915 gear ratios for 3.0 high butterfly MFI twin plug new build

[dwa917]

915 gear ratios for 3.0 high butterfly MFI twin plug new build

Trying to lock in on final gear ratios for new build 915 gearbox... Any thoughts to consider appreciated!
Hoping Bill V and other experts will chime in here!

Already acquiesced to having a high RPM at 70 - 80MPH cruising speed
Considering limiting top speed to 150-160MPH (theoretical) and optimising gearing down from there
Not trying to optimise gearing for a specific track
Keeping 8/31 R&P
Keeping stock 1st gear (11:35 AZ)

Engine is 3.0 high butterfly MFI... very close to RSR spec (deviation to a more street-able DC60 vs RSR sprint cam)... will post DIN TRQ and HP numbers... graphs... and thoughts on optimum gear ratios - looking at SCRS and SCRS Opt... (more on that build in a later car build thread – got to find the time)

Car is a '71 that is being built into a '73 RSR look… mainly to FIA spec including RSR brake system, 9 and 11 x 15s, twin plug with 8 pin, Michelin TBs, etc... Mainly for road use, but will be taken on circuits several times a year - Silverstone, Donnington, Brands Hatch, Spa, Zandvoort, etc... (more on that in a separate thread when have the time)…

Donor is a Mag case 915/61 from 1977… Changes will include clutch plate LSD (probably from GT)… and custom gear ratios for 2 through 5 (also probably from GT)… RSR cooling system…

So... Numbers and graphs…

As the car is mainly a road car – optimized to run with air filter (loss of about 7-10HP vs no filter), but a slight increase in torque, and a smoother torque curve

Final numbers with Air Filter



Comparison (No filter Dyno run only to 7,500 RPM)



Looking at SCRS or SCRS optional ratios… 8/31 R&P and Michelin TB15 26/61 (295x40x15) for rears… 615mm diameter (~24.2 inches) unloaded…

Not exactly perfect… So still considering options… Open to forum’s views… Let the games begin…

SCRS Ratios






SCRS Optional ratios

[aston@ultrasw.c]

Looks good to me.

Look at the RPMs after each shift, right in the BIG power ;-)

Nice flat power curve!

[MichaelSJackson]

Start by finding your actual HP and torque curves. This should give you your engine's desired MAX RPM.

As you noted, First is already fixed. Each up-shift should land you in the torque sweet spot. After that you need to determine how you want to drive? Going to be near the top RPM? Or cruising on the torque curve? Some combination?

You state "Already acquiesced to having a high RPM at 70 - 80MPH cruising speed", but "Considering limiting top speed to 150-160MPH". If 5th goes to 150 mph, then you're only using roughly half of your MAX RMP at 70 mph. Probably fairly quiet and good mpg? (Depends on the torque curve.) This could turn 5th into an "overdrive"? Then spread 2nd - 4th thru your expected driving demands?

I'm sure some real wizards will speak up. But start by getting the actual numbers and figure out your driving goals.

[lvporschepilot]

Great looking motor. This is a very timely thread as my very similar 2.8 engine (DC65 cam) has me scratching my head about gear ratios as I believe gears 2-4 are just a bit too tall. I believe the SCRS optional ratios per your graph is spot on for your engine. With peak torque realized at 5700rpm you have 1-2 gear shift point at 5500rpm, which is just right IMO. Your torque curve is very flat and you could shift at 5000rpm but from personal experience I think that makes for too tall of a second gear on up. 74mph+ in 2nd gear would be great if there were 375-400bhp, not 300. A 288GTO is geared tall this way and it works perfect for that car.

Almost, if not equally important though, is your chassis mechanical and overall grip *may* not play well with with such tall ratios. Spirited road driving with such tall 2-3-4 gears may leave you searching for a gear between the two, and the frustration of not maintaining high enough rpm to assure maximum 'poke' out of the corner is frustrating.

[Bill Verburg]

Originally Posted by dwa917

915 gear ratios for 3.0 high butterfly MFI twin plug new build

Trying to lock in on final gear ratios for new build 915 gearbox... Any thoughts to consider appreciated!
Hoping Bill V and other experts will chime in here!

Already acquiesced to having a high RPM at 70 - 80MPH cruising speed
Considering limiting top speed to 150-160MPH (theoretical) and optimising gearing down from there
Not trying to optimise gearing for a specific track
Keeping 8/31 R&P
Keeping stock 1st gear (11:35 AZ)

Engine is 3.0 high butterfly MFI... very close to RSR spec (deviation to a more street-able DC60 vs RSR sprint cam)... will post DIN TRQ and HP numbers... graphs... and thoughts on optimum gear ratios - looking at SCRS and SCRS Opt... (more on that build in a later car build thread – got to find the time)

Car is a '71 that is being built into a '73 RSR look… mainly to FIA spec including RSR brake system, 9 and 11 x 15s, twin plug with 8 pin, Michelin TBs, etc... Mainly for road use, but will be taken on circuits several times a year - Silverstone, Donnington, Brands Hatch, Spa, Zandvoort, etc... (more on that in a separate thread when have the time)…

