PDA

View Full Version : Gearing and drivetrain experts inside, please.



Ralph LoGrasso
09-22-2004, 08:55 PM
Matt and I need your help to settle a slight disagreement.

Here's the deal. Matt and I are trying to select the best rear gear for Fantom, in order to meet the project goals.

Project Fantom should make 600rwhp(Blown LS1 @ 12psi), hopefully more.

The goals for the car are as follows:
Mid-High 10 second 1/4 on either slicks or ET Streets.
185mph top speed

Here's the data
Power: 600rwhp
Max RPM: 6600 (est.)
Tire diameter for 1/4 duty: 28"
Street tires (top speed): 26.1"

Here is the gear ratios for the T56. I think these are the correct ratios for the LS1 T56.

1st: 2.98
2nd:1.99
3rd: 1.35
4th: 1.00
5th: .79
6th: .62

Basically what we need here is the lowest gear possible that will net the 185mph top speed.

You may find this link helpful: http://www.csgnetwork.com/rearendgearcalc.html

I, personally want to run a 3.55 or 3.73 gear. Matt recommends 4.33.

From a pure math perspective using that calculator, 6500rpms, 3.73s, .62 final gear ratio, and 26" tire equals a possible top speed of 217mph.

4.33s = 187mph.

My arguement is that, the Coefficient of drag is going to drastically reduce the possible top speed. I personally don't see the car running 185 with anything in excess of a 3.55 or 3.73, Matt obviously sees it differently. Matt knows much more than I do, so I am probably wrong, but we'd both like a second opinion..

Any opinions on what ratio to run /comments/expertise is welcome.

I asked Tony (nineball) his thoughts, and his recommendation was for a 4.10 gear for a 10 second car, with the T56.

Matt@RFR
09-22-2004, 08:59 PM
I’ve been hearing that some people will go to the trouble of installing a six speed, but only use 4 gears. What the hell is that??? They say that the car “tops out” in fourth gear. Well, it wouldn’t do that if they had the correct rear end gears!!

For Ralph’s car, 4.33’s look to be perfect for obtaining 185mph @6500rpm with the tires he’s going to run. (Will the motor pull 185? I doubt it…but we’re talking about gears, not horsepower)

3.73’s would bog the hell out of the motor in sixth, and to some extent in fifth too….might as well put a 4 speed in the car and call it good if he runs those 3.73’s.

Ralph mentioned “from a pure math perspective”. My point is that math is the only thing involved when picking gearing. If the motor will only pull the car to 150mph, then even shorter gears could be used.

This never came up in our conversation, but another area of concern is cruising rpm…don’t wanna be spinning that motor 4500rpm going down the highway!! 4.33’s in sixth gear would have him motoring down the highway at 80mph @ 2800rpm.

Bottom line is that 4.33’s are perfect for Fantom, and I just need help getting that through Ralph’s thick head!

dennis68
09-23-2004, 05:49 AM
All you are doing by running 4.33 in a 6 speed is eliminating the need for 1st and 2nd instead of 5th and 6th.

Ralph, you are not going have enough HP to pull top sped in 6th gear with 3.73. The car will actually go faster in 5th gear beacuase the engine will be able to pull the gear to a higher RPM.

Matt, we (Chrysler) tried doing the same thing on the new Jeep, re-gear the diffs so top gear (6th) would be able to actually be a usable gear, now 1st is worthless, only good till about 2 MPH.

6th gear is designed to be a freeway cruising gear only. It was never intended to be used for top end passes, that is why both 5th and 6th are overdriven, in 5th the engine can operated in or near its peak HP while approaching top speed. In 6th the engine is not near that powerband yet and the "wind wall" affect is already in place-the engine just can't pull the car into that "sweet spot".

We go through the same thing with boats, the vessle has to be able to break the coeffecent barrier, be it wind or water and at the same time turn the propellent, be it a prop, a pump, a drive, or a diferential. The key is to place your top end desired speed to match the engines peak HP then gear accordingly (using 5th gear ratio).

derekf
09-23-2004, 07:08 AM
Wow, this is some good info... this would seem to tie in with my post in Open about powerbands and fuel economy and gearing - http://66.70.234.198/forum/showthread.php?t=915

When you're gearing for performance, (assuming a 6spd since that seems to be the assumption in this post) how do gear ratios, RPM vs. Torque/HP curves, and vehicle speed relate?

