Roygarth

I recently had the following modification:
1)Cat Bypass - cat removed
2)Perfomance chip fitted by JZ Machtec
3)Air box drilled - noise alert!

Questions:
A) What is the new horsepower of my car? I'm looking to compare the figure which the hand book says is 250 as standard
B) What is new torque figure?

I realise that this varies from car to car ambient temp etc etc but I just needed a rough idea!
Thanks in advance!
Piers
1990 C4 Coupe 45,000 miles.

Christer

Piers

I understand that JZM don't have a 4 wheel dyno, so I guess they can't give you a before and after figure? AMD though have a 4 wheel rolling road. If you want to know I would pop up to them. I would ask them for 'at the wheels' horsepower reading as opposed to at the flywheel - it should be more accurate. Bear in mind that on this forum we assume a 15% powertrain loss so the horsepower of a 964 which is actually pushing out the full 250bhp after all these years should be showing about 210-213hp at the wheels as standard. If you believe JZM's claims, you should be near to 230hp at the wheels.

AMD's details are <a href="http://www.auto-amd.com/" target="_blank">here</a>

It would have been best to get this done a tthe time of the conversion.

A quick question: is the exhaust much louder now? Do you get any resonance at certain engine speeds?

Roygarth

Christer

Thanks for your answer

1) Do all cars lose 15% via the powertrain?
2) So the 250 figure is now up to 275 'ish?
3) The noise is only different under 'accelerative load' (i just made that up!) situations not when cruising. It is bl**dy head turningly loud off the lights but not OTT. I have not noticed the resonance that you ask about.
Piers

DaveK

Hi,
Isn't the power train loss higher for a C4? I thought I'd seen posts from Adrian that say more than 20% ( I think it was 22?).

I also had a cat bypass pipe fitted - didn't notice the car being louder or any resonance. Didn't actually notice all that much difference at all - perhaps felt a little crisper but I think that was more because the cat was collapsing.

Dave
90 C4

DaveK

Hi,
Power loss of 15% doesn't mean power was really 275. My understanding is that when Porsche quote power figures, these are engine figures - hence 250bhp is the max, measured at the crank (or wherever they measure these things). The 15% or so is then lost between the engine and the wheels.

Dave
90 C4

Christer

Yes Dave is correct. The 250bhp quoted when the car was new is measured at the flywheel. Maybe the C4 has a slightly higher transmission loss, certainly if Adrian says so then I would take it onboard.

Piers, what you would really need to do is to call AMD and book your car in. Ask them to rolling road your car for power at the wheels. Then we can discuss further (and involve other people with more knowledge.... <img src="graemlins/c.gif" border="0" alt="[ouch]" /> )

Roygarth

[quote]Originally posted by DaveK:
<strong>Hi,


I also had a cat bypass pipe fitted - didn't notice the car being louder or any resonance. Didn't actually notice all that much difference at all - perhaps felt a little crisper but I think that was more because the cat was collapsing.

Dave
90 C4</strong><hr></blockquote>

Hi Dave

It's the holes drilled in the air box which make the noise.
Piers

Roygarth

OK OK I think I understand!!

Original HP = 250 at flywheel, 210 at wheels.

HP now = 275 at flywheel, 230 at wheels.

Yes I know figures are approximate but it's all bl**dy comparitive anyway!

I'm going home now!

Christer - your bit on the Distributor disguised as a bodum coffee maker had me in stitches!!

George Stevens

Roygarth

A couple of points I feel you should be aware of.

The 964 was designed to produce the same BHP with or without the cat, so just removing it will not alone increase BHP. In fact removing the Cat can lower torque as some of the back pressure is lost.

The removal of the cat does make it easier to re-chip the car. I have had the same conversion carried out by JZ and it is the re-chipping which gives the modest power gain not the removal of the cat.

As with all 964's you can not guarantee if your car was developing a true 250 bhp before you had the conversion carried out so it is dangerous to just guess that it is now pushing out over 270 bhp

My RS being 2WD was measured before at only 220 bhp instead of 260. This was because of a poorly serviced engine and a blocked cat.

The engine was pressure tested and was confirmed to be in excellent health, even after the de-cat conversion and re-chip the power output was confirmed at 270 bhp which is only 10 bhp up on the standard 260 for an RS.

However the acid test is how it drives, not dyno graphs, mine felt explosive when I got it back and another Porsche specialist who drove it felt mine was quicker than another RS clubsport they had in with an AMD hotfilm conversion producing a claimed 300 bhp.

Adrian

Guys,
Data received from 4, C4 owners, 3 in the USA (different states) and 1 in the UK all show a transmission loss of 28% from flywheel to rear wheels. They measured rear wheels HP only, before and after. However do not forget 31% is sent to the front. All used AWD dynos, none had the front driveshaft disconnected.
Data from C2 owners shows a fairly consistent 15% transmission loss. To get an extra 25HP on the rear wheels of a C4 takes one heck of an effort and money. I have never actually seen HP figures or measured HP, on the the front wheels of a C4. I also believe the traction control system should also be disconnected for dyno tests on a C4. I never see if this is actually carried out on any of the test results. The traction control if it activates during the testing will mess up the data and I surmise increase the power loss across the transmission,
Ciao,
Adrian
911C4

PS: It only takes a measured wheel speed differential of 0-8kph to activate the traction control. Rear wheel slip will see power transfered to the front.

