15psi is always 15psi, but 15psi at 80 degrees is not the same as 15psi at 160 degrees; this being what compressor efficiency is measuring (heat added to the intake charge). A larger compressor will generally have better efficiency numbers higher up in the airflow range, and a wider efficiency island.

You aren't changing the "flow", persay. Given a certain engine condition with a better compressor, you're flowing the exact same volume of air (assuming you have the exact same pressure), but the better compressor will be pushing a cooler, and thus denser air charge. (Which would be mass, as opposed to volume).

This is why a MAP based system also requires an intake air temp sensor to correctly calculate the air density. A MAF system measures mass directly, and is probably more accurate under a wider range fo circumstances, but the restriction to inlet flow leads me to use a MAP/IAT system 90% of the time. I would choose a MAF for things like hill-climb motors where rapid changes in altitude would necessitate the wider operating range, and self barometric correction would be more desirable.

Sorry, but you are dead wrong.

TTT is right.
Pressure (i.e. "psi") is only one variable when talking about turbo potential.
AIR FLOW the other big variable.
To ignore air flow is downright stupid.
15psi with a T3 is not the same as 15psi with an HKS T51R SPL...

Air temperature is tied into the whole thing (see PV=nRT), but it's a relatively minor variable.
To emphasize air temps over air flow is downright...ignorant.


-Ted

You're in such a hurry to contradict me, you didn't pay any attention to what I actually wrote. You need to re-read what I wrote, and what I responded to, and rethink your statement. We're not talking about total turbo potential, we're talking about two turbos on the same engine, both capable of making a given pressure at a given flow rate (CFM).

Flow and pressure are directly related; unless you have enough flow to over-supply the engine, you can't make boost. Thus, increasing flow in a system increases the pressure. With a larger turbine, you can't "flow" any more air into the engine at any given boost level than with any other turbo capable of flowing the same amount (assuming, of course, identical hot-sides, and thus equal exhaust backpressure at that boost level). So where does the power come from when you swap to a larger cold side? Increased efficiency, and thus a cooler air charge.

Being capable of flowing more air doesn't mean you actually do, it just means that it takes less work for the compressor to flow the air you do need, and less work means less heat.

That is not possible unless we're talking about IDENTICAL turbos here.
(And even with two same turbo models, there is still miniscule differences in efficiency cause of production machining tolerances.)
AFAIK, there is no such thing as two different turbo models flowing exactly the same specs - do you have proof of such beasts?

That only holds true if we narrow our conversation to a single turbo.
You cannot say that when talking about different turbos - which we are.

I think you're overemphasizing air temps.
By nature, PV=nRT comes into play, and you're just stating the obvious.


-Ted

Again, we're not talking about total potential, we're discussing turbos in usage. Any two turbos that, on the same motor under the same conditions can make the same boost are flowing the same amount of air. Putting a bigger compressor onto the same engine doesn't flow any more air unless you raise the boost level. This is because the engine is only capable of consuming a static amount of air/fuel mix per revolution.

Let's take a hypothetical 12a engine, which has a static displacement of 550cc x 2 per revolution. With a hypothetical 96% VE (which means that in any given revolution, the chamber is 96% filled with a "good" air charge; both inadequate or over-aggressive port timing will hurt this, as the chamber either doesn't have time to properly fill, or as exhaust finds it's way into the chamber in place of fresh air.), at 7000 RPM, the engine is consuming 7000 * 550 * 2 * 0.96 cc of air per minute, or 7,392 liters, or 261.046 CFM.

As volume and pressure are two different measurements, this volume will be reasonably constant at a given pressure and engine operating range; that is to say, no matter what the compressor, at a given 15 psi and a given 7000 RPM, the engine will consume the calculated volume.

So what we have is, 260 CFM at 15 PSI (~200 kpa), which is of course much denser than 260 CFM at atmospheric pressure (~100kpa). Of course, the turbo has to ingest and compress somewhere less than twice that much air to pressurize it to that level, and this number changes. A more efficient compressor will have to consume -more- air to achieve a given pressure than a less efficient one, by virtue of the fact that the less efficient one is heating the air more, resulting on a less dense charge, which takes up more space, and thus achieves higher pressure with less airflow.