Matt@RFR
09-23-2004, 08:39 AM
All you are doing by running 4.33 in a 6 speed is eliminating the need for 1st and 2nd instead of 5th and 6th.

Ahhh, but Ralph is trying to build the perfect dual purpose car. From where I stand, with 4.33's, now he's got a useable 1st and 2nd gear at the drag race track, and will still run the mph he wants in road race trim.


6th gear is designed to be a freeway cruising gear only. It was never intended to be used for top end passes,

And '68 Camaros were never intended to run 185mph.

If he runs 4.33's, he'll have a decent chance at an ok launch at the drag strip, it'll go easier on clutch's on the street and be easier to drive on the street too.


Ralph, you are not going have enough HP to pull top sped in 6th gear with 3.73. The car will actually go faster in 5th gear beacuase the engine will be able to pull the gear to a higher RPM.

My point exactly.

dennis68
09-23-2004, 08:58 AM
Matt I still think 1st will be totally useless, how about a compromise? Take Tony's advise and run the 4.10's. Ralph, gearing is not really going to determine top speed, the car isn't going to make enough HP to overcome the drag coeffiecent at over 180 or so, not to mention that without some SERIOUS aero work it's just gonna go airbone anyway. A friend of mine died at 190ish in a Viper, left the ground, and it has mega aero work. Those are airplane speeds and lokking at side view siloette of acar they are somewhat shaped like a wing.

Matt@RFR
09-23-2004, 09:14 AM
Dennis, he's gonna need all the first gear he can get to get that pig moving and have a sub 3 second 60' time. lol

You make a very good point about aero. I knew it'd take a lot of motor to pull the car to 185mph, but never thought about the wing factor.

So maybe we should concentrate on figuring out what a realistic top speed for Fantom will be, then gear it from there.

Ralph LoGrasso
09-23-2004, 12:35 PM
Good info guys.

Dennis, I read that 4th gens usually top out in 5th gear as well. As for the 185 top speed, I agree that extensive work needs to be done, but the speed is still possible. I have a fully adjustable rear spoiler, and I would swap to some REALLY stiff springs for high speed runs. That's not an incredible amount of work, but Big red ran 222mph, and although the car was before my time, from looking at the one or two pics that I've seen of it, I don't recall extensive aero work. I'd like to think that 185 is definitely possible in a 68 to some extent.

About 6 months back, Tom estimated my top speed, with the same setup, only 430rwhp (before blower) at around low-mid 170s, safely. I'd think the car could pull an extra 10mph out of that with another 170rwhp, but I could be wrong. Where's Tom? He's the first gen road race man.

I'm not saying that you're wrong, I'm just looking at it from a different perspective.

dennis68
09-23-2004, 01:44 PM
I beleive Big Red had some belly pan work if I'm not mistaken. Some rear air management below the bumber will help quite a bit too.

Ralph, I didn't mean lots of "stuff" to put on, however aero tuning is trial and error and unfortunately difficult to do around town.

My buddy Tracy Castro at work is pretty good at the 1st gen high speed thing, he has been 167MPH at ther Silver State a few times. His LT1 RWHP was right about 400 I think. That is with a Tremec and .83 5th gear, 255/5016's and a 3.73. I'll go talk to him tomorrow about the combo to make sure I got it right. He says it feels good at 160, much more than that it starts getting air under it.

BTW, comparing Big Red to any of the other cars on this site is like comparing my bucket to Earnharts old Malibu-the one that one the Daytona 500. :rotfl: :rotfl: wait or maybe Yunicks Chevelle- :rotfl:

Ralph LoGrasso
09-23-2004, 02:23 PM
Dennis,

Please ask him. Find out if you could get me an email address or a phone number that I could talk to him at, if possible. I'd like to discuss a few things with him, specifically belly pan work, and other aero work that you mentioned.

JamesJ
09-23-2004, 03:56 PM
Here are two examples that may or may not help you. On our 65 Corvette I can run about 180 with a 3:55 rear end, 600hp big block 2,800 pounds. On the 63 small block 430hp I can get up to 165 also with a 3:55 rear end before I have to break. We run 225/60/15 which comes out to something like 26 ½ This is at CMS just passed start & finish. Not brave enough to keep it pined going into the turn.
Blue car http://www.hotrodhoodlumstexas.com/Images/FontanaRace%20(22).JPG
White car http://www.hotrodhoodlumstexas.com/Images/FontanaRace%20(10).JPG
Both cars have stock trans in them

justanova
09-23-2004, 03:58 PM
"Dennis, I read that 4th gens usually top out in 5th gear as well"


I can vouch for that, My brother has a 2001 SS and to get to the speed limiter you have to use 5th gear, it just won't pull 6th at that speed(that could have something to do with the 3.42 rear gear and the 275/45/17's)

Another thing to think about is, is the gear itself for 6th going to be strong enough to hold the horsepower its going to take to get to 180+? :dunno: that is going to be alot of strain on the gearset itself.

