James Bailey

Why did Porsche fit 2:20 gears in the U S ....for fuel economy Why is 5th effectively an "overdrive" ? for fuel economy. How often does a 928 race car with the 2:20 use 5th gear ??

We can now say this thread has been completely Kiborted......HP/TQ/gearing which sadly is not at all like aborted

GlenL

I always wondered why cars had an overdrive gear instead of just a lower final drive ratio. The answer, I've surmised, is that the 2nd-highest gear gains efficiency for being "straight-through" and then the highest gear gains efficiency by having a lower engine RPM. Still, who cruises the highway in the second-highest gear.

I'll use 5th in my current track car with the 2.73 rear but my new track car got a BW tranny with the 2.20 drive. I'll probably use it even if I could use 4th as I'll be using neutral throttle. This is in a banked turn with a 140-150mph entry speed.

My 944 has a huge step going from 4th to 5th. They wised up and figured that redline in 4th was anything anyone would need and then 5th was for highway cruising. At the track it's a problem.

Ducman82

Not yet..... No mention of oil products. :-)

mark kibort

i think more projection going on here jim-bo! someone just made a very interesting observation of a comment, that i reiterated with some chilling facts!

you ask an interesting question though, and to Gregs, point, the gear box as well, is dual purpose (as Gregs intake will be) . let me ask you, how often does a GTS transmission based race car (2.73:1) , use 5th on the race course.... not often how often does the corvette with the 3.42:1 use 5th gear not to mention its 6th on the race track?
FYI.. most race cars at most popular tracks, use 3 gears.. they are lucky to use 4...... unless we are talking factory race cars with close ratio gear boxes

the 2.2 uses 3 gears at the race track.. sometimes it can use 4, but its rare. (and its not 5th)

so the answer here is that porsche did a great job of making a race car like transmission and gear ratios, but also with an overdrive, so that you can comfortably dump the car into 5th at 70mph and cruise with no engine noise and good fuel economy. and the engine still had enough power to accelerate to get to higher speeds , not for passing , but without having to downshift if you didnt want to. i think it was one of the best gear boxes ever designed.

mark kibort

yep, the 2.2 is great for the track, until you get to 155mph...... then, you better not be racing next to anyone, as you just got a big drop in power. we dont get much over 140mph here so its perfect. full use of 2nd, 3rd and 4th.
but, go to cal speedway and its a problem. Still just about perfect for road america. never higher than 154mph there , unless you got 450-500hp, then you are in trouble. )

dr bob

By true definition, "overdrive" is a term that relates to the car's ability to reach peak horsepower RPM with a particular total gear-set and tire-diameter multiplication. If it can't get to peak HP RPM, it's overdrive.

There's a different colloquial definition that's become more "common".

GregBBRD

I'm not going to try or suggest that anyone try to get their 928 to peak horsepower in 5th gear with a 2.20 rear end, with one of my engines. That's a bit faster than my angel can fly.

These are GT cars....not specially designed race cars. Fifth gear works great, on the freeway.

Besides that, 7500 in 4th gear with a 2.20 rear end is moving the mail and is plenty fast for everyone I know.

James Bailey

Ah , clever reference to the Salsbury !!

mark kibort

yep, it was ideal for road america and im sure willow springs as well, and thats only 6600rpm and 155mph!

GregBBRD

"Chart" 7500....plenty fast for this old man.

GregBBRD

I really love the clever pop open door for backfires!

They thought of almost everything....

James Bailey

Then you spin it 7,500 and it is over 175 MPH in fourth gear. Well beyond put me in jail speed !!

Imo000

Saw a black and a white one on the shelf at the Cambridge Surplus yesterday. $69 CDN!! Even took a picture of them. That's practically free with today's exchange rate!!!

mark kibort

indeed!

im wondering now if you are talkling two things now..... survival of a 928 engine at 7500rpm as well as the intake... from the look at the CF dyno runs, it looks like the intake allows for the HP to keep pretty flat from 6000rpm to 7000 and doesn't look like it wants to fall... function of the intake as well as the cams, or just intake? compared to the behavior of the prior Joe and mark engine without the intake, it sure looks like mostly intake limits! ( stock intake showed peak hp curving around 6k, and CF intake was flat from 6000 onward)

either way, the intake kicked up the HP post shift and peak by huge amounts. should be the same or better with your design I would imagine

yep, 176 on 320 or 335 tires! now, will the hp be enough to get it to that speed, that is the question. at road America... I just tapped the rev limiter at all the straights in 4th.. 335rwhp was its limit there in 4th gear.

James Bailey

Mark your former Holbert "stock" S-4 did 176.3 clockwise and 181.8 counter clockwise at the Nardo ring in April of 1986 !!! Prior to taking it to Bonneville where altitude and tire slip took their toll. Speed was said to have been with stock exhaust including catalytic convertors....