a/x trans gear set options, what works? Options

[Trekkor]

What is your favorite combo.

I need to stay in the 4500-6500rpm powerband.

I can tach out at 7-7300rpm on the limiter.

I have a spare trans I can build up now and then install... whenever.

KT

[john rogers]

If the engine is a big four, say a 2.4L engine, then M-S-X work really well as that makes 2nd, 3rd and 4th close together and a 5th that can be used if running on bigger tracks such as Willow Springs or CA Speedway or Portland. I have a NEW MSX tranny with a "Q" limited slip if interested since it does not work with the 2L six unless I was to race at Bonneville or something!

[Aaron Cox]

im going flipped ZD - S - ZD ir maybe a flipped N for 5th....

and all in an AL case 901 (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[McMark]

Has your tranny arrived yet Aaron?

[Aaron Cox]

QUOTE (McMark @ Jun 15 2005, 09:15 PM)

Has your tranny arrived yet Aaron?

not yet.

soon.
thx mark
AA

[SirAndy]

QUOTE (trekkor @ Jun 15 2005, 07:49 PM)

I need to stay in the 4500-6500rpm powerband

ok, let's back up for a minute ...

what is your problem ? (this is a serious question!)
please describe your problem ...


i'm not sure about the trans i have right now (it's a loaner) but 2nd and 3rd seem like all i need for an AX and the gearing is great.
i have hit the rev limiter (set to 6k right now) in 2nd, but usually where it was possible to shift in 3rd.

i didn't seem like you were having problems with the gearing ...
(IMG:http://www.914world.com/bbs2/html/emoticons/idea.gif) Andy

[McMark]

Yeah, if you're at redline in 2nd and shift to third, what RPM are you it in third? Shift faster? (IMG:http://www.914world.com/bbs2/html/emoticons/happy11.gif)

[Aaron Cox]

to keep it in the powerband trekk?

[Marv's3.6six]

My opinion is that the gear sets depends on where your car makes the best power, what shift points you want, how wide the useable power band is, what wheel tire combo you are running.

I have been working closely with Jim Patrick to optimize my 915 trans with my new 3.6 and 930 wheel combo for my GT clone, we've calculated optimum RPM shift points, RPM drops between all gears, useable engine torque, blah, blah. Trans is done this week all rebuilt with many new goodies! can't wait. (IMG:http://www.914world.com/bbs2/html/emoticons/idea.gif)

[SirAndy]

QUOTE (McMark @ Jun 15 2005, 09:23 PM)

Yeah, if you're at redline in 2nd and shift to third, what RPM are you it in third? Shift faster? (IMG:http://www.914world.com/bbs2/html/emoticons/happy11.gif)

your shiftpoint should be about 500 rpm above max. *torque* !!!

but, you'll have to do a dyno run to figure out where exactly that is ...

there is NO point in spinning the motor any higher than that.
(IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) Andy

[North Bay 914]

Auto-X is a huge variable to be changing course to course. Second is fixed in a 914 Type 901. You can use different 911 type 901 shafts, but it is still machined as part of the shaft. GA for second is very popular 2nd gear choice, and what I will use. I have a 904 shaft (allows second gear to be changed) in my gearbox, but it is possible by using the 911 part.

A Sears Point set-up I was given by an old PORSCHE Racer;

Stock first, GA, KA(flipped 5th) Q, U. This is very short, as U is a low 4th gear on the 901 chart.....makes M,S,X look like a top end set up.

For Auto-X I would go short, but this will effectivly end driving the car on the street much, I drove mine on the street with M,S,X and it was terrible. Very nice set up I must say, but not really good for the street. Nice smooth acceleration with M,S,X. Would have been perfect with a 6th gear...

[J P Stein]

With the stock gears, 22.5 in dia tires & 7100 limiter I get
!st= 36 mph
2nd=58 mph
I haven't bothered to calculate 3rd, but the jump is bigger than the 1-2 spread.

IMO, 2nd is fine, 1st could be higher and 3rd could be lower.
I seldom use 3rd but I shft by ear into it at 6.5-7.0 K.
I have bought a 'B' first to raise the speed to 43mph and actually make it useful in places. I've enuff torque to grunt thru most all the sharper corners.
......but, what do you want for that trans, John?

[McMark]

QUOTE (SirAndy @ Jun 15 2005, 08:33 PM)

QUOTE (McMark @ Jun 15 2005, 09:23 PM) Yeah, if you're at redline in 2nd and shift to third, what RPM are you it in third?  Shift faster?  :evilgrin: your shiftpoint should be about 500 rpm above max. *torque* !!! but, you'll have to do a dyno run to figure out where exactly that is ... there is NO point in spinning the motor any higher than that. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) Andy

True, but ask Trekkor where he shifts. (IMG:http://www.914world.com/bbs2/html/emoticons/lol2.gif)

I admit that I'm all about going to redline as well. ZOOOOOOOOOOM!

