Lets Talk HP numbers / Engines

[YTNUKLR]

Yep, Steve is right, all ROW SC's had the large ports .

[safe]

Allright, news to me. Happy to learn something new today!

[joetiii]

Originally Posted by spuggy

Of all the money I spent, my vote for best "bang-for-the-buck" goes to the asymmetric lockup diff.

I already had 21/30 TBs, but the LSD makes a very useful difference under power, decelerating and through corners. Pretty much everywhere except cruising, really

what does the LSD do to whilst cruising?

[spuggy]

Originally Posted by joetiii

what does the LSD do to whilst cruising?

Well, to put it another way, if you can tell the difference between an open diff and a limited slip one whilst pottering about gently or cruising on a fixed throttle, you're a more sensitive driver than I am.

There are situations where it's obvious there's an LSD in the tranny, but it's completely undetectable to me in the above two.

Even kicking it hard in a straight line, there's no hint there's an LSD in the mix unless one (or both) wheels break traction.

[joetiii]

Originally Posted by spuggy

Of all the money I spent, my vote for best "bang-for-the-buck" goes to the asymmetric lockup diff.
Pretty much everywhere except cruising, really

please expound!

[spuggy]

Joe, sent you a PM.

[chris_seven]

The debate about HP and which transmission to use is interesting.

Surely it is torque that breaks transmissions rather than HP and I would be interested to know what torque figures are considered sensible for the various different boxes taht are used in early cars.

I wouldn't like to use a 901 with much more than around 180 lbsft.

The 7:31 used in the 915 also seems to be an issue but is this due to the amount of correction needed to obtain 'smooth' running with only 7 teeth on the pinion or due to gear case stiffness with mage cases?

Any thoughts?

[spuggy]

Originally Posted by chris_seven

The debate about HP and which transmission to use is interesting.

Surely it is torque that breaks transmissions rather than HP

Agreed.

This is probably the definitive "beef up my 915" thread:

The "How to beef up a 915" thread. Parts and costs?

Hayden says that controlling the differential and the two gear shafts are critical for the 915. WEVO sell a billet replacement diff cover that uses a 930 diff carrier bearing, and a one-piece intermediate bearing retainer plate.

Interestingly, he also cites a limited slip as a durability item by preventing radical rotational speed differences within the diff that breaks the gears on an open diff..

For track use, controlling oil temperatures (cooler) is essential. Ensuring oil gets where it needs to be (spray bars) intuitively seems good.

Mag-case 915's aren't strong. The alu units are stronger (and heavier) and got stronger again when the factory switched from the 3.0 SC to the 3.2 Carrera. Eventually, of course, the factory decided not to use the 915 for the 3.2.

That said, Jack O has a tweaked, mag-case 915 in BBII with the 3.6, and if I'm reading his response to "how long has it lasted" right, it's been in there for 5 years and countless track days...

Hayden's rule of thumb for a 915 with appropriate modifications is apparently 350 ft/lb & 5,000 RPM. In other words, if you make more than 350 ft/lb at less than 5,000 RPM, the 915 is just not going to last.

He also says that there is no guaranteed way to make a 915 live at these power levels. You can drive carefully around the limits and get good usage from the 915 for years, but you can also break it almost at will. And it probably doesn't much matter what you do to it internally either - it'll still break if abused.

and I would be interested to know what torque figures are considered sensible for the various different boxes taht are used in early cars.

I wouldn't like to use a 901 with much more than around 180 lbsft.

"Sensible"? LOL

Well the 914 guys who run small-block V8's with the 901 joke about changing them out with the clutch. Except they're probably not joking...

In that application, they either remove 1st completely or use 1st for moving around the parking lot only, and strengthen what they can.

They do prefer to build high-revving, low-torque motors for that application, but "low-torque" in the context of an SBC lump is, uh, considerably in excess of 180 ft/lbs..

The 7:31 used in the 915 also seems to be an issue but is this due to the amount of correction needed to obtain 'smooth' running with only 7 teeth on the pinion or due to gear case stiffness with mage cases?

Any thoughts?

The 7:31 is apparently an issue for any motor significantly more powerful than the one it was originally shipped with, and I understand that that is entirely down to the strength of the crown wheel and pinion itself.

It's not usually advised for a 3.0, much less anything with more grunt than that.

[Patrice911]

Originally Posted by spuggy

The 7:31 is apparently an issue for any motor significantly more powerful than the one it was originally shipped with, and I understand that that is entirely down to the strength of the crown wheel and pinion itself.

It's not usually advised for a 3.0, much less anything with more grunt than that.

I had a Mag case 915 with 7:31 for many years with my 3.2 with no problem. Doing track and AX with it.
Nothing special was done to it. I rebuilt it with new synchro and some bearing but that was before the 3.2 went in it. The dyno chart that I show up earlier in this tread was done with that tranny. I now got an 84 or 85 Alu. 915 with the 8:31 in the car. With the size tire that am running with, the 7:31 was too short gear, I was on the rev. limiter in 5th at VIR and Road Atlanta, not that fun. With the 8:31 I still got some room left in 5th, and during AX I don't need to go in 3rd that often anymore. Having an Alu. 915 now give me the possibility of making some change to the 3.2 to have more HP and still be on the safe side of the tranny.

[Brant]

My belief on the 901 trannys comes from local race experience.
I'd agree that its probably more a question of torque than hp.
but the local 914 race shop believes that 225-250hp is the max for a 901

this was based completely on cost effectiveness.
it became more expensive to rebuild 901's over a course of time than to convert to a 915 at the 250hp mark.
(this was an observation based on approximately 8 cars)

also these are high rpm race motors with out the torque of a V8.

I use an "F" first gear ratio in my 901/914 box and these have proven to be good up to 200hp, but again thats a high rpm race motor that is not built for torque.

the 914 guys commonly believe that a 901 will survive to 300hp on the street
(and I believe 250 in track conditions)

[Bill Verburg]

torque and heat are what kills a transmission, besides run of the mill negligence

what a trans will take for a short period of time is inversely proportional to what it will take for a long period of time due to the above factors.

that said factory ratings for a warranted trans are
sand cast case 901 used thru B series MY69 -138 ft-lb
die-cast & later mg case 911 used on C & D series MY 70-71 - 148.5 lb-ft
die cast mg case 915 used on E, F, G series MY72-74 181 lb-ft(for racing where durability isn't an issue & w/ cooler - 275 lb-ft)
mg die cast case 915 used on H, I series w/ 8:31 200 lb-ft
al die cast case 915 used on J - K series 225 lb-ft(250 lb-ft w/ cooler)

[Bill Verburg]

For street driving torque is everything, for race torque at high rpm w/ proper gearing is half the battle.

a 250 hp 3.0 isn't going to be as accomodating on the street as a 250 hp 3.6 because of the torque characteristics of each engine, gearing can trade revs for either accelration or speed but not both at the same time.