General Turbo Response Question, Guru Input Needed.

[GTRNUR]

Hi all,

So im not just interested in skylines. I am also in the process of building a new race engine for my other car, the vw pictured below.

The engine is entirely custom. Displacement is 2443cc (88mm crank, 94mm bore). Compression is 6.7 or 6.9:1 depending on what head gasket i choose. Cam is 278 degrees @.050", 112 lobe seperation. Everything in the engine will allow it to rev to around 8000-8500 rpm. The turbo setup i had chosen and already purchased was originally intended for a smaller engine (2332cc). I have a pair of garrett GT2554R's, rated at 260hp each. I would like to see about 500rwhp on a max boost tune. The engine is intercooled and injected as well. The heads are cnc ported and are huge. 48mm intake and 40mm exhaust valves. They flow 240cfm at 25".

Now here's the question. Based on what everyone knows about the Garrett GT2860-5/7/10's and HKS turbo's and how they perform on a RB26 is it possible to take a guess as to how quickly these turbo's designed for the GTR would work on this engine? Would response be better as the exhaust gas pulses per cylinder are 177cc larger per cylinder than the GTR, but there is one less pulse as 2 cylinders feed each turbo and not 3?

Ive seen that its possible to get 500rwhp from a pair of HKS 2530's provided supporting mods are there. I am considering upgrading to these turbo's for the project or even larger provided response will be reasonable still.

Cheers,

Ian

[paulr33]

horrible compression = flat & empty response

[R31Nismoid]

Why is the compression that low?

Given i dont know what motor your talking about... i would have thought you might want to up it if you have everything flowing much better?

[GTRNUR]

The compresion is that low to allow up to 2 bar to be run more easily. Factory compresion on these engines is around 7:1. The lower compression helps keep head temps down as they are air cooled. I know that compression is a factor for maintaining off boost response and torque/hp etc, but i am more concerned about when full boost will be achieved. The deck height of the engine will be 0.045" or 0.065". The pistons have 38cc dishes to lower the compression.

My last 2332cc turbo motor ran 7.4:1 with a GT28R. It made 10lb by 2100 rpm. But the driving response was easily twice that of a stock engine due to the increase in engine capacity. That engine wouldnt rev past 5500 rpm though due to the cam and turbine being too small. The car weighs around 830kg so the rate of acceleration off boost is still very quick.

The engine block is from a 2.1 water boxer vw but the water cooling jackets are machined off the block and its converted to air cooled using custom components. They call this an oxyboxer engine. Its a flat horizontally opposed 4 cylinder engine.

Cheers,

Ian

Edited March 19, 2008 by Vspec R33

[R31Nismoid]

Was 7:1 the reason though because of fuels used "X" years ago?

Considering you've ported and so on, cams etc and given the added decency of fuels (much more decent) it might not be that much of a worry running factory or more than factory compression i would have thought

[djr81]

The compresion is that low to allow up to 2 bar to be run more easily. Factory compresion on these engines is around 7:1. The lower compression helps keep head temps down as they are air cooled. I know that compression is a factor for maintaining off boost response and torque/hp etc, but i am more concerned about when full boost will be achieved. The deck height of the engine will be 0.045" or 0.065". The pistons have 38cc dishes to lower the compression.

My last 2332cc turbo motor ran 7.4:1 with a GT28R. It made 10lb by 2100 rpm. But the driving response was easily twice that of a stock engine due to the increase in engine capacity. That engine wouldnt rev past 5500 rpm though due to the cam and turbine being too small. The car weighs around 830kg so the rate of acceleration off boost is still very quick.

The engine block is from a 2.1 water boxer vw but the water cooling jackets are machined off the block and its converted to air cooled using custom components. They call this an oxyboxer engine. Its a flat horizontally opposed 4 cylinder engine.

Cheers,

Ian

I have a pair of 28/60 707150-5's on an RB26 running 435rwhp. There is no one at home at anything below 4000rpm but it is making its 15# boost by 4500rpm. 9# at 3900rpm. With your lower compression & smaller capacity yours will obviously be laggier. My cams are 260 degrees (Both)

On the up side the compressor on the dash 5 is very efficient which should help temps.

