GregBBRD

The following is my point of view!

We would put lower gear ratios/short ring and pinions in Turbo 911s, in the old days...at the customer's request.

The cars were always slower, since turbos like to build boost and keep that boost.

My naturally aspirated "hot rod" engines (which don't make anywhere near 750 horsepower) make so much torque that 7 speeds would be absurd. You'd be "skipping" so many gears, you'd do better with the 4 speed.

Perhaps, in order to make 750 horsepower, the ports get so huge that the velocity gets so low (in the port) that the torque suffers so badly that you will need a 7 speed. (Oval ports versus rectangular ports in a BBC come to mind.) The other obvious factor is that to make 750 horsepower, the camshaft is going to be so radical, that torque will suffer from that, also.

RedPanda

Interesting I never thought of it like that.
So a 5 speed auto depending on gearing would be the only "maybe" then? (minus any stroker+supercharged+twin turbo combos)

slate blue

I will give a quick response and maybe more detail later, with more gears it will depend on the speed of the change as to which option is faster. If you have fast changes, the car with more gears will win. This is demonstrated in the new C7 Corvette, the 8 speed auto is faster than the 7 speed manual and prior to rule changes in F1 governing how many gears you could have this was the case. Even now in F1 they are allowed only 8 gears and these ratios are set and not allowed to be varied from track to track. So more gears gives greater scope to deal with varying conditions and intended uses.

The other issue is the gear change sensor that is on the box itself. This is very similar to the sensors that sequential transmissions use to adjust the rpms to "rev match". This is one of my key reasons for choosing the 7 speed over the 6 speed. The engine is capable of more RPMs than a normal synchromesh box would like. So there is a need for the engine to match transmission speed as best it can to enable the syncro to do its work, this function assists the syncro.

I think if you look at my final/combined ratios you will see they are slightly shorter than the stock Porsche ones and given my engine has more RPM capability than stock roughly equivalent to the difference in the ratios, I will be changing at the same road speed but having more gearing to assist acceleration. I have to get back to now, coffee break is over.

Strosek Ultra

That is what people over here tell me all the time. Buy a Corvette or a Mustang and get rid of that old fashion Porsche 928. There is a huge amount of Mustangs and many Corvettes driving around in our local small town. I do not agree, I have been a 928 owner for almost thirty years and plan to keep the Strosek cars for another thirty years even if my wife is talking about a 997 Turbo. Will never buy american iron, those cars do not even have metric threads on the bolts.
Εke

slate blue

You could then ask why has the new 911 got a 7 speed? Yes having up to date technology is good for marketing

slate blue

Just in case there is any confusion I am not using the Corvette rear end/differential. I am using the Porsche 928 differential with modified PSD. So all the mountings are the Porsche factory mountings. There is no cutting and shutting envisaged. I am just updating an area of the car that is required with the extra power, revs and engine capacity.

In this process the torque tube will be changed for a carbon driveshaft at least. What I am planning is to do is investigate the use of the new Z06 full carbon tube, that is driveshaft and tube made from carbon. If this doesn't work out the tube will be aluminium. An advantage of the Vette tube is the lightweight design of their hydraulic throw out bearing. The flywheel will be all alloy running the Tilton clutch.

The one concern I have about my modified PSD setup is keeping the brake fluid cool in the pump. Maybe a larger reservoir? Maybe some ducting to the pump also. The differential will be also cooled but this is not a big big issue.

9x8

Easy: because salesmen had taught us (or the mass consumer, anyway) to always think that "more (or, in some cases, the inverse - less) is better", without questioning the reasoning behind that.

Why do we have 8-core smartphones that last only half a day on a full charge?
Why do we have 40-megapixel (but with so tiny sensor that those extra pixels only generate more noise) cameras?
Why do we have "audiophile" cables that can't be told apart from a coat hanger?

Why do we have any other extremely overcomplicated, "overkilled", "overdone" products everywhere? Because it helps to sell stuff, even if it's completely unnecessary! That's the way it works nowadays.


