Busa_rob

iTrader: (1)

3.08s in my ls foxbody, s475 lights off fast with a turbo 400 and 3500 stall. And it's worth mentioning at 70mph it's at 2600rpm.

It's all about keeping the engine in peak torque as long as possible with turbo cars. That's why they respond better, and usually run quicker times with a 3.08 vs a 4.10.

gsteele

That is true with any car isn't it? Not just with turbos. BTW I think it is at peak HP that you want to run isn't it? This is all just theory with me as I am running a 2.5 gear with a ford 9" in an S10 with street tires. No traction until well over the speed limit. When I got the rear from Currie the gear set was used but that is what I wanted so when I wear them out I think I will have to go 3.08 which will be fine. I did put a set of slicks on it for a couple of hours and on the street it was fun to see how fast the boost did build at the lower speeds.

Busa_rob

iTrader: (1)

Not exactly. That's why na and nitrous cars do a lot better with lower gearing.

codyvette

iTrader: (24)

Most nitrous cars are just set up and tuned like NA cars. With the right nitrous controller and enough stages a nitrous car can be set up to rely on torque over HP a lot like a turbo car.

Orr89rocz

iTrader: (13)

I know some wont agree... Lol but Turbos dont need "load". They spool with exhaust energy and that comes with right rpm and turbine. If converter is too low and turbine housing to large, you could leave in 3rd with a 2.73 and 33" tire and it wont spool any better.

Starting point for gear is based on tire diameter and mph you will achieve at the stripe and engine rpm like any other combo. Problem is turbo cars usually have tons of power and make it very difficult to maintain traction. Less numerically gearing helps.

Then its matter of keeping motor in the power band where max average power is applied down the track. Like any other setup. Cam heads converter trans and rear gear all have to play together. Play around to find what it wants. Broader power curve sometimes doesnt need alot of gear anyway. My bolt on ls1 na car ran practically same with 3.73 vs 2.73 and thats all motor.



And if your boost control is doing its job, boost stays same setting regardless of what rear gear.

gsteele

^^Bless you. It was getting lonely in here.

gsteele

"And if your boost control is doing its job, boost stays same setting regardless of what rear gear."

I guess I was assuming leaving from a stop with less than full boost the 4.56 car would build boost faster than the 3.0 car. Especially with a tight converter. Maybe not but I get your point.

Orr89rocz

iTrader: (13)

In my experience it does. When my combo was 2.73 gear and tight converter i was a dog off the line. Going to 3.42 got rpms up faster into the boost theshold rpm. Car moved faster as boost was made quicker. Still was not optimal due to tight converter

Now with a transbrake and proper converter i control launch hit boost via launch rpm and ramp rate with electronic controller. If i short shift first and go to second, i slow down lightly. More load didnt help pull second. I simply was not using all the average power available and thus ran slower

Forcefed86

iTrader: (4)

In my experience peak torque and peak boost in each gear usually go hand in hand.

As you say, a lot depends on the engines RPM range/power band. Most factory stockish turbo setups are RPM limited by OEM parts. So if you can get into boost earlier in the "stock" RPM range (without lugging the engine down) it will result in more average HP per gear and quicker overall acceleration.

A good boost controls setup should leave the WG shut 100% until the desired boost is reached. Boost control won't get you into boost any sooner if the gates closed. Adding additional load to the engine can.

For example: In my 93 talon I ran a 16g turbo with the WG literally pinned shut. Running the factory 25" tire I data logged my passes. Switching over to a 27" tire I'd see higher boost earlier in the RPM range in EVERY gear. In my case this net quicker ET's and higher trap speeds. There are of course limits. I know a 33" tire wouldn't perform better than the 27" Similar to the 2.73 gear, which is a pretty extreme example.

gtfoxy

Go throw in some 4.50's & get back to us how it works. I doubt you will like it much. Your very own example proves my point of, for the majority of given combos, a low-to-mid 3's works well. I can't see any need for even 4.10's in a typical 2500-3500lb car unless the tire diameter is north of 30" or the RPM is way up there.

