S15 trans vs S14, which can handle it?

[r1jlewis]

I'm trying to figure out if my stock S15 transmission should be able to handle around 450-500 RWHP? I was running a GT2871R and got 360 out of it, but since then have upgraded again to a hahn hybrid mitsubishi top mount (which wasn't very easy to make fit but I did) that's rated at 720hp. I'm waiting for some 720cc injectors to come in and then getting it retuned, but am kind of afraid that the transmission won't be able to handle it. A couple of people have told me to put an S14 5 speed in place of it. Any facts that I can go by??

[Hijacker]

unfortunately not many people stateside have tested how much power an S15 transmission can handle. Personally, I would leave the 6 speed in there for the time being. If you grenade it, throw in an S14 or S13 transmission. If it lasts, then you saved yourself some cash and still have your 6 speed.

[spider_slayer]

i have only read in tuner mags that s14 transmission > s15 transmission. but how often do the mags actually know what they are talking about? tenkana is right, jsut blow it up. and figure out exactally when and where it breaks so we can all figure out which is actually better.

[Onizuka]

If it were me I would swap it out. Mainly because an s15 6speed transmission is actually worth something where as you can get the s13/s14/sr/ka 5 speed cog swapper for $75. For the money you would make from selling the 6-speed you could get the 5speed and have it rebuilt with fresh seals, syncros and gaskets, plus have the gears and shafts cryotreated AND have money left for beer. Lots and lots of beer.

[r1jlewis]

Just bringing up the beer makes me want to swap it out right now. Do you guys know me, cause that's the way I think too!!! There is one other problem I'm running into with the 6speed. When I'm running it at the track, it reaches redline 7,200 right before I the finish. I thought about getting a little taller tire to see if it would offset it just a little. I wouldn't think the 5speed would have that problem but I'm not sure. Anyone??

[nrod17]

U were pushing it at 360 -- 400 is about the limit that a 6speed can handle. I have had a couple customers go against advice and prove this

[spider_slayer]

U were pushing it at 360 -- 400 is about the limit that a 6speed can handle. I have had a couple customers go against advice and prove this

am i missing something to this pic?

[teddy]

am i missing something to this pic?

yeah, I can only see like 1/7 of the pic too if thats what you're talking about. I guess I'll have to be creative and imagine theres a bombshelled transmission in the picture on fire.

[Silvia007]

This may not be a well researched response but being a S15 have a whole extra set of gears for it's 6th gear, wouldn't the gears be smaller to fit inside what seems to be teh same size transmission housing making it weaker? I think the hassle of a busted transmission either on the road or at the track is not worth it. Just swap in a S14 or S13 transmission.

[Dori Dori]

^Not necessarily.

[r1jlewis]

What exactly do you mean by not secessarily? I did try to email the guy that put the pic on, but didn't get a response yet. I'm kind of scared to see it. I'm in the process of installing a new clutch so I'm going to have the trans. out already. Does anyone happen to have an S14 trans. they are looking to sell? I just really don't want to put the car all back together then break the transmission that S15 transmission. Then I would be out double. How much do you think my S15 trans. is worth?

[naed240sx]

This may not be a well researched response but being a S15 have a whole extra set of gears for it's 6th gear, wouldn't the gears be smaller to fit inside what seems to be teh same size transmission housing making it weaker? I think the hassle of a busted transmission either on the road or at the track is not worth it. Just swap in a S14 or S13 transmission.

Thats correct

[r1jlewis]

I was checking into finding an S14 5 speed, but started thinking. Won't that totally throw off the gear ratio since i'm running the stock S15 rear chunk. I know that the ratio on it is suppose to be different than the others and it's made to match the 6 speed. If I keep that rear end, what will that do to the final drive ratio? I'm not sure what the ratio on the S15 rear end if though.
Modified by r1jlewis at 5:51 PM 1/16/2006

[95lstegman]

yes, that's correct, that the S15 6-speed's gears are skinnier and therefore would be able to transmit less power.

and as for using the S15 diff with integral speed sensor, changing the transmission won't matter. the driveshaft still turns the same number of times for a given vehicle speed.

[CursedGTR]

A friend and I used a S14 transmission with S15 Diff and it did just fine for responsiveness and power delivery. However, we blew up 3 S15 transmission's before we actually put the S14 one in out of necessity. He was pushing 420WHP and shattered the input shaft on the first one and destroyed 3rd gear on the second and third. Also alot of guys here in Japan that build high power SR's either go with the HKS or some oother after market mission or an S13 or S14 Mission, due to the S15 transmission's proven weakness under power. Hope this helps.

