Transmission/gearing advice

[DesertSn8ke]

Thought I'd ping the brain trust here. I'm planning on a 363 stroker/TKX combo. 18" wheels and 3.73 gears. Cam is 236/247 on 110 LSA.

Per the Tremec gear calculator:

.81 OD: 75 MPH=3000 RPM; 65 MPH=2600; 55 MPH=2200

.67 OD: 75 MPH=2500 RPM; 65 MPH=2200 RPM; 55 MPH=1900 RPM

Seems like the .81 is fine for 55-65 cruising, but freeways around my neck of the woods are routinely 75. Because of this I'm inclined to option the .67 OD TKX. I know this engine won't want to lug like a stock motor, but it seems to me like 2200-2500 should still be fine for cruising. Thoughts? I'm guessing that around 55 is where it might have more of an issue, and 4th gear might need to be used. 2700 RPM in 4th gives about 55 though, which seems more reasonable to me than long distance cruising at 75 and 3000 RPM with the .81 OD. I'll be going 55 a lot less than I will 65-75.

[Gordon Levy]

too much gear for the torque you will be making. You would plant the power better with a 3:31 or 3:55. You are pushing 2400lbs not 3400lbs.

[DesertSn8ke]

I have considered this. It is a takeout from a Mustang PP. I wanted the Torsen diff so it has the 3.73 in it. Gearing may get changed at some point.

Actual gearing aside, I guess what I'm really trying to figure out is what is an appropriate highway cruising RPM for the engine; 2000-2700? What should I consider upper/lower limits. As long as whatever gear/trans combo gets me in that range...

With a 3.31 the .81 does seem to become the better choice. With the 3.55 seems like you could go either way on the OD gear.

[Gordon Levy]

5th gear really depend on how you want to use the car. If you plan on doing track work then the .81 is a better choice because it is a usable, raceable 5th gear. If you never plan on doing track work, get a .68, is will be better for highway cruising.

[DesertSn8ke]

Ah, ok that is a helpful way of considering it. Or perhaps.... what is the ratio of highway cruising to track days :p (sorry, couldn't resist the pun)

[Gordon Levy]

The .81 is usable because the split is close to 4 and you are still in the power band. The .68 will drop the revs so far in the power band that you will have virtually no acceleration in 5th gear. The top of 4th gear (6500 revs) with a 3:73 with be in the low 130's mph.

[GoDadGo]

Gordon really nailed it with his explanations.

The deepest gear you'd want to run is 3.55 with a 5-Speed.
I'm running a similar cam grind with a 383 SBC with 3.73 rear gears.
I turn 6,000 at 124 MPH in 4th, which drops to 4,500 in 5th and 3,000 in 6th.
Unless you go with a 6-Speed it DOES NOT MAKE SENSE to run gears deeper than 3.55.

My Gear Ratios:
1st - 2.68
2nd - 1.80
3rd - 1.29
4th - 1.00
5th - 0.75
6th - 0.50
Reverse - 2.50

For the record, my .75 - 5th gear isn't worth a hoot on the interstate, but great on highways.
Try to pick your gears so that you turn 2,000-2,500 at 75 MPH since you'll be running a 363.
Finally, my engine is barely happy at 1,700 at 70 MPH and is much happier at 1,900 RPM's.

Again, Gordon really nailed it with his explanations.

Good Luck & Happy Wrenching!

[Cutter 54]

I found a very helpful calculator when deciding on gearing choices. I have a SBF 363 and TKX. Try this:

Blocklayer.com/rpm.

[DesertSn8ke]

@Cutter54 that link/address does not seem to work.

Thanks all for the input. Sounds like I'll go for the .81 OD. Can't say I'm excited about pulling the center section of the IRS out to swap gears though. But.... so it goes with projects. Still better to do it now than later once the body is on. Should have just done it before it went in. I haven't tried, but with that center brace for the IRS, it certainly doesn't look like the gears can be swapped with it in the car. Please correct me if I'm wrong.

[Gordon Levy]

You're not wrong, the chunk has to come out to change gears.

[DesertSn8ke]

Also, as long as I'm at it, looks like I'll probably drop it all the way to a 3.31. Which seems to give a nice combination all around. 55 MPH cruising becomes a 4th gear operation, with 65 and 75 working well in 5th. Plus the benefits of a usable gear on track.

[Jim1855]

Just a comment. Not sure what tracks you intend to run but I've found that my comfort zone was about 140mph and I didn't need to go much faster than that. This was at Grattan and VIR South track.

This was about 6,200 in 4th. TKO-600 .64 OD, 315/35-17, 3.27 gears but wouldn't be much different with 3.31 gears. 3.55s would be about 6,800 for 140 in 4th.

I found that the 4-5 and even more so the 5-4 shift at 130+ was uncomfortable and not a clean shift, real notchy with the TKO. It was far easier to just run it up in 4th.

I only used 5th as a cruising gear. 80mph at 2,200rpm worked well.

You may not even need the 5th gear on tracks.

