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From the Editor

10 Nov 1998

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Despite the belief to the contrary, some turbo car performance parts do little. Like all generalities, the following doesn't hold true for all cars - just for most.

Firstly, turbo blow-off valves. Basically, in lots of cars they do nothing for performance. For the uninitiated, a blow off valve vents the pressure build-up that occurs between the turbo compressor and the shut throttle blade on gear changes and when the throttle is suddenly closed. If you believe the publicity, this prevents the turbo slowing and also stops a pressure wave attacking the compressor.

Okay, but there are a few problems with the theory. When the throttle is closed there isn't much volume of exhaust gas being produced to keep the turbo spinning anyway. The result? - the turbo slows pretty quickly when the throttle is closed - with or without a blow-off valve. But don't many cars have factory-fitted blow-off valves? They do - and you need to read the workshop manuals to find out why they are used.

Firstly, factory blow-off valves reduce noise. Reduce noise? - aren't blow-off valves the ones that make the pppsssssht! noise on gear changes? Yes they do when the valves are vented to atmosphere, but all factory blow off valves are of the recirculating type, where the exhausted air goes back into the intake after the airflow meter. What these valves do is reduce the bbrrrrrt! noise of the compressor blades whirring into a dead-end of a shut throttle...

The other reason that throttle closed blow-off valves are fitted to factory turbo cars is to stop a build-up of pressure in front of the turbo compressor when it suddenly stops passing air. This pressure wave can result in a reversal of the flow of intake air back out past the airflow meter, which - if it is of the hotwire type - will record this as extra intake air that requires more fuel...

If you have a huge turbo, a blow-off valve may well benefit turbo spool-up after gear changes. But if your car is near-standard, more often than not a blow-off valve is a waste of money - especially if you're paying a heap of dollars for a special supa dupa huge valve. If you are blow off equipped and want to see how much difference the valve makes, just pull off the vacuum line leading to it. Let us know via the Tech Forum what differences you found in performance.

And if you've got a turbo car, you're sure to have been told that a turbo timer is a good idea. You know, to give time for the turbo to cool down and so stop frying the oil in the turbo bearing after you switch off. Well, I think that turbo timers are a total waste of money. I'll go further - they also make your car easy to steal.

So why are they a waste of money? If your car has a water-cooled turbo (all turbo cars of recent times), the heat build-up in the bearing after switch-off isn't so terrible - the water transfers the heat away. The use of synthetic oil also reduces the likelihood of oil coking - and all turbo cars benefit from the safety of running a synthetic. And finally, if you've been driving your car hard, exactly what is the problem in sitting in your car for a minute or two while it idles? You can then let the engine cool down for as long as needed - and it doesn't cost you a cent.

And easier for thieves? Remember that a turbo timer is an ignition switch bypass. That means that a thief spotting a turbo timer prominently mounted on the dash only has to pull out the wires leading to the box to have ready-made access to an ignition jump-starter. And that doesn't even include the vulnerability of leaving your car idling when you wander off!

In six turbo cars I've owned I've never fitted a turbo timer - and one is just passing 170,000 of mostly urban kilometres with the original (and boosted!) turbo still happily in place. In summary - why'd you bother?

Another furphy that has widespread currency is the concept that engines need back-pressure. Simply, there is no properly tuned engine where increasing exhaust back-pressure causes an improvement - in power, torque or fuel economy. One of the reasons that this idea has gained support is because when people change their exhaust they seldom check the air/fuel ratio or re-map the ignition timing to once again give optimal performance. For example, some MAP sensed cars drop substantially in power with a large exhaust fitted because they are then running lean.

Atmospherically inducted cars that use a tuned length system to improve cylinder scavenging (via extractors, for example) are sensitive to exhaust diameters within the tuned length part of the system. This means that the maximum effect of exhaust pulsing may come from an exhaust system that is small enough that some exhaust back-pressure is developed. However, that is a quite different concept to saying that engines "need" exhaust back-pressure! Turbocharged engines require as big an exhaust as possible, with the same applying for naturally aspirated cars once the tuned length part of the exhaust is passed.

Few tests have been done that clearly show the affect of changing back-pressure. Most muffler and exhaust comparison tests change more than one parameter simultaneously, making the identification of exhaust back-pressure as a culprit difficult. However, Wollongong (Australia) mechanic Kevin Davis is one who has done very extensive testing of varying back-pressure on a number of performance engines. These range from turbocharged Subaru Liberty [Legacy] RS flat fours to full-house traditional pushrod V8's. In not one case has he found any improvement in any engine performance parameter by increasing exhaust back-pressure!

The tests came about because Kevin has developed a patented variable flow exhaust that uses a butterfly within the exhaust pipe. He initially expected to use the system to cause some back-pressure at low loads "to help torque". However, he soon changed his mind when any increase in back-pressure proved to decrease torque (and therefore power at those revs) on a properly tuned engine! What increasing the back-pressure does do is dramatically quieten the exhaust.

One of the engine dyno tests carried out by Kevin was on warm 351 4V Cleveland V8. Following the extractors, he fitted a huge exhaust that gave a measured zero back-pressure. Torque peaked at 423 ft-lb at 4700 rpm, with power a rousing 441hp at 6300 rpm. He then dialled-in 1.5 psi back-pressure. Note that very few exhausts are capable of delivering such a low back-pressure on a road car. Even with this small amount of back-pressure, peak torque dropped by 4 per cent and peak power by 5 per cent. He then changed the butterfly position to give 2.5 psi back-pressure. Torque and power decreased again, both dropping by 7 per cent over having zero back-pressure!

And if you still believe that exhaust back-pressure improves performance, simply block off part of your exhaust outlet and see if your car goes any faster!

Julian Edgar

editor@autospeed.com

PO Box 175, O'Halloran Hill, South Australia, 5158, Australia

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