RaveDisc

Hi everybody,

I have seen quite a lot of reviews of the 964 (c4) and also videos that show things like it's top speed and stuff like that.
However, I don't understand why are there some 964 (stock) that can push the revs up to 7'000rpms in 5th, at 270km/h (168mph), or some "only" 158mph.
http://www.porsche964.co.uk/rev_rep/...P1~2_small.jpg

Some say the rev limiter is at 6'700rpms, other say 6'800rpms... Mine goes up to 7'100rpms with stock eprom.

Why am I talking about this ?
I tested mine (and I have to say it was a very nice experience!) :
In 3rd it can run up to more than 7k rpms @ 110mph, in 4th also (at 7k it goes 220kph/137mph).
In 5th gear @ 5'800rpms : 240kph / 150mph.
and @ 6'200rpms : 258kph / 161mph.

And... that was it, it didn't go further . Why ? Is the car too "weak" with apprx. 285hp to push the revs more than that in 5th ? I mean, if it had 300hp, would it be able to rev up to 7k in 5th.

Or is it possible that my 16" wheels are limiting the potential of the car at high speed and that 17" wheels would improve it ?

I know I should be happy that my 18year old car is able to go that fast, with on top of that a stability that is inspiring a lot of confidence, but I have the feeling it can go faster (I still have a chip and a MAF that I may install, it would build the power up to 300hp and the max rpms at 7'500),
so, why not...

Thank you for your time,

Have a nice day !

Andy

crg53

I am no expert on this, but my understanding is, that unless you do internal modifications to the engine, like rods/rod bolts, lighter pistons and so on, the rev limit should stay at 6750rpm, or there is a very good chance you may blow the whole thing to bits. Just my 2C.

Tom W

Andy: Are you sure you have a stock chip? You could check Adrian's book if anything came from the factory with a higher rev limiter, but I don't recall it. Since you specify kph as your normal speed, I assume you are in the EU and not USA and you might have a variant that we never got here (but I doubt it).

I'm told that valve float becomes a significant problem with stock engines above 7k rpm and you run an increased risk of bad things happening (like valve to piston contact). My engine can rev to 8k rpm, but it has been built for it with stronger parts.

Regardless of the rev limit, a stock dyno makes most it's power much lower in the rev range and running at the higher rpm simply increases wear on the engine at reduced power (in lower gears than 5th). You might want to dyno your car to see what power it really is making (at what rpm) before you conclude that higher revs are better.

Oracle

Something is not right.. you probably don't have a stock chip or the rev. limiter is really off.
Stock 964 is Steen said 6700 RPM.
Maybe your Rev gauge is off??

BlueHeeler

Do you have a stock wheels and tire combo?

RaveDisc

@Tom W :
It should be stock, because I have a modified chip and a "racing" MAF that I can install on the car, the previous owner gave them to me. Yes I am in EU but I don't think we had different versions of the car, except some minor differences for US and EU specs.
Its the first time I tried to push it to the max, to check things, but even if 7k is unsafe (and I don't know why it can run up to 7k...) nothing makes it impossible for me to change gears at 6'500 or 6'700rpms.

@Oracle : I thought about the same thing, maybe the gauge is showing a little more than reality, but I am not sure in that. How may I check it ?

@Blue Heeler : Now I am running on 16" cup 2 and regular tires (205/55/R16 front / 225/50/R16 rear), when you say 'stock' you mean the D90 wheels ? Yes I have them.

My car isn't stock. It has an additionnal exhaust (left side), BMC air intake and lightweight flywheel. May some of the difference in rpms be linked to some of those equipment ?

However, my concern was more based on the top speed and the revs in 5th gear. Do you have any clue about why it stops at 6'200rpms in 5th ?

Thanks,

Andy

ABQ Slim

Along those lines, what is it that "governs" the rev limiter? Specifically, is it possible to run past the stock max revs with a lightweight flywheel, all other things being equal?

RaveDisc

I don't know, you tell me ? The eprom should be the instrument that "governs" the rpms, but since it's a stock one and the rpms are higher than stock 964, I'm just trying to figure why is that.

wellcraft290

you proably max out with wind resitance. everything has a terminal velocity due to air.

Geoffrey

There were two different ring and pinions in the 964 series car. As far as manual gearboxes, a G50/03 has a 3.44:1 and a G50/05 has a 3.33 but have the same 5th gear ratio at .868:1. So, a G50/05 will have a higher theoretical top speed, however, it may require more power than the engine has. I don't know as I've never had my street car over about 85.

The ECU governs the rev limit by cutting ignition timing or fuel or both. If you have momentum, like in 1st gear, it is possible that the engine will swing past the redline even when the ECU is cutting it. This is where damage occurs on stock chipped, stock valve springed engines.

Bill Verburg

Geoffrey is refering to US 964 from '92 -94 which had the 3.33(9:30 cwp) in their g50/05 transmissions , RoW from '90 -94 used g50/03 w/ 3.444(9:31 cwp)

w/ 255/40x17 the theoretical speed @6700 is
G50/05 166.6
G50/03 161.3

Geoffrey

Bill, what is intersting is that my 92, and Jeff Burger's 92 both have a G50/03 trans, however, the gearing appears to use the 3.33 R&P. In the case of my car, it is the original trans to the car and looking at the other one, it appears to be original as well.

EDIT: I should add, that both of these cars have a VIN stamping with ZZZ rather than the WPOAB vin although they are both US based cars.

RaveDisc

@wellcraft290 : It's true that I could feel some wind resistance. But since the 964 has such a Cx I thought it would go through the wind like nothing, haha, just joking. Maybe that was the problem, and if it is, then I'm ok with that.

@Geoffrey: Thanks for the info about gear ratios.
About the momentum thing, I would never use such slaughtering method on an engine, if it has a limit there is a reason, so no need to "trick" it and get over it. I mean, if my rpm gauge is accurate and it can really go up to 7'100rpms, I don't see the need of doing that on a stock engine, because the maximum power is at about what, 6'200-6'500rpms ? Even when I drive fast I change gears at 6k.

@Bill Verburg: That's interesting! I should have the g50/03 then, you see my top speed is also 161mph. The only thing that's strange is that I'm doing 161 at 6'200rpms, not 6'700.

Have a nice day,

Andy

Henry964

according to the user manual that came with my 1993 C2 the Final ratio of 964
C4 and Turbo is 3,44:1 for the C2 is 3,33:1 and for the Tip 3,56:1

So C2 are faster

Bill Verburg

As you know the facory never threw anything away, and switched parts when they ran out of old parts. I've seen the same sort of things w/ the change from older to newer spec parts several times.

All the factory docs spec as I posted, which is all I can go by.

964 C4 used a totally different trans, the G64. There is little, if anything, in common w/ a g50

to make matters even more confusing Switerland got the g50/04 w/ 9:31 but different 1-4 gearing