Jim

[DesertSn8ke]

Yes, prior to this conversation I figured 5th was really only for highway use, hence my pre-disposal to the deeper OD. Either way the 3.31 seems to be a better option for the rear. The .81 OD does still get the revs up a bit for 75 MPH. 2650 to be exact, even with the 3.31. I also figured most tracks don't really see speeds over 140-150, which, with the 3.31 is top of 4th, more or less, as you mention. The 3.73 ends up a bit short on that front. I would be curious what tracks see speeds upward of 150 in these cars.

[Gordon Levy]

Tracks you can do over 150, Road America, brainard, road atlanta, Attesa (600hp), VIR full, NJ Motorsports park, Daytona, Firebird main, Kent, Lime Rock.

[FracAG1980]

Thought I'd ping the brain trust here. I'm planning on a 363 stroker/TKX combo. 18" wheels and 3.73 gears. Cam is 236/247 on 110 LSA.

Per the Tremec gear calculator:

.81 OD: 75 MPH=3000 RPM; 65 MPH=2600; 55 MPH=2200

.67 OD: 75 MPH=2500 RPM; 65 MPH=2200 RPM; 55 MPH=1900 RPM

Seems like the .81 is fine for 55-65 cruising, but freeways around my neck of the woods are routinely 75. Because of this I'm inclined to option the .67 OD TKX. I know this engine won't want to lug like a stock motor, but it seems to me like 2200-2500 should still be fine for cruising. Thoughts? I'm guessing that around 55 is where it might have more of an issue, and 4th gear might need to be used. 2700 RPM in 4th gives about 55 though, which seems more reasonable to me than long distance cruising at 75 and 3000 RPM with the .81 OD. I'll be going 55 a lot less than I will 65-75.

186322186323186324186325

I am not a racing expert by a long shot, but I am an engineer and at least understand the principles in place on gearing and engine performance. I would believe that you would want maximum torque available throughout a lap. Therefore, the gearing should be such that you don't drop below max torque rpms when shifting. I also think you should study video from the particular track you plan to run your car on the most. Look at the maximum straight away speeds and minumum turn speeds in between. Then select the gear combo that gives the speeds you need with the fewest shifts. The last thing you want is to have to shift up for just a second or two of racing and then have to down shift for a turn.
I have a spreadsheet that I use to look at this. The pics attached are the output from that spreadsheet. Put in your rear tire diameter, rear ratio and transmission ratios and the graphs are generated. My local track has a maximum speed (at least the way I'll drive it) of just over 120 mph and a minimum turn speed of about 40-45 mph (which comes right after the maximum staright, if running the track clockwise. So I selected the TKX(0.67) 5th with a 3.15 rear end. I can run the entire lap in 2nd & 3rd and stay at maximum torque (except for a second or two just below max). For me 5th gear is just for cruising and I'll be doing it around 2000 rpm.

"Just my opinion, I may be wrong" - Dennis Miller

[FracAG1980]

Thought I'd ping the brain trust here. I'm planning on a 363 stroker/TKX combo. 18" wheels and 3.73 gears. Cam is 236/247 on 110 LSA.

Per the Tremec gear calculator:

.81 OD: 75 MPH=3000 RPM; 65 MPH=2600; 55 MPH=2200

.67 OD: 75 MPH=2500 RPM; 65 MPH=2200 RPM; 55 MPH=1900 RPM

Seems like the .81 is fine for 55-65 cruising, but freeways around my neck of the woods are routinely 75. Because of this I'm inclined to option the .67 OD TKX. I know this engine won't want to lug like a stock motor, but it seems to me like 2200-2500 should still be fine for cruising. Thoughts? I'm guessing that around 55 is where it might have more of an issue, and 4th gear might need to be used. 2700 RPM in 4th gives about 55 though, which seems more reasonable to me than long distance cruising at 75 and 3000 RPM with the .81 OD. I'll be going 55 a lot less than I will 65-75.

186322186323186324186325

I am not a racing expert by a long shot, but I am an engineer and at least understand the principles in place on gearing and engine performance. I would believe that you would want maximum torque available throughout a lap. Therefore, the gearing should be such that you don't drop below max torque rpms when shifting. I also think you should study video from the particular track you plan to run your car on the most. Look at the maximum straight away speeds and minumum turn speeds in between. Then select the gear combo that gives the speeds you need with the fewest shifts. The last thing you want is to have to shift up for just a second or two of racing and then have to down shift for a turn.
I have a spreadsheet that I use to look at this. The pics attached are the output from that spreadsheet. Put in your rear tire diameter, rear ratio and transmission ratios and the graphs are generated. My local track has a maximum speed (at least the way I'll drive it) of just over 120 mph and a minimum turn speed of about 40-45 mph (which comes right after the maximum staright, if running the track clockwise. So I selected the TKX(0.67) 5th with a 3.15 rear end. I can run the entire lap in 2nd & 3rd and stay at maximum torque (except for a second or two just below max). For me 5th gear is just for cruising and I'll be doing it around 2000 rpm.

"Just my opinion, I may be wrong" - Dennis Miller