Big Will_

Just been doing some calculations/simulations in choosing my new turbo and it's thrown up some interesting questions over delta pressures.

Now, delta pressure is the difference between the boost created and the exhaust backpressure, so - for example a delta pressure of 5 would be achieved by having 25psi of boost and 20psi of backpressure in the turbine housing. This is good because it shows efficiency of the turbo.

A delta pressure of -5 is the other way round, so for the same 25psi of boost, there's 30psi of back pressure in the turbine housing.

Now, i'm looking at a delta pressure of -15 at high revs (6000rpm+) on one particular turbo, but the next size up in compatible exhaust housings is a BIG step up, but takes my delta pressure to -4.

So - the question is, what is an 'acceptable' delta pressure and is it worth changing!

Fudgey

iTrader: (4)

if only 95% of the people on here knew what you were on about

phil_focus

fook knows

but interesting subject

JonnyBravo

I am sure it could be quite handy to know the answer to it but in all honesty most people just use a average exhaust and get the results they want without spending loads on development.

Didn't Euan do a bit of development with his exhaust system ? Maybe he is his tuner would have some good info.

dojj

iTrader: (3)

i think what people would be more intrested in finding out would be why the boost pressure developed at the source (the turbo) isn't the boost pressure seen t the reciving end (the inlet manifold)

if you can find a turbo that does this wihtout overboosting itself to oblivion and can find a nice and simple way to get this figures, i'd be intrested in reading about it

for example, i know that with 3 boost pressure gauges all hooked up (twin turbo) the turbo's were boosting at nearly 30% more than the inlet was seeing

and there were no "leaks" in the system

as for your delta stuff, i think that it's not a matter of one turbo being more efficient than the other, it's about one turbo having a worse designed housing than the other from what you've put

JamesH

iTrader: (21)

As said I think you're over doing it a bit. I just wouldn't really want a back pressure of over 50kPa. But steady state is also different to transient. Back pressure will creep up an extra few kPa over a minute or so if held steady state

J1mbo

iTrader: (1)

Ideally pressure for pressure but acceptable is just into the negatives IMHO you don't want to be looking at -15 especially running 30psi

What boost do you think it will run?

J1mbo

iTrader: (1)

Dojj its not all about the exhaust housing but the exhaust wheel design aswell

Big Will_

Bearing in mind my engine is going to spend most of its time at high engine speeds on track, the question is, in relative terms - do I take a GT30 exhaust housing with backpressure beyond 6000rpm (and i'm revving to 8000rpm) which will in turn reduce volumetric efficiency as the intake charge will be fighting leftover exhaust gases that are unable to freely escape. OR do I take a GT35 exhaust housing with no noticable back pressure and the ability to flow better at higher engine speeds.

For arguments sake we'll leave the compressor wheel and housing the same so the boost pressure run (around 25psi as my engine is over 9:1 compression) remains constant and the boost curve is tapered to aid traction so the 'lag' isn't the be all and end all consideration.

J1mbo

iTrader: (1)

As you say the gt30 is poor for the back pressure, even with the biggest exhaust housing, a gt35 isn't my fave turbo, if you can get over the surge issue (Mark will be here shortly ) then the exhaust wheel and housings are not as bad, but there are far better turbos about for all factors, back pressure, surge lines and response, look away from the gt series is my opinion!

Big Will_

Yes Jim - fully agreed mate

I was saying 'relatively' to put it into terms people would be more familiar with - i'm looking across the whole board at the moment as going turbo only with my setup...

J1mbo

iTrader: (1)

Your going turbo only now will? Or am I missing it lol

Big Will_

Yes mate Basically, for Time Attack I decided it would be simpler, more reliable and more suited to go turbo only so i've sold my supercharger and GT3076R combo.

Getting a Jenvey inlet plenum and looking at doing a twin scroll turbo setup - just trying to make sure I get the turbo right first time!

Student loans don't stretch to covering mistakes in selection

J1mbo

iTrader: (1)

Fair enough! What power are you hoping for now?

Big Will_

Would like to see mid to high 400s. Maybe touch 500 at the top, torque around the 400lb/ft mark but most importantly I want to keep it driveable! Not silly huge boost spikes etc. - it's front wheel drive and high comp!

J1mbo

iTrader: (1)

That's where id be at power wise, get some big tyres on there and make it useable!

If you wasn't so high comp and revving it so high a 3076 would have been lovely but I'm not sure just over 500

The BW EFR turbos are looking good, I think there is one a smidgen bigger than a 3076 might be worth looking into,

scoooby slayer

iTrader: (4)

what about a new 3076 gtx ? dont they show 55lb ?

J1mbo

iTrader: (1)

It's not about the amount of air they flow, thats ot the problem, its the exhaust wheel and housing that is un changed from the non gtx if I'm not mistaken

Big Will_

Jim, it's the internally gated twin scroll EFR7670 with the 0.93 AR exhaust housing that i'm considering.

The EFR7064 just seems to run close to choke and build higher delta pressures at high engine speeds. Even if I go for the externally gated version!

There are others out there I need to explore. Was just keen to get a feel for how far delta pressures can go before becoming a big issue!

