Technical(ish) Turbo Discussion

[cooper1896]

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You can hit the wastegate pressure without going WOT if the engine is under enough load. You can also easily have high revs and be making no boost if you don't have the throttle open much.

When you refer to wategate pressure, i presume you are meaning the point at which maximum set boost has been achieved, and then the wastegate opens?
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Boost availability is entirely dependent on LOAD level, not throttle position (though generally throttle position has a major effect on load).

In Zossy’s post he mentions load a lot.
Can we define engine load?
Is load essentially a factor of throttle position?

Q: With reference to turbo charged motors, 'spool up' is the time it takes for a turbo to reach the maximum programed psi. 'spool up' and 'lag' are the same thing.

Sort of. You are right about spool, but lag is the delay in engine response created by slow spool and is characterised by a steep torque curve as the turbo begins to spool at higher RPM. The longer it takes a turbo to spool, the greater the lag. The reality is that all turbos lag, but it is less noticeable (or sometimes not noticeable at all) in smaller, better flowing and/or well-tuned turbo systems, or with bigger engines, as the turbo is spooling effectively at a lower relative engine RPM.

Ok, so lag = amount of energy (exhaust gas) that are required to get the turbo to spool.
I presume this is a factor of how much friction the turbine wheel has, what angle or shape of the turbine wheel blades.

Q: Does increasing boost psi increase or decrease lag?

The lag effect, being the delayed torque response resulting from the rise in boost, will be magnified by the increase in boost. But this doesn't necessarily mean more "lag" as you call it. The nature and intensity of this effect will depend on your turbo and tune, and your boost level. All things being equal, the engine will produce the same power up to 4psi, but from there the 20psi setup will just keep on giving right to 20psi from which point the torque curve will rise at a more uniform rate.

So you are saying the it will take the same time to reach the 4psi of boost (and hence the same lag up to that point) but it will still continue to increase the rate of torque / power up until the wategate opens at 20psi?

[NitroDann]

Basically yes to everything.

Load is also affected by things like gear ratio (final drive AND what actual gear you are in), how heavy the car is, and if you are going up a hill.

Basically anything that requires you use more throttle opening to maintain the same acceleration.

An ecu determines load using a Throttle Position Sensor only. So throttle position must be pretty close to right haha.

Dann

[cooper1896]

Q: With reference to turbo charged motors, 'spool up' is the time it takes for a turbo to reach the maximum programed psi. 'spool up' and 'lag' are the same thing.

Lets get this straight-
Boost threshold is the lowest RPM that a setup starts making boost. Just 1psi counts.
Lag is the amount of time the car takes when above the boost threshold to achieve enough boost to make good torque.
If a cars boost threshold is 3000rpm, and by 4000rpm its at full boost (say, 12psi) it takes 1000rpm to hit full boost. However at 5000rpm it may take only 400rpm to hit full boost, so lag isnt quite as clear cut as many make it out to be.
People also commonly refer to boost threshold as lag.
Also, if you are running a smaller turbo at a very high boost level, say a Nissan S13 T25 at 20psi then at 20psi its FAR out of efficiency range. You may do this to get the last 10 hp out of the system over running 16psi.
In a setup like this its not important to achieve full boost quickly, as half of full boost will net you 80% of the torque.
SO... the LAG between 0psi and 20psi, above the BOOST THRESHOLD isnt important, the LAG between say, 0 and 10psi is whats important.

Ok so from your response, I am assuming that at very low levels of engine load, ie idle or low throttle applications, there is so little exhaust exiting the engine, that these gasses can move through the exhaust turbine without causing it to spin?

Q: With relation to a turbo charger, what is flow? Can turbo's of the same size have different flow capacities?

Zossy's explanation is pretty good. Of course a bigger intake wheel or a larger a/r will flow more but it weighs more and in the case of a bigger a/r it takes longer to pressurise the larger housing. Many people dont understand why big turbos make more power at the same PSI, stating that they flow more air. They dont usually, they usually flow more exhaust. Its the small exhaust housing that restricts power most.

What do you mean with A/R?

Hope this helps,
Dann

Yes it does, I think I am actually starting to get a clearer and more in depth understanding of turbocharged systems.

[cooper1896]

This is great discussion (well for me, I haven't found it yet on any forum discussion).

Lets move this on to a more mx5 relevent discussion.

I would like to know why the SE/Mazdaspeed turbo cops such a pasting compared to other turbo chargers?

Numberous articles I have read state that the turbo is not noticable until it is above 4.5K (presumably they mean the turbo making good boost). How can this be the case?

[NitroDann]

It sure does spin at idle, but there isnt the pressure or energy to spin it hard enough for the intake side to make any pressure, when cruising along the turbo may help the intake move, for example at 100 cruising on the highway you might use a few % less throttle due to the compressor side 'helping' blow the air forward, and effectively lowering pumping loses, but boost will not be achieved without load.

A/R can be googled but basically the snail looking shell part of the turbo, the housing, has exactly what you think on the inside, a gap in the centre thats circular for the wheel, and a cone shape that spirals out towards the exit, this is the same for both the compressor, or cold side, and the turbine, or exhaust side. The larger the A/R is the larger this cone spiral shape is, basically it will flow more air, but slower.

