2nd Gear Swap for a Street 915 Transmission - Better Acceleration?

[Gordo2]

Folks,

After loads of research, I've pulled the trigger and bought a different 2nd gear set for my 915/63 transmission. Meanwhile, I feel like I'm entering unchartered waters.

Current Trans and 2nd Gear Limitation

I understand my transmission has a 18:32 / 1.778 2nd gear set. In normal street driving, I feel as if I'm not getting the acceleration that I would like out of this gear. If I'm red-lining from 1st to 2nd, it's reasonably good; it seems to drop RPM to the 3.5k range and spins up relatively quick. Meanwhile, if I accelerate from a slightly lower RPM range (not winding out 1st before shifting to 2nd) it feels somewhat sluggish.

Options Considered / Why a 2nd Gear Ratio Swap

I've picked up a 15:30 / 2.0 2nd gear set to swap in. Based on a gear ratio calculator, it would appear this gear ratio should get me faster acceleration, with a bit of trade off (slightly more RPM drop as compared to stock) when shifting to 3rd. The 2.0 gear set at redline also appears to keep my shifts/ RPM drop slightly above 4k RPM (in the peak / flat torque curve). I'm currently planning to keep the 3-5 gears stock.

For a while, I considered swapping in a 7:31 differential. Meanwhile, I didn't like the idea of 1st gear being even less useful (even shorter time in the gear). I also didn't like the idea of lowering 5th and running higher RPM's on long trips. Additionally, I understand the 7;31 isn't as durable, requires a good shop to install / setup ($$) and can be somewhat of a challenge to find / determine that it's in great shape (buying used).

I also considered a shorter/closer ratio stack (2-4 w/stock 5 for cruise), but I really like 3rd gear as-is. I'm in 3rd for a good majority of my local driving and it provides pretty good acceleration from ~45 mph to redline.

My trans is due for a refresh (200k+ miles) and I figured while I have it apart, I would see what I could do to improve the street performance.

So what's the Downside?

I've researched & reviewed more 915 gearing threads than I would like to think about.
The majority of discussions quickly evolve into details regarding gearing for a track car and certain tracks/road courses. Meanwhile my performance requirement is relatively straight forward: I want to get into second and spin it up to max MPH in the shortest time/distance possible.

Am I off my rocker only swapping 2nd gear?

Gordo

--------------------------

For those running gear calculator's, here's my spec's:
- 3.2L engine built for better low RPM grunt. PMO's with smaller than normal venturis/chokes, SSI's, DC GT2-102 cams, JE 9.5:1 pistons, rev limit set at 6.5k RPM (ARP's bolts etc). I'm hitting ~ 197 ft/lbs (wheels) at ~ 4k RPM
- 255/40R17 rear wheels.

[G450X]

My 915 is sitting in a shop (Chris's German Auto Service near Seattle) for a re-gear.

Based upon much research, I decided against a 7:31 R&P swap. I chose my gear sets to compliment my anticipated rebuild (3.0 late SC). I plan some engine mods on my base SC to have a higher rpm redline, I hate when my SC falls off cam at around 5k rpm. Anticipated engine mods include "early" SC CIS and heads, Max Moritz P&C's, 964 cams, lightweight flywheel & clutch kit, SSI's and an M&K 2/1. I hope for a safe 6800 rpm redline.

Gear sets are 2nd: 15:30, 3rd: 21:31, 4th: 24:27, 5th: 28:24. I debated on spending the money on 5th gear, but with the other gear changes, I felt in was necessary to prevent too much of an RPM drop from 4 th to 5th.

I don't think that changing one gear set (in your case 2nd) will provide you the results you seek. The RPM drop from 2nd to 3rd will increase and your "nice" 3rd now will become a cog too distant from 2nd. Gear set swaps require careful planning for their overall impact on performance. I still feel it is a better overall modification than the 7:31 R&P swap (my concern was that 1st and 5th would be too low), but it is costly.

Good luck with your decision, I suggest a full gear set change (leave 1st, it's low enough) and try to save money by sourcing good used gears. I have been told that GT gears are robust, but a bit louder in operation (I would rather have the whine if they last longer). I sourced 2 GT sets and 2 Albins sets, all were new except my GT 3rd which was in excellent shape.

