gear ratios - 986 Forum - The Community for Porsche Boxster & Cayman Owners

The Radium King · 10-12-2012, 09:15 AM

so, if one were to put a larger motor in their car (i.e., more torque, hp) what would be a better transmission for the track - the 5 or 6 speed? apparently the 6 speed can handle a bit more power, and has closer gearing so that rpms don't drop as much between shifts. the 5 speed ratios are not as tight so that you'll have more rpm drop on shifts but the torquier engine should be able to mange it. further, with longer legs in each gear you wouldn't be rowing as much, which might be a benefit on a more technical track?

JFP in PA · 10-12-2012, 09:35 AM

Five speed.

The Radium King · 10-14-2012, 08:07 AM

and, five speed is lighter and has better axle orientation than the 6 speed ...

BYprodriver · 10-14-2012, 01:12 PM

I wonder if anyone has done a chassis dyno comparison with 5speed & 6speed ?

JFP in PA · 10-14-2012, 01:35 PM

Quote:

Originally Posted by BYprodriver

I wonder if anyone has done a chassis dyno comparison with 5speed & 6speed ?

I doubt that running a comparison on a chassis dyno would show anything useful. Problem is the dyno is measuring torque and imputing HP at any given RPM, neither of which would change as the engine is the same, they would just shift in relation to the wheel speeds. Plus most chassis dyno's don't like gear shifts under power as the car tends to start moving about.

One of the single biggest advantages of the five speed is the difference in weight Radium noted, which is considerable (to the tune of about 100 lb. all in), and the dyno has no idea how much the car weighs.

Bigsmoothlee · 10-15-2012, 06:36 AM

I have a 3.4, and I will be installing an early 5 speed transmission in my car.

The final drive is 3.89:1, so acceleration would greatly improve. Ive always felt that the later 5 speed and 6 speed had gears that were a bit too long...

jaykay · 10-15-2012, 06:46 AM

Doesn't the 6-speed transmission have a higher power transmission capacity?

During my limited track time, I have never used six.....perhaps due the large elevation change on the one long straight I have encountered

Topless · 10-15-2012, 09:07 AM

With a 3.4L I would add a motorsport LSD and trans cooler to preserve reliability. Wheel spin kills em quick.

The Radium King · 10-15-2012, 09:37 AM

yeah, i was looking at the trans cooler components at california motorsports. thinking draw from the drain plug, through a filter, pump, cooler, return to the fill plug. manually control motor with a switch in the cabin (repurpose the less than usefull refrost switch). cooler would probably go in rear bumper opening.

what do the spec guys do?

Topless · 10-15-2012, 01:49 PM

This thread shows how a trans cooler is being plumbed on the 5 spd:
http://986forum.com/forums/boxster-parts-sale-wanted/32117-boxster-spec-5-spd-transmissions-sale.html Eric could fill you in on the details.

At 170rwhp and no wheelspin a stock 5spd is pretty reliable in Spec cars. Add a 3.4L or overly stiff rear springs and get a lot of wheelspin and a hot, unhappy trans. Adding a cooler and LSD does seem to improve reliability but a wild-eyed driver slamming gears in the heat of battle can still do a lot of damage.

My trans was replaced at about 96k with another stock 5spd. It had a failed pinion bearing that was likely a result of inside wheel spin on corner exit.

The Radium King · 10-15-2012, 02:46 PM

i resemble that wild-eyed driver. from what i've read the 'proper' way to do a cooler is to draw from a new opening closer to the diff where the fluid gets the hottest, then return to the drain or fill. this appears to be what Eric has done. just wondering if the quick and dirty approach that doesn't require any machining might suffice.

BYprodriver · 10-15-2012, 03:28 PM

Quote:

Originally Posted by JFP in PA

I doubt that running a comparison on a chassis dyno would show anything useful. Problem is the dyno is measuring torque and imputing HP at any given RPM, neither of which would change as the engine is the same, they would just shift in relation to the wheel speeds. Plus most chassis dyno's don't like gear shifts under power as the car tends to start moving about.

One of the single biggest advantages of the five speed is the difference in weight Radium noted, which is considerable (to the tune of about 100 lb. all in), and the dyno has no idea how much the car weighs.

I realize that. I was thinking about axle angularity effect on driveline loss?

Jittsl · 10-16-2012, 08:08 PM

What is the intended use of the car? I wouldn't rush to put a cooler on a street car unless you drive like a complete a..hole. Cooler only comes into its own under extreme heat, constant high speed and beating on the car.

In answer to how spec racers are set up- mine is exactly as you described other than I have a non-return valve to stop pumps etc. from draining back overfilling the tranny when not turned on. I bought the required parts on eBay and through Summit and spent about $500 on the whole set up. Kit probably not needed.

The Radium King · 10-16-2012, 10:46 PM

street/track use. 3rd gear gets very vague when things heat up at the track; hoping a trans cooler would fix it. that's why i don't want to get too fancy with a temp sensor triggering the pump or anything; just turn it on at the track and off on the street. also don't want to be drilling the case; if i can fix the issue just by repurposing the drain and fill openings then i am happy. given my intended use a check valve seems a good idea; thanks.

jaykay · 10-17-2012, 06:10 AM

+1 6-speed 2nd to 3rd on a tight technical track is just impossible after things heat up. As I recall, the last two sessions of the day can be aggravating.
....as in coasting trying to find third on occasion.

Not sure if this could be due to shifter cable heating as well

Jittsl · 10-17-2012, 10:49 AM

My spec Boxster (obviously trak car) used to have the stiff and vague shift issues when very hot. Cooler hooked in and out at the fill and drain holes has resolved the issue. My suggestion would be make it as easy as you can on yourself and just go with the existing holes and don't do anything more extravagant than the basic set up you described. For track/road car I think the nonereturn would be critical. You might also consider a faucet on the return line so that you can ensure no overfilling. If you do not I would run it every now and again because nonreturn valves often let oil slowly creep by.

When you are building your setup consider ways to make sure that as little oil as possible will want to return under gravity. For example most people mount the cooler with the in and out holes at the bottom. In my case, I mounted the cooler so the nozzles go upwards through the trunk floor and oil is sucked through by the pump. The consequence is that, when not used, the oil stays in the cooler rather than wanting to return to the tranny. Likewise the filter (before the cooler) and pump (after) can both be sloped so that the low end is at the cooler. In the end (in my case) the non return valve (mounted at about the level of the oil in the tranny on the line leading from the drain) only has to stop a small amount of oil contained in the pipe rising up to the filter and the rest simply remains where it is until the pump is run - that is except the oil that was in the return tube which I calculate as part of the standard oil requirements IE when it has drained back the oil is at the correct level or slightly high)

Hope this helps and makes sense, if not send me an email address by PM and I'll get some photos over to you.