Gearing for Road Courses

[Scootsmcgreggor]

So the gearing on most S550's is quite long unless you have the power to push 200MPH+ and also run on tracks that fast. Not most of us I'd imagine. In my case this is for an ecoboost, but looking at gearing of GT's and even GT350's it seems to be the same story. So I'm thinking that when I build a new diff I'll lower the gearing as well. In my case 4.09 would still net 180+MPH in 6th, and freeway cruising RPM's will still be just fine. So is there a chance that shifting more (6spd manual in my case) will cost more time on a road course than you gain in torque multiplication and faster acceleration? I'm coming from 3.55's so a 15% change. There's a lot of info on gearing impact for drag racing, and it seems that the benefits seen there should translate mostly to track use, but just curious for anyone with experience with this. Of course in a real race car you have a quick change rear end and adjust for each track you run, but alas that's not me, not now. Looking forward to any thoughts or comments.

 

[blacksheep-1]

I think you are right on the money with the 4.10 ish ratio. pushrod motors used to like 3.7 to 3.9, but the cammers really like the extra RPM. Realize also that the IMSA cars (before all this GT4 mickey mouse) hit around 170 at Daytona, so I'm thinking 180 should be plenty.. lol. One more important consideration is the gear spacing in your trans, the gears you will probably use most are 2 through 5th, so if that 4.10 doesn't keep you in that proper rpm range for your engine, you'll need to reconsider. Also you will have to recalibrate the car after the gear change, I would recommend HP tuners, they are a bit more complex than other tuners, but they allow a wider range of tune.

 

[Apex Wheels]

Shifting takes time to execute on track, and shifting all the time can also get a little distracting. I've generally seen 3.31 and 3.55 work best on track for 5.0 powered cars with the MT-82. You'll accelerate faster in lower gears than you would in higher gears so keep that in mind too.

For example, at Thunderhill I had consistent 2-3mph faster speeds down the front straight when running a 26.3" rear tire vs 25.2" rear tire. I was able to stay in 4th gear longer with the taller tire.

At the end of the day, use some calculators online and what speeds you hit on track to choose a ratio that works best.

 

[Scootsmcgreggor]

Thanks BS1.

For example, at Thunderhill I had consistent 2-3mph faster speeds down the front straight when running a 26.3" rear tire vs 25.2" rear tire. I was able to stay in 4th gear longer with the taller tire.

This is interesting, and is a good example of why discrete gearing for a particular track makes sense I suppose. Out of curiosity, were you faster overall with the larger OD tire?

For reference here’s the charts for current and 4.09

3.55:


4.09:



2nd gear:
Full boost by 26mph, which is plenty slow for any corners I can think of. 50mph upper limit seems like it would be workable. If I have a corner at say 45mph that needed more room in the gear I’d still be at 4K rpm in 3rd which is full boost so still good.

5th gear: 125mph top speed. Not that high so would need to grab 6th on some longer straights.

It seems like for my local tracks (Buttonwillow, Laguna, Sonoma, thunderhill) I’d be shifting my normal gears from 2, 3, &4 to 3, 4, &5 with some 6th on long straights.

This is all theoretical for me though so interested to hear any and all thoughts on this.

 

[67GTA]

Thanks BS1.



This is interesting, and is a good example of why discrete gearing for a particular track makes sense I suppose. Out of curiosity, were you faster overall with the larger OD tire?

For reference here’s the charts for current and 4.09

3.55:
View attachment 60850

4.09:
View attachment 60851


2nd gear:
Full boost by 26mph, which is plenty slow for any corners I can think of. 50mph upper limit seems like it would be workable. If I have a corner at say 45mph that needed more room in the gear I’d still be at 4K rpm in 3rd which is full boost so still good.

5th gear: 125mph top speed. Not that high so would need to grab 6th on some longer straights.

It seems like for my local tracks (Buttonwillow, Laguna, Sonoma, thunderhill) I’d be shifting my normal gears from 2, 3, &4 to 3, 4, &5 with some 6th on long straights.

This is all theoretical for me though so interested to hear any and all thoughts on this.

