Calling on all “gear heads”

MZ3 SBC · MZ3 SBC's Garage

The car..1979 930 dawe 3.4 twin plug with SC cams k27 7006 hybrid making .9 bar at around 3750 RPM. I plan to install the K27 7200 Raptor to get spool sooner....but I want to see how the 7006 with larger billet wheel feels first.

4 speed with stock gear ratios ....and 8.41 Ring and Pinion.
This car is solely for driving pleasure ...zero track duty. Zero drag strip duty.

Trans is being rebuilt so now is the time to consider any gear ratio changes.

I like the 8.41 however I when driving on the highway at 65-70 mph the car begs for an overdrive gear. Acceleration in 4th is great- but not really necessary . I don't mind shifting to 3rd.

Let’ me know what you guys think ...but no I won’t do a G50 conversion lol....

I have attached an old dyno sheet made on a mustang dyno with a tired turbo which has since been rebuilt and modified. ( the dyno over lays a stock 930 with BB exhaust on same dyno performed by dawe as a point of comparison for powerband improvements.....(please - this is NOT a discussion in dyno / power / or anything other looking at powerband shift points please )

Looking a changing the gearing.....ideally would like to optimize the power band...looking at torg curve / hp curve . My sense is the short gearing helps get the car humping before boost....

Example - with my 8:41 ratio and 255-50-16 tires ...shifting at 6500 RPM at 110 mph. It will step down to 4600 RPMs in 4th and pull like a train...but I would trade off a little less gearing for a cruising over drive gear. thinking it would be better if the 4th gear ratio were .514 ( when paired wit the 8.41 matches the oem 930 4th gear ) the .514 would be at 3800 at 110 mph ...still in the power band to pull if I need it

If you look at the speeds in 4th with the 8.41.....you really start humming at 75-80 mph.....quite loud with no muffler........ in 4th at 80 MPH the car is buzzing at 3400 RPMs vs 2750 if were the .514.. ( or a 9.38 with the .625)....and thats likely where I will wind up. Back to the 9.38 and then change 1 gears 1-3 to match what I have now.

I love 2nd (8.41 with 1.389 ) and 3rd ( 8.41 with .89 ) .......I would prefer to leave the 8.41 in there and look for a longer 4th for the highway. But as I understand there is nothing tall enough available to pair to the 8.41 to get me where I want to be.

but separately...if YOU could design a gear box for your 930 to match the power band.....what would it look like?? What are your thoughts....

__________________________________________________ __________________________________________________ _____________________________________________
79 930 3.4 Dawes Motorsports Sport, Bosch Twin Plug, Dual CDI, J&S Boost Control, SC Cams, Euro Fuel Head, Evergreen K27 7006, TIAL .9, Fabspeed Long Neck, Fabspeed Headers, Rennshift Short Shift, KEP Clutch Package, 8.41 R&P, Elephant Racing Suspension, Weltmeister

MZ3 SBC · MZ3 SBC's Garage

Soeed / Gear / RPM
1st line is stock 930
2nd line is my 930 with 8.41 ring and pinion
3rd line is a taller 1st and 4th...considering taller 4th for highway,

Mustang dyno pull of my 930

Mustang Dyno pull of stock 930 with BB exhaust ( same / same )

RarlyL8 · RarlyL8's Garage

I've found that every mod you do has a different affect on use of gearing and that the more mods done the better the fit of the stock ratios. Don't change the gearing, grow into it with the appropriate mods. There are a lot of ways to reduce turbo lag. It would be tragic to change the gearing then do a mod and find you no longer like the new gears.

Restore_84 · 12-29-2020, 01:08 PM

Agreed, great advice Brian.

lite75 · lite75's Garage

Have you driven your car or one with the 9:38 R&P?

MZ3 SBC · MZ3 SBC's Garage

Quote:

Originally Posted by lite75

Have you driven your car or one with the 9:38 R&P?

Sure, several. from fully stock to a mint 88 with Brians Headers .9 Tial and Raptor 7200 turbo...its really grea and that modified 7200 makes boost so much lower than my 7006....... but my 3.4 with cams headers twinplug and only a 7006 and 8.41 pulls much harder ...the issue is the casual highway driving.....it never relaxes and on long drive I wish it had a 5th gear.

those stock R&P feel nice at 65-75 mph just cruising along and mine want to scream...hell the 9.38 really feels nice everywhere. Especially with a rennshift and stomski coupler...like magic....but the question is have you driven an 8.41?

