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

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!

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

and all in an AL case 901

Has your tranny arrived yet Aaron?

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

Has your tranny arrived yet Aaron?

not yet.

soon.
thx mark
AA

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 ...
Andy

Yeah, if you're at redline in 2nd and shift to third, what RPM are you it in third? Shift faster?

to keep it in the powerband trekk?

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.

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?

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.
Andy

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...

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?

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. Andy

True, but ask Trekkor where he shifts.

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

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...

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.

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.

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


KT

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...}

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

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

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...

Hi Trekkor,

I saw Randal at lunch yesterday and he said he e-mailed you my gear spreadsheet. That should help you in looking at the power band.

To give you an idea on my "Q" tranny, 99% of the time I would use 2nd and 3rd during Bay Area auto-x's. At longer courses like Alameda, sometimes 4th, but that was VERY rare. I would spin my motor to 7200 and like you are thinking, if I needed more rev due to the course design, I'd switch my limiter selector to 7400.

Your 901 has a 7.3:1 R&P, a 911 box normally has a 8.3:1 (at least in 915 boxes). In other words a 911 usually has a taller gear box. At many of the tighter courses, the taller 2nd gear makes it hard to keep the motor in the torque curve, and when they shift to 3rd, it's too high, creating a flat spot in the torque curve.

For "The Beast", we are regearing the box to make the RPM drops between shifts very close, allowing the motor to be spooled up all the time.

From my experience, regearing a transaxle is a bigtime gain in performance. Once I did that, my car was normally in the Top 5 of a Zone 7 auto-x and on the track we could run down 3.2 carrera's, and Doris and I both own "Y" class lap records at Buttonwillow (CW direction) for PCA-GGR Time Trials (Admittedly, we had a warmed over 2.7 ).........

Basically, when you put a revving six in there, match the gearing, and sort that suspension, you can get fast in a short period of time. Doesn't make for a great cruising on the street, but my car was track only, and yours is getting there.....

Oh...and did I mention you have to start cutting metal out of the doors, f/g hoods, etc., to get your car light?.....

Feel the power of the dark side....young Trekkor.....(insert Darth Vader's breathing here).....

Bill P.

Trekkor,

I agree with Demick's comments with autocrossing you need to make compromise decisions etc...

So... What problem areyou trying to solve? Your call for rpm drops is a solution, which can be achieved by gearing - but we have not heard you state the problem. Is the problem at the last event or is it at each event?

When you state the underlying problem - you may find that gearing is not your solution - but rather the solution may lay elsewhere.

Steve

QUOTE (nine14cats @ Jun 16 2005, 11:39 AM)

Your 901 has a 7.3:1 R&P, a 911 box normally has a 8.3:1 (at least in 915 boxes).

just a couple of tweaks for technical accuracy and completeness...

the R&P ratios are 7:31 (4.429:1) and 8:31.

Type 901-914-911 transmissions (IOW - 900-series pre-915) including the 914.6 used the 7:31 R&P (except for Sportos and a couple of other special-order options...)

QUOTE (ArtechnikA @ Jun 16 2005, 05:04 AM)

with a 1000 rpm drop between gears

first of, i said ~about~ ... each motor will be different. 1000 rpm drop seems too much. i have seen as low as 500 and as high as 800. so maybe my "about 500" was too low ...

let's make it "about 700" ...

happy now?
Andy

QUOTE (SirAndy @ Jun 16 2005, 01:16 PM)

happy now?

not really, especially after a review of the gearing charts showed that there's closer to 2000-2500 rpm drop between gears.

if you shift 2->3 at 4500 rpm (500 above peak torque) you enter 3rd gear at 2000 rpm. the engine just doesn't run there.

if you'd said 500 over peak HP, i agree completely...
(e.g. - on that same shift, 2->3 at 6500, you enter 3rd gear right at 4000 - peak torque...)