Donor is a Mag case 915/61 from 1977… Changes will include clutch plate LSD (probably from GT)… and custom gear ratios for 2 through 5 (also probably from GT)… RSR cooling system…

So... Numbers and graphs…

As the car is mainly a road car – optimized to run with air filter (loss of about 7-10HP vs no filter), but a slight increase in torque, and a smoother torque curve

Final numbers with Air Filter



Comparison (No filter Dyno run only to 7,500 RPM)



Looking at SCRS or SCRS optional ratios… 8/31 R&P and Michelin TB15 26/61 (295x40x15) for rears… 615mm diameter (~24.2 inches) unloaded…

Not exactly perfect… So still considering options… Open to forum’s views… Let the games begin…

SCRS Ratios






SCRS Optional ratios

KLooks like you did your hW and nailed it

For a road car I like ~160mph top which w/ you setup is the std SCRS ratios

w/ that gearing everything looks just about perfect

[targa72e]

When I was choosing ratios for my gearbox I thought about things a little different. I had no intention of using the car on the track. I thought more about the speeds I would be driving most of the time and what I wanted the engine RPM to be instead of trying to equally spit the ratios. 1st is stock to just get me moving. I keep a very tall 5th gear because if I am in 5th I am on the highway and want to be at comfortable cruise RPMS.. Most of the other roads I am likely to drive are going to be 40-60 MPH speed limits. I wanted to be able to drive at a comfortable RPM (2500-3500) in 3rd or 4th at those speeds and also be only one gear away from the meat of the power band. This car is a 914-6 conversion so about 2000lbs, RS P&C with Mod S (DC40) cams and webers and headers. Dyno numbers are at the wheels.



I am running 195/60-15 tires and these are the gears I selected



And here are my speeds in gears.



I find these ratios work very well driving on the street and thru the mountains with the power band of the engine. Overall I am quite happy with the ratio selection. 90% of the time I am using 2,3,and 4 on spirited drives. If I were to change anything I might make 2nd a little bit lower as it sometimes feels too close to 3rd.

john

[dwa917]

Originally Posted by Bill Verburg

KLooks like you did your hW and nailed it

For a road car I like ~160mph top which w/ you setup is the std SCRS ratios

w/ that gearing everything looks just about perfect

Thanks Bill... Was thinking of trying to fine tune in-between the two... Perhaps the following? Limits top to ~155MPH... but shortens up 3,4 & 5 to tighten up the drop from 2nd to 3rd... Thoughts?

[dwa917]

Originally Posted by targa72e

When I was choosing ratios for my gearbox I thought about things a little different. I had no intention of using the car on the track. I thought more about the speeds I would be driving most of the time and what I wanted the engine RPM to be instead of trying to equally spit the ratios. 1st is stock to just get me moving. I keep a very tall 5th gear because if I am in 5th I am on the highway and want to be at comfortable cruise RPMS.. Most of the other roads I am likely to drive are going to be 40-60 MPH speed limits. I wanted to be able to drive at a comfortable RPM (2500-3500) in 3rd or 4th at those speeds and also be only one gear away from the meat of the power band. This car is a 914-6 conversion so about 2000lbs, RS P&C with Mod S (DC40) cams and webers and headers. Dyno numbers are at the wheels.



I am running 195/60-15 tires and these are the gears I selected



And here are my speeds in gears.



I find these ratios work very well driving on the street and thru the mountains with the power band of the engine. Overall I am quite happy with the ratio selection. 90% of the time I am using 2,3,and 4 on spirited drives. If I were to change anything I might make 2nd a little bit lower as it sometimes feels too close to 3rd.

john

Thanks John...
I thought about a cruising 5th, but know I won't be happy when on circuits like Spa and Silverstone... And I will be taking it to many circuits... It is really meant to be a dual purpose car... So I'm willing to compromise cruising 'comfort' RPM for tighter ratios on circuit... Nice setup for road use though...
Dave

[dwa917]

Originally Posted by aston@ultrasw.c

Looks good to me.

Look at the RPMs after each shift, right in the BIG power ;-)

Nice flat power curve!

Thanks! really pleased with how the engine turned out...

[dwa917]

Originally Posted by lvporschepilot

Great looking motor. This is a very timely thread as my very similar 2.8 engine (DC65 cam) has me scratching my head about gear ratios as I believe gears 2-4 are just a bit too tall. I believe the SCRS optional ratios per your graph is spot on for your engine. With peak torque realized at 5700rpm you have 1-2 gear shift point at 5500rpm, which is just right IMO. Your torque curve is very flat and you could shift at 5000rpm but from personal experience I think that makes for too tall of a second gear on up. 74mph+ in 2nd gear would be great if there were 375-400bhp, not 300. A 288GTO is geared tall this way and it works perfect for that car.

Almost, if not equally important though, is your chassis mechanical and overall grip *may* not play well with with such tall ratios. Spirited road driving with such tall 2-3-4 gears may leave you searching for a gear between the two, and the frustration of not maintaining high enough rpm to assure maximum 'poke' out of the corner is frustrating.

Thanks. Your combo sounds like it will be nice... I almost went with a 2.8SS, but had a 3.0 donor, so ended up going that path... Pleased with the end result... Thinking somewhere between the two...

