at 5500 thru the traps instead of 4650.....

but you should gain a couple of mph, lets say you raise your mph from 108 to 112, then

(112/108)*5500=5700

So I would think 3.92's would be best if you can hook & shift

Good luck



BTW your torque in 1st gear would increase by a factor of (3.92/3.33)= 1.177 or about 17.7% greater

Razor89

Thanks Bud

I agree with your numbers. 392's for drag racing would work maybe.

I do a lot of street driving and it is already embarrassing when I light them up to 6k and haven't traveled 20 feet yet. 17% more tq would be allot of fun.

Lichen

IMHO, you're fine with the gears that you have.

88BlackZ-51

I dont see how the motor pulls hard past 5500k, when at 5500 plus the torque band is dropping like a rainbow, and the hp is really starting to tale off as well. It looks like at 4000 your making around 380rwtq, and at 5500 around 260rwtq. I do agree that you did at quite a few more rpm's with those mods, but its still the typical rwtq tpi rainbow effect.

So what gear are you going with?

Razor89

I wish I had a more current dyno graph. The one in my profile is from the week after I did the H/C swap. The car runs a little better now but I can only see it in the GTech graphs that I have.

I have two tachs in my car and they both have no problem climbing above the 5k line and the car is still pulling. Did you watch the movie file? I know it is hard to see but both the rpm's and mph were still climbing all the way to red line.

LT4BUD

The old question of how to determine the proper shift points....

Here is his dyno curve from his profile..



He has the ZF transmission whose ratios are 2.64, 1.78, 1.30, & 1.00.
Looking at 5500 as a shift point, here is what happens..

On the 1st to 2nd shift the rpms will drop to 5500*(1.78/2.64)=3708rpm. Look at the horspower at 3708 vs 5500....just about the same, so this is a pretty good shift point......notice if he would shift any soon the rpms would fall back below 3700 and his horsepower would be much less than if waits til 5500 to shift...

On the 2nd to 3rd shift the rpms will drop to 5500*(1.30/1.78)=4016rpm. Again looking at the horsepower curve the power will go up just a little...not a bad shift point, but a slightly lower shift point would be better..

On the 3rd to 4th shift the rpms will drop to 5500*(1.00/1.30)=4240 rpm. Again looking at the horsepower curve the power goes up even a little more...starting to be reving too much for optimum, so definitely should shift a little sooner..

Basic rule for shifting is to always be in the gear that puts the engine at the highest horsepower point. This maximizes both rear wheel torque and rear wheel horsepower.

This post is getting too long...

88BlackZ-51

I did watch the movie a while back. My car sounded like it was making power all the way up to 5000, but when you looked at the graph it was clear that the peak was from 4000-4500. I still think with your car reving it past 5500 is past the power band.

Razor89

Well 5500 maybe a little stretch but the GTech graph is a more up to date then the dyno graph. I have changed a few things since the dyno was taken.

Don't ask what I have done Rick, I'm not tellin.

staugur

Like most you confuse HP with torque.

tequilaboy

Your gearing is nearly optimized. Choose the gear ratio that puts you at or just past the peak HP at your expected trap speed. LT4BUD is also on the money regarding shift points.

I created this graph to illustrate the effect of 3.33 vs 3.73 gearing. It is based on your dyno data (up to 5500 rpm) with speed vs gear data obtained from an online f-body gear calculator. The plot is cut off at 108 mph to fit your trap speed.



HP vs speed has been plotted for each gear. 3.73 in blue and 3.33 in red in this example.

You can see that 3.73 gains initially, then there are nearly equivalent gains and losses for each gear, with 3.73s loosing out above 95 mph or so. The low speed gains are likely lost in the real world due to wheel spin.

The point at which the like-colored curves cross is the ideal shift point. The ideal shift rpm tends to decrease with each upshift, as is shown by the extra length of the curves remaining after the lines cross.

tequilaboy

Here's the same data, expressed this time in terms of axle torque vs. velocity for the 3.33 gears:



It should be noted that both methods result in the same ideal shift points.

The same chart for 3.73 gears:

Edit: I need to correct the chart. The 3.73 is using the wrong speed data. Here's the corrected chart:



The 3.73 is only superior at low speeds through the mid part of first gear. You can see this on the HP plot where both are overlayed. This level of axle torque likely exceeds the available traction, so the losses that exist at higher speed in 4th gear and lower shift speeds will outweigh any benefit of the increased launch torque with 3.73s.

This also illustrates why higher numerical gearing changes generally only improve 1st gear performance when there is available traction (assuming 4th gear is already in the ball park which is typically true for L98s). Higher revving cars that are over geared (lower numerically) in 4th may benefit on both ends LT1s and LT4s are an example. Flame suit on.

Razor89

Thanks tequilaboy for taking the time to plot this out.

If my car would have been built in 2nd half of 1989 I would not be having this discussion. I think the early 89s sticks have the right gears in them.

I think I have taken care of the 25 to 30 hp loss between 4600 and 5700 rpm but all I have to judge that is my sotp feeling and a before and after GTech graph. I still need to dyno tune the car.

Still on the fence with this mod.

Who knows It maybe only a few trips around the block before this motor gives out. Car is running great now and I plan on driving it hard until she does.

Future plans will require 3.73 at least.