xschop

I'll take the overdrives, Something about 1400 RPM at 70mph long trips makes too much sense....A Race box and Commute box all bundled up.

dprantl

Sure it's worth it. You can't compare the combined merits of both 2.2 and 2.73 928 gearboxes when comparing to the vette 6 speed. Choose one. With more power and a 2.73, 1st gear is useless and 5th gear is annoying on the highway. Over 3,000RPM cruising with a 5.0 liter V8 for a car that claims to be the ultimate GT is dumb IMO. On the other hand, the 2.2 doesn't have enough gears to be able to afford that long 5th gear even with more power, so there ends up being way too much space between 4th and 5th. And if the vette transmission can handle more power, that's another huge reason it's worth it.

Dan
'91 928GT S/C 475hp/460lb.ft

mark kibort

that points to the S4 gear box. 1-4 gears for the track at speeds 40mph to 155mph, and then a comfortable gear to drive 4 hours home from the track , lugging around at 2200rpm at 70mph.

i just poked out some of the gears. you can do the ranges for yourself. Yes, and sure you can pick out the gears or at least gear pairs, becuase are you ever going from 0 to 155 or 0 to 200mph? no, you are not. generally, we all have a sweetspot of speed ranges. SO, with this knowledge, and understanding, you can see that its really the spacing that is important, and the c5 box is near the spacing of our gear boxes. the c5 gear box really doesnt amount to more than just a GTS gear box with an overdrive. and a GTS box vs a S4 box is no different , REALLY, for how its used, mostly, on the race track AND the race track approximates how you will probably use the car on the street. 1st burns rubber, and 2-4th with an S4 box can keep up with any other gear box with no other changes in a varied amount of speed ranges. i.e. 40mph to 155mph, the S4 box works best, with some advantages going to other gear boxes in a slight way, depending on other ranges of speeds, or narrowed operation.

so, there you go . plenty of gears and proper spacing to run from 40mph to 155mph. and you dont drive over 155mph often, do you? didnt think so. SO, there is your answer. AND with a broad HP curve, there is not even a need to have closer gears because you are always in the peak HP range. closer only creates more shifting, for marginal gains. why do you think the Audi R8 didnt have more than 5 gears? it didnt didnt need it.

with my race car , if i could have anything at all, the 6 speed would be the very last on the list, and only if it tighten the ratios up a little. now, sequential shifting,........ another story all together!

want to see what happens in real life??? go look at my car running against a 505hp vet with this transmssion against me for 30 min, at speeed ranges of 40mph to 130mph. even with a 50hp advantage, it sure didnt buy the vet too much over me, now did it?

Mark

mark kibort

http://youtu.be/dj6zCJpsAxY

rear cam video with the c5 Z06 ncorvette racer

xschop

What is the Hiway MPG of a C5/C6 compared to the 928?

I have a 951 box with 3.375 R&P and .72 5th from the NA box. I would die for 1400 RPM at 70, whereas I'm at 2600 RPM now. The T56 is a stronger box and it is not scarce. I think Dinsdale said he picked his up for $1500 including the TT. Speaking of Torque Tube, The C5/C6 is a whole lot easier to rebuild than these steel ones.

dprantl

C5/C6 get's 27 - 28mpg highway...

Dan
'91 928GT S/C 475hp/460lb.ft

Jon B.

Yep. I actually averaged 29-30 during a trip from Texas to North Carolina.

The strength of these vs the 928 is HUGE also. It's not just the added gear.

mark kibort

i have to try pulling out the o2 sensor on my next hyway cruise. they say it can bump up the MPG by 2mph. right now, with the race car going 80ish, im at around 20mpg. not bad.

dprantl

Just pulling out the O2 sensor won't get you much unless your MAF is old and reading lean. And you also don't have a wbO2 installed so you are just insane if you try that. I drove to SITM and back running 17:1 - 18:1 AFR almost the whole way and got just about 24mpg with the 2.73 pinion and a supercharger. I could probably get close to 28mpg with a 2.2 pinion, but that is not a fair comparison since I could get over 30mpg easily doing the same thing with a C5/C6 transaxle in my 928.

Dan
'91 928GT S/C 475hp/460lb.ft

Mike Simard

The Vette gets good highway mileage because it's a modern aerodynamic design. The engine revs matter little.
Any car from the 70s is a bread truck aerodynamically.

dprantl

C4 corvette 6-speed gets 27mpg highway, has a 0.33 - 0.341 coefficient of drag and 19.0 - 19.5 sq/ft frontal area. 5.7l V8, 300hp and 340lb.ft torque.

A 928 S4 has a 0.34 coefficient of drag (with the cooling flaps closed, I think) and 21.28 sq/ft frontal area. 5.0l V8 316hp and 317lb.ft torque. I would think they are pretty close...

My former '86 coefficient of drag was much worse than an S4, yet with the 2.2 rear I was able to get between 25 and 26 mpg on the highway. From what I have heard, 928's with 2.73 rear get between 22 - 23mpg at best. I can get ~22mpg with mine without messing with AFRs.

Dan
'91 928GT S/C 475hp/460lb.ft

blown 87

We got a measured 29.7 MPG on my procharger equiped 87 C4 back when gasoline was all gasoline, this was on a long highway trip.
That is close to double what my 87 928 gets out on the highway, I think I have gotten in the high 18's once in it.

RKD in OKC

At Poker game last night heard of local guy that has been dropping chevy motors in 944s is now putting corvette 6 speeds in 928s. Only details I know so far is that it requires a change to the bell housing.

xschop

This was Doug and Dinsdale's madness. When my new home is finished and I start the 928 hybrid, I'll be going with the Vette 6 speed and a couple extra cylinders.


piranhabrothers@hotmail.com

Renn_Thrills

can anyone elucidate? did someone succeed in stuffing the vette tranny in a shark?