On topic: I've been driving my GT exactly like Rob said: often going 1-3-5. It's not the optimal way, but it's way more convenient with all that torque.
I'm also driving my Audi RS6 (converted to 6sp) like that as well: 1-2-4-6, or 1-3-whatever.

Also, more gears with MT won't necessarily improve your acceleration times due to extra shifts. It works with AT, DSG or sequential trannies, but how fast can you change gears with MT? How fast are you going to go anyways?

That being said, I'm impressed by your project and I wish you best of luck with it. While your reasoning might be a bit off (a more sincere explanation being "because I can!") - the project itself and amount of thinking behind it is truly amazing!

123quattro

Because the base Carrera engine makes 287 ft-lb of torque. More gears help it. The base C7 engine makes 460 ft-lb of torque. It doesn't need a 3rd overdrive. It's just for marketing and a good sticker fuel economy. I used to work in GM Powertrain and am friends with the guys on the Corvette development team.
Absolutely. My 200 20v with a big turbo is faster in 2nd gear than first.

UpFixenDerPorsche

My thoughts as well. I'll never criticise a new development just because ... but the practicality of 7-speeds manually shifted ... arrr geee. Anyway whatever floats your boat.

After driving my GT for a while in day-to-day suburban traffic I got really antsy abt the short gearing making me shift so much.

I didn't like clutch slipping to start off in 2nd, and 1st was just a 'rrrmmm' to 5mph and then into 2nd. Spent more time shifting than accelerating.

But, as it happened the GT needed new rubber, and the S4 (on stands) had next-to-new rubber. but with a compromise: the rears were 50 profile instead of 45. (One of those emergency situations where I had to take what was available).

OK. 50 profile = larger rolling radius (enough to correct the VDO speedo error. yay.), that also resulted in slightly taller o/all gearing.

This might not sound like much but the taller gearing made the car far more pleasant to drive in stop/start suburban traffic. Also got an improvement in highway fuel economy, while on twisty mountain roads it was great to let the engine rev longer through the taller gear ratios.

Also, I have a v/steep driveway and the car easily 'grumbles' it's way up with a minor amount of clutch slip.

The OE ratios had me totally flummoxed. Even if you were tracking the car every weekend, why would you want such close ratios? You'll spend more time shifting instead of accelerating. Doesn't make sense unless U were running between say 5000 to 7000 rpm on an open sweeping track, but who does that with their average GT?

Back to my GT: the current rubber won't last forever so what are my gearing choices?

(a) - taller diff ratio. (= $$$$$$$)
(b) - 3-piece 18" wheels plus appropriate profile rubber. Have done this before. (= $$$$)

Open to suggestions, as long as they don't = $$$$$$$$$$$$$$$ LOL.

FBIII

Back to my GT: the current rubber won't last forever so what are my gearing choices?

Your original gearbox is worth its weight in gold and if severly damaged, with limited parts availability, might be unrepairable. Maybe find a late gearbox with the borg warner syncros and the 2.20 final drive and squirrel the original gearbox away.

slate blue

Quote:

Originally Posted by GregBBRD

The following is my point of view!

My naturally aspirated "hot rod" engines (which don't make anywhere near 750 horsepower) make so much torque that 7 speeds would be absurd. You'd be "skipping" so many gears, you'd do better with the 4 speed.

I think that less gears may well apply for the street at low speeds. Certainly not highway speeds, this is where the 7 speed comes into its own. I think we should ignore the fact that the TR6070 has 3 overdrive gears also, I know you didn't bring that point up but it is combined ratio with the diff that counts. Depending on how the clutch reacts at street starts I may be able to start off in 2nd as second is lower than the 2nd in the 928. In which case first will be used to get up my driveway and starts that are on a incline. If we look at the Ferrari 599 it has a very low first gear much lower than what I am proposing and I suspect that is because of the F1 system may be hard on clutches. So the point being again, I may need that slightly lower gearing to stop excessive clutch wear.

