Flipping gears in a 901

[Burg]

Hello Folks,

I am currently rebuilding an ex 914 gearbox for my 550Spyder project. The car will be powered by a 96*71 Typ4 engine with 7500rpm max. The problem is, that the standard gearing in a 901 is way to high giving a calculated top speed of about 160mph when using 215*60*R15 tyres. The tyre size can not be changed for a certain reason.

I have read about "flipping gears" in a 901 tranny meaning that I could use the std. 5th gear flipp it and use it for a third gear, use the std. 4th gear for the 5th gear without flipping and buy a shorter 4th gear from a 915 (???) or another 901 tranny such as a Q gear or close to that.

This should then give me a short geared race box with a top speed of around 130mph which is perfect for the car and will hopefully keep the engine on cam.

Can anyone tell me what he thinks about those ratios and what I do not understand is what "flipping the gears" really means. Does it mean that they will just be turned around or what? The 2 gears of a pair do look different to me so I have a hard time believing that they will just be turned around.

Also, anyone around who has a "Q" gearset for sale?

Many thanks, Burg

[Grady Clay]

Burg,
First, welcome to Pelican
You will find a lot of help here.

Yes, you can flip the 5th gear as you describe.
Search Pelican on my name as I posted a good set of images describing the vector forces that change.
While I am normally adverse to this, your application should be satisfactory.
Your high-rev VW engine should be well within the transmission torque limits.
Be sure and rebuild the transmission properly for longevity.

915 gears are NOT interchangeable with 901/902/904/905/906/911/914/925 gears.
You must find a '901' gear-pair.


Tell us more about your 550 Spyder project.
I owned and raced RS-60 718-055 for several decades.
I ‘test-fitted’ a 901 engine and 914 transmission a few times.
I races with a 587/2 engine and the 718 transmission.

Again, WELCOME.

Where are you located?

Best,
Grady

[Zeke]

To make Grady's post clearer (and he's the consummate expert here), you place the 2 gear set in the 5th position on opposite shafts in the 3rd position. When you do this, you will note that the bevel of the gears will now be oriented in the other direction putting force on the shafts not against the intended thrust washer. What Grady is saying is that your proposed HP and torque set up probably will not hurt the tranny.

Do this with a high torque larger motor and it will not last.

[TimT]

I think I have a Q gearset, I'll check when I get home.

[Matt Monson]

Q 3rd gears are very common. Q 4th gears, as found in the 4spd gearbox are a bit more rare.

I've never had a problem flipping gears, even on some high powered applications, like v8 conversions.

[Burg]

Thank you very much for the nice welcome! I live in Munich only a few yards away from the Wiesn...

The spyder is a 550 Beck Spyder, I bought a couple of years ago from a guy named Dave A. I believe he still lives in the near of New York. He did some very nice mods to the rear suspension of the car which is the reason I bought it from him. I will try to post a picture. Its not the real thing though, I wish it would...

Grady, I just found your fantastic post about flipping the gears and the problems that could be attached to it. Since I won`t be making my hp´s using lots of torque I guess I will be fine though.

I understand that I have a short 3rd when flipping my 5th and that I also have a short 5th when using the unflipped 4th of my 914/901 tranny.

All I would then need is a Q gear set coming out of a 901 which I could then use as a short 4th. Are there any differences in between the Q-sets or could I use any one coming out of a 901? Also, what is your opinion concerning the gear ratios? Cam will be a pauter with about 320degrees so something between a Webcam 86b and a Webcam 86c?


Many thanks again, Burg

ps: here is m spyder....

[Burg]

Tim, yes please check if you have a Qset laying around. My ratios would then be:

1st: 3.091
2cnd: 1.889
3rd: 1.408
4th: 1.125
5th: 0.926

What do you think?

