FWIW - I had 4.10s and recently reinstalled 3.73s. I had specific reasons for doing so related to AutoX and Road Racing. But none the less, I have lots of experience with three different gear ratios: 3.55 (stock), 3.73 and 4.10.

IMO 4.10s make first gear very short, i.e. the shift to 2nd is quick. 4.30s would make it worse, its almost as if you'd turn the car into a 4 speed because 1st would be so quick...you could easily start in 2nd.

My advice would be - if you are really considering 4.30s, find someone who has them and try them out. I'm sure they have a lot of "punch" right off idle but ask yourself the question can you live with them day to day. If you put them in and don't like them, its a 5-6 hundered dollar mistake.

In comparing 3.73 to 4.10s I'd say the following: right off idle the 4.10s have a little more punch and throw you back in the seat a bit more. But once you get going there is no difference in the above 3500 rpm. Keep in mind, all shorter gears do is get you into the torque band quicker, once there they do nothing different. To me the 3.73s are much more versatile.

 

It's all a matter of balancing HP required to overcome total drag with HP delivered. HP delivered depends on the engine's torque curve, tire size, gearing, and a few other factors.

If your actual top speed happens to be "power-limited" with (say) 3.55's you'll never see the peak power rpm in top gear. Can't get there. So you end up actually using perhaps only 285 hp vs 300 (I'm using advertised numbers for simplicity).

Suppose you swap to 3.90's or 4.10's (doesn't matter which for the purposes of illustration here). It might be possible that you've now brought the road speed to match the rpm at which the engine develops its full 300 HP. Now that you've got all 300 hp available, with the above numbers it might be worth all of another 2.5 mph. 152.5 vs 150 isn't much more, but it still is more.

Most people do not look at the whole picture, everything (or at least most things) involved, and link it all together. Having just one or two scattered pieces of it is about the right amount of "knowledge" required to keep these discussions from ever ending. Not intended as flame, just that I've been tinkering with this whole topic for longer than you'd likely guess.


Norm

 

Thanks for all your input.. AGAIN i ask and specifically NORM! with your enginering knowledge would 430s not create MORE stress on the drive line? more multiplication= more stress???? more wear and tear???
Thanks

 

why not just get 5.10s, where the tach never leaves 5-6k with every gear change? lol.

 

I think you need to divide the question right at the ring&pinion itself, since the torque multiplication only applies to the "downstream" pieces (ring gear, differential/spool, axle shafts, wheels, and tires). A r&p does not multiply the engine output itself; only the torque delivered to the wheels.

If you then define whether the driving either requests the same torque output from the engine (resulting in slightly greater acceleration) or less (in order to match some particular value of acceleration seen with the numerically lower gears). WOT falls into the first case, normal driving in heavy traffic approaches the second.

In the first case, everything ahead of the r&p will experience the same stress as before, ignoring differences in friction and efficiency with the 4.10's vs that with the numerically lower gears that they replaced. Everything downstream will be under somewhat greater stress.

In the second case, the downstream pieces are being stressed the same as before, while the upstream pieces are under less stress. Same deal concerning r&p friction & efficiency.

That's the general situation assuming that you're in the same gear with either axle gear. For those relatively brief ranges of mph where you are not normally in the same gear with both axle gearsets, you'd consider the transmission internals as another place to divide the analysis. Engine/clutch or TC/input shaft vs output shaft/U-joints/driveshaft/pinion gear, and considering the appropriate r&p effects. ± friction & efficiency details here, too. Plotted against mph, the arrangement that stresses various drivetrain parts more can be expected to jump back and forth a bit.

I have intentionally ignored the possibility of wheelspin. Even though this effect defines an upper limit to the axle torque actually delivered to the wheels (and hence can be thought of as a "torque fuse" in that it limits axle/wheel/tire stress), it complicates the discussion more than is necessary here. But it does suggest that you're likely "over-geared" for your tires and pavement conditions if you can get wheelspin from pure engine torque in more than one gear - IOW, from just squeezing the throttle down and not using clutch-drop/loose TC effects/other means to "help" get them spinning.


Norm

 

hey Norm, sorry to sound like a broken record-

but what are your opinions of 4.10s with a high stall(2800/3000) converter on an automatic?