Perhaps this is what you were trying to imply, and if so, I apologize, but if so, you didn't make your point well.

/\ This is all very interesting stuff, thank you for the sensor explanation...

As far as tuning is concerned, does it make it easier to tune a car if you keep the MAF, or is it a significant enough intake restirction that it's advantage is overshadowed by the intake restriction?



PS: also, would purposefully creating an intake restriction help things such as boost creep at all? If this works, maybe someone could make a restrictor for the people who still run stock turbos + exhaust w/o doing wastegate porting? This is a just an idea i got from the hill climb/ rally racing comment...

Tuning the fuel map with the MAF is very easy. It is also very easy to tune the fuel map with a MAP sensor. Ignition Timing with a MAF is a little more tricky, especially on a turbo engine. Boost enrichment is also difficult with a MAF tune.

Ususally, OEMs that run MAF-based systems on turbo cars also have a MAP sensor to control ignition timing and fuel enrichment more accurately.

The beauty of the MAP sensor is that you can tune all the maps from the same sensor, without any need for the MAF as long as you have the ability to adjust your fuelling for your engine (basically anything but a stock ECU).

The MAF does make restriction in the intake. I have never quantified it on a dyno, but it meant the difference between spinning the tires going into second or just chirping them.

Creating restriction on the turbo inlet as a means to control boost creep is dangerous at best. That can cause the turbo to go into surge and/or overspeed. Wastegate porting is a much better idea.

One practical advantage of running a MAP system is as long as you have no vacuum leaks after the throttle body the car will still run and drive where a MAF car wont. This is kinda nice on cars with front mounts because if you intercooler piping or couplers get a leak you arent stranded until you can plug the holes.

Also, in a properly setup and tuned speed-density system using a MAP and IAT to calculate air density, and a VE table to calculate cylinder filling, swapping turbos, even with very large efficiency differences, results in very small tuning changes, and most of that is from hitting areas of the map that the old turbo wasn't capable of hitting.

NoDOHC is dead on about tuning spark advance with a MAF, too. If you're flowing a moderate amount on a turbo engine, you wouldn't be sure if you were near your redline and just about to start making boost (relatively high spark advance) or if you were at 3500 and already at max boost (very low spark advance); but it knows how much air is there, and adding the appropriate amount of fuel to it is semi-trivial. It is possible (and this is how the MS works if you use a MAF sensor) to map MAF vs RPM for ignition timing, which is almost as good as a MAP. Although, the MS has a MAP on board, so, it's all hypothetical.

So, with that said, I once I get the drivability map all figured out, I wont have to mess with it too much, and if I will swap turbos, i will only have to mess with tuning under boost?

In theory, yes. As long as you're running the same or less boost, you'll probably find your old map is reasonably close, too, as long as you're tuning VE tables and not direct injector PW tables.

cool! I was looking into a BNR upgrade, or possibly something with ball bearings and an aftermarket manifold...

Great discussion.
You guys are world class!

Megasquirt looks like it *might have been* a good choice for a guy like me to gain some hands-on knowledge, but time consuming.
One drawback to my Haltech E8 is that it seems there are no tuners in the San Francisco - San Jose area. Not sure about Megasquirt tuners.
The other drawback is that although I can connect the E8 to my laptop and see the HalWin screens, I'm afraid of screwing-up the current tune. Would like to smooth the idle.
Any suggestions?

Think a Haltech tuning for dummies or beginners might work for me!!
I'm the original owner of a '88 very with a pinned and half-bridged 13B-RE Cosmo that has a MP 60-1 Turbo.

Dave Segalla built that yes?
For Haltech guys, contact Illicit Performance, they actually are between SJ and SF. I know the owner has plenty of experience with that particular ecu

i might be researching the haltech for the merkur project.