[Bill Verburg]

Originally Posted by Gordo2

Folks,

After loads of research, I've pulled the trigger and bought a different 2nd gear set for my 915/63 transmission. Meanwhile, I feel like I'm entering unchartered waters.

Current Trans and 2nd Gear Limitation

I understand my transmission has a 18:32 / 1.778 2nd gear set. In normal street driving, I feel as if I'm not getting the acceleration that I would like out of this gear. If I'm red-lining from 1st to 2nd, it's reasonably good; it seems to drop RPM to the 3.5k range and spins up relatively quick. Meanwhile, if I accelerate from a slightly lower RPM range (not winding out 1st before shifting to 2nd) it feels somewhat sluggish.

Options Considered / Why a 2nd Gear Ratio Swap

I've picked up a 15:30 / 2.0 2nd gear set to swap in. Based on a gear ratio calculator, it would appear this gear ratio should get me faster acceleration, with a bit of trade off (slightly more RPM drop as compared to stock) when shifting to 3rd. The 2.0 gear set at redline also appears to keep my shifts/ RPM drop slightly above 4k RPM (in the peak / flat torque curve). I'm currently planning to keep the 3-5 gears stock.

For a while, I considered swapping in a 7:31 differential. Meanwhile, I didn't like the idea of 1st gear being even less useful (even shorter time in the gear). I also didn't like the idea of lowering 5th and running higher RPM's on long trips. Additionally, I understand the 7;31 isn't as durable, requires a good shop to install / setup ($$) and can be somewhat of a challenge to find / determine that it's in great shape (buying used).

I also considered a shorter/closer ratio stack (2-4 w/stock 5 for cruise), but I really like 3rd gear as-is. I'm in 3rd for a good majority of my local driving and it provides pretty good acceleration from ~45 mph to redline.

My trans is due for a refresh (200k+ miles) and I figured while I have it apart, I would see what I could do to improve the street performance.

So what's the Downside?

I've researched & reviewed more 915 gearing threads than I would like to think about.
The majority of discussions quickly evolve into details regarding gearing for a track car and certain tracks/road courses. Meanwhile my performance requirement is relatively straight forward: I want to get into second and spin it up to max MPH in the shortest time/distance possible.

Am I off my rocker only swapping 2nd gear?

Gordo

--------------------------

For those running gear calculator's, here's my spec's:
- 3.2L engine built for better low RPM grunt. PMO's with smaller than normal venturis/chokes, SSI's, DC GT2-102 cams, JE 9.5:1 pistons, rev limit set at 6.5k RPM (ARP's bolts etc). I'm hitting ~ 197 ft/lbs (wheels) at ~ 4k RPM
- 255/40R17 rear wheels.

In general changing one gear will rarely make things better
here is a stock 915/63 run w/ 6500, 245/45x16 tires, 255/40 x17 doesn't afect the drops it is just like a cwp change to taller gearing so your speed in gears will be a tad higher
stock, it does have that obnoxious drop from 1 to 2, but otherwise it's pretty good


w/ 2.0 2nd, now there are 2 obnoxious drops 1 to 2 and 2 to 3

[Kemo]

out of curiosity, why did you opt not to go with the 7.31 R&P?

[Bill Verburg]

Originally Posted by Kemo

out of curiosity, why did you opt not to go with the 7.31 R&P?

Here's why you wouldn't want a 7:31, same drops but speed in each gear drastically reduced, cruise rpm through the roof

[Walt Fricke]

Well, the torque curve on my stock US '82 SC motor shows that I should upshift at about 6,250 RPM in every gear (other than 1st, and I don't use 1st on the track, and don't push the car that hard on the street). This will reduce those drops. The motor is strong at 4,000 RPM, too, so a drop to that speed isn't bad.

I know that the 7/31 isn't favored if you do much long distance highway cruising, though the one trip I took with this gearing in a later transmission wasn't disagreeable, though it was back in the days of the 55 mph speed limits, so I tended to keep below 65. My wife asked what I had done with the engine, as the car seemed so much peppier. I think this is a nice modification for a track car. There aren't a lot of tracks where you will get to 135 mph at 6,500 rpm, and the US rev limiter cuts in at 6,700. If you don't have racing class rules which disallow it, you can up the rev limit to 7,000 for 5th gear, or for saving an upshift at the end of slower straights.

The 8/31 is said to be a stronger gear pair, as the gear roots are larger or some such gear technical stuff. However, if you run a stock 3.0, I don't think you are going to be overpowering the diff gears the way you might if you plopped in a highly modified 3.6.