[icarp]

Wow well done , I like it .
I vote for the 2.059, 1.476, 1.167, .0962, with max TQ starting @5100rpm, and the tq multiplier for 2nd gear you will gain for the drivability of this choice. Any chicane type turn . Better for acceleration for the 2nd gear, -2mph is not going to hurt you . Then 3rd,4th are just perfect, 5th is a compromise just because , but still in the sweet spot
I always choose the middle close ratios over the ultra close in a sports car . If you run in the rain then the gears are in your favor . You can always drive smoother and better than your competition .

[dwa917]

Originally Posted by icarp

Wow well done , I like it .
I vote for the 2.059, 1.476, 1.167, .0962, with max TQ starting @5100rpm, and the tq multiplier for 2nd gear you will gain for the drivability of this choice. Any chicane type turn . Better for acceleration for the 2nd gear, -2mph is not going to hurt you . Then 3rd,4th are just perfect, 5th is a compromise just because , but still in the sweet spot
I always choose the middle close ratios over the ultra close in a sports car . If you run in the rain then the gears are in your favor . You can always drive smoother and better than your competition .

Thx Ian! thinking along the same lines... trying to find the 'best' compromise...
Cheers
David

[gled49]

These were my race ratios, with a 315/18 tire, good to about 150 @8000. Put the car back on the street, no more track use, now with 295/18 and I left the ratios alone nice street ratios.
8:31
11:35 3.18
17:35 2.05
21:31 1.47
23:28 1.217
25:26 1.04

[Bill Verburg]

That's a very healthy motor, it compares favorably w/ a stock 964, yours is the dashed red line, the 964 was choked by having ssi's on it when it was dyno'd, and not far off from a stock 993 nvram



I added aero effects on these doesn't change the choice much std SCRS is better for a street and all around car, option is better for a pure track car, be sure to use an effective lsd, an asymmetric like GT is best




the shorter the gears the more shifting you need to to, at it's extreme it's like riding a bicycle where you are constantly shifting among the 21 choices, w/ a PDK type trans that's probably not an issue but w/ our old fashioned boxes every shift is another pause in acceleration

The main thing to avoid is crossed tails, gaps in the tails of 1 & 2 are always going to be there

[Jonny042]

I think you're on the right track.

I went with the following in the box for "Project Heavy Metal", big bore 3.2SS MFI. More of a GT car than yours, however.



It's very close to std. SC/RS ratios 1st-4th with a cruising 5th.

I think for road/track (especially the fast tracks you plan to visit) the Std. SC/RS ratios will be a good start, you will definitely love them, and you can shuffle the ratios a bit later if needed (say if you run out of gear or find 2nd to be a bit too low).

Do you have a build thread on the car, or the motor? I've been eyeing up that high butterfly MFI for a future project...

[dwa917]

Originally Posted by gled49

These were my race ratios, with a 315/18 tire, good to about 150 @8000. Put the car back on the street, no more track use, now with 295/18 and I left the ratios alone nice street ratios.
8:31
11:35 3.18
17:35 2.05
21:31 1.47
23:28 1.217
25:26 1.04

Thx Gordon... Looks nice... Lots too think about...

[dwa917]

Originally Posted by Jonny042

I think you're on the right track.

I went with the following in the box for "Project Heavy Metal", big bore 3.2SS MFI. More of a GT car than yours, however.



It's very close to std. SC/RS ratios 1st-4th with a cruising 5th.

I think for road/track (especially the fast tracks you plan to visit) the Std. SC/RS ratios will be a good start, you will definitely love them, and you can shuffle the ratios a bit later if needed (say if you run out of gear or find 2nd to be a bit too low).

Do you have a build thread on the car, or the motor? I've been eyeing up that high butterfly MFI for a future project...

Looks like a nice combination for road use... If I wasn't going to circuits, I would opt for a more 'cruising' 5th for RPM noise and MPG... Imagine that's a sweet ride...
Will do a build thread and also for the engine... just need to find the time to do it justice... and find the right balance... Don't want to start something I can't finish...
Happy to chat solo if you helpful and you are thinking about building soon...

[dwa917]

Originally Posted by Bill Verburg

That's a very healthy motor, it compares favorably w/ a stock 964, yours is the dashed red line, the 964 was choked by having ssi's on it when it was dyno'd, and not far off from a stock 993 nvram



I added aero effects on these doesn't change the choice much std SCRS is better for a street and all around car, option is better for a pure track car, be sure to use an effective lsd, an asymmetric like GT is best




the shorter the gears the more shifting you need to to, at it's extreme it's like riding a bicycle where you are constantly shifting among the 21 choices, w/ a PDK type trans that's probably not an issue but w/ our old fashioned boxes every shift is another pause in acceleration

The main thing to avoid is crossed tails, gaps in the tails of 1 & 2 are always going to be there

Thx Bill. Always appreciate the extra thinking through drag, etc... I'm really leaning toward SCRS Standard, maybe slightly tighten up 4th and 5th to next gear... and then trial and error from there... also completely in sync on asymmetric LSD... and GT for that and gears...