Zetec Mods Dyno Comparison

[Seawulf]

Time to sticky for future reference.

Thanks BP for the list.

[ButtonPuncher]

Cool.

I haven't been over to this forum in a while.

BP

[pwhitelock]

How come a performance intake manifold reduced your torque so much? I thought it would increase torque as you would have less resistance to the flow of air, hence you getting more air in your engine and hence more power!
When i fitted my focus central 65mm throttle body i seemed to feel more torque. So by that chart the intake manifold probably reduces torque quite considerably.

[ButtonPuncher]

Actually longer and narrower ports give you more low RPM power and torque. Shorter and wider ports give you more high rpm power and HP. Stock, the Contour/Cougar Zetec intake has really long runners. That's why we actually have a decent amount of torque stock.

HTH,
BP

[pwhitelock]

Ah ok.

[kpeeks]

If we could do 2 of both styles it'd be nice. Getting low end tq & high end hp....but it'd screw up compression. At least I'd think it would or with 2 of each it'd prolly do nothing.

[pwhitelock]

Not particularly wanting to get into a physics discussion here, but how exactly does more restriction in the intake give more torque? Is it something to do with bernoullis theorem? IE: like in a pipeline if the diameter is reduced the pressure increases but the flow decreases? A bit like a shower head:



A bit like a turbo increasing the pressure of the air entering the engine. So in theory if you had a slimmer intake RAM pipe for your induction kit and T/B and a huge filter it would be better? Just slower throttle response?

[Yofavcracka]

imagine the exhaust, if you go bigger piping than 2.5 the exhaust starts to not have enough back pressure to give you horsepower, so the piping is decreased for the amount of displacement that your engine creates which gives you torque. too big - doesnt flow fast enough to give you that torque.

[bdiddy]

Originally Posted by Yofavcracka

imagine the exhaust, if you go bigger piping than 2.5 the exhaust starts to not have enough back pressure to give you horsepower, so the piping is decreased for the amount of displacement that your engine creates which gives you torque. too big - doesnt flow fast enough to give you that torque.

horrible example. backpressure and exhaust is crap. I made a post about this a while back in garage talk

[bdiddy]

here...

One thing that I get tired of hearing 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!