[Burg]

Here is the gear calculation:

RPM 1.gear 2.gear 3.gear 4. gear 5.gear
0 0 0 0 0 0
1000 5 9 12 15 18
1500 8 13 18 22 27
2000 11 18 24 30 36
2500 14 22 30 37 45
3000 16 27 36 45 54
3500 19 31 42 52 63
4000 22 35 47 59 72
4500 24 40 53 67 81
5000 27 44 59 74 90
5500 30 49 65 82 99
6000 32 53 71 89 108
6500 35 58 77 97 117
7000 38 62 83 104 126
7500 41 66 89 111 135


Top speed would be 135 which is more than enough for a spyder. They become very light at the front I heard...

[Grady Clay]

Burg,

These will make very nice gears.
A, 11:34, 3.091:1
F, 18:34, 1.889:1
1/ZD, 1/31:22, 1.408:1
Q, 24:27, 1.125:1
V, 27:25, 0.926:1

The good news is that you can change them again if you want.

What does your car weigh?


I like the rear suspension mod.
More pictures please.

Now, you just need this in place of the VW
.
Type 547/4
8000 44 70 95 118 144
Available, running, less than 150k€ FOB Denver.

Best,
Grady

[Burg]

Grady, my car weighs around 1100-1200pounds. The gears you listed are precisley the same ones I had in mind. Is that a "k" behind the 150? If not whats your phone number?

Here a few more pics of the rear suspension:

[Burg]

I thought this could be of interest too....




Cheers Burg

[Matt Monson]

Nice looking car. Good choice of ratios too...

[Burg]

Thanks Mat, you mentioned that Q sets on a forth are a bit more rare than Q sets on 3rd. What is the difference between those two?

Cheers Burg

[TimT]

I checked, and must have sold my Q gear set..

I built an AFKQV box a few years ago and it was great for short tracks like Lime Rock.. When I ran the car at Watkins Glen.. the gearing was so wrong I only turned a few laps..

1/ZD is a K if iirc.

anyway nice looking car...

[Laneco]

NICE! Really diggin' the rear suspension, Burg!

Of course, I'm a little biased toward spyders - they're all beautiful!

angela

[Grady Clay]

Burg,

Be sure and read the ‘fine print’ (between the two 911SCs) in Angela’s post above.


Keep in mind that the 1st gear in a 901 is a starting gear to get the car moving and not for ‘jackrabbit’ starts.

The reason is the first gear (Fixed Gear I) is cantilevered off the end of the input shaft.
The unsupported shaft can break off.





This was in Paris last month.


Here are the ratios for comparison:

A, 11:34, 3.091:1
F, 18:34, 1.889:1
1/ZD, 1/31:22, 1.408:1
Q, 24:27, 1.125:1
V, 27:25, 0.926:1

A, 11:34, 3.091:1
F, 18:34, 1.889:1
K. 21:30, 1.429:1
Q, 24:27, 1.125:1
V, 27:25, 0.926:1

I have used AFKQV in my street 911 and enjoyed it in the Colorado High Rockies.



901 gear compatibility:
The first gear set (3-pieces) is unique to 901 1st gear.
All three pieces have matching numbers.
There are more than five choices for 1st gear ratios.

The second gear on most street transmissions has Fixed Gear I cut on the input shaft.
Both the input shaft with Fixed Gear I and the Free Gear II have matching numbers.
The two common choices are: F (18:34) and GA (18:32).
There is also a 904 input shaft that accepts splined gears unique to 2nd gear.
There are more than seven choices of 904 2nd gear ratios (including removable F and GA).

Third gear is unique to 3rd gear position because of the angle of the gear teeth.
You discovered this issue researching my post on flipping gears.
There are more than nine choices for 3rd gear ratios.

The 4th and 5th gears are interchangeable.
As you noted, you can ‘flip’ a 4th or 5th gear to use it in the 3rd gear position (and vice-versa) but the ratio is reversed and the gear angle is incorrect.
There are more than 16 choices for 4th and 5th gear ratios.
Search Pelican. There is a lot more information.


There are Q gears for 3rd and Q gears for 4th & 5th, both are 24:27 ratio but are very different parts.
The third gear has Gear I fixed on the input shaft and Gear II is free on the pinion shaft.
The 4th-5th gear has Gear I free in the input shaft and Gear II fixed on the pinion shaft.
If you use a Q 3rd gear (24:27) in a 4th-5th gear position, it will have 27:24 ratio (same as W ratio but gears cut the incorrect direction).




