2_point8_boy

I need to know if anyone has a chart of some kind showing the power curve of a STOCK 2.8L

I'm going to be getting my tranny rebuilt and am considering a taller OD gear than the .76 that's in there. I'm thinking of stepping up to a .63 ratio for freeway cruising and I need to know if that'll be too steep for this little motor to handle.

Thanks.

Fast355

iTrader: (2)

You might consider it, if the area you live in is very flat.

Thunderfest

I would not recomend that at all. Why?
If you do that then the trans will downshift at the lightest load (say freeway crusing onto a small section up uphill grade) it will modulate the rpms high in 3rd and back down low to fourth- you will contantly be modulating.

It is far better to drop the rearend gear and keep the ration in the trans in sync.

2_point8_boy

Well, I've got a manual trans, so it better not be shifting unless I tell it to...lol

Thunderfest

Yes, My bad Generally when I see OD I assume someone talking about an auto.

Right now if you have 3.42 rears, you are cruising 1:1 in 4th gear 70mph @ 3100/ then drop to 5th @2350rpms

That change in 5th gear from .76 to a .63 will drop 70mph 5th to 1950 rpms. You have to now do about 85 to match the same rpm levels as you were in the previous 5th gear ratio. Its much too tall for the little V6 and you'll render 5th useless.


I have a .70 OD with 3.23 rears and I can not drive 70 in OD without it lagging. I stay in D to keep the motor purring in a decent powerband and gasmilage at that speed. I have the 3.23's because I mostly cruise about 85 in this car @ about 2500 rpms

edit- I will also add that 1:1 (Drive) for me @ 70 is 2950rpms since I have the 3.23's. (85 is 3600, 110mph is 4600) 1:1 is a good torque gear and is very usable in my car.

The real powerband in these cars stock is about 3000-4200 and thats it.

Naft

QFT.

Dale

iTrader: (12)

While were on the subject of gears and rpm bands n such.

Stock 5sp, but I'm running the auto 3.23 gears. I forget what RPM I'm running at 70(in 5th), but it seems alot.


I want to verify, that once I put the "proper" 3.42, my rpms will lower in 5th, but slight raise in all other gears correct?

2_point8_boy

The Lower the rear end gears, the higher the RPMs will be.

The whole reason I want to go to taller gears is to get milage. 5th gear is not a passing gear, so I'm not too worried about being able to get on it and move, if I want that, I'll downshift. I just want to turn less than 2500 while doing 70mph. I just want to make sure that on most roads, I'll be able to stay there. I don't mind the occasional downsift to speed up.

Fast355

iTrader: (2)

With the tires/rear-end gear ratio/.7 OD of the 700r4, my 1987 GMC Jimmy would pull just fine in OD going down the highway on FLAT road with no headwind. The engine would turn about 2,200 RPM @70. That is in a 4,500 lbs Brick of a 4x4 Jimmy.

Thunderfest

Jay, The misconception of gearbox ratios is the thought that above 1:1 increases rpms and below 1:1 decreases rpms- It is false. The 1:1 barrier is simply in essence a direct drive ratio, But anything above and belo this ratio will proptionately either climb together, or drop together. One does not go one way and the other the other way aove and below 1:1.

Here's the deal- Any ratio below 1:1 (aka OD) with have a sufficiant torque loss due to gear reduction loss. Torque to the rear wheels is increase the higher above you go from 1:1 through the gearbox. So for example with 6 sppeed trannies, a 6 speed is better geared with a 5th 1:1 rather than a 4th 1:1 like a Richmond in compared top a T56. With a Richmond, you have to run a lower rearend gear to compensate, but you recieve overall less torque loss to the rear wheels overall through both the gearbox and the rearend. SO any time you can keep the car higher than 1:1 (without going into a lower OD gear) you are keeping more TQ to the rear wheels- BUT that torque also slows the car when you lift the throttle.

Dale

iTrader: (12)

Bummer, I thought it would lower the rpm.

O well, gm says it should have 3.42's, it will get them along with a LSD... someday when I find someone to install it all for me.