Magurk · 12-29-2020, 08:30 PM

Ive got an 8.37 R&P with shorter 2nd and 3rd gears and i wish 4th was longer. its a rocket ship of course. Depends on the type of driving you are into and what you want to achieve. I inherited my gearing and it is great for being quick up to 80mph fast but for freeway driving like you i'm just too wound up in 4th. If i was doing a tranny build i would not hesitate to modify 4th. I cannot however give you a specific 4th spec as i am not experienced with what would perform well with your 8.41. Doing 110mph at 3800 rpm sounds good. Just like you my car also runs no mufflers, a zork off of 935 headers and keeping the noise down and just being as incognito as possible is sometimes desirable.

RarlyL8 · RarlyL8's Garage

I was thinking in terms of gearing to optimize torque, not gearing just as an overdrive to stay out of boost while cruising. That's a different deal. Do you normally run at 110mph? The typical mods we do have the boost all in by 3000rpm. The engine stays out of boost while cruising at 70mph but can pop on boost real quick which I like. At 110mph yes you'd be on boost all the time. I had a similar problem with the 915 in the old Red Rocket. Cured that with a custom 5th gear. I guess if you want to use 4th gear for cruising the same could be done to the 930 trans. The only caveat is during events where you are doing a power run to top end. I did that often in the Red Rocket and found the large gap between 4th and 5th to be the worst place possible resulting in a loss of momentum when air resistance was nearing it's max.

Porsche 935 · Porsche 935's Garage

It also depends on your powerband and rpm range. I kept the original ring and pinion and changed 2nd, 3rd, and 4th. Stays in powerband but pulls longer in each gear. So basically 4th is like overdrive but when wound up still falls no further than 4000 rpm but good cruising rpm at 75mph.

356911930 · 356911930's Garage

Get a gear chart off of the internet for free and start plugging in gears to see what rpm changes you would like. Start with stock gears and then start switching them out. Guard Transmissions has examples of stock and custom gear ratios that you can use to plug into the chart. The stock transmission first to second gear change always annoyed me. I have a spare 930 short box that I rebuilt and changed gears 2 and 3, 1 and 4 are stock. I have never installed this transmission in my car. This spare transmission has the following gear changes for 2nd and 3rd.

1st 2nd 3rd 4th
16 17 26 32
36 26 27 21

Best of luck,

Rahl

Ollies930 · 12-30-2020, 10:41 AM

What you are planning on doing is the equivalent of having a 5 speed and removing 4th gear. You will not be happy with that large of a space between gears. Like Brian mentioned, the worst place to have a large jump in gearing is on the top end. Your 8:41 R/P is intended for racing on shorter tracks in an 8000rpm 935, not for highway cruising in a street car. What it really boils down to, are you willing to deal with the buzzing and droning on the highway, or are you willing to sacrifice a bit of acceleration and gain comfort, mileage and top speed by switching back to a stock R/P?

MZ3 SBC · MZ3 SBC's Garage

Quote:

Originally Posted by 356911930

Get a gear chart off of the internet for free and start plugging in gears to see what rpm changes you would like.

1st 2nd 3rd 4th
16 17 26 32
36 26 27 21

Best of luck,

Rahl

thanks - but I guess you missed my first 3 posts of this thread....maybe my chart was too confusing...sorry about that

The chart I put up shows the stock 9.38 R&P with stock 1-4 gears.....and my 8.41 with stock 1-4 gears.......comparing speed in each gear from 2000 RPM to 7000 RPM

If anyone is interested ...expand the chart and you can see how swapping the 4th gear to a .514 paired to a 8.41 will match the stock .625 paired to a 9.38 that wi drop the RPM for me by 500 RPM or so on the highway and still have enough roll on acceleration under boost at higher speeds.

the math is interesting

MZ3 SBC · MZ3 SBC's Garage

Quote:

Originally Posted by Magurk

Ive got an 8.37 R&P with shorter 2nd and 3rd gears and i wish 4th was longer. its a rocket ship of course.
.

thats interesting....if I am understanding you.....not only do you have the shorter 8.41 R&P...but you also have shorter 2 and 3 than the 1.3 and .89?