My car had 4.10’s in it when I bought it - S197 5.0 with the stock MT82, so clearly not apples to apples but relevant. The MT82 for the EcoBoost is geared slightly lower in 1-4, and I’m sure both motors make their power & torque in different ways. I would run out of gear on AutoClub Speedway front straight at around 130, and shifting into 6th was just way to big of a jump. With aero drag at that speed, it just wouldn’t pull. When the MT82 broke, I made the switch to a Magnum XL. While it’s a costly change, the 4.10 gearing is recommended for the Tremec and helped towards justifying the swap because the rear gear was already done.

Gearing in the Tremec is 2.66, 1.78, 1.30, 1.0, 0.8 & 0.63. Now I shift from 4th to 5th near the start/finish line, and with the 0.8 5th gear, it still pulls up to around 150 by turn 1. Again, the aero drag from the wing becomes the limiting factor to higher top end speeds, however more than makes up for it by allowing higher overall speeds through the corner. A bit of give-take, but I’ve had several Porsche’s catch me by turn 1, only to put the gap back on them by the brake zone of 3 because of the aero advantage.

Long story short, I wouldn’t consider 6th gear being an option at all. With the EcoBoost, I would assume the fall off between gears would be at least comparable to the 5.0, if not more drastic.
Maybe run the same modeling with a 3.73 gear - if that gets your 5th gear direct-drive speed closer to 135ish at 6500rpm, you should be about perfect for any Cali track but ACS, and the car will accelerate harder in your high-use 3rd & 4th gears. Had I kept the MT82, I would have swapped to 3.73’s to be able to stretch 5th gear a bit more.

 

[Fabman]

I'm 4:10 gears as well, which was pretty good but I found myself a little short of gear in a few critical turns where I needed a little more to get to the next corner without shifting up. I have the magnum XL 2.97 trans so I went to the 2.66 trans because 3rd gear is higher and that will hopefully get me to the next corner, plus the ratios are much closer. 3rd to 4th gear shift was a really long run with the 2.97 so the 2.66 is a much better choice for those who are looking at these. For reference I would hit 5th gear just after the start/finish line at Laguna.

 

[67GTA]

For reference I would hit 5th gear just after the start/finish line at Laguna.

It will be really interesting to compare the boosted 3V, wide ratio setup vs your new coyote and close ratio Magnum XL. The higher RPM range should get you T1 brake zone in 4th.

 

[Fabman]

It will be really interesting to compare the boosted 3V, wide ratio setup vs your new coyote and close ratio Magnum XL. The higher RPM range should get you T1 brake zone in 4th.

Hard to say, I wont have the power that got me up to speed so quickly and only about 600 more RPM (7200 vs 7800) but I'll also have better suspension and improved aero so its going to be a crapshoot. Stay tuned.

 

[Scootsmcgreggor]

Thanks for the input and reference points guys. Just what I was looking for. I guess 3.73 will be good to try first, especially since I can just buy a used loaded GTPP diff and be done with it for now. 3.73 will net me ~137 in 5th.

 

[Grant 302]

discrete gearing for a particular track makes sense

Every shift you save...

Is actually a pair...up/down.

 

[Fabman]

Every shift you save...

Is actually a pair...up/down.

True story.
Another side of this is that you are sometimes forced to pull a gear that is too high or over wind your motor in a gear that is too low. Sometimes the rev limiter will decide that for you, which is where I was at.
My son has a 50th anniversary PP s550 w/3:73's and the gear spacing is perfect. I can't believe how hard that car runs with 100% stock everything.

 

[Bill Pemberton]

So much depends on the tracks one runs, and I know I really find the launch I get out of tighter corners running a tire with around a 25.5 to 25.6 inch diameter works the best for me. The fact that these are usually wide muthas, also helps, but because few of the tracks I run hit much over 130mph as their top speed, I have different parameters than the guy who runs a track hitting 140-150. Plus where I normally run has quite fast sweepers and the gearing using a 315/30/18 just seems suited to rocketing through and out such corners. Surprisingly , though the top speeds are not super high the overall averages are quite quick -- two have corners exiting at 100 + and one is blind.

Great conversation and the gearing or tire size will often come down to what works best for a driver at the tracks he frequents the most.