[ArtechnikA]

QUOTE (SirAndy @ Jun 16 2005, 12:33 AM)

your shiftpoint should be about 500 rpm above max. *torque*

that can't be right...

your typical 6 has peak torque around 4000, peak HP around 6500.

with a 1000 rpm drop between gears, if you shift at peak torque + 500, you spend at least half your time on the side of the curve where both torque and HP and low.

what studies have shown is that you want to shift when the power in the new gear is no less than the gear you are leaving. this can even be above peak HP and a couple of thousand rpm over peak torque.

running the engine between peak torque and peak HP is the sweet spot.

there may be a study showing low ET benefits from operating below peak torque - i'd love to read it - but it will be with a skeptical eye...

[mskala]

QUOTE (ArtechnikA @ Jun 16 2005, 07:04 AM)

SirAndy,Jun 16 2005, 12:33 AM] your shiftpoint should be about 500 rpm above max. *torque*

I was also going to say that can't be right.

You have to account for the fact that shifting to a higher gear lowers the
torque at the wheels by the new gear ratio. So you'll want to be way
higher than the torque peak in second when you shift to third, to get
closer to the torque peak, but reduced by more at the wheels.

Of course in autocross where things happen so fast and sometimes
you don't want to shift (and my 901 is not a quick shifter), you can
benefit from having a higher redline even though the torque drops
off.

[Trekkor]

My goal is to be able to use 2nd gear all the way up to redline if the course is tight and not enough time to go to third and then back to second again.

On long courses I would like to come out of second at around 6500 and enter third at 4500 rpms.

Is this even possible?

As to shift points. Yes, I take it to redline. ( When I can )
My car drives in the 911 power band. 911's usually just stay in 2nd...

I use the 7000 rpm limiter most of the time.
If there is a stretch of course that has a straight before a hard braking zone and I limit out before I hit the braking zone, I'll pop in the 7100 or 7300 limiting "pill".

The higher the rpm in any given gear regardles of hp results in a higher speed. No surprise there.
Sometimes, there's just no time to benefit from the next gear as with my current trans, It drop below the "Sweet Spot" as Rich called it. Tranlation is I would go slower than if I just kept it in 2nd. (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif)

Sears Point a/x last year was a good example of this.


KT

[ArtechnikA]

QUOTE (trekkor @ Jun 16 2005, 10:16 AM)

On long courses I would like to come out of second at around 6500 and enter third at 4500 rpms. Is this even possible?

sure it's possible, but i think you'd find a 2000 rpm drop between 2nd and 3rd undriveable, especially on the street. *see edit

are you still using the /4 'F' mainshaft? the 914.6 'GA' mainshaft is a good bit taller for a reason, and it will have to be a course with a v-e-r-y long straight before 7300 is not enough... (they exist, but 7300 in 2nd is just shy of 70, and if you're going 70, the big-HP cars can be over 100, which is faster than most organisers like to have in a parking lot.)

you can run even taller than GA in 2nd, but now you're talking big bucks, because you'll need a 904 mainshaft or one of those Sportomatic R&P...

{*edit - okay - i checked the owner's manual and the stock /6 gearset has about 2500 rpm drop 2->3. so a shorter 3rd gear would minimise the drop and keep you in the powerband better. that might leave you with a big drop 3->4 but if you figure that AX is a 1-2-3 game, and big tracks are 4-5 it might not be too bad...}

[Trekkor]

I don't know the details of my stock '74 FOUR trans.
It is a correct sideshifter, but the internals are unknown.
My spare is also unknown.

Thanks for the ideas.

KT

[Demick]

Trekkor

The gearbox will always be a compromise - especially for AX where every course is different. Gearing up in 2nd to be able to go faster on some straights without shifting to 3rd is a trade-off you will pay for when you are coming out of a low speed turn in 2nd - and visa versa.

You've only pointed out one concrete thing that you would like - a 2nd to 3rd drop in RPM from 6500 to 4500. But your stock /4 tranny already pretty much does that (I calculate 6500 in 2nd shifts to 4340 in 3rd).

So like Andy says - exactly what problem are you having that you would like to solve or improve? You haven't made that clear.

Demick

[ArtechnikA]

QUOTE (trekkor @ Jun 16 2005, 10:34 AM)

I don't know the details of my stock '74 FOUR trans. It is a correct sideshifter, but the internals are unknown.

unless someone went to a lot of bother to change to a mainshaft that's not well suited to the car it was in, there's a REAL good chance it's the F mainshaft. GA mainshafts are out there.

i was at -1- AX with a straight long enough to make me think about upshifting (but i didn't...). i used to go to -1- big track where i'd use 2nd. (airport course, long 180's near redline...) and i went to one track with a long enough back straight to be in 5th long enough to write a letter home to mom. that track is a shopping mall now.

here in the East, there are a few serious top-gear tracks tho...