Say you have 2 cars with the same TT/engine setups but one has 4.10's and other 3.42's. If both are at WOT, will the 4.10's have a significantly better spool time than the 3.42 car?

 

The difference will only be in first gear from a stop. The time difference to spool is the difference of the car with the 4:10s to hit roughly 3000 rpm (full boost) vs. the 3:42 car to hit the same rpms. Shall we say 1/4 of a second difference?

 

with auto trannies, I would say that the 3.42 geared car would spool the turbos quicker than the 4.10 geared car. the 3.42's generate more load. it would probably be a wash with manual trannies.

 

The reason why im asking will probably get alot of WTF's from people...My target is eventually 700 RWHP but will be a street car. I'm not looking to break any ET or lap records, just a combo to cruise around but have fun once or twice a month (Auto-X, track, or drag strip.) I'm curious on how 700 TT'd RWHP would be with an A4, 2.73, ss3200. For road racing, I would think keeping the 2.73 I won't have to worry about the A4 shifting as much as the MPH range with 2 and 3 is very broad. At the same time, 3.42's with 700 RWHP would roast the tires anyways (at 70-140 speeds) so why not take advantage of the broad MPH range with a 2.73.

With a 2.73 I can maybe even get 25+ MPG highway with the built 346 motor as well (not that important but a plus.)

I'm in a brainstorming mode on what combo's I want to do so any comments or feedback is appreciated! But I don't think I've seen anyone try a TT'd 2.73 so I'm sure there's good reason!

 

I think spool time depends on the stall speed of the T.C. I can get the turbos spooled and making boost 5-7# before leaving the line. On the street from a roll with a 2800-3000 stall I can hit peak boost instantly 13-14#'s but not sure what good it does since the tires just break loose and start spinning.

Not sure how it works with a a M6, but it would seem to me the 4:10 with traction to create load would spool the turbo's faster based on the higher rpm range they would have over the 3:42.

 

My guess is that whatever you gain as far as reduced spool time with 4:10's in a TT M6 you loose as soon as you smack the rev limiter. :-)

Mark

 

Nah dude..the higher gears create more load..lower gears just get through the rpms quicker to the rpm where your max spool up occurs.

With a manual, my spool varies a noticiable difference between 3rd and 4th, and 4th and 5th.

For example,
From a roll in third gear the boost comes a little later...but gets to that rpm quicker because it is geared lower..because thats what lower gears do is get the revs up quicker..so it is actually faster.

When in 4th from a roll (at the same speed as I was in 3rd) the boost comes in almost immediately (not full spool) but has more present at a lower rpm but it takes longer to get to full spool because it is a taller gear, thus taking longer to get to the rpm where full spool up occurs..its really not that much different then a NA setup...there is just a lot more tq with a Turbo setup. Lower gears simulate more power because they push through the gear faster...much like having more tq to push the car forward fast.

Tq gets you moving...lower gears get you moving same as having more tq.

If you have ever owned a 4X4 and lifted it and put bigger tires on the vehicle you can certainly tell the difference. My previsous truck was a lifted Dodge Ram. I lifted it and put 35" tires on it, but kept the same gears. It was a total dog because it took more power (tq) to turn those big tires (3:73:1 GR 318cid motot). Well I took it back in and had them install 4:56:1 gears...then it launched very hard out of the hole and power was more readily on hand (or foot).

Now I didn't increase the hp any..just the gears..gears simulate tq low and get into the upper rpms quicker because they are shorter.

Its no different for any other application..except most turbo setups make great tq..so lowering gears is like adding more tq to great tq at a lower rpm...this means traction becomes even more of a problem.

But whether it be a SC/Turbo or a NA setup...lower gears always get you moving harder and quicker out of the hole and keep the rpms up in each perspective gear when cruising around.
I think there are some pros and cons that suit each application differently.

The pros for lower gears on a Turbo setup, to me is the same as for a NA setup. Lower gears serve the purpose of getting through the gear faster and into the higher rpms, and they provide power like adding more torque.

I don't like the word lag...mainly because I feel that it is not lag..it is just not in the boost.
When you press the gas pedal down the time between the rpm the boost comes in at..is ofter referred to as lag..as far as I'm concerned it is just the time the car is running NA....thats why I like having a turbo system that builds boost over a larger rpm range before it gets to full spool...vs a system that just hits like a shot of NOS at the lower RPMs and then dies out in the upper rpms. Anyway.

Having lower gears regardless of NA or FI setup is going to put you in the higher RPMs a lot quicker and takes less power to do it.

But I really enjoy the way my cars have driven on the street with 3.42 gears. If I want to cruise around out of boost the car drives like a stocker until 2200-2400rpms...with eventual max spool occurrying at 3400rpms. If

If I wanted to launch in boost I can release the clutch later 2600rpms and feather the throttle to get into full spool. From a roll I can either pull whatever gear I am in and wait for the boost to kick in...or I can drop it down a gear and be in boost immediately.