To be honest with your set up I would be looking to use something other than common or garden 98 octane. Otherwise it will just be laggy & therefore slow.

By the way (sorry if this is already obvious) 10lb/min airflow from the compressor equates to approx 10hp at the flywheel. This is usefull as you can plot how the motor & compressor work together by plotting the airflow on the compressor map.

[GTRNUR]

Was 7:1 the reason though because of fuels used "X" years ago?

Considering you've ported and so on, cams etc and given the added decency of fuels (much more decent) it might not be that much of a worry running factory or more than factory compression i would have thought

Yes thats definatly part of the reason for sure. Engines setup for 92 octane. Yes it will be 98 octane on the street and should make around 300rwhp on 12-14lb, street tune. For the high boost tunes it will be Sunoco Fuel of some sort. I havent checked on whats going to be easily available in my area though.

Generally the highest compressions being run by the medium hp engines (300-350hp) of this type is around 8:1. If you want to push higher power than that the compression always comes down. Unless your running methanol that is. The methanol forced induction guys are running around 10:1 from what i've read.

With the CR at 6.7:1 - 6.9:1 I should still be able to run 30-32 degrees of ignition on 1 bar of boost. With that boost and CR it will be running simular to a 5lt engine at 12:1 cr.

I have a pair of 28/60 707150-5's on an RB26 running 435rwhp. There is no one at home at anything below 4000rpm but it is making its 15# boost by 4500rpm. 9# at 3900rpm. With your lower compression & smaller capacity yours will obviously be laggier. My cams are 260 degrees (Both)

On the up side the compressor on the dash 5 is very efficient which should help temps.

To be honest with your set up I would be looking to use something other than common or garden 98 octane. Otherwise it will just be laggy & therefore slow.

By the way (sorry if this is already obvious) 10lb/min airflow from the compressor equates to approx 10hp at the flywheel. This is usefull as you can plot how the motor & compressor work together by plotting the airflow on the compressor map.

Just been looking at the compressor maps and trims between the GT25 and GT28 ranges on the turbobygarrett website. I guess i'll just have to try what i have, see how it performs, and then decide where to go from there.

djr81, about the temps side of things. Porsche took that approach with the early 911 turbo's. Laggy turbo response also meant less heat as they didnt start intercooling till the mid 80's. I shouldnt have to worry about that as much as they did. The engines cooling system should be pretty sound.

I realise that its exhaust gas expanding from a high pressure area to a low pressure area accross the turbine side of the turbo, but there is more science to it than that. What i was really after was some sort of technical answer on what would make a turbo spin up faster. Assuming an RB26 has 1.3lt/3 exhaust pulse events feeding its turbine, will it spool faster than or slower than a 1.3lt/2 or 1.3lt/1?

Thanks for the feedback so far everyone... i realise talking vw's on a skyline forum is walking a fine line to being flamed...

[djr81]

Just been looking at the compressor maps and trims between the GT25 and GT28 ranges on the turbobygarrett website. I guess i'll just have to try what i have, see how it performs, and then decide where to go from there.

djr81, about the temps side of things. Porsche took that approach with the early 911 turbo's. Laggy turbo response also meant less heat as they didnt start intercooling till the mid 80's. I shouldnt have to worry about that as much as they did. The engines cooling system should be pretty sound.

I realise that its exhaust gas expanding from a high pressure area to a low pressure area accross the turbine side of the turbo, but there is more science to it than that. What i was really after was some sort of technical answer on what would make a turbo spin up faster. Assuming an RB26 has 1.3lt/3 exhaust pulse events feeding its turbine, will it spool faster than or slower than a 1.3lt/2 or 1.3lt/1?

Thanks for the feedback so far everyone... i realise talking vw's on a skyline forum is walking a fine line to being flamed...

I was just trying to indicate that the more efficient the island you are on for the compressor the less heat the compression of the inlet air will generate thereby easing the load on your intercooler and lessening the temps into the engine. Sorry if that is obvious or known but I can never work out at what level to pitch answers at on the forums.