J1mbo

iTrader: (1)

That sounds nice with the big exhaust housing

The thing with a track engine your not hanging it in forever, it does get a big of a break, but when we measure the pressure in exhaust housings if its much over lb for lb then its not great 5psi or so would be acceptable IMO but I'm no expert to be fair just from what I've seen

Big Will_

Well, hopefully these will shed a little more light!

These are the input data values i've used:


This is the 'smaller' turbo compressor map with the 6 points plotted on:


And the figures it 'claims' - the delta pressure being at the bottom:


Now for the larger turbo - compressor map first.


And the figures:

RWD_cossie_wil

iTrader: (9)

Best topic on PF in, like, ever

TimC

iTrader: (1)

Finaly back to some proper tech, Lets hope Cossie4i dont come in here and start ruining it with T38 talk.

Will how did you come up with all the figures you have inputted? Is it based on knowng the exact amount of air your head/cam choice can consume at particular boost presures etc.

J1mbo

iTrader: (1)

If those delta pressure figures were achieved when put into practise it would be very good

But we know that it dosnt always work that way lol be interesting to compare the data you get after mapping compared to the calculated that's for sure!

CossieRich

iTrader: (1)

Far to complicated for my pea brain to understand all this

Makes good reading and i will attempt to understand it though

scoooby slayer

iTrader: (4)

how do you know exactly what data to input though ? the results can only be as accurate as the initial data is.

Big Will_

Initial data values are currently a 'conservative best guess'. It's catch 22 really as until i've bought a turbo, fitted it to the car, got the car running and can put it on a dyno for a few days to test everything - I can't get accurate data.

But then i've kind of negated the point of the testing in making my turbo choice as i'll have bought one by then lol.

I'm tempted to turn this into my dissertation for uni next year now - but don't really want to keep the car off the road that long

ian sibbert

Will,

Personally I would not be too unhappy with the pressure figures you are seeing at such high revs..turbine efficiency and work rate are not really affected by ΔP, turbo's are primarily driven by waste heat. ΔP and ΔT are all phenomena of the turbo doing it's job ΔT being as, if not more important than the mass air flow of the exhaust gases into the nozzle...at least that's what I have always thought...lol.....

I would have thought some of factors such as cam choice, and exhaust design would see a significant in change in ΔP

Big Will_

Well, i've got Variable Valve Timing on my inlet cam, so have a 40 degree window to play around with that in.

There's a part of me that still thinks the big turbo is 'too big' and i'll be using it at only a part of its capability.

Chip

iTrader: (3)

It also implies that the turbo is probably going to be laggy, so it depends what your requirements are.

Im sure Jimbo will be able to confirm that T4's score better that GT turbos based on backpressure, but they spool slower too.

As with most things in life, its a trade off.


Not much data around for exhaust manifold pressures for our applications (although I believe from what Mike was saying years ago its something SCS have monitored?), its not something I have personally monitored on any of my engines, but its effects are certainly indirectly very visible, for example a simple swap on twitchells astra from .63 to .82 housing made a massive difference to both power and EGT, and thats only going to have come from a reduction in exhaust manifold pressure.

gjh

This threads way out of my depth, still makes very interesting reading though.

Big Will_

Circuit car with a short ratio box, will hit 145mph at 8000rpm in 6th so it's never going to be subjected to 'top speed run' loads.

Chip

iTrader: (3)

Agreed, by definition any big power car on a circuit isnt ever going to see sustained load at one point in the rev range, as the speed is ALWAYS greatly transient at full throttle as you arent going quick enough for their to be enough wind resistance to slow your progress drastically through the rev range.
Different story in a 100bhp car on the bedford back straight of course, where you could easily be sat at near peak rpm for long periods of full throttle! lol

J1mbo

iTrader: (1)

CHip, your right, the t4 is better for back pressure but still not perfect, also a bb t4 is not very far behind a gt30 at all really response wise, (comparing bb to non is unfair IMO)

The t4 exhaust wheel is also a lot bigger than needed for the compressor wheels capabilities, so could be made better

Chip

iTrader: (3)

Thats one of the reasons for the GT series AFAIK, to tackle the mismatch in the context of what people are doing now with a T4 (was designed for trucks originally IIRC, lol) although possibly they have gone too far for your specific applications, they are really suited to engines that rev better than most YB's I believe (honda engines seem to be a match made in heaven for the GT for example), but thats a whole other discussion.

Big Will_

Honda engines on GT30s tend not to run big boost though. Most of the ones i've seen have been between 14psi and 20psi.

I wonder if any of the Honda tuners have any information on backpressure at 9000rpm...

Chip

iTrader: (3)

Im sure they do, but wether they will share is another matter, lol

Ask rickylee?

ian sibbert

The IHI turbo on the World Car has the turbine blades cut back, small housing, low compresor trim and high pressure ratio....absolutely no information on it lol and I've never seen a curve, I'll have to measure the backpressure when I get it running again...again ignorantly i'd always assumed the turbine wheel being cut back was to reduce the backpressure in the small housing to optimise flow/reduce pressure...

ECU Monitor Enthusiast

iTrader: (1)

Agree with both comments in terms of that its unfamiliar territory for me

Big Will_

I'm glad its of interest to people

There are some very clever people on here and I was keen to hear their input on the matter. If others can learn from it too then great!