If we are talking performance, again you would need to look at the final/combined ratios, mine as I put them up in the first post are just slightly lower than the 928 and slightly wider when you consider the first four gears which will be used on the race track a lot. Remember also that the engine has a higher usable RPM range as such you wont be changing anymore than in a 928, in fact I would say slightly less. Fourth gear will be capable of around 150 mph/250 kph. It is just that the 7 speeds offers more flexibility, the next step to 5th isn't such a big jump and 6th is only slightly taller and designed for this car to set its top speed in. Seventh gear is purely economy.

Quote:

Originally Posted by GregBBRD

Perhaps, in order to make 750 horsepower, the ports get so huge that the velocity gets so low (in the port) that the torque suffers so badly that you will need a 7 speed. (Oval ports versus rectangular ports in a BBC come to mind.) The other obvious factor is that to make 750 horsepower, the camshaft is going to be so radical, that torque will suffer from that, also.

This engine is not going to be a perfect engine, many items are absolute state of the art however, the engine was only going to be 6.0 litres and the throttle bodies and headers were purchased with that capacity in mind. Those parts are very expensive, if somebody wants to buy 45 mm world's best throttle bodies let me know, they will make your 5.0 litre sing



However they may not be so good on the 7.5 litre, the company that makes them has had customers get 330 hp from 2.7 litres (4cylinder engine) They have also flowed them on a bench and they lost less than a couple of cfm of flow over a completely clear tube, as such the flow rate from these small throttles is around 345-350 cfm i.e the same as the projected heads. The air is flowing straight an fast at that point of the intake tract. However again the intake is bent to get the approximate 14"/355 mm that is required for the engine to be strong in the 7000-7500 range.

The intake port needs to be in the 2.6 to 2.7 sq" range to flow 350 cfm at around 300 FPS. The reason I say there is a range, we will need to test the intake and if the intake has lost a lot of flow we will taylor the intake port to a smaller size given the reduced airflow. The heads may flow 350 cfm but if the intake can only do 325 cfm that is where we will be.

I think you may have been a bit hasty to assume that the port will be so big and lazy like BBC. The Porsche 928 port or in this case the 944 port is a very downdraft port as such it responds differently.



The above except from David Vizard explains this best. In the upper part of the same page, you can see the idealised intake tract. This is very similar to my intake, which starts off with a 55 mm runner however the bell mouth is even larger, at the 90 degree turn (this is a modified spider intake) the runner tapers to the 45 mm throttles and then at the port is expands again to around the equivalent of approx 47 mm. The plenum is as large as I can make it and is many times in capacity of the original factory plenum. It is a tribute to that design though.

The Tri-Y headers also help build a broader torque range than 4 into 1 versions do. Also the valvetrain is as close to a roller cam train as I can make it. It will use the radius tappets, which weigh 30 grams and use Ti intakes that weigh less than 70 grams. So a quick valve action should be possible. This again builds torque.



Lastly I would say that this engine uses lots of friction reducing technology as such it may not need to make as much power as a conventional engine, that is it has greater efficiency. Anyway we will see in good time.

danglerb

Marketing likes more gears, more is better right?
MPG likes more gears.
Drag strip racing likes a really low first gear, so more gears for normal driving.

mark kibort

engine torque is not the issue, its rear wheel torque. quite simply, optimizing hp. if you have 750hp and it's a flat curve up top, you only need gears that allow you to stay in that range. close ratio gears are needed to optimize the HP curve, if it is peaky. it does no good to have more gears if you are already optimizing the HP curve. e.g. if you have a 25% drop in RPM and the HP curve doesn't fall with the 25% drop in RPM, then closer gears buys you nothing.
High engine torque vs HP is only an indication of a flat HP curve and that's desirable for a given HP.

