• If you currently own, previously owned or want to own an Avalanche, we welcome you to become a member today. Membership is FREE, register now!

Performance Curves??

BooTheRedneck

BooTheRedneck

SM 2003
Full Member
Joined
Apr 29, 2002
Messages
459
Location
Birmingham, Alabama
Does anyone have the performance curves (horespower and torque) for the 8.1L?

Does anyone know the gear ratios in the 3/4 ton transmission?

Gonna do some redneck engineering!! >:D
 
Funny you should ask - all questions answered in this comparison between the truck and marine version of the 8.1L General Motors? new Vortec 8100 V-8 is a combination of tradition, power, and technology. Great read and info - third article has the dyno data.

The gear ratio is either 3.73 [90%] vs 4.10 - they should have all been 4.10's IMHO, much better match for the power curve of the 8.1L and weight of the truck. Of course you get better MPG with the 3.73, but what is MPG with a 7000 lb truck. ;D

from Annual Transmission Review
The view from GM Powertrain

"For 2002, we've introduced the 4L85, an upgrade of the current 4L80, which came about because of the 8.1 liter engine," said Steve Ford, assistant chief engineer at GM Powertrain.

"We had to do some significant torque management limiting the engine control to protect some gears," he explained. "What we've done with the 4L85 is take the turbine shaft that seizes the input torque to the gear box, apply premium plastics, and induction-harden the shaft to handle the torque of that engine."

The overdrive roller clutch assembly also was upgraded so it could handle that increased torque, Ford said. And in the torque converter area, he commented, "We physically had to go in and have the turbine blades glazed to get the strength to handle that power level. We upgraded the gear sets as well by changing the traditional 4-pinion type carrier to a 5-pinion configuration, also to handle the additional torque."

The result allows drivers to use all of the torque the engine makes to launch the vehicle, said Ford. "With the 4L80 we had to torque manage the output, and that did impact some of the performance of the vehicle. Now, rather than using, say, 90 percent of the torque, we use the full 100 percent," he said.
 
I know, here is the ratio's for the 4L65E 3.06:1,1.62:1,1:1,.69:1 - yours will be similar - still looking - patience. ;D

4L80E 2.48 1.48 1 0.75 - best I can find. :rolleyes:

You may want to look at this

"The baddest of the bad, this is a non-electronic 4L80E transmission. Some of the features are: Full manual shifts - the transmission will be in the gear you select and changes to any gear you choose when you move the selector, no computer or wiring needed, Red Eagle clutches and band, Kolene steels, rated at 900 + horse power and has full throttle 3-4 up-shifts. It comes with a non-lock-up torque converter rated at 600 horse power and the core charge is included in the price. This is the transmission that off-roaders, big horse power streeters and racers have been waiting for. Something with an overdrive gear that you can beat the snot out of without giving it or you a runny nose. It just gets comfortable at 900 horse power. The shift kit alone costs us $250.00 and takes a full day to install. All of this can be yours for $2695.00. It doesn't get any better than this folks. If you have over 500 horse power you may want to consider this transmission."
 
From GM:

Transmission
4L60-E ? ? ? ? ? ? ?4L85-E
? ? ? ?
:)1):
? ?
First:
3.06 ? ? ? ? ? ? ? ? ? ?2.48

Second: ? ? ? ? ? ? ?
1.63 ? ? ? ? ? ? ? ? ? ? 1.48

Third:
1.00 ? ? ? ? ? ? ? ? ? ?1.00

Fourth:
0.70 ? ? ? ? ? ? ? ? ? ? 0.75

Reverse:
2.29 ? ? ? ? ? ? ? ? ? ? 2.08


 
I ran the performance curves and the gear ratios in the transmission on a spreadsheet last nite and this is was I inferred from my 'figurin':

1) to stay within the most efficient range insofar as horspower, the 8.1 should shift at 5500 rpms thru all the gears.

2) to stay in the most efficient range insofar as torque, the 8.1 should shift 1-2 = 4000 rpms, 2-3 =3500 rpms, and 3-O.D. = 3500 rpms.

I have always been under the assumption that shifting should occur at the point where the declining horsepower in the present gear would match the rising horsepower of the next gear. Is this correct thinking?

