Is boost constant throughtout the inlet system?

[Davezj]

Has any one put boost gauges just above each turbo to see what happens with relation to each other and one before the throttle valve. This would be to see what pressure is lost through the intercooler and soft ducting pipes, etc.

or can someone tell me what happens.

it just so happens i have a few pressure gauges 0-30psi and i was thinking of doing this just to see the results. unless it has been done already.

I was going to weld a nut on the outside of the turbo pipe then drill through the pipe to create the opening, this way i will be able to blank it off afterwards.

What do you think?

[Nick Mann]

I think the pressure will be the same just after each turbo. Even if they are not working perfectly in harmony, they are pushing air into the same system, that has no restrictions between the two turbos. I guess flow would be more interesting there - especially with the VR4 that has unmatched turbos!

It is possible that you could have a pressure change accross the intercooler, I guess, depending on how bad a design it is. And it is certainly possible to get a pressure difference between the manifold and the turbo pipework - there is a throttle butterfly to cause a restriction!

A couple of years ago, one of my turbos became temperamental. I'm sure a couple of times it stalled altogether, as even with my foot flat to the floor, I couldn't build boost. A very sluggish car ensued, but with the addition of a whooshing air sound. Not like a leak. I think the front turbo was pushing air into the system, but it was flowing the wrong way through the stalled rear turbo. It explained the sound, the car that was sluggish with no boost, and the fact that the fuelling was still correct. Lots of air flowing forwards through the front turbo, but it was easier for it to flow backwards through the rear turbo than it was to build pressure for the intercooler/manifold.

[Davezj]

so do you recon any of the tests like the ones i am suggesting have ever been carried out before.

i would have imagined that the preassure developed just after the turbo must be greater than that measured at the inlet manifold.

[Nick Mann]

In theory, if there is no restriction, then the pressure is equal. I guess it depends on how much of a restriction the pipes and intercooler cause. Thre will be a lag, but it will be almost unmeasurable, I would have thought. I'm not an authority on flow dynamics, though, so maybe somene can help a bit more?

[Davezj]

i have thought about this a while ago so when work were scrapping some of the mass specs we make, i have been collecting bits and bobs, gauges pipe work regulators, solenoid valves,metal plate, etc.
so i think it is just down to my welding skills.

[Turbo_Steve]

As Nick says, unless there is a significant restriction, you won't see a pressure difference, just some gauges after the restriction reacting slightly slower....we're talking maybe a second if it's a serious restriction (which I think you're unlikely to see). It's easy to forget just how much a turbo flows, and how little volume there is between the throttle body and the compressor: we're talking hundreds of cubic feet a minute, whereas the inlet tract measures maybe 4 cubic feet on a bad day.

So only a SERIOUS restriction (e.g. something akin to a complete blockage) is going to cause significant impediment as 300(ish)cfm try and rush into a space of less than 4 cubic feet...and with constant pressure behind it, it will only delay the pressure equalising.

If you happen to have a load of electronic sensors, which we could meter against time on a computer simultaneously, and then ideally graph, you'd probably see any restrictions. Even then, I doubt you'll find anything significant until you start seriously turning up the boost....1.3Bar or higher.

Nice idea, but I suspect a lot of work for no real benefit.

[bradc]

most intercoolers do talk about having a certain psi loss at a certain airflow, so over the entire system, I'd bet that the boost would be highest just after the turbo, and lowest in the intake manifold, or just before the throttle body.

Actually thinking about it, wouldn't it be interesting to have a boost gauge before and after the throttle body, I bet some interesting things would be shown, especially at half throttle openings

[Eurospec]

The rally teams will tell you it does make a difference where you measure boost.

Iirc, the original rule was that you could only run x boos t pressure. This was of course tested. Teams got round it by flaring out their i/c pipes to a huge size and taking the reading there. If you think about the mass air flow going through the pipe, where it broadens flow is going to slow and pressure is going to drop.

Eventually the ruling body went for inlet restrictors. Afterall you can only get so much air through the hole on the suction side.

The teams then put massive compressed air tanks in the boot to get round that one! Sure you gan only get x air in per sec, but nothig to stop you compressing what you are not using and then storing that for use when you really want to boot it!

Cheers,

Ben.

[phosty]

Nick - what do you mean when you say the VR4 has unmatched turbos? I thought they were the same?

Or are you refering to the different pipe routings?

I do notice that the outlet pipes from my turbos are different temperatures - one is almost too hot to touch but the other I can comfortably hold my hand on it. Just upstream of where the pipes mix again.

[Nutter_John]

someone has fitted a bigger turbo to the rear bank but not changed the front

[Nick Mann]

Yeah - what John said. The VR4 does have matched turbos, until you make just one of them bigger!

I can't remember who it was now though?

It wa only a couple of days ago too.... /pan

[Nutter_John]

http://www.clubvr4.com/forum/showthread.php?t=29390&highlight=rolling+road

[bradc]

I just wonder how long it will be before the engine blows up

[Turbo_Steve]

At normal boost levels it'll probably last quite a while: He'll end up grinding away the crank bearings or something similar due to the abnormal load.

