Exhausts

[GTSBoy]

208,

Pretty much any pressure gauge can be used. You can buy guages from all sorts of places, from auto shops to engineering supply places.

You will need one that reads up to about 1 or 2 bar (15 or 30 psi, 100 or 200 kPa, 30 or 60"Hg etc etc etc) if you find that you have very high back pressure, but remmeber that using a pressure guage that is scaled much higher than the actual pressures you are reading means that you are trying to use on the bottom end of the scale and so you get reduced resolution and accuracy in your measurements.

you need to attach a short length of copper tube or similar to the exhaust and hook it up to the pressure gauge with rubber hose of some sort. If the copper pipe gets too hot and melts the hose you need to lengthen the pipe (perhaps coil it up) to give it some space to shed it's heat before the hose touches it.

cheers

[Warpspeed]

You can use any pressure gauge with a suitable range. Carby type fuel pressure gauges, and boost gauges will work fine. Something that goes up to 20psi would be ideal.

The figures given in that article were for a particular SUPERCHARGED engine, and may not apply to a different sized normally aspirated engine.

Remember a positive displacement supercharger stuffs the same amount of air into the engine per revolution almost regardless of back pressure. As you increase the exhaust restriction, the boost also rises by almost exactly the same amount. For this reason the power drop off will not be as severe. But the engine will not be happy though.

I still stand by my figure of 1% per psi for any normally aspirated four stroke engine.

[208_Fireball]

Thanks GTS,

I guess i'll get one of those 0-15psi fuel pressure gauges - then i can keep an eye on the efi fuel pressure when i'm finished

Cheers,

Dave

[208_Fireball]

Warpspeed,

Thanks for the info - so boost gauges will also work as fuel pressure gauges too then by that assumption?

I wasn't trying to challenge your figures btw, just thought that the info would be appropriate

Cheers,

Dave

[Warpspeed]

Dave, thats o/k. It is something that has interested me for quite a while, and that figure is not something that is widely known.

The theory is, that in most typical (fairly stock) car engines the cylinder pressure on the power stroke falls as the piston goes down the bore. At the point of exhaust valve opening the cylinder pressure is roughly about 100 psi.

Now if you keep increasing the exhaust back pressure less exhaust is able to actually get out during the exhaust stroke.

If you fitted some sort of adjustable control valve to the exhaust, and a pressure gauge, you would find that as you turned off the valve, and increased the exhaust back pressure, power would fall.

At full wide open throttle you could actually reduce the engine to idle speed ! and have zero power output. The exhaust back pressure might not be much below 100psi at that point.

This explanation is a bit Mickey Mouse, but you get the idea.

They actually do this on the diesels fitted to intestate trucks. when descending long hills they use the Jacobs engine brake, which closes off the exhaust creating massive back pressure. They can then control the downhill speed by using the engine, without burning out the trucks brakes. It makes a frightful bloody noise though when they back off!

[upgarage]

i just got a quote for a 2.25" catback system with a stainless steel muffler for 340$? how does that sound? is the price good?

thanks

[208_Fireball]

When they say cat-back, i am gathering it does *not* include another cat?

The cat is also a restricting factor in the exhaust, and some types can be modified, by chopping off the ramp out to the cat, and welding on a freer flowing entrance, as can be seen in the attachment...(sorry for the artwork)

Cheers,

Dave

[upgarage]

no worries, cheers

i have another question regarding exhausts. wats a resonator and what does it do?

thanks

[208_Fireball]

On your typical cat-back exhaust system, you'll see a couple of bulges in the piping that are apparently mini-mufflers out to help the big muffler that hangs out back. These are called Helmholtz Resonators and are very similar to glasspacks. The main difference is that firstly, there is no sound-absorbing fiberglass or steel wool in a Resonator. And secondly, their main method of silencing is the reflective principle, not absorption. An easy way to tell the difference between a glasspack and a true Helmholtz Resonator is to "ping" one with your finger. A glasspack will make a dull thud, and a true Resonator will make a clear "ping!" sound.

- Quote from some website

Cheers,

Dave

[208_Fireball]

or...http://auto.howstuffworks.com/muffler3.htm might explain it better....

Cheers,

Dave