L8 Apex

Just put my car on a dyno for the first time today. I had a blast!

The "reason" (read: excuse) for the dyno run was to determine whether to rebuild my 901 transmission or update to a 915 gearbox. The engine is a 2.7L MFI built on a 911/53 case. Not sure what P&C set was used although I do know that they were replaced when the engine was built. I found a note written in the owners manual that said the original 2.7 had a 10.2:1 cr. Not sure if the previous owner changed this ratio or kept it the same at the last rebuild. It has "S" cams and the MFI tag has been restamped as a "19" when it was built. I found that this designates a 2.7 "S" spec. Anyone have an idea what the redline would be for this, or does it depend on valve / spring combination? I've been keeping it below 7k for safe measure.

The dyno results are below. So, for all of you gearheads out there, the question is, do I rebuild my 901 with the billet intermidiary plate or do I definitely need to go forward with a 915 swap? Not planning on taking the car to the track although I wouldn't mind attending a DE event someday. I'd rather keep the 901 because frankly, I like the shift pattern and I already have the box, so I could spend my money on upgrades (billet plate / closer gear ratios / LSD). Otherwise I have to source the 915 box (more $$) or hope to find one that is a fresh rebuild with the upgrades that I want. Would a 915 box with stock gears be ok, or would a more closely stacked box be very desireable?

Max hp was 195, and max torque was 175. Picture isn't very clear.

Whatcha think?

GrantG

Looks good. FWIW, the stock redline for a 2.7 RS MFI was 7,300. To run higher, better valve springs and retainers are recommended. My 2.7 runs strong to 8,000, but I try to keep it to 7,700 to preserve the flywheel mounting and general longevity. Don't you just love a good running MFI motor

r911

I have not broken mine yet (with 250 hp and lots of torque). I'm told you'll be ok in a street car if you don't do any jack rabbit starts - be careful who drives your car....
If it's really a 911 not a 901, you will have a bit stronger trans. anyway.

Can you post more info on the 901 with the billet intermediary plate? I've heard of it but not seen one and don't know who makes them.

L8 Apex

It is a 911 tranny, not the 901. Steve Weiner (sp?) at Rennsport Systems is who turned me on to the billet intermidiate plate. If I rebuild the 901/911 I intend on having him do the work. I also saw a picture of one in the Performance Products catalog. My understanding is that it just keeps the gear shafts / operating sleeves (not sure of the terminology here) from flexing, or the holes in the magnesium plate would become "oblong". Here in Arkansas we would say "They wuz all wallered out!" Not exactly sure which of these two functions the plate performs but it has someting to do with stronger alignment.

I really like the MFI. It's quieter than carbs but you do feel more connected, more throttle control when applying gas during a corner or for short bursts. It is pretty responsive. I had a '72 T that also had it but I don't think that it was properly set up, or the car just didn't have enough power to recognize the true benefits.

Steve Weiner-Rennsport Systems

Dr. Randy:

901/911 trannys that are going to be subjected to high torque loading and/or raced, need the billet 7075 T-6 Intermediate plate.

This helps hold the transmission shafts in alignment to prevent them from spreading apart and permitting gear teeth to be sheared off. The OEM magnesium ones lack strength & rigidity and are simply too flexible.

r911

Thanks Steve. I'll be calling you for hollow t-bars first -- unless I get a big windfall of $$ soon....in which case, I'd do both.

Curious to see a pic tho.


L8 - you might want to change the title of the thread so others can find this info easier.

Steve Weiner-Rennsport Systems

I do not have a billet Intermediate Plate here at the moment, but when I get another, I'll be sure to shoot some hi-res pics and post them on the website.

L8 Apex

Steve,

Thanks for the info. I need to give you a call to talk about gearsets and talk about pricing on the rebuild & intermidiate plate again. I still think that 1st and 5th are too low. The car just sounds wound out while cruising at 75 on the highway, it's at about 3800 rpm. I know you said this is ok it just doesn't feel comfortable at all.

I also found a 915 box out of a '74 that has a mag case and mechanical speedo drive. Are the stock gear ratios the same in it as my 911? It's $850 and will also need a rebuild. It's on the west coast so I would probably have it sent straight to you if I went that route.

The mechanic who did the dyno for me said that the car would make some more power if the air/fuel was leaned out a little more, but that it would run hotter and this may sacrifice the longevity of the pistons in an air cooled application. Would it be worth leaning it out or should I leave well enough alone?

r911

You realize you have to beat that panel upwards to fit a 915 box, right?

Steve Weiner-Rennsport Systems

I'd need to know what size rear tire you have to see what is going on, but if you are turning 3800 @ 75 MPH, thats kinda low unless you are doing some DE events. Someone might have already changed some gears.

The '74 915 will work, but its gearing is far from ideal for a 2.7 RS-spec motor. As Randy said, the transmission tunnel would need a little tap to clear the 915, but its no big deal at all.

I'd need to see your A/F curve to know if its OK or not.

L8 Apex

Steve,

The a/f curve is at the bottom of the dyno sheet located above. Thanks. I'll give you a call today...

DGaunt

I understand the other issue with the 901 is that first gear is furthest away from the flywheel and a relativly light, elegant, but long input shaft is prone to wind up on hard starts. I'd be interested in Steve's opinion as to what maximum torque it will take before it 'pretzels'.

Steve Weiner-Rennsport Systems

Dave:

The issue is that 1st gear is overhung and away from any supporting bearings at the case or intermediate plate.

Max torque is all about technique. I know people with a 2.2 that have trashed these things from abuse and I know people with a 2.8 that are gentle.

Its all in the hands of the driver.

That said, I would not use a 901 with anything bigger than a 2.7 and I'd be very careful at that.

L8 Apex

I don't launch the car from stoplights, but, it sure is fun to put down some power in first or second after rolling. Is this too harsh? Or does it require dropping the clutch from a standing start to do the damage...

The reason it needs rebuilt at all is because second gear is essentially nonexistant. All of the other gears seem solid, including first. I've read that second gear is part of the mainshaft so the entire component has to be replaced, but some of the receipts I have indicate that the 2nd gear dog teeth were replaced about 10k miles ago. Are the dog teeth the same as the gear itself? The receipts also show 5 synchro rings, 2 op sleeves, and a speedo gear drive. This was done at 102,398 on the odo. At 103,875, the first gear op sleeve was replaced. The car is now at 113,500 and second is out again.

If the 911 tranny is no good for this application (assuming I'm not very disciplined / gentle), and the '74 915 w/ mag case is no good with the stock ratios... then I'm thinking my only / best option is to buy a 915 and have it totally regeared? It does have the 7:31 final drive... how much do gear sets run for the 915?

BTW, I'm running 205/55's.

Steve - tried to call you around 4:00 Pacific but no answer. I'll try back tomorrow.

Thanks,
Jason

L8Apex

Jason,

Nice dyno! I like your screen name too!