7 speed gear boxes are more beneficial to a peaky HP curve. optimize the HP you put down to the ground and you optimize the torque that the wheels see. most often, this means you NEVER see or want, peak torque used from the engine.

mark kibort

...........................….Corvette…driveline……..Porsche….driveline
First gear...............…2.97…….10.90………….3.764….…10.26
Second gear.............2.07……..7.60…………..2.515…….6.85
Third gear.........….....1.43………5.25…………..1.79………4.88
Fourth gear............…1.00………3.67…………..1.35………3.69
Fifth gear.................0.82………3.01…………..1.00………2.73
Sixth gear................0.68………2.50……………..8% less rpm
Seventh gear......…….0.48……...1.76……………35% less rpm
Reverse...............….2.90

in actuality, it is , as was already said, about marketing. this 7 speed gear box , for all practical purposes, is a 4 speed gear box, that you will use 3 gears at the track, so its a 3 speed for the track, and 7 speed for leaving the supermarket and going onto a hyway in Montana.

1st is usless on the track, but great to inch along in traffic in LA. 2nd is a decent 2nd gear with a top speed of about 80ish. 3rd is close to the stock S4 3rd as well, top speed 115mph, and 4th is close to the S4, albeit a little lower (10%), so top speed 140ish. the rest?????? 5th, same as the S4 with a 2.73, and then you have two more gears, one to give you S4 2.2 hyway RPM and then some super overdrive to put you at 700rpm at 70mph. both bad ideas for cam pitting, and carbon build up.
with sequential injection of the Corvette, that's less of a chance, but lugging around a 928 motor, especially a big one with batch fire injection, is probably a bad idea.

so, the net net of all this. 7 speed sounds cool...... functional, no. stronger? yes.

slate blue

Quote:

Originally Posted by mark kibort

...........................….Corvette…driveline……..Porsche….driveline
First gear...............…2.97…….10.90………….3.764….…10.26
Second gear.............2.07……..7.60…………..2.515…….6.85
Third gear.........….....1.43………5.25…………..1.79………4.88
Fourth gear............…1.00………3.67…………..1.35………3.69
Fifth gear.................0.82………3.01…………..1.00………2.73
Sixth gear................0.68………2.50……………..8% less rpm
Seventh gear......…….0.48……...1.76……………35% less rpm
Reverse...............….2.90

in actuality, it is , as was already said, about marketing. this 7 speed gear box , for all practical purposes, is a 4 speed gear box, that you will use 3 gears at the track, so its a 3 speed for the track, and 7 speed for leaving the supermarket and going onto a hyway in Montana.

1st is usless on the track, but great to inch along in traffic in LA. 2nd is a decent 2nd gear with a top speed of about 80ish. 3rd is close to the stock S4 3rd as well, top speed 115mph, and 4th is close to the S4, albeit a little lower (10%), so top speed 140ish. the rest?????? 5th, same as the S4 with a 2.73, and then you have two more gears, one to give you S4 2.2 hyway RPM and then some super overdrive to put you at 700rpm at 70mph. both bad ideas for cam pitting, and carbon build up.
with sequential injection of the Corvette, that's less of a chance, but lugging around a 928 motor, especially a big one with batch fire injection, is probably a bad idea.

so, the net net of all this. 7 speed sounds cool...... functional, no. stronger? yes.

Basically i agree that it will be the first four gears, Bathurst and some other circuits will see a top speed around 180 mph which is where the slightly lower 5th gear is useful, 6th gear as mentioned is for setting the top speed of the car.

The engine is a special build using the highest quality pieces and technology available, so I will not be using the factory injection system. It will be the Motec 150 Build ECU. This allows the user to write their own firmware like an ECU manufacturer would. The injectors that will be used will be Injector Dynamics, said to be the most advanced in the world and according to the machinist who builds the worlds most powerful rotary engines from billet. "They are the best for performance, idle and economy. Turn them up to 120 psi and you can achieve wonders."

http://youtu.be/xFOTtSrFugM

As to cam pitting, the billets are steel, old designation being EN40B or currently known as 722M24 and has the following qualities;

EN40B is suited for applications that require excellent resistance to wear and abrasion combined with high fatigue strength. Typical applications include shafts, extruders, gear wheels, drills, guides, pins, spindles and bolts. Widely used in the automotive, textile and general engineering industries.

It will be superfinished and the tappets as seen in a previous post are DLC coated. I would say the likelihood of excessive wear is very low.