"6th gear is designed to be a freeway cruising gear only"

6th works great for cruising. personally I would gear for top speed in 5th. with a 600hp ls1 it will still launch hard :drive:

good info though, keep it coming

Ralph LoGrasso
09-23-2004, 11:17 PM
James,

Those vettes look awesome!! My Dad would love to have one of those, he had a '64 that he sold many years back. He wants to own a 67 again one day.

Justanova,

The T56 will be a G-force built version. I think it's rated at 800 or 900hp. The standard version comes with upgraded 1-4th gearsets, but I'll be using the upgraded overdrive 5th and 6th gear sets as well. I agree, I think it will still launch hard with the 3.55s. :)

edit: Dennis, I'm working on getting some pics of the underside of big red to see the belly pan work.

XcYZ
09-24-2004, 04:44 AM
Ralph, if you're going to have a T56 built by G-Force, why would you use the stock 5th and 6th gearsets? You'll definitely want to tighten them up.

Using my spreadsheet, plugging in 26" tire, 6500rpm, and 4.33 gears, I get:

Axle ratio-> 4.33
Tire radius-> 13 (from Tire Radius sheet)
RPM-> 6500

GEAR GEAR RATIO Speed @ 6500 RPM
1 2.66 43.6 mph
2 1.78 65.2 mph
3 1.30 89.3 mph
4 1.00 116.1 mph
5 0.74 156.9 mph
6 0.50 232.2 mph



I say drop in .86 5th and a .76 6th gear with a 3.70 rear:

Axle ratio-> 3.73
Tire radius-> 13 (from Tire Radius sheet)
RPM-> 6500

GEAR GEAR RATIO Speed @ 6500 RPM
1 2.66 50.6 mph
2 1.78 75.7 mph
3 1.30 103.6 mph
4 1.00 134.7 mph
5 0.86 156.7 mph
6 0.76 177.3 mph

XcYZ
09-24-2004, 04:55 AM
Ralph, I forgot to include this for comparison. Here are the numbers for 3.73's, stock T56 gearset, and 6500rpm:

Axle ratio-> 3.73
Tire radius-> 13 (from Tire Radius sheet)
RPM-> 6500

GEAR GEAR RATIO Speed @ 6500 RPM
1 2.66 50.6 mph
2 1.78 75.7 mph
3 1.30 103.6 mph
4 1.00 134.7 mph
5 0.74 182.1 mph
6 0.50 269.5 mph

Ralph LoGrasso
09-24-2004, 12:21 PM
Scott,

Thanks for the spreadsheet data. That's really helpful, I couldn't find anything like that online, just final speed calculators with the overdrive, and the rear end ratio.

I'm going with the upgraded overdrives, but I think I messed up in my post and included the stock overdrive ratios, instead of the upgraded ones. Sorry 'bout the mixup. Thanks again for the data.

Matt@RFR
09-24-2004, 01:45 PM
Scott, thank you. I'm seeing that this conversation is pretty useless until Ralph nails down exactly what transmission gearing he'll be using.

dennis68
09-25-2004, 10:34 PM
Raplh, I talked to mt buddy at work, I'll post a new thread to discuss the specifics-this is not the correct forum for it. Check in open discussion.

TurboLark
09-26-2004, 08:21 AM
I think one problem is the tire choices for each goal. If you pick the gear for top speed, you will really kill 1/4 mile performace with the switch to the taller tire. And vice versa. I would suggest using a 26" tall tire for both events, and running a 4.10 rear gear.
The 4.10's will at least allow you to pull the overdrives better than 3 series gears, plus give good 1/4 mile performance also.
Trying to calculate the 1/4 miles stuff is a bit hard without knowing the weight. i was guessing 3600lbs. With 600hp 3600lbs you should be able to get into the mid 10's at around 128mph. Calculating that back for gearing would get you to a 3.90 gear with a 26" tall tire. So a 4.10 would help you off the line a bit more, and help pull the overdrives.