Since torque is what you actually put to the pavement, would it not make better since to shift as stated above but in regards to the torque curves instead of the horsepower curves? :cautious:
 
That is interesting, got to think about it and do some research. You have to have the actual dyno information with the mods that you have to take that approach - my curve looks a little different due to the Whipple and TOG headers. ;D
 
I have a vague, old-school recollection about shifting at points that will put your rpm at the top of the torque curve in the next gear. Clear as mud? In other words, if your torque peak is at 4000 rpm, you should shift at a point that drops your rpm to 4000 rpm at the start of the new gear. At this spot you will usually be building horsepower. Ideally your shift point would also be your peak horsepower rpm. YMMV.
 
Thats about right steelheadchaser.., You want to keep the Engine in its power band between shifts.., so it can pull quickly to its peak for the next gear..., sometimes a little trial, and error is needed to fine the sweet spot.....
 
I'm not sure that the shift point should be at the top of the torque curve. The reason is that if your torque curve falls fast after reaching its peak, then rpms above the peak torque would having diminishing returns. I think shifting should occur when the torque in the next gear would be better than the torque in the present gear. Here is an example:

In second gear (and other gear for that matter), peak engine torque (490 ft-lbs) would be developed at 3500 rpms. Shifting from first to second will reduce engine speed by 40 percent, so the rpms in first gear would need to be 5800 rpms to have a second gear rpm of 3500.
Now the problem is that the torque in first gear at 5800 rpm is down to about 290 ft-lbs.

By waiting to shift out of first into second on the other side of the curve, i.e., the declining side, the engine will be producing better rpms and better torque values in the next gear. But I think you can wait too long.

Shifting out of first at 4000 rpms instead of 5800 rpms, torque in first gear would be 475 ft-lbs, second gear rpms would be 2400 rpms and torque in second would be 480 ft-lbs.

In summary, by waiting to shift so that the rpms in the next gear are at the peak of the torque curve, the present gear may be falling on its face.

To find out where to shift from first to second insofar as torque, one could graph out the torque curve for the two gears. The curves would be identical, except one be lagging on the horizontal scale by 40 percent. Where the two curves meet should be the optimum shift point.

Clear as mud????


Now that I have confused even myself, that brings me back to horespower. Horsepower is really just a function of torque (or vise versa). In theory, shifting should be centered around torque and not hp. So why the obsession with hp and not torque?
 
It gets interesting Boo.., because Transmission gearing..., and Rear End gearing come into play here.., because they are Torque Multipliers..., so things dont always work as they seem they should in the Real world..... As an example.., the engine in My Drag Race car.., made its Peak Torque at 8500 R.P.M....., but for Best times at the track...., I left the line at 11,000 R.P.M.....,and Shifted at 10,500 R.P.M......... I ran a Doug Nash clutchless 5 speed Transmission, and 6.50 Rear End gears..., the Minimum R.P.M drop was on the 1-2 shift, would drop to 9800 R.P.M....., the Maxium R.P.M drop was on the 4-5 shift..., would drop to 8900 R.P.M..... You'll notice.., All the shift points are Above the engines Torque Peak..., but Gearing, and the 5 Speed Trans are a big factor......
 
O.k., forget trying to figure this out on paper, which I'm sure it could be done, but would require a different engineering degree than the one I have. :(

I think as gandolphxx eluded to the best and most efficient approach, have it dyno'ed.
 
BooTheRedneck said:
O.k., forget trying to figure this out on paper, which I'm sure it could be done, but would require a different engineering degree than the one I have. :(

I think as gandolphxx eluded to the best and most efficient approach, have it dyno'ed.
Click to expand...

I believe both shift points could be correct, depending on what you are trying to accomplish. After roadracing for many years, I know for maximum accelleration, you want to shift as the hp begins to fall, so that the next gear the hp is just climbing to the peak again, but for towing or hauling, you could pull easier by shifting around the tourque curve..
 
That is one of the reasons for the detailed "evolutionary" instructions in the HPP3 manual on shift points, they will be different for each vehicle, tires, mods, etc ;D
 
You must log in or register to reply here.
Back
Top