LS4 / F40 swap - fieroguru (Page 128/216)
Rickady88GT JAN 02, 09:23 PM
Thanks, so are the 6th gear speeds acual or theoretical?
I just don't like the top gear theoretical numbers. Of coarse that is with an unloaded condition, but on a dyno what do you think it will change to?
When I had my LS4 4T65EHD on the dyno, it was run up to 180mph. I will never drive the car that fast but I can say that the drive train has done it at least 5 times.
The reason I am asking is because I want to build a manual transmission for my LS4. But I want it as close to the same ratios and feel of my 2006 LS2 GTO. That would be perfect for me.
fieroguru JAN 02, 11:10 PM
Most dynos are ran in whatever gear gets it closest to 1:1 ratio. So your LS4/4T65 should have been dyno'd in 3rd gear. I dyno'd my LS4/F40 in 4th gear (151 mph @ 7000 rpm). With the final drive swap, when I dyno it the rear wheels will spin 176 mph.

Maximum vehicle speed will be reached in 5th, not 6th, so any maximum speeds are theoretical because they simply can't be reached under normal conditions (and assume no clutch slip, no tire expansion, and power to reach those speeds).

I did some digging and the 06 GTO came with 245/45/17 tires (25.6"), 3.46 final drive, and gear ratios of 2.97, 2.07, 1.43, 1.00, 0.84 and 0.56.

The maximum mph at 6000 rpm in each gear of your GTO are:

code:
Tire Dia	RPM	1st	2nd	3rd	4th	5th	6th
25.6		6000	44	64	92	132	157	236


Using my rear tire size (24.9"), the same 6000 rpm, and the stock G6 F40 gears and final drive (3.55). Notice 2nd, 3rd and 4th are about the same as the GTO, with 1st and 6th being much shorter and 5th taller (swap in the 0.85 5th gear and the MPH would drop to 147 making it also shorter than the GTO's). So running around town in 2nd, 3rd and 4th - the two transmissions with slightly different tire heights are very similar (but the Fiero is 1000 lbs lighter).
code:
Tire Dia	RPM	1st	2nd	3rd	4th	5th	6th
24.9		6000	33	61	95	132	165	202


Keeping the 24.9" tire, the same 6000 rpm, and the G6 F40 ratios with the 3.09. Now 1st and 6th are closer between the two transmissions and 2nd, 3rd, 4th and 5th are all longer. With a car that weighs 1000 lbs less, the longer gears are not really an issue.
code:
Tire Dia	RPM	1st	2nd	3rd	4th	5th	6th
24.9		6000	38	70	109	151	189	232


If you wanted to make 3rd - 5th less long, you can swap in the short gear ratios for each and end up with this combo:
code:
Tire Dia	RPM	1st	2nd	3rd	4th	5th	6th
24.9		6000	38	70	105	137	170	232

[This message has been edited by fieroguru (edited 01-02-2018).]

Rickady88GT JAN 02, 11:41 PM
I have 255 45 17 and should be about 26.04 dia. That would change the numbers quite a bit.
But I think the taller 1st and shorter 6th should work best for my style.
I don't need ultimate top speed or fuel economy, just a comfortable cruise RPM in 6th. Something around 1900-2100 at about 65-70.
The first gear should get to at least 40 and the rest of the gears should be smooth enough to comfortably and evenly fill the gaps between with even and consistent RPM drops. This would make the best use of the LS4 power band. Of course a modified LS4 powerband. Something like 30-40 MPH per gear would be perfect.

[This message has been edited by Rickady88GT (edited 01-03-2018).]

Rickady88GT JAN 03, 01:07 AM
I found a gear ratio calculator that I can input the gear ratios, axle ratio, tire diameter and RPM for a calculation of MPH in each gear. I input the taller 1st gear of 3.77 and kept the rest of the gears stock for the F40 including the stock 3.55 final drive and the end result was a near perfect 40mph first and about (theoretical) 40mph gain per gear after that.
6500 RPM: basically stock with bolt on bits and dyno tuned.
Final drive ratio: 3.55
1st gear: 3.77 = 38mph
2nd gear:2.04 = 70mph
3rd gear: 1.32 = 107mph
4th gear: .95 = 149mph
5th gear: .76 = 187mph
6th gear: .62 = 229mph
This would make the cruise RPM of 1900
5th gear: 55mph
6th gear: 67mph
And a 1600RPM cruise in 6th at 55mph.
Figure in aerodynamic drag and engine power limitations, and the gear shifts (mph) get closer together as the car goes faster. Starts out about 40mph in first and progressively gets down to around 30 - 20mph or less per gear change with a top speed drag and power limited to about 180. Just a guess, could be way off, but hopefully you understand what I want to accomplish with a gear set change.
The drag coefficient of a Fiero is about.377
The frontal area is about 20 Sq ft.
I guess the weight to be around 3000.
This will require about 320hp to do 180mph.
JUST ROUGH ESTIMATES for bench racing purposes; )
I would like the 6th gear to be the highest speed, not 5th. A high top speed in 5th just makes 6th an economy gear. I would rather have a true close ratio transmission. But most importantly I would like a more practical 1st gear out of the F40. The original stock 1st gear is a joke for an LS. Might as well start off in 2nd. BUT honestly, I have never driven an LS F40, only read comments from others who have. I am trying to figure out the best ratios for my engine, car, driving preference combo and trying to match my GTO.