I am sure these individual gear sets are probably hard to find.....other than say Albin....they are probably impossible to source......

MZ3 SBC · MZ3 SBC's Garage

Quote:

Originally Posted by Ollies930

What you are planning on doing is the equivalent of having a 5 speed and removing 4th gear.

If that was the case I would agree with you ...take a look at the chart and you will see that pairing a .514 gear set to the 8.41 R&P is mechanically recreating the oem 4th gear ratio.....and the step from my shift point in 3rd at 6500 RPM /110 mph will go to 3900 RPM/110 in 4th and that will still be at full boost .9 bar so its not awful

equally important optimizing 110 to top speed is of zero importance to me.

i appreciate all the comments...helps me think through

356911930 · 356911930's Garage

If thet is the case then...

Highway crusing is about 80mph or so...

What is your 3rd gear rpm at 80mph and what will it be when you shift to 4th with the .514 gear? Isn't that what you are interestedin?

Rahl

Ollies930 · 12-31-2020, 11:16 AM

When shifting at 6500rpm from 3rd (.893) to 4th (.513), not accounting for drop in speed due to time off power between gears, you would end up at 3734rpm. Consider the dead time between shifts and you might end up under 3700rpm. Personally, I would absolutely hate that.

kamaro · 01-01-2021, 02:07 AM

in your case, its impossible to get the best of both worlds. I got a stock gearing 930 and I LOVE it, good enough acceleration, and nice quiet high way speed.

MZ3 SBC · MZ3 SBC's Garage

All good thoughts......this should more clear of what is happening with an 8.41 /5.125 Ring and Pinion with stock gearing. I really like the gear calculation tool at retro-sport.

https://retro-sport.com/calculator/porsche-gears

You can swap in any gear ration for a number of different Porsche gear boxes and choose between several ring and pinion rations.

As I mentioned the objective was to drop RPM at highway speeds...in the real world for me that's 65-80 mph.

By swapping in a taller ( longer ) 4th gear paired to the 8.41...that is what will happen. So what is optimal is a trade off between comfortable noise decibels and staying near the power band...the step down. For me I think the .514 ratio paired with the 8.41 will do that. And numerically the output on the wheels will match speeds / rpm of the stock 9.38/4.122 with the stock .625 4th gear.

If you look at the torque curve my motor makes power high in the power band by design with cams and 7006 turbo...so yes I will step a little lower but the car still pulls hard from 3000. I will be swapping out the 7006 for a raptor 7200 and that will help fill any dead spot.

the other way I found is to use the 9.38/4.22 Ring and Pinion,,,and select custom gears 1-3....if my gears were shot I would do that but its a more expensive route since I already have the 8.41 which is a strong and $$ part. Full customized gear sets is probably the way to go as an 8.41 is also a tad noisy IMO

Anyway,,,,hope this is clearer. I appreciate the comments. this is 1/2 the fun of ownership....driving and chatting with like minded enthusiasts.....

Outlaw3.4

https://www.instagram.com/p/CJfPBMMlLeD/?igshid=1peoto4j7caoq

Happy New Year.

quattrorunner · quattrorunner's Garage

Quote:

Originally Posted by kamaro

in your case, its impossible to get the best of both worlds. I got a stock gearing 930 and I LOVE it, good enough acceleration, and nice quiet high way speed.

Me too. I found the 4th gear a bit of a gap but love the cruisabikity of it on the highway.

MZ3 SBC · MZ3 SBC's Garage

this is a cool tool...you will need your gear ratios, final drive, torque curve and HP curve. Check it out.....

https://glennmessersmith.com/shiftpt.html

"For any point in time, the highest acceleration is acheived with the highest torque at the wheels. Gears multiply torque, so generally, the lowest gear provides the highest acceleration.

As long as the output torque in 2nd is greater than what you would get from 3rd for any given road speed, you should continue to accelerate in 2nd. Depending on your car's torque curve and gearing, there may be a point where, for a given road speed, you can get the same torque in 3rd as you are getting in 2nd. This is where you would want to shift, as the torque curve is falling, and because of the multplying effect of gears, the output torque in second is falling faster than it would be in 3rd. In this case, more torque is acheived in 3rd gear, so you should use it."