At the Bay Area meet last week, one owner who is planning a 3.2 liter six conversion also talked about substituting a flipped HB (19:31) gear in fifth in place of the standard ZD (31:22) in order to lower engine revs at highway cruising speed and increase fuel economy. Does anyone have any experience with this transmission modification?

Dr. Evil would be the guy to answer this question.
I have a box that has the 3 rd & 5th gear swapped around in it.
It makes for a short 3 rd & 5 th gears. I had it in my 70 914 1.7 in the early 90's.
It was ok for the auto x and short race tracks. I feel that it wasn't good for street driving as 5 th gear was short and gas mileage suffered.
I plan to hit up Dr. Evil when it's time to get my 6 back on the road.
I'd like a nicely geared side shift box to go with my 2.7 RS spec engine.

Talk to Dr. Evil
I'm doing the same I thing H is the tallest gear and HB is 2nd choice.
This is a freeway or MPG gear choice for the big engines.

thats an interesting idea, i hadn't heard of that but that makes total sense. now instead of the ZD 0.709 ratio, the flipped HB would be 0.613 ratio

so given 205/50/15 tires
ZD @ 3000 rpm = 68 mph
flipped HB @ 3000 rpm = 78 mph


makes kind of a big jump between 4th to 5th, you could add a Y gear (0.8214) from 4th to have a smoother transition to the flipped HB. Y was used in many 901's for 5th

I think it was an H gear that was used in mine. Anyway, it lowered the rpm about 800 from what it was previously. Made it much nicer to drive on the highway. At 60 mph, it runs about 2200 rpm.

How long has the gear been in?

I've been thinking of doing this. From what I've read with flipping the gear set reverses the helical and changes the load on the gearstack.

I had an H gear with my V8 and Turbo rotary.

With the V8, you don't use first so it's just like driving a 4 speed. I found myself wanting a gear above 5th to shift into.

With the rotary, I used 1st-5th. It worked great. A true 5 speed and not a 4+1 like the stock 901 setup.

I recommend this mod for any engine that has enough torque to push a taller 5th.

I guess it's HA and HB, mine is the HB which is a bit lower ratio when flipped. I think peeps call it an H gear when really it's an HA?

Anyone know what my final ratio and RPM's would be with a flipped HB and 205/60-15 tires?



Edit: I have corrected my mistake, I did have HA and HB the wrong way around.

It's gonna get you back close to stock gearing, maybe a touch taller. I was running 195/60/15's and that's about 4.7%under driven.

There is a gear chart/rpm/tire size XLS chart in the archives somewhere that Aaron cox posted a long time ago that you can change the tire size and it will give you a final output. The someone was able to also add flipped gears to the chart to give a taller gear ratio and different tire size ratio's.

If you can find it it will tell you what your ratio's will be.

I remember that chart, I can't find it now and IIRC it didn't tell me what ratios a flipped HB would be.

this is a site i look up gearing

http://www.blueskymotorsports.com/index.ph...17&Itemid=6

and a calculator i plug data into to determine rpm / speed

http://www.teammfactory.com/gear-calculator

There also threads via research which does discuss and gives a variety of gearing information including several gearing charts. Doc should be able to provide source/thread information.

Had Doc rebuild my 901 trans with: 1st-A; 2nd-F; 3rd-O; 4th-X; 5th-Flipped H. Although I have had very highway time on the 3.2/trans conversion, it appears to perform very well for the street.. The fifth gear appears to hold 70mph at approximately 2500-2700rpm.

The total trans meets my needs and approval. The final ratio 0.59:1.00) (may have this reversed). The HB Flip is a higher ratio but is the alternate for the flipped H; it will meet most needs for highway travel and lower rpm. The other gears are some what short in the lower end but the 3rd and 4th match to the flipped gears and does meet my needs.

Again, Doc can provide additional info.

Steve