This page hosted by Get your own Free Home Page

Very Low Budget Performance

300ZX Turbo Performance

I came to the conclusion that the best measure of performance would be that measured over the full range of usable engine speed, and relying on no driver skill whatsoever. Therefore the method used is that of acceleration from a steady 2000 rpm (35 km/h) to 6000rpm (105 km/h).

The test technique is simple. Just me in the car, 1/2 tank of fuel, windows and roof closed (see temperature if you wonder why). Find a straight, level stretch of road, cruise with the tacho on 2000rpm, and floor it.

This test not only takes into account the performance under boost, but also the lag time required to achieve boost. This is a very significant factor in street performance of a turbo car but one that is ignored by most measures of performance such as a slowly accelerating dyno pull.

I choose to perform this test in 2nd gear, as this restricts the maximum speed to something unlikely to get me into too much trouble. :)

Modification	Conditions	Time	Average	Power: Wheels/Engine	Comments
Stock	At night, ~10°C	6.52 6.83 6.56 6.72	6.66	88/155kW	The engine power figure is derived from the vehicle specifications. The wheels power figures, and the subsequent engine powers are calculated as explained below.
75 mm High Flow Exhaust	At night, ~10°C	6.64 7.32 7.35 7.15 7.44	7.2	81/143kW	Arrgh! The scum went and put a chicanne in my test strip. My time achieved on other stretches either shows that a 3 inch system has marginally slowed my car, or that my original strip was slightly downhill.
75 mm High Flow Exhaust AND new sparkplugs	At night, ~13°C	6.18 5.86 6.40 6.10 6.40	6.19	95/167kW	Or maybe both my test strips were flat and the problem was old plugs. Goes to show that the basics are still important. And that a 3 inch exhaust DOES help matters.
As above, with boost increased from 5 psi to 7 psi	At night, ~13°C	6.95 6.90 6.95	6.93	85/149kW	Hang on, that's not supposed to happen! Maybe I'm bleeding off too much air, that would make it too rich. I don't know what's up but I'll take the boost back down and see what happens.
As above, with boost decreased from 7 psi to 6 psi	At night, ~17°C	5.68 6.29 6.04	6.00	98/172kW	That's more like it! I'll try to up the boost while dumping as little air as possible from now on. The secret is to have the coarse adjustment valve very close to closed. Later No, the problem was far more basic, I was starting to get knocking and the computer was reacting by retarding the timing. Time to intercool.
With the new intercooler installed. 3.5 psi.	At night, ~27°C	10.9 10.5	10.7	55/96kW	Wastegate set to standard 5 psi but a 1.5 psi pressure drop over the intercooler plumbing giving a manifold pressure of only 3.5 psi. No water in intercooler yet to actually cool the air. A much hotter night so the figures aren't quite comparable to earlier. Nonetheless there is an obvious power loss even compared to a atmo engine. There are obviously problems.
With the new intercooler installed. 3.5 psi. Leaky Pipes Fixed (?)	At night, ~25°C	8.27	8.27	71/125kW	As above but with the plumbing redone and some serious leaks fixed. At least now I've got more power than the atmo engine. There is still some hesitation on takeoff so I think I've still got some problems.
Intercooler removed for yearly roadworthy check. Stock boost	At night, ~29°C	6.77	6.77	87/152kW	Nothing else was changed between this and the 71 kW result with the intercooler, so the restriction is responsible for the 16 kW drop. Improved plumbing should reduce this but I really need to add some water and see what happens.
120 cc/min water injection. 10 psi boost	At night, ~10°C	5.97 5.86	5.92	99/174kW	As I said above it was time to add some water, but the intercooler gave me so much trouble that I added the water directly. Still seem to get some knock related ignition retard around the torque peak so I'll increase the water flow.
120 cc/min water injection. Working properly this time. 10 psi boost	At night, ~10°C	5.56 5.67	5.61	104/184kW	Actually when I went to increase the flow I found a hose clamp missing and the water leaking out. I wasn't getting the full 120 cc/minute. So I replaced the clamp and tested it again. More power and much smoother.
250 cc/min water injection. 10 psi boost	At night, ~10°C	6.67 6.53 6.15	6.45	91/160kW	If some water is good then more water must be better right? Maybe not. And the results are less consistent too. Definitely not what I wanted.
250 cc/min water AND 20% ETHANOL injection. 10 psi boost	At night, ~10°C	5.56 5.55 5.50 5.58	5.5475	106/186kW	I didn't expect that much of an improvement! Not that I mind. This has much the same power as 120 cc water injection but should be safer with respect to detonation. And check out the consistancy.
500 cc/min water and 20% ethanol injection. 15 psi boost falling to 10 psi after 2 seconds	At night, ~15°C	5.44 5.40	5.42	108/190kW	More water, a bit more boost but only down low: Result a bit more power. Only to get this took months of fixing the boost leaks.

So what does this mean in terms of power? Well going back to primary school physics (high school for those who went to state schools :)

Kinetic Energy E = M/2 * V^2 where

M is the mass of the car (plus driver) and
V is the velocity of the car

So a change in kinetic energy DELTA E = M/2 * (U^2 - V^2)

U is the final velocity of the car

And power is Energy/time, so the power applied to accelerate the car in time t

P = M/2 * (U^2 - V^2) /t

Now as stated above,

the starting velocity V is 35 km/h = 9.72 m/s
the finish velocity U is 105 km/h = 29.17 m/s
Mass with a 100 kg driver is 1550 kg

Stick all these numbers into the equation and we get POWER = 586/time.

These power figures are listed in the above table.

NOTE: This is NOT THE PEAK POWER AT THE ENGINE. This is the power at the wheels averaged over the rev range from 2 to 6 thousand.

This is a lot less than the headline figure at the engine. This is, however, a much more realistic idea of how much power you have available to push the car down the road.

To convert my calculated power to the ordinary "peak engine power" a bunch of fudge factors have to be used. These are as below:

Transmission losses. These are usually estimated at 30%. So divide by 0.7 or multiply by 1.42.
Air and road friction. I don't know. 10%?????
The ratio of average to peak value. This varies with the shape of the power curve but based on the 6.66 seconds figure for the stock car, which is supposed to have 155kW, we can estimate this as 1.1. (Which seems pretty flat to me!)

This gives a total fudge factor of 1.76. Which for the stock car, giving 88kW at the wheels, gives 155 kW at the engine which is no surprise as that is how we calculated the factor. But it does allow use to calculate some sort of power for the modified car.

As a reality check we can look at the figures for an Imprezza WRX. This too claims 155kW at the engine. When measured on a chassis dynometer a figure of 88kW is achieved at the wheels. Now THAT's a coincidence. It can be pointed out that a 4wd WRX will have more frictional losses than the RWD Zed, but in return the road acceleration test has air friction to contend with, and the averaging function, both of which the chassis dyno does not. It is just luck that they cancel out to within one kilowatt.

Reality Check 2: The Cartest program allows you to enter all the data for a car and then it will simulate the performance under a range of tests. When the 1986 300zx turbo is selected, and power set to 170 kW, the 2 grand to 6 grand in 2nd gear is given as 6.0 seconds. Looks pretty close to me.

Go To Patrick's Homepage