Exhaust manifold pressure reading
Turbo Mini Forums

So I've been speaking to a Chris from chris isaacs race cars about my custom manifold.

I asked how to find out if it's restricting power and he said I need to monitor the back pressure in the manifold before the turbo.

Does anyone do this? If so what have you used?

He said the EMAP should rise fairly evenly with the boost, and obviously if it gets a lot more then it's becoming restrictive.

Cheers

Dave the Cake

compression fitting(or similar) in exhaust, length of metal pipe so the gasses can cool down a bit (remember theres no flow just pressure so you dont need to go nuts) then normal hose and a suitable pressure transducer.

All that said I dont see how that will tell you if the manifold is restrictive, itl only tell you if stuff downstream of the pressure tap is restrictive, i.e turbo/exhaust. perhaps he meens monitor back pressure at close to the head? or even better, at head and turbo ? I dunno. quite a complicated dynamic to try and measure.

EDIT
Infact the more i think about it, the more i think you simply cant determine an exhaust manifolds effectiveness by simple pressure taps. I meen, if you measured the pressure at the exahsut port, then measured it at the collector (pre-turbo) as the area in the collector is larger itll give a lower pressure, and thus the appearance of a restriction. and god knows what all the pulses and still burning gasses and all that do. The only way really is to go by rules of thumb, and if your loaded/desperate, make two manifolds and try em on the rollers to see what direction to go in.

Edited by evolotion on 25th Feb, 2015.

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

On a similar note, I would like to measure exhaust pressure on our Formula Student car, as it will be running a turbocharged single cylinder, and I'm interested in the effect of the single pulses on turbo speed.

Is there a way to monitor the exhaust pressure as accurate as possible, as using a long line to reduce temperatures will also dampen out the pulses I suspect.

Use a transducer thats rated to a very high temp and a nice short metal line?

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Humm I really don't know, he definitely said to take the reading from the collector. He seems like a very knowledgable bloke and I didn't understand all of what he said.

I was hoping someone has done it on here and could shed more light lol

Dave the Cake

What you would actually be measuring is the turbine inlet pressure.

I have this, although I've never rigged it up to a transducer:


I have an array of the MS map sensors setup on an IO Extender to collect the pressure data from 4 different places.

I did some heat transfer simulations recently and actually found that the length of pipe, in stainless, does not need to be more than 100mm long:


However, this would not give you any information regarding the performance of your manifold. It would just tell you the exhaust pressure required to generate your set boost.

The cycle average pressure would normally be 1 bar above boost pressure. However, through a cycle this could swing 1 bar either way. So if you are running 1 bar boost, the exhaust pressure will swing between 1 and 3 bar during a single engine cycle.

The important thing in turbo manifold design is not to make it less restrictive, but to time the pulses, by adjusting runner length, so that you get the low pressure pulse at the back of the exhaust valve when its open. After all, that's what an LCB does.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Similar bit of pipe to Paul's, for the same reason, and also waiting to be installed...
The usual Freescale sensor is only 2.5bar(a) but they are available in higher pressure ratings.
Re. the Formula Student question, stainless bodied sensors are available at a reasonable price but the electronics inside (and plastic terminal connector) would probably still die so why not make the tube the largest practicable diameter. That would avoid the pulses being damped and, because of the greater surface area, could possibly be even shorter and still dissipate the heat.

Schrödinger's cat - so which one am I ???

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

i was shocked to look down the metro t3 and see like a 15mm hole that the engine must breath threw instantly thought how the heck will this make power pushing the exhaust past that then i thought well its only going to flow threw there till its at full boost then it gets bypassed in the waste gate so not all that bad really but defiantly the biggest restriction to bulk flow in the whole exhaust system

As said above, its the turbo that is going to cause the restriction as long as the manifold is something like sensible. But I think measuring the back pressure in the manifold is still a useful exercise, it could maybe point you towards running a slighly "larger" turbo at less boost pressure to make the same power for e.g.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Any combination of pipes will "work", but if you want the most power for a given boost, then the design needs to be pretty precise.

Particularly with the 5-Port. The centre siamese exhaust port is sensitive to poor design, in my experience.

Material costs for a decent manifold is less then £100 and if you have the welder, then it is money well spent in terms of hp/£.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

on the subjecto of manifold fabbing this series of vids is quite useful,

stick with it ( all 6) theres a few obvious but useful hints I hadnt thought of.



https://www.youtube.com/watch?v=j9knDsi-td8

Thanks for all the replies!

Here's my manifold



It made 140bhp and 160lbs at 15psi

From memory they are 1 1/2" pipes

Dave the Cake

There is no way ever that would restrict power on an A-series.

The scroll in the turbo is probably less than half the diameter of any 1 of those tubes.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Fair enough :)

Dave the Cake

Pipes are too large on diameter and too short in length for maximum power.

Just saying.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Makes sense Paul and I understand you've got the simulations to back it up. But 95% of OEM manifolds are short, uneven lengths - is it really so bad?

If you wanted to go with the same shape as Custard's, for packaging reasons, what's a good diameter tube to use?

No, it's not bad just not the best way to do it, particularly with the 5-Port.

I'm way past just simulations, just need the cars sorted to get some real data.

As you suggest OEMs don't do it because of packaging reasons.

If you are putting the turbo at the back of the engine, then the 38mm/1 1/2" pipe is best because it creates low amplitude pulses that do not have too much negative impact.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

So if you can't tune / lengthen the runners to make use of the pulses, then just make the pipe large enough that it has no +ve or -ve effect ?

I cut up an old LCB to make a temporary turbo manifold. I'm certain it will work fine, but will be one of the first things I change once the car is up and running so looking forward to seeing the results Paul.

Edited by PhilR on 1st Mar, 2015.

haha,

If you want maximum power from a given boost use smaller, longer pipes to generate pulses to improve cylinder scavenging and filling. This invariably means a remote/side mounted turbo as the space is not available.

If you put the turbo down the back of the engine, there is not enough space for the longer pipes, so make them bigger to reduce the pulse amplitude otherwise the small pipes could have a negative impact on cylinder VE.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Measuning exhaust pressure on its own is meaningless. You neet to compare it to inlet manifold pressure. This gives you engine pressure ratio. Anything above 1:3 is considered poor, while 1:1 is excellent and achievable, it requires careful consideration of complete engine design.

Isn't that what that nut-job Mini1071 used to bang on about?

He was aiming for less that 1:1 iirc? But that might need magic to achieve!

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

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