At higher boost levels, I think there will be that nasty "tnock-thoop" sound, an oily smell and a new block required. Hopefully it won't catch fire, though.


Going back to the orginal topic:

Yes, if you measure boost at a wider part of the tract, you will see lower pressure...however the flow rate will mean that as you move to a narrower part, the pressure will increase again.

Here's the thing about pressure. It's not that relevant to the power output: effectively you're measuring the engines resistance to forced induction.
Hence a bigger turbo will make more power at the same pressure: it's simply running more efficiently at that pressure, meaning the charge is cooler, and chances are it's denser. You can see this on the MAF readings between turbos on the same car. A typical example is a scoob running a VF34 on a 2.5 lump with 2.0 heads.
Boost controller set at 1.0Bar. With the VF34 on there, it quickly reached it's maximum flow rate, hit 1Bar at 2700rpms, and saw a MAF reading of (made up number) 3.8. At the same pressure (1.0Bar) with a GT38, the car made another 180bhp, had a MAF reading of 4.6.

A change of heads to higher flow heads reduced the boost pressure to 0.8Bar. But it made an extra 32bhp.

This gets even more complicated around the intercooler: constant pressure through the intercooler will ensure it removes as much heat from the charge as possible. However a small reduction in pressure, depite reducing the charge temp due to pressure loss, will mean more time exposed to the intercooler, dropping temps further. However as the air leaves the intercooler it must increase in pressure as it enters the narrow piping again. This can result in a temperature increase. Good intercoolers (and the Greddy's are especially good at this) will show a slight increase in pressure across the core. This aims to minimize the impact on flow, whilst ensuring the maximal cooling effect.

This is one of those "the more you know, the less you understand" models, where you need a really good fluid dynamicist to best understand where the losses are. Suffice to say that most cars entire inlet tract is ideally suited to the turbo(s) it's fitted with.

The exception to this tends to be the intercooler, which tends to saturate early. If you're running higher boost, you can often run more advance with a bigger intercooler.

Charge temperatures are more revealing than pressures in this instance (do GCMS's have lots off thermometers in them).
The place (as mentioned) I'd be most interested in measuring pressure is pre and post throttle body at WOT: this is the place you're most likely to see "lag" of any significance.

[Davezj]

are you say Gas Chromatograph Mass Spectrometers.
we specialise in Quad, TOF and Qaud/TOF mass specs with the long serving magnetic sector instument throwen in for good measure. Organic mass spec.

anyway back to the topic, you do have a lot of good info in your post and it is much appreciated you put it on this site cheers.

we don't have a lot of thermometers in our systems but i do a an 8 way thermocouple reader so i could do some measurement. unfortunatly it is not a decent one only designed to monitor a PC case. but it will probably do to tack a few measurement. where do you suggest.

[Turbo_Steve]

Ohh now this will be interesting!
I'd do:
1) Airbox
2) Intercooler input
3) Intercooler output
4) Throttle body
5) Inlet manifold

The first three are probably the most relevant, as it will reveal the shortcomings of the factory intercooler.
The pre & post throttlebody temperatures are interesting as the throttlebody is actually heated to prevent it icing up. I'd be interested to know what impact this has on temperatures in the manifold, as I always remove it.

[Davezj]

It will probably take a while to get round to doing as i have a few other things on but i will take note of you suggestion and do all those places.

One thing to add is i have taken out the the rubber strip at the back of the engine bay to allow air out from under the back edge of the bonnet. so i will do a temp mesurement without rubber stip and fit it again do another measurement to see if it the right thing to do.

[Turbo_Steve]

That makes sense: there should be an area of negative pressure just at the base of the windscreen, so it should pull air through.
One of my concerns tuning the VR-4 is the lack of ventilation, which is compounded even further when you realise that most of the holes cut in the front bumper are just there for show!
Coming from the 300ZX, an evo and a number of scoobs, on all of which every hole channels air towards something, this is a touch concerning: no brake cooling, no transmission cooling, intercooler, oil, water, steering and ventilation all go through the centre two grilles.

[Nick Mann]

People have done that before. Although I think you can then get vibration from the plastic trim at the bottom of the windscreen. It's on my list of jobs to do, but I want to be sure that the plastic doesn't move around, and that when I wash the car I don't soak the gubbins on the bulkhead!

[Davezj]

rubber strip has been off for the last 6 months and i have had no vibration and had no other issues.

takes 5-10 to remove and can be fitted back on if necessary.
I took the rubber off the clips first and then removed the clips afterwards.
the bit of the clip that holds the rubber on is like a grain of rice shape so you have to pull the rubber on one side to streach it off the tip then the other side for each clip.
You will see what i mean when you pull on the first one.

The only thing i have noticed is a slight movement of the middle of the bonnet when you are driving but i did know if it did that before i removed the stip or not. this could be because you have just removed all the support along the back edge of the bonnet, but the stip is so floppy and squishy it could not give much support at all.
you tell me if the back of your bonnet show slight movement up and down when you are driving.

hope this helps
Dave