Measuring Exhaust Manifold Pressure

[grim32]

Ok i can weld a fitting on my manifold just before the turbo and im sure i can get a high temp line made up from someone, but what sort of gauge would i need to handle the heat?

[The Mafia]

yeah I am interested to know this too

[ebola]

Ok i can weld a fitting on my manifold just before the turbo and im sure i can get a high temp line made up from someone, but what sort of gauge would i need to handle the heat?

<{POST_SNAPBACK}>

Why?

(I don't know how, either)

[MattSR]

Backpressure is as important, if not MORE important that boost pressure when it comes to sizing a turbocharger. Myself and a few other guys tried this in a thread over in another forum - ill post a link when I get home.

Cheers,

Matt

[discopotato03]

Some people braze a coiled length of fine copper tube to a bung and screw it into the exhaust manifold ie O2 hole . The copper tube will conduct out a lot of heat , enough to clamp silicon hose to it and join it to a boost gauge . Its a fixed volume so flow and heat is usually not an issue .

Cheers A .

[grim32]

Backpressure is as important, if not MORE important that boost pressure when it comes to sizing a turbocharger. Myself and a few other guys tried this in a thread over in another forum - ill post a link when I get home.

Cheers,

Matt

<{POST_SNAPBACK}>

Thanks mate ill appreciate it.

Russell.

[grim32]

Some people braze a coiled length of fine copper tube to a bung and screw it into the exhaust manifold ie O2 hole . The copper tube will conduct out a lot of heat , enough to clamp silicon hose to it and join it to a boost gauge . Its a fixed volume so flow and heat is usually not an issue .

Cheers  A .

<{POST_SNAPBACK}>

Thanks so a boost gauge should be ok with a long copper line.

One more question, on a car with high backpressure how much psi would there be? I no its very difficult to answer as every car will be different but should a 30psi gauge be ok.

Another one, sorry, what would the ideal ratio of backpressure vs boost be?

Russell

[MattSR]

OK, heres the thread and the results,

http://www.performanceforums.com/forums/sh....php?t=67189028

And heres a bit of tech theory as to why its important (well, its mostly opinion but its still quite interesting)

http://www.performanceforums.com/forums/sh....php?t=67188787

Cheers,

Matt

[MattSR]

Just found this too

http://www.performanceforums.com/forums/sh....php?t=67188787

[grim32]

OK, heres the thread and the results,

http://www.performanceforums.com/forums/sh....php?t=67189028

And heres a bit of tech theory as to why its important (well, its mostly opinion but its still quite interesting)

http://www.performanceforums.com/forums/sh....php?t=67188787

Cheers,

Matt

<{POST_SNAPBACK}>

Sweet, thanks for that Matt.

Russell

[Warpspeed]

I have done this on several cars.

Usually the easiest way is to find a convenient boss somewhere on the exhaust manifold casting. Sometimes there are threaded holes provided to mount the heat shield for example. They may typically be tapped for an 8mm bolt, and the thread most often does not go right through into the exhaust manifold.

So what you do is drill out the hole so it is larger, and goes right through into the manifold. Make damned sure you don't leave any metal chips inside the manifold !! Best to take the manifold right off the car first. Then blast out the chips with compressed air or a garden hose.

Tap the hole for a convenient thread, 1/4 BSP is easy to get fittings for.

Then go around to a hydraulic hose supplier, or somewhere you can get a steel fitting that goes from 1/4 BSP to 1/4 flare, and get them to supply 500mm of steel bundy brake (or fuel) line, a matching flare nut, and get them to flare one end of the steel pipe for you while you are there.

You then have a 500mm length of quarter inch steel pipe sticking out of your exhaust manifold. A piece of rubber tubing pushed onto the end will work fine, and will not get hot enough to burn. You boost gauge can then be used to measure the exhaust back pressure if it is not too high.

When you have finished, you can replace the fitting with a 1/4 BSP blanking plug, and that will also hold your heat shield in position, after you have filed out the hole in the heat shield to suit the larger diameter.

A typical stock factory turbo at factory boost level, you can expect the turbo to reach rated boost at maybe half redline Rpm, and the exhaust back pressure will come up to twice the boost pressure and stay pretty much at that while at full boost.

If you wind up the boost to maybe 15psi, expect around 30psi back pressure with a small stock turbo and factory exhaust system. That is fairly typical.

Fitting a monster turbo will lower back pressure a lot, but boost will then not hit until fairly high up in the Rpm range. The lower back pressure will definitely raise top end power, we all know that !

If you built some sort of freak turbo with big compressor and small turbine, trying to get a low boost threshold, it will not go as well as you hoped. The increased back pressure of the small turbine will kill power, and the big compressor will do nothing but surge.

Exhaust back pressure is a problem. But knowing what it is can be an excellent guide to sorting out other problems. Lower exhaust back pressure will also reduce detonation by allowing the heat to get out of the engine. Those red hot manifolds and turbine housings are trying to tell you something.

Low back pressure = big horsepower, it is that simple.

As a guide, each 1 psi in the exhaust manifold is costing you 1% engine power. So find out what it is and be truly surprised.

There is far more power to be had by playing around on the exhaust end of the turbo, than the compressor end. It is also an excellent way to compare turbo efficiency between different turbos. Remember the turbine has an efficiency figure as well as the compressor.

Another thought, the total back pressure is turbine back pressure plus the pressure drop along the whole exhaust system to the back of the car. We all know a big exhaust goes better, but most of the problem is in the exhaust turbine itself not the rest of the exhaust system.

Find out what your back pressures are, and you can then identify the weak link in your system.

[grim32]

Thanks Warpspeed that was very helpfull.

Russell