Response

Performance

In The Best Performance Determinant, you've nicely worked out the measurements that correlate best with power/weight, and shown that this correlates nicely with a couple of other common measures.

What you haven't done, is show that this is the BEST Performance Determinant, rather than just the easiest to do calculations with.

This is more of a philosophical question, but it is really quite interesting. What is the measure that correlates best with how quick a car is, or alternatively feels, on the road? My own experience is that my Nissan 300zx running 18 psi of boost and giving about 520 Nm of torque at 3500 rpm and a peak power of 200 kW felt like a much faster car than a similar twin cam car with slightly more peak power but heaps less torque.  This is not a result that shows up in any of the basic performance measures.

Patrick McAuliffe
Australia

New Car Tests

Hi, really enjoy your stuff.  Don't read all of it, but you guys provide a unique offering that I really like.  Any chance of getting back into new car reviews.  I know you've got issues with some of the manufacturers, but can't you rent something for a day once every month or 2, borrow friends cars etc?  Cheers.

John Palmer
Australia

Boost Control

Thanks for some great articles. I have a question that really goes back to the articles you did on the Audi Boost controller,   Brilliant Boost, etc.

Referring specifically to the Audi Boost controller, I do not understand how part of the pressure control works, as I will now explain: To make the question easier to explain, let’s leave the relief valve out and deal just with the pressure regulator. We have the compressor on one end, then the regulator, leading to the actuator of the wastegate. Now when the wastegate actuator gets around 7psi or so of pressure the wastegate will (start to) open because of the factory spring setting. But if the regulator has been set to say 11psi, wanting to control boost at that level, how will the boost pressure in the line ever get up that high, because the wastegate will always open as soon as 7 psi is reached in the system, and never get to the 11 psi before it opens. So the way I see it, the regulator is always fighting the 7psi setting of the actuator and the pressure will always be bled off before the higher pressure is reached. I'm clearly missing something in the mechanics of this. I'm sure the answer is fairly simple.  Please could you explain, as its driving me mad trying to conceptualise how it works.

Andrew Bosch
South Africa

You are assuming that when the pressure regulator is set for 11 psi boost, its outlet pressure is 11 psi. It isn’t. The pressure regulator’s outlet pressure might only be 2 psi but we say “11 psi” (referring to manifold boost pressure) because it’s the turbo boost pressure level that concerns us, not the pressure regulator outlet pressure.

Compression Ratios

I'd like to start with the usual "I'm a big fan of this site" stuff as really it is the most interesting tuning website around.

I'd like to request an article on the subject of compression ratios. My track car uses a ford cologne v6 engine which I have added a pair of turbochargers to.  I've just recently put a big hole in the pistons 5 and 6 due to them being crappy cast items with poor support on the ring lands which leaves me with an interesting problem.

I've been speaking with a company in the states called JE pistons, they custom make pistons and will make me some to any compression ratio I desire, the only trouble is I really don't know what I want!  I'd like advise on how low to take the compression ratio (I'm of the understanding that lower is better for full-throttle use) and how to go about taking measurements that will allow me to spec the size of the bowl to put into their crowns.

Luke Nicholls
United Kingdom

We could do an article on compression ratios but it would be so lacking in any specific recommendations, it wouldn’t be very helpful. The required compression ratio in a naturally aspirated engine depends on the combustion chamber design, head design, cam design, fuel octane, engine management and the climate the car will operate in. In a turbo car you can add to that the shape and magnitude of the boost curve. There are simply so many variables that anyone who just looks at the engine on paper and suggests a best CR is fooling themselves. Look at what others have successfully done with the engine you are modifying...

New Turbos

Re New Tech Turbocharging. I remember reading an older book that looked at the last of the piston engines being developed for airliners just before jets made the development obsolete. They had gone to diesel and ran massive boost pressure, I recall reading up to 90psi. Even the Super Connie had the turbo shaft linked to the crankshaft output.

The book concluded that the piston engine simply became a clumsy way of producing hot gas for the turbo. I wonder if the motor vehicle industry will go the same way? Maybe in ten years time we'll be driving gas turbines that charge a capacitor operating an electric engine that turns the wheels?

Matt King
Australia

Turbo’d for Torque

I've read your 'Turbo'd for Torque' article (great!) and I think it’s the answer I've been looking for. I have a 2003 BA XR6 sedan with 4spd tiptronic which I was hoping to turbocharge, there in lies the dilemma... The auto is said to handle a max. of 350fwkw (though I’ve heard of it breaking at the standard   240fwkw mark). So modifications and turbocharging to approx. 350 –   400fwkw like I was planning is no longer an option. Then I read your article. What if I select a turbo to supply a max.  270fwkw (still better than a XR6T) and have heaps of low-mid rev torque (as it’s a daily driver)? Beautiful! But... Is it the onslaught of torque or the +350fwkw that’s going to kill my transmission??

Steven Brown
Australia

Unfortunately transmissions are almost never power limited – they’re torque limited. People erroneously talk about the power rating of a transmission because the numbers make more sense to more people. So if it the gearboxes break with standard torque, you won’t be able to safely increase it a great deal.