In my 914-6 (both street and race) I converted the axle CV joints to the ’69-’71 108 mm type that use M10 bolts.
The small 914-4 joints won’t take much abuse.
This conversion uses a 914-6 (only) axle shaft to have spline compatibility.
I suspect the only ones available today are aftermarket.


In your ‘tinkering’, you might consider plumbing the fuel system as a circulating system rather than the existing ‘dead-head’ system.
This will circulate cool fuel past the carburetor intake and return the excess to the fuel tank.
This keeps cooler fuel available to the carburetor and reduces the chance of ‘vapor lock’ and carb fires.
Here is a good thread:
Weber fuel recirculation- Questions
There are a LOT more good threads.
Many have direct application for your Beck Spyder.

Are there thermal insulating spacers between the intake manifold and carburetor body?


More pictures please.

Best,
Grady

PS: You have PM.

[djpateman]

Burg, yes unfortunately for you that is a "k" in front of the "€".
In the transmission two gears make up a set, one is 'fixed' (splined to the shaft) and the other is 'free' (rotates on bearings) until it is selected. Fourth & fifth gears are paired the same way, but opposite to the third gears. The cut angle of the gears is the same for all on the mainshaft, and of course opposite for the pinion shaft. So a Q3 is different from a Q4 because the fixed and free gears are reversed. If you were to flip them, not only would the angle of the gears be reversed but so would the gear ratio (it would no longer be a Q ratio). Gears M, N, O, P, Q & R are all available as 3rd gear sets and also as 4/5th gear sets.

[Burg]

Thanks guys, you have been a very great help to me! I was offered the following Q gear set. Still waiting for the sellers response if it is a 4th or a 3rd. From what I can see it should be a 4th but I want to be sure. What do you think? Many thanks again,

Burg

[Burg]

Looking again at the picture, ups and now its looking like a 3rd to me;-( Anyone who has a Q 4th then? Burg

[Grady Clay]

Burg,

Here is what I see:




On a ‘Q’ (24:27) gear, Gear I is 24-tooth and Gear II is 27-tooth.

The top gear is a 24-tooth (green numbers) Free Gear.
Note the bearing surface (teal arrow) for the needle bearing (not there).

The engagement dogs (red arrows) seem attached to the top gear (flat side toward gear) as it should be for a Free Gear.

We cannot count the teeth for the other gear but we can see the splined center (violet arrow) of a Fixed Gear.

If this were a 3rd gear-pair, the Free Gear would be Gear II on the input shaft – that is not the case.
This is a 4th gear-pair with the 24-tooth Free Gear I on top and (assumed 27-tooth) Fixed Gear II underneath.

I also see a “Q” etched on the gear. This is etched in an unusual place (for Porsche) as it is on a machined bearing surface.
Porsche usually etches on the side of a tooth when there is minimum space.
There should be ‘matching numbers’ on both gears.
Typically there are dates and a manufacturer’s trademark.



Now comes the more difficult part ‘by image’: deciding the condition of everything.
There are three main areas of concern.

1. Condition of the gear contact faces.
2. Condition of the bearing surfaces.
3. Condition of the engagement dogs.

Getting good images of the contact faces is very difficult when you are trying to assess the gear face condition.
There should not be rust, pits, chips, scratches or missing metal.
The faces should not have any signs of ‘burning’ from over-temperature or lack of lubricant.
The edges (radial and axial) of the faces should be a radius, not ‘sharp’.

The bearing under the Free Gear should appear smothe and free from damage.
The usual damage is from an out-of-round needle bearing causing axial marks on the surface.

The condition of engagement dogs has been discussed in many places.
1970 (911/01) transmission refurb issues

So … if the unseen gear is a 27-tooth Fixed Gear II and the condition is acceptable, it may be a suitable Q 4-5 gear set.

Best,
Grady