You have to be a bit carefull with the exhaust flows. If you look at the graphs for the turbines what counts is the pressure differential (as you say) - given the maximum efficiencies of the turbines are only about 70% the more more consistant the gas flow the better the outcome would be my best guess. I would expect a higher PR would result in a lower efficiency hence lower compressor performance. So a more consistant flow ie 3 pulse events, not 1 would give a better outcome. But that is only supposition.

[Dale FZ1]

Thanks for the feedback so far everyone... i realise talking vw's on a skyline forum is walking a fine line to being flamed...

If anything, the question really tests what ideas/knowledge/experiences are out there.

VW performance is something a lot of the now generation don't experience, but they can be very fast. Also a bit fragile unless done right, so I understand the lower CR approach. Adequate engine cooling, oil cooling, and different level of combustion efficiencies are all things to factor in. A hot running VW will probably have less knock resistance than a modern 4 valve Nissan for instance.

I'd be leaning towards the GT2554 especially if you wanted a torque band that starts early enough to be useable on the street.

Can you answer these questions:

what is the car being used for (ie. is it intended to be a registerable drag bug? )

why not look at a single GT30

have you considered water injection

cheers

[discopotato03]

Take 2 , lost another longish post to wifi .

CR too low IMO , modern turbo with free breathing tracts can do much better than dinosaur bits from the 70's .

Parallel twins not worth the weight and complexity on a I4 or flat 4 , most boxer 4's have the wrong firing order to hang a turbo off each head .

IMO a TS single with twin external gates (very easy to do with flat 4 tubular headers) would give the best results .

If regs allowed possibly water/methanol injection , highest octane fuel available and as much timing as it takes to make the most torque everywhere .

Lots of effort needed to get airflow around the heads/barrels and effective oil cooling to keep it alive .

My 2c , A .

[GTRNUR]

Ive built a new chassis that is going to be engineered and hopefully approved for registration next week. The goal is motorkarna and drags depending on the tune.

Yes i had considered single and i have run two diferent styles of single turbo motor in the car previously. I guess i wanted to try twin turbo to try something diferent. I enjoy building something and working around the engineering chalanges as much as i enjoy driving the car. Space for a large turbo is a big problem too. As it is you have to cut away parts of the body to clear engine and gearbox changes to make things fit, but thats pretty much considered normal practice for vw's these days too. I didnt want to go completely overboard with the cutting away of metal with this car though as the body is near perfect.

Yes water injection is definatly on the cards. I was considering a CO2 spray system for the intercoolers too, so if I am pushing the compressors out of their efficency range the air temps should remain relitavely sane. Only time will dell i guess.

[GTRNUR]

Take 2 , lost another longish post to wifi .

CR too low IMO , modern turbo with free breathing tracts can do much better than dinosaur bits from the 70's .

Parallel twins not worth the weight and complexity on a I4 or flat 4 , most boxer 4's have the wrong firing order to hang a turbo off each head .

IMO a TS single with twin external gates (very easy to do with flat 4 tubular headers) would give the best results .

If regs allowed possibly water/methanol injection , highest octane fuel available and as much timing as it takes to make the most torque everywhere .

Lots of effort needed to get airflow around the heads/barrels and effective oil cooling to keep it alive .

My 2c , A .

A big part of what im trying to achieve is keeping the car semi streetable. Finding the balance between drivability, reliability and perforamance is the challange and there has to be a compromise made somewhere. Keeping the CR down should help with streetability to a degree by keeping the off boost response more managable. When its on boost i expect its going to be a handful. When at what RPM that happens at is my main concern.

My last turbo engine (2332cc, 84x94)with low compression had twice the throttle response off boost that my GTR has. I'd put that down to the displacement increase and the lightened flywheel. Its rod ratio was low at 1.61 as well so that was probably a factor with the low end torque it made. The new motor is 1.65 but has 4mm more stroke than the previous one.

Whats a TS? and why the twin wastegates? My previous setup ran a 38mm external gate with the GT28R-320 (0.6/0.64) and it had no boost regulation problems?