I have a CD vid that VR shot from the drive down to A&As party last month. From a roll me another 04 Z06 are side by side in 4th. I told him to go ahead...in the vid you see him pull out a couple of cars lengths...then you just see me accelerate past him. I could have dropped it into third and fought traction to blow past him a lot faster..but instead I just left in 4th and went past him very smooth.

So I like the higher gears for normal everyday use. A Turbo cars makes great tq so lower gears are not needed IMO for normal use. If I had a dedicated strip car with a solid rear end..I would be running 16# of boost with 4:10 gears all the time.

I am not sure how this applies to an Auto setup. I have always had manuals. But I have heard and read..mainly on the Buick applications that Turbos and Autos work really well together..especially with the right lock up converter because you can load the turbo right in its sweet spot and then blast off.

Oh and by the way I tried my car out at various boost settings over what my wastegate springs are set at.

Anything over 9#s on the street is almost useless. I took the car out on 14 and it would just burn out regardless of how well I can manage my pedal.

As far has having a setup that makes over 700rwhp/tq. The dyno in my sig was with 3 broken spark plug wires and some misfiring...I am redynoing it down at A&A hear shortly for some more testing.

The first run will be at 16# again to make sure the misfire has been addressed. It will be interesting to see the power difference. Then I plan on doing an 18-20# run to just see what it will do at that boost level.

Then I will just turn it all back off again and leave it a 9#s.

 

I think I got what you are saying about the spooling (how gears relate to acceleration I get). It sounds like I didnt think RPM's were needed as much to spool the turbos to full boost (at least for twin T3/T4's to 12?? PSI.) Do you think the TTiX kit will take too long to spool with a 2.73 A4? I guess you could say I'm trying to make the most of my gearing versus losing traction (wasted HP) during acceleration from a road racing perspective. (Take 2 cars, one who has more HP but a 2.73 and one with lower HP with a 3.42 A4) If both cars accelerated at the same rate I would think higher HP with 2.73's (giving you a greater range of speed for each gear) would be better than lower HP with say 3.42's (again given they accelerated at the same rate.)

So in a nutshell, if you can't put the power to the ground, can you benefit by going to a shorter rear gear to get more out of your tranny gears (without significantly more spool time)?

Add: BTW, nice car!

 

Sorry I rambled too much in the above post.

I am an advocate of gear swapping for performance. I think they are often one of the biggest over looked mods. Back when I had my NA H&C 97 Coupe..I often told myself afterwards that my first mod should have been swapping my gears for 4:10s.

Paul's Z06..he has a TT Cartech built for him, tried running 2.73s (I think, I know they were taller than the stock 3.42s) at the track..in the end they busted too. He now has a solid rear end in his car..just as I predicted he would need on his first TT post.

I would stay with the taller gears (stock 3.42s) on street tires, you will get more out of them and you will not have to shift as much or as soon. A Turbo car has a lot of TQ. As far as the TTix is concerned, I can only comment on what I have seen (others dyno charts). When I compare Phil's 346cid TT graph to mine, the graphs look similar. However, the TTiX T3/T4s appear to reach full spool quicker than my T4 PT52s. That is a tradeoff in favor of T3/T4 on a 346cid.

This is why I think the A&A TT will be the most versatile of all the TT systems being offerred. His V band clamp design at the header flange is going to allow easy swapping of different size flanges (T3 and T4) without having to change the header. Guys with 346s can choose T3/T4 turbos and guys with 400+cid can choose T4 size turbos.

Even though my PTK setup works fantastic and I am completely satisfied with the power it makes. The exhaust energy it takes to spin a T4 is greater (the same AR on a T3 is going to be smaller than on a T4). It takes more energy to put more mass in motion. So this means even though I have a smaller AR, the wheel/ trims are still larger. So it takes more exhaust energy..and that comes in the form of RPMs which is related to full spool up.

I would not use lower gears with the TTix setup for normal driving. The Turbos are going to spool up fast enough and produce boost that it will not be an issue.

So in a nutshell I would keep 3.42s with an M12 or 3.73s with an M6.

If I had a S/C setup (Centrifugal) I would run 3.73s with an M12 and 4.10s with an M6.

Reason being, the power you make with a Turbo does not incur the parasitic loss the SC does. So you don't need the lower gears on the Turbo setup as much as you would on a SC setup.

 

Oops...sorry, I used the 6 speed 4.10 and 3.42 examples as they are more common (and to throw out the torque converter factor)...I have a 2.73 A4 but feel that if I go to something like 3.42's, when I road race, it will shift too much not to mention have too much power from the TT. But if I keep the 2.73, I'll can get better MPH ranges out of gears 2 and 3 with less shifting between the 2 going on. And on top of that, the big hp numbers ill get out of the TT will keep the car from bogging down. To my understanding I can't use anymore than a SS3200 converter as anything higher will have over-heating issues.