It all boils down to intended application. In your application a 3.08 may be perfect if you have the tire to make it hook. Maybe even 3.55's... It boils down to where you want top speed to be. If you want it at full tilt at the drags on stickies would require one gear but say you wanted to go run 200mph+ on a 25.5" tire with a 1:1 final tranny gear, depending on your RPM range, that same gear won't work out too well.

There is ofcourse a limit to how shallow, or steep for that matter, a gear you can practically get away with.

Not sure of the combos in your sig but I doubt you would run 5.10 gears in your 8second ride. If your 2.73 to 3.42 change was in that car, and depending on your RPM, tire size & tranny gearing, etc I could see that change helping.

Torque converter slippage ofcourse plays a role here & can give a leeway up or down in gear ratio, but at the same time that in itself needs to be set to work with the package as a whole. Too tight & it will take too long to build boost regardless of gears, to loose & it's no shift recovery RPM & good-by top end pull.

Now take the slippage of the TC out of the equation with a manual & that control of being able to get more into the rpm range goes away & gearing being right becomes a bit more critical.

As it stands I think, & I have said & most agree, low to mid 3's just seems to get the job done in a typical car weight range. I am in no way saying people should run 2.40's or something like that unless they are on dinky tires like 23" or something crazy.

Now if you increase vehicle load though mass increases, say a 8.1L turbo in a 1ton dually hauling a 18Klb trailer, your gearing needs change dramatically & 4.56 gears may be what the doctor ordered. I wouldn't try it with 3.08's, that's for sure.

gsteele

"Go throw in some 4.50's & get back to us how it works. I doubt you will like it much. Your very own example proves my point of, for the majority of given combos, a low-to-mid 3's works well. I can't see any need for even 4.10's in a typical 2500-3500lb car unless the tire diameter is north of 30" or the RPM is way up there.

It all boils down to intended application. In your application a 3.08 may be perfect if you have the tire to make it hook. Maybe even 3.55's... It boils down to where you want top speed to be. If you want it at full tilt at the drags on stickies would require one gear but say you wanted to go run 200mph+ on a 25.5" tire with a 1:1 final tranny gear, depending on your RPM range, that same gear won't work out too well."

Who said I would? I have been responding to your first post on turbos needing "load." That is the point I have been responding to. The comment on 3.08 maybe working better if I have the tire to make it hook is spot on. It has nothing to do with "load." Read your first post.

RedRocketZ28

iTrader: (19)

I have had a 2.73, 3.00, 3.25, and a 3.50 gear in my car. The car has run its best pass with the 3.50 gear. Now, my example isn't 100% accurate as some things did change slightly when the gearing did. With the current engine combo and the 3.00 gear it went 9.9 @ 142 on 16-17 lbs. Can't remember off hand what the 60' was but it was in the low 1.5 range leaving on 6-7 lbs. off the 2-step and footbrake. When the 3.50 gear went in the trans was then rebuilt and the converter was sent back to upgrade to a triple disc. Nothing else was changed with the converter is what I was told. The car went 9.5 @ 147 on an average of 15.5 lbs with a 1.43 60'.

With that being said, I don't think the converter change, or trans rebuild, added that and the car has now been faster on SLIGHTLY lower boost levels. The car leaves much harder for obvious reasons. If I had a transbrake I think the 3.00 gear could have worked. If I ever go TH400 then a gear change will be needed as I drive the car a lot and won't be able to stand the RPM cruising down the interstate. For now though, it's perfect.

Not all combos will work the same. I found that a lower numerically gear was good with the small T4 turbo I ran at one time, and with the bigger turbo my combo liked more gearing.

gtfoxy

Umm you seemed to think that steeper gears would be better, because you said, incorrectly...

& You're still not getting it.

I know what I said & I stand behind it. I even said, in my quote below it takes the "Right" amount of load. Then I even later specified a gear ratio range & a tire diameter. There's your load range, for most applications. Then when someone even said they went from real steep gears to the range I said you came back to them with even more rabble.
If you have said,"hey I have 2.73's & I'm thinking I want to jump to 3.08's. Will I gain anything?" I would have dug deeper & found out more about your combo & what you may need to get the best performance. But you never did that.