My understanding of wastegates is that it needs to be large enough to flow enough gas to maintain a constant boost pressure without having the boost pressure rising toward the top end of the rpm range, yet also be small enough that it can properly regulate boost during the early stages of when boost is being achieved. If its too big boost could fluctuate when the wastegate starts to open. Is that why 2 wastegates... a first and second stage to boost control?

The engines cooling is covered. It uses water jacketed heads and a porsche 911 turbo fan/alternator with a custom shroud. Twin front mounted radiators regulate water temps and a water/oil heat exchanger is used to maintain oil temps. The engine is dry sumped as well with the case running a vacuum. It doesnt use oil squirters to cool the pistons but i've had Competition Coatings in brisbane do the ceramic/teflon coatings treatments on them.

[Dale FZ1]

With an 850kg body to move, a low comp air cooled engine of 2440cc is not going to feel sluggish in the way that a Skyline running under 7:1 would. Run with what is done/known to assist with reliability. The effective compression ratio if running 2.0bar is quite significant too, and will put you close to detonation.

Room probably would be a problem (as always ) so where is an intercooler packaged into the back end of a bug and still get good airflow? Both drag and motorkhana are short-run events, so for effective detonation control and simplification of the inlet tracts why not eliminate the intercooler and concentrate on a really good port water injection setup? With that power level you should be able to access a range of nozzle sizes that will work well.

500rwhp is going to take some pretty big compressor flow - more than the combined 520hp of those GT2554. And there would be no real reason to push them over the edge of their efficiency maps. No sense in doing that if there is a better spec available. You'd want something able to run 600hp worth of flow - so if a single was possible to package then try a GT3582 and go external gated. Physically they should fit? Slightly downsized, a GT3076 running a EW 1.06 housing would definitely be a thought. Very free flowing top end, and make use of that low down torque:weight you have while allowing it to not be choked as the revs rise?

I agree that a split pulse layout could be a really good thing with the horizontally opposed engine, and maybe something from a Fuso (MHI) truck would give you the goods. They do split pulse turbos in the size you'll need. Likely available cheaply too, so you might be able to easily try some variations there.

[GTRNUR]

Im using twin air to air intercoolers that will be mounted in the fronts of the rear guards. There will be a big hole that opens to the intercoolers in each guard. They are widened 100mm so there is about 220x400mm of space for the air opening. The cores are larger and will be ducted to the engine bay, so the 911 fan pulls air through the intercoolers and ducts at idle and when the engine is running.

Its doubtful i'd be trying to drive the car with a max power tune in a motorkarna event. Even with a light-medium tune its going to take some getting used to. I have a heap to get familiar with, with regards to the suspension setup of the new chassis as well, so im going to start with a low power tune and play with the chassis till I become good enough a driver to be able to handle driving it with more power. Considering how infrequently events happen near where i live, this will take a while.

I have never been able to find any good reading material to explain the real benefits of split pulse turbines. I understand how they feed one set of cylinders to one side of the housing, and another set to another part of the housing. I am guessing that the angle the gasses hit the wheel are optimised for response? Is this the reason for the twin wastegate setup...? one for each turbine intake?

Is that why the T51 setups on GTR's ive seen use a pipe from each side of the turbine housing that T's back to a single wastegate? Shouldnt they be using twin gates as well, so the exhaust pressures from the front 3 cylinders cant balance to the rear 3?

Edited March 20, 2008 by Vspec R33

[Dale FZ1]

It's all about the effect of pulses travelling down the headers and into the turbine, and minimising reversions that affect scavenging of neighbouring cylinders. There's a fair bit of science in it, and I'd say that twin wastegates would be required to nail it technically.

There was a thread that ran last year. Helped resolve a few things in my mind. http://www.skylinesaustralia.com/forums/Sp...;hl=split+pulse

She's obviously going to be a multi purpose car. Big difference in power requirements for drag and motorkhana. High or low boost, should go like a cat shot in the ass. Sounds like fun.

[raver1987]

why not run a smaller turbo for lower boost whilst running a bigger turbo for higher ranges