CAMAROBOY69
10-08-2004, 04:02 AM
Dont forget about "The Twister" it also ran over 210MPH. Ralph will have a lot more HP than the Twister if memory serves me correct. The Twister had 460Hp and 450TQ So Ralph you should have way more than enough HP to push your car to over 180. The Twister had 3.89 gears.

ballistic69
10-08-2004, 07:44 PM
Here is a calculator I used to decide what rear gear ratio to use. You can enter tire size, gear ratios, max rpm and the calculator will give you mph at in all six gears.

My line of thought when I chose my gears (3.50) was if I want to run the 1/4 mile and use gears 1-4, use a calculator based on HP and weight of your car to estimate your trap speed. Play with gear selection until you have a trap speed in 4th gear about where you calculated your mph to be basted on your weight an HP.

You should be able to run in an "Open Road" race with 5th gear an reach drag limited top speed. The 6th gear is for good highway mileage.
(Doing 90mph and turning 1500rpm.)

http://www.geocities.com/z_design_studio/transmission_z28_6.html

Here is a link to more useful calculators, sign-up is FREE, 2 calculators require subscription to. Use the wind resistance calculator to determine Cd. It lists an '69 Camaro, so a '68 Cam should be close. Happy number crunching.
http://www.smokemup.com/auto_math/index.php

Ralph LoGrasso
10-08-2004, 09:05 PM
Kirt,

Thanks for the reply. I found the exact calculator two days ago. It's pretty sweet! At this time, I'm looking at just running two seperate center sections, and swapping one for open road racing. Probably 4.10s on the Street/drag race, and then something like 3.08 for ORR. The racer that I talked to said he runs 3.08s, in his 4th gen, and he has run 197 before he was DQ'ed. So I'm thinking that the 3.08s will pull me to 185 hopefully, since I'm much worse off in the aero department. I'm still up in the air about what to do though.

How's your car coming along?!

ballistic69
10-09-2004, 08:43 AM
Base on the "ideal" gearing:

Trans 1st gear X rear gear = 10
(Z= rear end ratio)
2.66 X Z = 10
Z= 3.7594 (This is the "ideal" best ratio for acceleration, remember tire diameter will affect this)

Your est HP & weight:
Vehicle Weight - 3200 (lbs)
Wheel HP - 600 (RWHP)
Trap Speed - 139.6 (mph @ end of 1/4 mi.)
E.T. - 10.17 (seconds @ end of 1/4 mi.)

For a 28" tall tire use 315/40/18.
I think you should use an 3.89 or 3.70 a 3.08 is way too tall.

My $0.02

Ralph LoGrasso
10-09-2004, 10:45 AM
Kirt,

You're saying the 3.89 or 3.70 for 1/4 mile runs, or top speed runs? For top speed runs my tire is a 335/30/18 (26.1" tall), it is however 28" diameter for 1/4 mile duty.

ballistic69
10-10-2004, 11:55 AM
For both. Also if you get a 345/35/18 tire, which I did since my slicks are 27.95", you won't have to worry about having to set your car up for 2 different tire hights since 345/35/18 are 27.5" tall.

Ralph LoGrasso
10-10-2004, 11:59 AM
I wish I could fit those 345s! Unfortunately I don't have enough room. 335 is really the max width that you can go on a 67/68 without relocating the frame rails. I've mocked up my 335s, and they're pretty tight as it is, I don't think the 345s would fit. I agree, having two tires the same diameter or relatively close would make things much easier.

RobM
10-11-2004, 09:56 AM
to me the need to swap out center sections kind of defeates the purpose of a 6 speed all together, because you could do the same with a 4 speed and have the same results! because basicly from what im understanding from this thread is the two sets of gears you would use would either make the first two gears usless or the top two gears usless. and that basicly gives you a 4 speed any way!

Ralph LoGrasso
10-11-2004, 01:16 PM
Rob,

They won't really be useless. 1st and 2nd will be short with the 4.10s, but I'll still trap in 4th, and 5th/6th will be used for highway driving. A relatively high top speed will be attainable (155/160 or so).

With the 3.08s or 3.25s, all gears will be perfectly useful, and the car will reach top speed in 5th. The acceleration won't be as great as with the 4.10s, but it will still be plenty fast. 4th Gen LS1s have relatively tall rear gears from the factory, so the 3.08s will be pretty similar to a factory 4th gen setup.