But hey, I've only been building & tuning turbo cars for almost 20yrs... You probably know better than me.

gsteele

And what was the above quote referencing? This previous quote:"What you are calling load is a illustration of the fact that to maintain a certain speed or acceleration and you drop a gear or two you have to increase the torque. Lets say you have two cars one with a 4.56 rear and one with a 3.00 rear gear. Let's say they are identical and have the same engine and transmission(automatics) and are both turbo charged and are not traction limited so they have some huge slicks. Which car is going to accelerate the quickest? The one with the 4.56 gear. Now depending on the set up, the 4.56 gear car may not get to max boost in first gear but it will get to full boost as soon as the 3.00 gear car with regard to time. Maybe faster."

Under those circumstances and assuming you do not run out of gear on the top end do you still think the 3.00 car will be faster?

Forcefed86

iTrader: (4)

The examples being used are so far off base it’s ridiculous. Why not use realistic examples? Traction is ALWAYS an issue. And you aren’t going to be deciding between a 4.56 or a 3.0 gear for a quarter mile car…

Gearing has a massive effect on the converter so the auto trans is a bad choice for comparison. To get good data we need to eliminate the traction issue. So do a test from a rolling start. Once all the variables are stripped away you can get down to a test that that gives actual data that’s worth something.

Use an average LS turbo setup:
2 identical 3500lb F-body’s
Same engine and turbo kits Revving to 6500 RPM
Both engines peak at 700whp (with gate wired shut) no boost control.
28” tire
Manual trans.

Use common gear choices:

One using a 3.55 and the other a 3.73.

Both cars start out rolling at in 3rd gear, 4k rpm, using the same 6500 rpm shift points. Which will be quicker to 140? Why?

I’ve got the back to back logs to prove the lower gearing (taller) was beneficial in my specific setup. Made a pass bolted on 2” taller tires, made another pass. Taller tires were both quicker and faster.

gsteele

Entirely possible. I believe you. There is the problem of course of the cars not being able to start off at the same speed because of the different gearing so there is some guessing here but I take the point. I took the example from a dead stop to eliminate that variable and used an automatic as a proxy for a perfect transmission (no shifting etc) and so you do not lose boost between shifts but as you point out there are problems with the converter etc in the real world. I think the answer to why in your case would be answered by taking your shift points and laying them over the power curve of your engine and seeing that the taller gears put your engine in the fatter part of the curve. You were developing more average hp because of all the variables that make up that curve and gearing is not one of them. I guess that is the nature of the controversy here. Does gearing change the shape of an engine's power curve and I do not see how.
I can see how a manual would change the curve because you would lose boost between shifts. Maybe that is part of the why with your car. I think that is another reason automatics are faster than manuals especially in boosted vehicles. Thanks for being civil. I freely admit I have not tested this out and do not have track data. I just think talking about power curves etc is more profitable than talking about "load."

3 window

iTrader: (1)

At gsteele, the OP asked why you see so many people running higher gears (lower numerically) with turbo setups? Because that's what the typical setups seen here require to run best. Thinking that the lower the gear makes the car accelerate faster isn't always the case. If you do believe that, put the 1970 Hot Rod magazine down where everyone is running a 4.56 or 4.88. Next will be that we all need straight axles to make our suspensions work!

gsteele

No kidding. Thanks for that brilliant insight. Are you sure you are not responding to something that was said in another thread? Or another website for that matter? In my previous post I just said why I thought higher gears(lower numerical) made that specific example faster. I know reading is hard but give it a shot some time.

gsteele

That is correct and the question I am attempting to discuss is why. My contention is that it is not because of how the engine is "loaded" but the shape and position of the power curve.

3 window

iTrader: (1)

Nope, I read it here, right where you typed it. And you're right, it wasn't hard. I do see your point to some degree. Take a naturally aspirated engine and run it on a dyno and get a curve. It would get geared accordingly. Now boost the same engine with 20 psi and get a curve. The curve would be way different and would allow you to get away with less rear gear. Now, the harder the engine has to work, the more heat it's going to build and the more energy will be produced. Heat builds boost, boost builds power. So load does have an effect on building power.