 

It's been a really long time since I drove a V8/automatic anything, but I suspect that you'll have to learn to be gentle with the throttle at least in low-speed street driving. The stalled torque multiplication is probably greater than 2.2, so you're going to have WAY more WOT torque at the tires in at least first gear than their grip can hold on any normal dry(!) road surface. 2nd is pretty deep in the 5R55S, and even a brief 1.25-ish torque multiplication is like temporarily having 5.88's in the axle with 2nd in the TR3650 manual. Really close to 1st gear in the TR3650 with 3.55's in the axle, actually. Nothing that most enthusiasts can't learn to deal with, though.

I think I'll have to let the guys with recent automatic tranny and any drag racing experience chime in with respect to how that much TC behaves under various conditions when you suddenly give it a bunch of throttle. And on how harshly the downshifts come in. This may be affected by any tune that you might have.

If it's a lockup TC, top speed should not be affected, nor the cruise rpm at any given speed (mph). But if it isn't a lockup, the slip required to actually transmit enough torque to go at whatever speed will reduce the top speed that you can actually achieve from what the simple math predicts (and will run the cruise rpm's up a bit, too).

Late edit - you may develop enough extra heat to require additional tranny fluid cooling. Burnt fluid isn't a good thing.


Norm

 

ha ha wow ok norm thank you for your super reply only can you please dumb it down a little for me i got your part of nothing before the rear end but i cant really break the tires loose in first from a rolling start with 355s unless im in a turn. also i get a slight chirp powershifting 1-2 and 2-3 on the drag track.
So if i may try to summerize what you say that the only increase in stress on the parts infront of the r&p is from being in a higher rpm in one case and better in the other and vise versa in the later. wow im confused.. norm i wish you were my neighbor i could learn alot from you. could you please help us all out with a physics lesson and break it down simpler if possible... Im a little slow sometimes (paramedics you know)

 

ok wow i just read it for the 10th time and i got it.. sorry long day ok so now whats up with torque fuse??? and also what your saying is as long as im not romping on it its easier on the car but if im on it its harder ok ok i got it makes sence too. now according to my thoughts it would be a little steep going to 430s but im in canada and at like 4000ft elevation and so ive got less power up here then most other places so i think with the right tires it would be all good. also i was thinking relocation brackets for the lca's wouldnt that help with makin them hook? but ive been warned it would make the car swirly on the turns. but how so... I drift on ice for fun and it hassnt bit me in the butt yet so couldnt i get away with it besides i only hammer on it in the straights really. I dont like taking turns too fast. things can go really bad really quick.
thanks again norm for sharing your brain.

 

Just an analogy. Like the electrical ones in an older house limit the maximum current, tire grip only allows a certain amount of torque and no more. In either case having some sort of limit can protect other stuff (think axles, U-joints or other things that might break in your powertrain instead of frying a circuit in your house wiring).

If you're not romping on it and just driving at your normal pace, it's easier on the stuff in front of the r&p and about the same for the stuff behind it. If you are getting on it, it'll be about the same for the stuff in front of the r&p and harder on the stuff behind it.


Relo brackets can help the launch or the very beginning of a sudden stomp on the gas (initial load transfer is more rapid through the suspension linkage than it is through the springs and shocks). Mostly what they do is make it more difficult to get wheelspin in the first quarter to maybe half second. Obviously useful at the dragstrip, and they also cover the driver for a little too much enthusiasm (or clumsiness) with the throttle on the street.

The downside that you've apparently been given a hint about is that it changes the rear axle steer geometry and tends to make the tail "looser". Yes, the rear axle actually steers very slightly in turns due to the fact that the car rolls a little and because the suspension links force the axle ends to travel in arcs rather than straight up and down. Not a whole lot, maybe a quarter of a degree toward the inside of the turn, but it's enough that you might well feel a difference under some conditions if it's changed to a different value. Certainly you'd notice if the direction that the axle actually steers is reversed (steering the axle toward the outside of the turn, aka "loose steer"). I doubt that any of the commercially available relo brackets would make the car undriveably loose all by themselves, but you can expect any of them to give a less stable to slightly "nervous" feedback to you if you ever do have to corner a bit harder than normal. Your ice experience could be helpful (keep in mind that, sometimes, poo happens ).


Norm