GEARS with STS TURBO

[creederado]

Take two identical trucks,

both with bolt ons, 3000 stall and sts turbo, 29 " tall tires.

Only differance, One has 4.10s and the other one has 3.73.

I have seen the higher ratios seem to go with turbos better, although I don't understand the concept. Would the 4.10s wind up too fast not taking advantage of the low end torque curve produced by a turbo?
What is the concept?

[Silver_Whistler]

The 4.10 gear ratio is going to get the truck moving faster off the line than the one with 3.73. Also the one with 4.10 gears will get the rpms up higher, faster, getting the turbo to spoolup faster. Then finally, when the truck shifts, the rpms dont drop as far as the one with 3.73's. That will keep the turbo spinning faster.
Faster spin=faster you get boost.

[Defunct SO]

4:10's, stall, the torque of a turbo- it will be a buch of tire smoking fun, but hard to hook up!

[farmtruc]

The turbo needs load. The concept behind taller gears is, the more you load a turbo the more it will spool up. Take a diesel for example, when they start to climb a hill with no load on the trailer, they don't spool up as much and you don't hear the whistle from the turbo. Now load that truck down, and as the truck is trying to climb the hill the turbo will spool up , the engine isn't necessarily making more rpms, it's just being loaded more. Just my opinion.
On the truck you speak of, I've actually been in a race where two identical trucks, even the same color, one with 3.73's and the other with 4.10's, the 4.10's would take the other off the line about 1/2 a truck length, then as mid 1st gear came along, the 3.73 truck would pull ahead a truck length, because he was able to stretch 1st longer, it was like this all the way through third, the 3.73 stayed a head but very little.
On the highway, the 3.73 would pull harder and win. I can vouch for that, my 4.10's were ok on the highway, but nothing compared to the 3.42's I used to have.
Matter of fact, my 3.07's would beat the 3.42's on the highway.
Now add a turbo to that race. I think it would be almost the same, but with the 3.73 truck ahead even more. The 3.73 truck would pull longer and harder, not having to shift, now add a turbo that loves rpms and load.

In my opinion, the 3.73's would win in the 1/4 mile. I think a 3.42 truck with a turbo would beat a 3.73 truck with a turbo in the 1/4 mile as well.
You said the trucks would have a 3000stall converter, that makes up for the take off the 3.73's would lack.

Either way, both will haul ass.

good luck,

allen

[farmtruc]

Just one more thing to prove that load creates boost and not necessarily rpms.

On my dyno, it's a loaded dyno, controlled acceleration, etc. I can put more resisitance than any truck can take, basicacly I can load the drums with enough resistance that your truck will not go anywhere when the pedal is to the floor.

Now on to my point.

I can run a turbo car/truck through a dyno run and choose to run an inertia only run. NO load. The drums are fee spinning with no resistance. The run is over in a heartbeat and the turbo will finally start to spool up at the end of the run. The RPMs rise very quickly, but the torqure curve climbs instead of being flat. Not a true reading.

Now, say I apply or simulate a couple thousand pounds to the drums, the rpms rise a lot slower, and the turbo spools up.
The run takes several seconds and I get a good torque curve on the dyno because I was able to load the turbo.

Just another thought, I'm sure this can be debated, but this is just my experience and why I think load will get the turbo spooling up and why the 3.73 truck will win.

later,

allen

[Tydriver]

wOOt wOOt... Finally a topic I feel qualified to answer. IMO, Farmtruc has the perfect explanation.... Turbos like to be loaded..

One other thing to consider.... I assume we are talking AUTO transmissions by the comment made about 3000 stall speed.

I have a '92 GMC Typhoon and we deal with this issue all the time. The Typhoons run a 700r4 and a fulltime AWD setup. The trans has a 3.06 first gear ratio. Because of this, and the stall convertor its possible to leave with quite a bit of boost (I love the 10# launches). The fact of the matter is, with the steep ass first gear and something like 400hp its quite easy to haze all four tires (BEEN THERE DONE THAT) from a dead stop. While its somewhat fun to do this, its really not beneficial to the 60' times...

Anyhow, cut to the chase, Typhoons come with 3.42 ratio from GM, for the most part 99.97% of these trucks still run that ratio, I have a friend however that swapped in a set of 3.08's front and rear..

The concept is, TURBOS make TORQUE (moreso than horsepower). Anyhow, why not LUG the motor down a bit and put that torque to good use instead of hazing the tires off it. Make use of the mechanical advantage in 1st and 2nd gear instead of skating through first gear into 2nd....

Obviously a fullsize truck is somewhat heavier (typhoons weigh in around 4000#) and may benefit from a gear change to get the heavy ass truck moving. While I'm sure a higher ratio gear will increase the 'sporti-ness' feel of the truck I'm questioning the actual time slip numbers.. (each case will be different)..

I know its not conventional thinking but I can tell you my friend picked up like 3 mph at the big end with the 3.08s so thats gotta be worth something.. Oh, and I forgot to mention, he regularly manages 1.4x 60's as well on drag radials

[creederado]

Just what I figured but wasn't sure.


Thanks for the explaination.

Might go back to my 3.73 if I go the boosted route.