Herbie

About trasmission loss:

When speaking of 28% loss, the question is: 28% of what? From previous posts it seems that, ex. a 250HP car leaves 70 HP (28%) in the trasmission.

But making a simple comparison:

A C2 and a C4, both with 250hp engine, with, respectively, 15% and 28% of power transmission loss.

So:

C2 puts on the wheel 250(1 - 15%) = 212hp;
C4 puts on the wheel 250(1 - 28%) = 180hp;

But C2 and C4 street performance, acceleration and maximum speed, are very similar.

So, something is wrong in this calculation.

An hypotesis: I'm not sure that transmission loss is a fix part of the total power.

I guess that there are other variables, for example the absolute speed, influencing the total count. And the transmission loss function isn't a simple constant but is a complex function of 2 or 3 variables.

I'm very curious about this.

Roygarth

However the acid test is how it drives, not dyno graphs, mine felt explosive when I got it back and another Porsche specialist who drove it felt mine was quicker than another RS clubsport they had in with an AMD hotfilm conversion producing a claimed 300 bhp.[/QB][/QUOTE]

Dear George
Could'nt agree more. Mine is a completely different beast now. A Diablo driving friend uttered 'what the f**k have you done to this car' first time he went in it after the mods!

Thanks to you all!
Piers
1990 C4 Coupe

Roygarth

PS: It only takes a measured wheel speed differential of 0-8kph to activate the traction control. Rear wheel slip will see power transfered to the front.[/QB][/QUOTE]

Is this why the small yellow light flickers (near the traction control button) in particular when I drive hard on windy roads? I find in distracting! The handbook says it flashes when the system is working!? This is opposite to most warning lights?!? Is it warning me I am driving too near the limit? (I doubt it as I am a novice!)
Thanks in advance!
Piers
1990 C4 Coupe

Adrian

Dear Herbie,
The C4 has a fixed, never can be disconnected (unless you remove the front drive shaft) AWD system. It has three differentials. A C2 only has one. On the C4 the centre diff distributes the drive. 31% forward and 69% rearwards. The centre diff is a wonder of engineering. You will have to have less HP on the rear wheels. Why is performance the same or very similar. The power distribution is to all four wheels not just 2.
The C2 and C4 cannot be compared in simple figures terms. Try to look at it this way. The C2 flywheel is connected to the gearbox via the clutch. The gearbox drives a shaft connected to one differential and loses 15% over this system.
The C4 on the other hand has a flywheel connected to the gearbox via a clutch. So far the same. However the gearbox is tied to the centre differential. The centre differential drives the drive shafts. Have wean extra loss through the drive of the centre diff? Yes. Not much maybe but still a loss. On the C2 we have around 15% HP loss to the rear wheels through the rear diff. Back to the C4. What is up front? Another driveshaft and another differential. Only 31% of the drive is heading forward so the losses will be less.
I hope you can see that this is just not simple anymore. However in the end the C4 is driving all wheels with a certain amount of HP and the C2 is only driving the rear wheels (pushing the rest) with a certain amount of HP.
Very different principles and ways of achieving the same end.
I must remind you that it takes HP to drive the front wheels. They are actually being driven. If you have 275HP on the rear wheels of a C4 then the engineoutput is going to be, let us say rather higher than normal.
Now getting back to my previous post. I am going to spend some time this weekend going through the data I have been supplied. I will try and work out how this data was obtained. I looked at one set last night and this data could onl yhave been achieved by disconnecting the front drive shaft.
I also need to determine if AWD dynos, AWD Rolling roads or 2WD Dynos were really used. I am sure I cannot come up with any hard and accurate numbers because of the variables invloved.
Ciao,
Adrian
911C4

johnfm

Adrian

have to back up Herbie on this one. 285 power loss for C4 not possible. The maths (which is simple) simply doesn't stack up. The on the road HP of any car can be very accurately arrived at from accurate acceleration figures (ie 0 -100, etc) if you know the vehicles weight during the test and the frontal area.

If C4 and C2 top speeds are similar, they are pushing similar HP (allowing for weight difference. I am gueassing their frontal area is virtually identical).

If you haven't already done so, check out <a href="http://www.pumaracing.co.uk" target="_blank">www.pumaracing.co.uk</a> for a brilliant bunch of pages on engine power analysis.

Also, there is a company in Scotland who make a thing called the road dyno. You can plug this into the car an it measures engine revs etc. Supply it with the cars mass, atmos pressure and temp etc and it will give very accurate HP at the wheels. It now has an accelerometer add on for logging acceleration etc for testing purposes. Sounds like a good use of £130 to me!!