[r1jlewis]

Thanks for all the input. It's looks like I should go ahead and find an S14 and sell my S15 while it's still good. If anyone knows some one interested in my S15 transmission, or also might have an S14 transmission for sale, please have them contact me at [email protected]

[95lstegman]

something else you might want to look into if you're going big power would be the Z32/RB25 transmission swap. it can be done, and there is a kit out there now for like $800 or less to do it, and the Z32 5-speed is good for at least 700whp. the gears and shafts are practically twice the size of KA/SR/CA/RB20 transmissions.

[spider_slayer]

hmmm, has anyone ever had their s13 transmission cryo treated? if its able to withstand anywhere in the neghiborhood of 500rwph wouldn't that be alot more cost effective then a $800 z32 transmission swap? you minght even save enough money to cryo treat 2 of them.

[Dori Dori]

yes, that's correct, that the S15 6-speed's gears are skinnier and therefore would be able to transmit less power.

and as for using the S15 diff with integral speed sensor, changing the transmission won't matter. the driveshaft still turns the same number of times for a given vehicle speed.

Can you confirm that or is this a guess? Subaru people operated on the same mentality until someone actually took both transmissions apart to see that the change in housings allowed for not only a 6th gear but also larger 1-5 gears & better syncs. If you can't confirm what you're saying it is not necessarily true hence my original reply. I'd love to know for sure though so please prove what you're saying!

[95lstegman]

sorry to bring this back from the dead, but i wanted this said for posterity's sake.actually, i can confirm this. the gears aren't themselves actually any smaller, so i don't think it has to do with the length of the shafts being longer. notice you need a shorter driveshaft for the S15-R 6-speed. perhaps less stability/more sideloading in the middle causes worse gear interactions. either way, if you're seriously thinking about 500whp, the big momma Z32/RB25 transmission is definitely your choice. the SR/KA/RB20 transmission's input shaft bearing isn't really up to snuff for that power level and the corresponding high-powered clutches you'd need, and the gears nor the shafts are going to tolerate it long, either, unless you baby it. but then, what's the point of 500whp if you can't drive the snot out of it? that would be like having 500whp on a stock radiator. you can use it for like 5 seconds, then back down. why bother then?

[r1jlewis]

Wow, you did bring this back from the dead. I ran into a couple of other major problems that I'm working on right now. I was told by a couple of people down in miami that they have done 500hp out of stock s15 transmission's before and had no problems. I'm sure as long as I don't have drag radials on there and dumping it at 7 grand, it will probably last for a little while. We had it doing 120mhp (spinning 1st-3rd) in the 1/4 before I ran into the block problem.

[95lstegman]

and that only confirms my beliefe that the input shaft and bearing are responsible for the failures. too strong a clutch will kill it by asking it to transmit more power. slowly easing the clutch transmits less power (same torque over greater time = less power), so the shaft lives. if you dump it, esp. against slicks, it has to transmit that same torque almost instantly and SNAP it couldn't take the power. it's one of the things you take into consideration in power transmission in early engineering courses. if i had one in hand i could probably calculate some fairly accurate numbers for how much wheel torque and how quickly the clutch can be dropped (although i couldn't shift based on such a goal anyways; "hey, don't shift faster than 220milliseconds or you transmission will blow up!"), but i doubt that will ever happen.

[89'ClubK's]

Aussies' tell me that the S13 is the strongest gearbox of them all. Don't know if it bolts on direct, but hope that helps. Good Luck!

[r1jlewis]

I have been told that the Z32 transmission can handle 750hp, so I'm actually considering on going that route. I know it's not a direct bolt up, but have heard of it being done before. It's still on the back burner until the motor is back together though.

[180fan]

the s13 transmission will bolt up to the s14/s15 sr's. the z32 transmission needs work done to the bell housing and a hole for the shifter moved back a little bit as well.

[89'ClubK's]

Z32=Heavy. Will the S13 transmission not hold enough power for you?

[l8apex]

I am pushing slightly over 400 whp with mine, and i beat the **** out of it every weekend.

if you read in the aussie forums, its all in the gear oileveryone uses redline anti-shock fluid.

and fyi- the greddy s15 has well over 500 whp and they run a stock transmission with a twinplate

[95lstegman]

you sure they run a stock gearbox? that company does happen to market (although i don't think they actually make for themselves) a slightly beefier 6-speed gearbox with straight-cut gears.