Imagine you have a twirly fan from the fair from when you were a kid, that you blow and it spins, if you blow a little bit, with a garden hose, nothing happens, but if you blow the same amount through a straw it will spin. Now if you blow really hard the little straw is a restriction so it wont spin a lot more, however the garden hose flows a lot so spins the straw very hard.

A/R is like this, the little housing on a small A/R (eg. 0.48) is like the little straw, spins the wheel up with a little flow, and so spools the turbo at lower rpm/load, but restricts at the top, where as the larger A/R (eg. 0.86) will not spool at low rpm/load but will flow heaps with a lot of exhaust gas pumping through it.

Clear?

Dann

[NitroDann]

The MSM turbo is of generally poor design, it has a low flow capacity so is unable to make a lot of power, but also spools quite slowly especially in stock form with the crappy intake and exhaust on it. So it has hardly and turbo advantage (power) and lots of disadvantage (lag and high boost threshold).

Dann

[cooper1896]

So is really a fault of the turbo unit, or the surrounding parts?

stock turbo + well designed exhaust + intake + ecu Lets say it makes aprox 200hp

For the same max power & talk, would a different turbocharger produce better results?

[NitroDann]

Yeah,
A T28 off an S15 will spool 1000rpm earlier and make 300rwhp.

Dann

[Lokiel]

Yeah,
A T28 off an S15 will spool 1000rpm earlier and make 300rwhp.

Dann

FM would disagree with this Dann:

"For better response than stock with a similar maximum horsepower level, we have the small GT2554R. For similar response to the stock turbo yet with the potential for approximately 60 hp more power, we have the GT2560R. For maximum power (and built engines), there`s the big GT3071R."

FM found that the T28 doesn't offer any benefits over the GT2560R or GT3071R; it is slower to spool than the stock IHI turbo which is why they recommend the GT2560R for similar performance.

ie. the T28 will make more power but will NOT spool up 1000rpm earlier.

I plan on upgrading to the GT2560R since my car is not a dedicated track car and I'd hate to lose the standing-start "punch".

[NitroDann]

For all intents and purposes the T28 IS a GT2860r.

And yeah I exaggerated. But the point remains.

Dann

[Lokiel]

Dann, I only commented because your comment that I quoted above was clearly wrong, your original comment:

"The MSM turbo is of generally poor design, it has a low flow capacity so is unable to make a lot of power, but also spools quite slowly especially in stock form with the crappy intake and exhaust on it. So it has hardly and turbo advantage (power) and lots of disadvantage (lag and high boost threshold)."

was correct - I assume that is the "point" you're referring to?

[NitroDann]

Yes, I exaggerated regarding how much better a T28 is, but essentially the point I was making remains, its response to power production ratio is FAR better than stock MSM turbo.

Dann

[cooper1896]

Yeah,
A T28 off an S15 will spool 1000rpm earlier and make 300rwhp.

Dann

FM would disagree with this Dann:

"For better response than stock with a similar maximum horsepower level, we have the small GT2554R. For similar response to the stock turbo yet with the potential for approximately 60 hp more power, we have the GT2560R. For maximum power (and built engines), there`s the big GT3071R."

FM found that the T28 doesn't offer any benefits over the GT2560R or GT3071R; it is slower to spool than the stock IHI turbo which is why they recommend the GT2560R for similar performance.

ie. the T28 will make more power but will NOT spool up 1000rpm earlier.

I plan on upgrading to the GT2560R since my car is not a dedicated track car and I'd hate to lose the standing-start "punch".

Lokiel, do you plan to upgrade the internals of your engine?
If not, wouldn't the maximum power of the GT2560R be more power than a stock motor can handle?

Also, do either the T25/28 or GT2554R/GT2560R turbo's bolt onto the oem SE turbo exhaust manifold?

As a general question;
for cars that are focusing on track work, would't response be more valued than overall power?
I would of thought that a responsive turbo shortens the time to go from partial throttle to full throttle, and there for ful power, (ie corner exit). You are getting more of the use of the turbo, more of the time - assuming the track doesn't have a 4km straight

[Lokiel]

I wasn't planning to upgrade my internals and will be happy to sit on 250rwhp (I say that now) which should be safe.

You will need to change your exhaust manifold too and I'm planning on using the FM "Mazdaspeed FMII conversion kit" (see http://www.flyinmiata.com/index.php?dep ... r=22-10050) - lots of $$$ so will have to wait until next year. MINX would say that's a good thing since it will give me a chance to learn to drive my car as it is

If you want to keep your exhaust manifold, look into the BNR upgrade option. See http://www.mazda-speed.com/forum2/index ... 209.0.html. bitrusty here has one of these turbo upgrades so he might be able to give you some speifics on it.

Dann will have his own ideas on this and it's worth listening to them (sometimes he can give great advice).

[NitroDann]

Im about to build 2 tubular manifolds utilising a stock SE turbo position, but flanged for T25/T28.
One car IS an SE, who is making the upgrade to T28, the other is not an SE but Ive decided that the SE location is pretty damned good for a full street car with a/c and p/s etc.

Thats not an expensive option. Infact a manifold alone is very cheap.

I feel that on a stock bottom end that a T28 (GT2860) is about perfect.

Dann