But I join the chorus singing that just making 2d shorter doesn't seem like a worthwhile modification. Expensive, too, even adding it to a synchro and operating sleeve 200,000 mile rebuild.

[Gordo2]

Thanks for the graph's Bill, agree completely with all your comments.

Meanwhile, I think it should be a fun experiment (I can always swap it back next winter).

Besides, I'm assuming the 2nd gear syncro dog teeth currently in my trans are pretty much worn out ~ $140ish & up to replace. The ones on the 15/30 I just bought are in very good condition ( Guard GT 915 short gears ) . If nothing else, with a few more parts - it should shift smoother.

I ran the graphs before I bought the gear set, and thought it kind of spread the RPM drop across 1-2-3 as opposed to focusing it narrowly in 1-2. Nothing's free - impact to how it transition's to 3rd gear will probably make or break my overall impression.

What pushed me over the edge to press ahead was how the graph showed the 2nd gear swap kept the engine above 4k RPM throughout all gears - a pretty sweet place to be:

My engine dyno (built to run strong at lower RPMs)


Itching to make it happen, unfortunately I'm not planning the trans rebuild until this winter.

Thanks again for the input folks.

Gordo

[Matt Monson]

Gordo,
You are right,when not racing, rpm drops are not the end all. It won't be optimised for acceleration but it should work out decently. It would be very sweet if you got your hands on something like an LT 3rd. But if you don't I don't think you will hate it.

[ganun]

Bill V graphs are part of the story, generate the tractive effort cascades for another way to look at the gear spacing

[Joe Bob]

R&P swap is a better method.

[Walt Fricke]

Gordo - if the dog teeth are worn out, chances are good that the operating sleeve is also. Those parts are often replaced together. New synchros, of course.

[Bill Douglas]

I think it's a good idea Gordo. I also find if I don't give my SC death in first gear it bogs down a little going into second. My SC is quite a high reving beast and doesn't like low revs so your sort of changes appeal to me - keep us posted

[Gordo2]

Closing this thread out with a few comments / observations regarding swapping the 915 transmission's stock 18/32 (1.778) second gear set for a 15/30 (2.00) second gear set.

My 915/73 trans rebuild went sideways at some points, but it's back together and working well: Gordo's 915 Transmission Rebuild

--------------------------------------------------

Street Driving Observations / Comments

I've only had the car & rebuilt trans back on the road for 3 days at this point, however I'm already very pleased.

Today was my first stoplight / stop-and-go / heavy traffic commute (Northern Virginia I95 corridor with choked side roads). Plenty of situations where I would normally be dreading the 1st / 2nd gear choice.

The trans rebuild / refresh definitely made it less dramatic (traumatic?) to get into 1st if I needed to, but I found that I didn't feel the need to put it into 1st unless I was sure I was going to be inching forward for a while - 2nd gear felt comfortable in a ~ 10-15 mph crawl and more responsive if I needed to accelerate to yo-yo with the accordion. A definite positive.

The second gear ratio swap didn't produce as drastic of a change to the driving / shifting feel as I would have expected - I would go as far as to say it's hardly noticeable. Meanwhile, I haven't driven my car since I dropped the trans about 5 months ago, so my frame of reference for what a stock 2nd gear felt like is a bit cloudy. Bottom line, it just feels right - like it was / or should have been a factory option.

Accelerating in 2nd gear is nice - it definitely feels like it pulls harder and faster from lower RPM's to redline - so I would say this is also a positive.

Shifting from 2-3 is smooth, with the feel of the slight increase in RPM drop from 2-3 (as compared to the stock 2nd gear ratio) negligible - another positive.

Overall, even though I've only had the trans / car back on the road a couple of days, I'm confident in saying - it's a keeper and a good modification if you can find a reasonably priced 15:30 / 2.00 gear set to swap in while the trans is opened up for a rebuild.

Thanks,

Gordo

(Bold/italics font used for those who don't really to read full posts - but feel compelled to comment)

[Joe Bob]

Good for you. I rarely use 1st gear as well. Having a low enough gear or the low end torque to use a stock gear is great for bumper to bumper traffic. The only thing better is no traffic.....glad you are back in the saddle again.

[Matt Monson]

Was an option on an sc-rs. Haha. Glad you are happy with it.