USAZR1
10-11-2004, 02:16 PM
Here are two examples that may or may not help you. On our 65 Corvette I can run about 180 with a 3:55 rear end, 600hp big block 2,800 pounds. On the 63 small block 430hp I can get up to 165 also with a 3:55 rear end before I have to break. We run 225/60/15 which comes out to something like 26 ½ This is at CMS just passed start & finish. Not brave enough to keep it pined going into the turn.
Both cars have stock trans in them

My 90 ZR-1 with 420rwhp LT5 would run 155mph in 4th gear(1:1 ratio)with 26" tall tires and 3.45 rear gear. Rpm at that speed was 7000.
To run 165 or 180mph with either of the above cars would require spinning their engines to around 8000rpm. I'm curious about your engine combos.

USAZR1
10-11-2004, 02:23 PM
Rob,

They won't really be useless. 1st and 2nd will be short with the 4.10s, but I'll still trap in 4th, and 5th/6th will be used for highway driving. A relatively high top speed will be attainable (155/160 or so).

With the 3.08s or 3.25s, all gears will be perfectly useful, and the car will reach top speed in 5th. The acceleration won't be as great as with the 4.10s, but it will still be plenty fast. 4th Gen LS1s have relatively tall rear gears from the factory, so the 3.08s will be pretty similar to a factory 4th gen setup.

It's really hard to beat a T56/4.10 gear combo. Running a T56 with 3.08's or 3.25 cogs suck. I'm running a 3.25 cog in my Fab9 now and 6th gear is pretty tall,unless you like cruising at 80+mph.
4.10's or 4.30's will be going in the car,soon.

Ralph LoGrasso
10-11-2004, 03:29 PM
It's really hard to beat a T56/4.10 gear combo. Running a T56 with 3.08's or 3.25 cogs suck. I'm running a 3.25 cog in my Fab9 now and 6th gear is pretty tall,unless you like cruising at 80+mph.
4.10's or 4.30's will be going in the car,soon.

Clint, I agree. I was doing some calculations to see what rpm I would be at with some of the taller gears, and it's pretty damn low. The 4.10s will be in the car 90% of the time. They will see the street and strip duty, the 3.08s or similar will only be for top speed events, so they won't really see all that much use.

Blown353
10-27-2004, 05:33 PM
Don't forget driveline critical speed, something which I didn't see mentioned here. With low rear end gears this can become a major issue at high speeds. You don't want to jumprope a driveline!

Decide what gears you like, and then find out what kind of driveshaft RPM you will be spinning at the highest speed you plan on driving the car and pick a driveline that will give you some margin as far as critical speed goes-- the more margin, the better. Critical speed is a function of material, OD, wall thickness, length and a couple of other properties. Any driveshaft manufacturer should be able to provide you with this data, but here's a handy chart on Mark Williams' site: http://www.markwilliams.com/pdfcat/pg69.pdf

In a general sense, the following factors have the greatest effect on critical speed:
Outer diameter
Stiffness of material
Overall Length

Since the length is fixed per the application, your best options for increasing the critical speeds are to increase the driveline OD and also to change the material.

A carbon fiber driveshaft offers the best critical speed properties, but I wouldn't recommend one on a street car that racks up any miles. A piece of road debris that would likely bounce off a metal shaft can nick and grenade a carbon fiber piece. I had a friend lose a brand new CF driveshaft in his 4th gen Camaro to a very small piece of wood in the road. The nice thing about the CF driveshafts is that they shred and don't beat anything up when they fail... except your wallet.

Troy

spanky the wondermuffin
10-28-2004, 08:04 AM
there is a discussion at openroadracing.com on this subject under the heading 'a little math help'

kball
10-31-2004, 06:17 PM
I agree with zr1, in my case i'm running a 640hp 496 w/a tko600 close ratio. Unless you've got a really mild cam, the engine won't be happy in 6th gear below 100mph. Some consideration has to be given to what the minimum rpm the engine will like to cruise at. Thats the reason I chose the tko600 with the .82 od rather than .64 The engine in my chevelle won't like much less than 2700 rpm, so it's pretty impractical to have a .64od. That would put it at around 2100 rpm@65mph with 315/35/17 and 3.73's. It makes for more use of all the gears. Right now, with .82 and 3.73 gears, its 2550 @65mph. I'd love to drive 150 all the time, but that gets expensive quick. It still leaves me with a theoretical redline speed of 175 mph @ 7000 rpm and a first gear that's sort of useable with some throttle finesse.