 

[blacksheep-1]

Let me toss out this blast from the past, the M22 (rock crusher) was considered the best transmission that GM ever built in the 60s, virtually every Z28 came out of the factory with that, even the drag racers used it because it was hella strong and had a bearing for the mainshaft instead of the bushing. (I don't recall any of the gearing anymore) this was all well and good until someone broke their M22 and had to run an M21 wide ratio trans at the drags. Remarkably, the M21 made the car go faster, even though the gear spread was actually wider than the M22. Reason was, the M21 had a higher first gear than the M22 and in drag racing Zero to 60Ft is life, they then dropped the diff gearing even more and the car went even faster. Point is, trans gearing is every bit as important as diff gearing to decide on one, without considering the other will be a problem.

Oh what the heck..if I had my way, we'd be running the old Doug Nash 5 speeds, easy to fix, super strong, based on the Ford GT MKIV transaxle. 5th gear was 1 to 1 so you could run say.. a 3.55 for top end and change all the trans gears to accommodate that diff gearing. (they slid right off the shaft) I guess it made way too much sense.

 

[Bill Pemberton]

Bit off subject , but thank you Ford for listening to the folks who track Mustangs and putting the T3060 in the Mach 1 instead of having it share the MT82 like in other Blue Oval Ponies. That is one major contribution for a much stronger track machine than the PP1 or PP2s. Maybe not a GT 350, but quite possibly GT 350 lite?!

 

[Grant 302]

Point is, trans gearing is every bit as important as diff gearing to decide on one, without considering the other will be a problem.

Yeah, a lot of people miss this point. It’s part of the reasoning when I suggested to Matt to run a taller gear and use 2-3-4 instead. The spread of the ratios is wider than 3-4-5 in the MT-82s.

 

[steveespo]

You aren't going to do any better than the 3.55 gear set on a road course, maybe 3.73 but raise rev limit to 7000. I don't know what kind of power your Eco Boost is making but 6th gear will not pull much past 145. 4.09 won't help things unless you track on short courses with max speeds of 120 or so. Big tracks like VIR, Watkins, Road Atlanta, Summit Point Main, even Mid Ohio, you will lose so much lap time due to limited top end. Keep what you have.
Steve

 

[Scootsmcgreggor]

Thanks for the advice Steve. Only making ~350whp so yeah prob not pushing past 145-150 with any gearing.

 

[Apex Wheels]

Thanks BS1.



This is interesting, and is a good example of why discrete gearing for a particular track makes sense I suppose. Out of curiosity, were you faster overall with the larger OD tire?

For reference here’s the charts for current and 4.09

3.55:
View attachment 60850

4.09:
View attachment 60851


2nd gear:
Full boost by 26mph, which is plenty slow for any corners I can think of. 50mph upper limit seems like it would be workable. If I have a corner at say 45mph that needed more room in the gear I’d still be at 4K rpm in 3rd which is full boost so still good.

5th gear: 125mph top speed. Not that high so would need to grab 6th on some longer straights.

It seems like for my local tracks (Buttonwillow, Laguna, Sonoma, thunderhill) I’d be shifting my normal gears from 2, 3, &4 to 3, 4, &5 with some 6th on long straights.

This is all theoretical for me though so interested to hear any and all thoughts on this.

Good data. To answer your question, I was quicker time wise with the shorter tire but that was due to differences in compound. The shorter tire had more grip.

I could see 3.73 working out for this application but I wouldn't go any higher. You do not want to mess with 6th gear at all on track. Honestly the spread you have there now with 3.55s looks great for the tracks you mentioned. You don't get into 5th down the front straight at Thunderhill East? You have plenty of power to go high 120s into the braking zone at T1.

 

[Scootsmcgreggor]

I could see 3.73 working out for this application but I wouldn't go any higher. You do not want to mess with 6th gear at all on track. Honestly the spread you have there now with 3.55s looks great for the tracks you mentioned. You don't get into 5th down the front straight at Thunderhill East? You have plenty of power to go high 120s into the braking zone at T1.

Thank you, noted on 6th gear.

Doing Thunderhill East for the first time in 2 weeks so we’ll see.

 

[steveespo]

Honestly I wonder if 6th gear can be removed from the main and countershaft of the MT-82 during a rebuild. Would save some rotational weight and friction. It is a useless gear for track use.