QUOTE (trekkor @ Jun 16 2005, 07:16 AM)

It drop below the "Sweet Spot" as Rich called it.

how can you hit the rev limiter in 2nd (at 7000 no less), upshift to 3rd and be below the "sweet spot" ???

your engine would have to drop 3k rpm or more between gears for this to be true ...

a 1k rpm drop between shifts already seems to much (see my previous post), again, after upshifting, you want to be close to peak torque in the new gear ...

i dunno where peak torque is on your motor (do you?) but you'll have to find out before you even consider any gearing changes for your tranny.

that is, of course, just my humble opinion ...
Andy

QUOTE (SirAndy @ Jun 16 2005, 01:22 PM)

after upshifting, you want to be close to peak torque in the new gear ...

aha. now it all becomes clear...

at first, you said shift -at- 500 over peak torque, so that was the source of the confusion...

we're all on the same page now...

QUOTE (ArtechnikA @ Jun 16 2005, 10:27 AM)

aha. now it all becomes clear... at first, you said shift -at- 500 over peak torque, so that was the source of the confusion... we're all on the same page now...

the confusion is the difference in opinions on how much the rpm will actually drop inbetween shifts ...

i have to go to a meeting at 11:30, i'll let you guys know what i find on my way there ...
Andy

QUOTE (SirAndy @ Jun 16 2005, 01:32 PM)

the confusion is the difference in opinions on how much the rpm will actually drop inbetween shifts ...

fortunately, that's not a matter of opinion, it's well documented :-)

i shouldda brought my "speeds in gears" chart from the owner's manual with me, but i didn't, and there's no scanner here anyway...

the drop between gears in a STOCK gearbox gets closer through the box. 4->5 is only about 1000 rpm.

from a GA 2nd to O 3rd is ~2500 rpm.

for those of you with Anderson 2nd, the charts are on p 238...

We had a discussion about shift points a couple of years back.

Shift point discussion

Here is one of my posts from that topic which discusses optimal shift points based on the torque curve. Although the example is based on a /4 torque curve, I think it applies to Andy and Richs' discussion:


If you want to be scientific about your shift point, here it goes:

First, you need a torque curve for the car. I will use Geoff's curve that he posted (I reposted it below so you don't have to look back and forth).

The 914 torque curve is almost flat up to about 4300rpm (I will assume it is flat at 108 ft-lbs).

Second, you need to know how much your gear ratio changes for the shift. For autocross, people are most interested in the 2nd to 3rd shift point, so that is what I will use here. Stock 914-4 transaxles have a 1.889 ratio 2nd gear, and a 1.261 ratio 3rd gear. This means that your gear ratio changes by about 33% when changing from 2nd to 3rd gear. Two important things happen in this change: 1. Your RPM will drop by 33%, and 2. For a given torque generated by the engine, your torque at the rear wheels will drop by 33%. Since it is the torque at the rear wheels that actually accelerates the car, this is a very important number. This also means that we need to make 33% more engine torque in order to maintain the same acceleration when we shift from 2nd to 3rd.

Now, back to the torque curve. Pretty much wherever we make the shift, we will drop back into our 108 ft-lbs of torque region. In order to have gained 33% torque in this transition, we need to have had about 81 ft-lbs of torque before the shift. On the graph, this corresponds to about 5400 rpm. This is the ideal shift point from 2nd to 3rd gear to maintain maximum acceleration.

Working through the same math, ideal shift points for 3rd to 4th is 5300rpm. 4th to 5th is 5250rpm.

Note, this applies to Geoff's '76 2.0, and will change dependent on the torque curve for your car, and any difference in gear ratios you have.

Makes sense?

Demick

Rich - you cannot just make a statement that a set of gears as an X rpm drop between shifts. It is dependent on RPM.

For instance, on a stock /4 box, the difference between gear ratios between 2nd and 3rd is 1.498. So if you shift from 2nd to 3rd at 3000rpm, it will be a drop of 1000 rpm. If you shift from 2nd to 3rd at 7000 rpm, it will be a drop of 2300 rpm.

Demick

How would a lightened flywheel change things if at all?

Would/should you take the light f/w effect into consideration when selecting gears?