For what my goals are, to me it makes sense to do it this way (or use 3.15's at the most) but was curious whether the spooling characteristics were extremely laggy or not. Also for feedback on why or why not to do this.

If I did my math right, here are the in gear speed ranges between a 2.73 and 3.42 using a 6000 RPM shift and 27.7 tires. 2nd and 3rd look like too much gear but with 600-700 RWHP I would think you could still rip the tires at medium speeds.

2.73
1st=0-59
2nd=59-111
3rd=111-181
4th=181-258 (highway OD)

3.42
1st=0-47
2nd=47-89
3rd=89-144
4th=144-206

 

I would probably go with 3.42s, but 3.15s would be an option too.

Really since this is an Auto Turbo (Coverter question). I am finished because I have never owned that setup or had time to work on one.
This really needs to be deferred to somebody who has a lot experience in this area. I'm thinking Phil could answer these questions, he was/is involved with Stenod (Harlan's car)
and I'm sure could make a recommendation based on the criteria that you want to use your car for.

If he doesn't I would steer you over to LS1tech..there is a boat load of Turbo guys over there with Automatic Trannies. Way more than on this board. Typically you don't see a lot of F Body Turbo Autos running around on the road course. But certainly there is a lot of information on that board regarding stall converters and Turbo setups.

Good luck...let us know what you find out. I would like to know so that I can quote an example.

Oh by the way.
The recommended stall coverter (as a Single Turbo application) for my Turbo (PT52) on a Buick 3.8liter is 2900rpms.

I have two of them and if you divided my motor in half..it would be like a 2.8 liter powering a single PT52..so you can see why the boost spools to max at 3400rpms.

 

For one thing you get to try the combo you have for free!! Even though if it is a stock A4 and 700 rwhp it will be a short time. Probably enough time to see how you like it. I do know that lingenfelter 8.90 car was running 3.15 gear with a big converter but they were making big HP. I am running a 3.73 with a 422 9psi TTIX pump gas setup there is no turbo lag and I just ran a 10.53 129 mph 1/4 mile on way less then 700rwhp
Chuck

 

If you're talking about an auto trans, use the lower ratio, with a higher stall - bingo - instant boost, lower ratio to load turbos more, and pull longer in gear.

 

If there is a way to I would. I would like to survive 20+ minute session driving events. To my understanding an SS3200 or comparable is the highest I can go without the tranny risking overheating issues.

 

What do you mean by spool time?

To my understanding there are two sides to turbo lag:

1. Where in the rpms boost is generated, ie the turbo kicks in. With lower gears like 4.10 you will be able to reach this on boost level sooner, but the detriment is a lower top speed in each gear. And as you loose time during shifting (esp manual cars) it can (not is, can) often be quicker to have a higher gear (numerically lower). As if you have the power to drive it you'll be able to reach a specific speed with less shifts.

So a rear end change will not directly affect how the turbo opporates but it will alter the character of the car.

2. This is the area most people forget about. Lets say your turbo will be producing full boost by 3000rpm and red line at 7000rpm.

If you are at WOT @ say 5000rpm, the exhaust gas flow will be sufficent to spinn the turbo impellars fast enough to compress the intake air and produce boost.

If you then lift completley off of the throttle, the TB will close and there will be a massive drop in exhaust gas flow. This will effectively 'stall' the impellar.

If you then reapply WOT there will be a momentary 'lag' before the exhaust gas flow is sufficent to once again spin the impellars fast enough to create boost. All of this could happen well aboue the 3000rpm boost level. And this really is the achillies heel of pretty much all turbo setups, which can be a hinderance for road use. (not really relavant on a circuit as you are either at WOT or hard on the brakes, there is not usually much middle ground).

Dump valves (I think you call them BOV but I'm not sure) try to eleviate this problem, but they are only part successful and on a mild boost level are often quite unoticable except for the noise they make.

The only way I know of getting around LAG is the use of the anti-lag systems that WRC rally cars use. They make some strange noises (imagine a squirrel stuck in the exhaust and its about the right sound). Performance wise these setups are great as they offer all the benefits of high nd turbo power and all the benfits of instant low end grunt as a supercharger does, but the down side apart from cost is they are disatrous on fuel efficency and massively increase strain and wear on the engine and turbo. But the 2.0 liter 4 cylinder rally cars do produce (with the FIA air restrictors removed) in the region of 650bhp and 650lb ft which is rather impressive. If you could tune a 5.7 liter LS1 to the same level you would have 1853bhp

A rear end gear change will have little or no direct affect on spool time due to a stall incident. Obviously if the engine can pickup revs qucker then the exhaust gas flow will also increase quicker when the throttle is opened. So it may reduce 'lag' time by a bit. But to be noticable I would think you would need a bigger variance than 3.43 to 4.10 (although this is only a guess and I have no data to back that up).