If I got this correctly, I know now what I am looking for.

[This message has been edited by Rickady88GT (edited 01-03-2018).]

fieroguru JAN 03, 08:24 AM
The 3 challenges are:

1st and 2nd gear are cut on the input shaft and must be changed as a pair.
The G6 1st and 2nd (3.77/2.04) are less than ideal, but they are the longest ones available. The other options are 3.92/2.04 and 4.17/2.13.
If you want 6th to be max vehicle speed, you will want to run a the .740 6th, but this will negate most of the cruise rpm benefit.

High torque V8s in light cars don't need a tight ratio transmission as they have a near flat torque curve and can pull well with a wider spread. This allows 6th to be a cruise only gear with the primary focus being fuel economy and comfort.

This is why I went with the 3.09 final drive to make 1st and 2nd longer (along with all the other gears because my long term goal is to go turbo and they like taller gears) and the car now feels more like a stock V8 car vs being an over geared swap. When you find yourself driving in 6th gear in town (45 mph) your gears are too short.
FieroWannaBe JAN 03, 09:33 AM
Off topic, but, If one wants a 6 speed trans-axle with a very free ratio selection, check out here:
http://eliteracingtransmiss...-sub-menu/fwd250-6s/
This is a sequential (think motorcycle shifting) gearbox and is not a bad price considering. 350ft-lb torque capacity isn't terrible, its better than the F40's rating from GM, although i doubt the criteria for those ratings are not the same.

I own a 2004 M6 GTO, and of the T56 range, a GTO has a close ratio spread from the Z06 in 1-5 with the tallest 6th available from Tremec, and top speed is reached in 4th, with these transmissions depending on aero and if the limiter is turned off. 5th and 6th are overdrive gears, for cruising, since a car spends like 90% of its life below 3000 RPM (unless you drive a 4 speed manual).

For generic performance, Ratios should be chosen so that the RPM drop between gears does not leave the engine outside the "power band". for track use, you need to tailor your gear ratios for your fastest corners, so that corner exit occurs at the start of the "power band" when exiting, so less time is spent shifting.

How OEM manufacturers choose ratios is a mess, and they have many requirements to meet, including cruising, acceleration, towing, starting, stopping, vehicle mass, range, emissions cycle, durability, mileage.

Rickady88GT JAN 03, 11:32 AM

quote
Originally posted by fieroguru:

The 3 challenges are:

1st and 2nd gear are cut on the input shaft and must be changed as a pair.
The G6 1st and 2nd (3.77/2.04) are less than ideal, but they are the longest ones available. The other options are 3.92/2.04 and 4.17/2.13.
If you want 6th to be max vehicle speed, you will want to run a the .740 6th, but this will negate most of the cruise rpm benefit.

High torque V8s in light cars don't need a tight ratio transmission as they have a near flat torque curve and can pull well with a wider spread. This allows 6th to be a cruise only gear with the primary focus being fuel economy and comfort.

This is why I went with the 3.09 final drive to make 1st and 2nd longer (along with all the other gears because my long term goal is to go turbo and they like taller gears) and the car now feels more like a stock V8 car vs being an over geared swap. When you find yourself driving in 6th gear in town (45 mph) your gears are too short.



You are correct on the gear sets, but whats better is that you know what you want. That is huge.
If the numbers I punched in are accurate and the calculator is correct, then that is as close to what I want as I can ask for. True that 6th will not be the fastest gear, given the power limitations, but the ideal cruise RPM is on target. And a taller first gear is a must. A 3.55 final drive brings down all of the final drive ratios 1-6, but with my wheel combo, everything worked out,.....on "paper".
FieroWannaBe JAN 03, 01:56 PM

quote
Originally posted by fieroguru:

When you find yourself driving in 6th gear in town (45 mph) your gears are too short.



I do this all the time in my LS1 GTO (with its factory cam), Engine speed is about 1200RPM, but the LS1 doesn't complain and just churns along down county highways.
Rickady88GT JAN 17, 11:57 AM
I think it is time for an update
fieroguru JAN 17, 11:32 PM
Not much to update. I live in the snow/rust belt so the car has been parked in the garage since mid November. I try to take it for a drive when the temps are above 50 and there is no salt on the ground, but that hasn't happened in the last month.

Also contributing to the lack of progress is that for the last month, I have been traveling for my day job 3-5 days per week, every week, and I expect that to continue through March, possibly longer. This week started as a 3 day trip and was extended to a 5 day one.

The only upgrade I want to get done over the winter is to get the car painted so it is ready for the 35th, but I am hoping I can pay someone to do that for me.