Paul

QUOTE (Demick @ Jun 16 2005, 01:45 PM)

Rich - you cannot just make a statement that a set of gears as an X rpm drop between shifts. It is dependent on RPM.

yes - of course. but we were talking about shift points, and the example in question was 6500 rpm.

i agree not all the posts were totally nonambiguous, but there's a limit to how much i'm willing to recap prior posts in each new post :-)

so - for completeness - we are talking about the optimal shift point for low ET... i believe this implies we're talking about higher, rather than lower, rev ranges...

the lightened flywheel effect does two things (trying to stack a couple of responses here...)

one is improve the "engine braking" effect when off throttle.
the other is to make sure that the engine has fully dropped to the revs it's going to be at when you upshift.

in our 2->3 *high rpm* example, if you can shift faster than the time it takes for the engine to coast down ~2500 rpm, letting out the clutch will drag the engine down and unsettle the driveline. it also makes shifts smoother if all the rpms are matched nicely

So, I'm just gong to flip 3rd and 5th hope for the best. And keep a stock 5th from the donor trans. Because I still got to drive it to the events.

This is really making my head spin. Pretty cool none the less.

Here's the problem I have with my current set-up.

Engine pulls very strong up to 7300 rpm in 2nd if I want it to.
When I go for 3rd, there is a short delay while the engine revs up again into the "sweet spot" before it accelerates at the same rate as when I was in 2nd.

I'll have to drive the car when it's dry again and note RPM drops and such.

thanks.

KT

AFKQV... AFKQV...AFKQV

hehehe

QUOTE

When I go for 3rd, there is a short delay while the engine revs up again into the "sweet spot" before it accelerates at the same rate

this is why you want to be higher in the powerband on every upshift.. It get exponentially more difficult to move the car through the air the faster it is going..

so every time you shift from a lower gear to a higher gear, you want to be climbing the torque/hp graph..

If your aims are primarlily doing A/x's with some track time.. and less street time, build yourself a short gearbox...

If the car will be street driven, build a short 4 speed, with a long 5th

I recommend using one of the many gear calculators available to figure out a set up... this will be a comprimise of course..

Trekkor,

I agree with Tim.

AFKQV or the flipped AF"KA"QV....(I think that's the flipped 5th gear). Your call will haul ass. The 2nd to 3rd gearing in your car is a tad spaced and your getting lag until the cam rolls on and the power hits.

It will be worth a ton of saved time on the Auto-x course and still will get you 132 MPH with 22.8" slicks.....



Bill P.

O.K.

Now here's the $____ question:
How much should I expect to pay to get the gears to assemble a short shifting secret weapon.

KT

QUOTE (trekkor @ Jun 16 2005, 08:08 PM)

O.K. Now here's the $____ question: How much should I expect to pay to get the gears to assemble a short shifting secret weapon. KT

The B gear set I bought was 400 bucks.

Common gears are much less expensive. The 3\5 swap is free(if you do your own trans work).
Save your money for wider wheels

The 1974 915 is 7:31

If I ever swap a 915 into my 914, it will be my 1974 915....

Here's some shift point data for you.

Ran 2nd gear up to 7000 RPM's.
Quick shift into 3rd and it's at 4000 RPMS.

Little lag, then the power comes on strong right beween 4500 and 5000. Then it's .

I did this several times to be sure. Same every time.
This is fun and feels great, I'm just leaving some time out there with the lag.

I think what I'm looking for is the gear that puts me at around 4600-4700RPM's out of 2nd. ( Maybe 5000 )

KT

you need a shorter 3rd gear.. it will keep you in the meat of your torque/hp curve..

play with some of the gearing spreadsheets.. and you can get close to what you really need gearing wise

How does this flipped 5th gear deal work?
Is it the same as a "k" gear.

I think I like the "k" gear.

That would drop me in third after spooling up to 7K in 2nd at 5200 RPM's in full song.

stock 4th and 5th for street or track days.

KT

trekkor

a flipped ZD is said to be bitchin....for third...
but then the gap to 4th is too big, so i scored an S gear.
4th to 5th will be a little bigger...but doable

AA