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I remember someone (Brat4by4 I think) mentioning that we don't hit full boost in 1st gear.

Is this supposed to be?

The reason I ask is that I happened to glance down at my gauge in 1st gear and read it as full boost.

If we AREN'T supposed to hit full boost in 1st gear, then why am I? And why aren't we supposed to?

In my experience, sometimes you do and sometimes you don't.

It makes sense that the ECU would limit torque a little lower in first gear in order to make the transmission last longer.

It might just depend on your speed, though, rather than the gear. The ECU might approximate "low boost in 1st gear" as "low boost below 15 mph" or something.

Ahh, good point. I never "launch" the car (because AWD makes that expensive after a while ), so I usually just roll into the gas

I generally get 9-10 psi in first, 11-13 psi in second, 13.5-14 psi in 3rd and 4th, and 14-14.5 psi in 5th.

But in your case, Matt, you're no longer using stock boost control.

I guess you don't hit full boost in lower gears because you're not in the gear long enough to spool up that high?

Hmmm, I'm not sure that's it. I get spool up really quick. I thought it was more of a load issue.

Yeah, I wasn't convinced by my guess either, since even a stock car has the potential to spool up way past 15 psi in 1st gear if the boost control lets it.

What do you mean by a load issue?

I think he's thinking that since it takes more power to move the car in a higher gear, you get more boost in a higer gear. am I right matt?

Is that really true though when you're accelerating at wide-open-throttle?

Well the next time you're driving pick a point like 3k-6.5k and time it in all gears. Or if you think about it, in 1st gear for every one rev then tranny is reving 3.55 times, in 2nd for every one rev it's 1.95 times, etc. Obviously to keep accelerating at the same rate you're going to need more power.

Right, but the engine makes the same power (or torque) at that point in each gear if it gets the same boost.

The car doesn't accelerate at the same rate in different gears. You get the best acceleration in 1st gear and it drops off as you go up through the gears.

I was alway under the impression that you made less power in lower gears, NA or FI.

Right, That's what I was trying to explain

You get less torque to the wheels in higher gears, yes, but the same power.

What I'm saying is, I don't understand why loading is the issue. The traditional definition of engine load is manifold pressure, anyway, right?

Hehe, well by load I mean that when in lower gears it takes less power to move the car because of the lower gearing. When in higher gears it takes more power to move the car. Now, the power that the engine is producing (if at the same boost level) may not be any different but it still puts more strain (load) on the engine, right? I mean, in 5th gear going up hill the engine isn't producing any more power then on a flat road but it's under a heavier load. Am I making any sense?

Oh, I forgot to add that on a hill in 5th I can get .5-1 psi more boost then on a flat road.

Well, yeah, I intuitively see what you're saying, Matt, but I'm trying to figure out if it's really true.

As far as the engine is concerned, what is different between the flat road and the hill? The engine will make the same torque, but it will accelerate faster on the flat road. Does slower acceleration somehow equate to greater strain on the engine? I'm not sure it does.

Also think about reving your engine at idle. You can barely make any boost because there is no load on the engine.

Well, that in fact is a case of not having enough time to build boost. If you disconnect the wastegate actuator (in order to get maximum spoolup) and snap the throttle wide open in neutral, you'll see good boost for a split-second before you hit the rev limiter.

(Note: I don't recommend trying the above.)

I understand exactly what Matt is trying to say.

The engine will make the same power given boost, that's very true. But actual load on the engine is determinant on gearing (it would seem).

Accelerating in first, the gear is so low that there isn't much of an actual load as there is in 2nd or 3rd (or our sky high 4th and 5th).

Kind of makes sense.

BUT, exhaust gas should be about the same in 1st or 4th. The engine is still firing at 3000RPM's, and it's still adding fuel proportionate to air. So a boost-cut in 1st gear HAS to be electronic (as boost climbs, more fuel is squirted, and more exhaust is produced which cycles through the turbine and creates more boost).

So it would be reasonable to believe that this "low-boost" would be an issue with the wastegate actuator holding the gate open at a certain point, only letting a few PSI through.

Matt's case is odd since he's using an MBC. I'd like to agree that you might not be in the gear long enough under WOT to fully spool all the way to 15psi. 1st gear does only last until 32mph afterall. You SHOULD be hitting full boost in 2nd through 5th. Why you aren't could be a load issue, or could also be a time issue like Vikash mentioned.

So if 1st gear boost is eletronically controlled, then why am I hitting full ~8psi in 1st gear? Could it be related to the malfunctioning Wastegate Controller that also causes a 5psi cap in any of the other gears?

Could it be a sticking wastegate? This doesn't seem TO reasonable as I NEVER have gone over peak stock boost.

BAC5.2 wrote:The engine will make the same power given boost, that's very true. But actual load on the engine is determinant on gearing (it would seem).

But what is this thing you're calling "load?"

Accelerating in first, the gear is so low that there isn't much of an actual load as there is in 2nd or 3rd (or our sky high 4th and 5th).

I don't think that's true. The high gear ratio in 1st multiplies the engine's torque more, but that makes you accelerate faster.

BUT, exhaust gas should be about the same in 1st or 4th. The engine is still firing at 3000RPM's, and it's still adding fuel proportionate to air.

...as long as airflow is the same.

So a boost-cut in 1st gear HAS to be electronic (as boost climbs, more fuel is squirted, and more exhaust is produced which cycles through the turbine and creates more boost).

It doesn't have to be electronic if the turbo simply doesn't spool up fast enough.

So if 1st gear boost is eletronically controlled, then why am I hitting full ~8psi in 1st gear? Could it be related to the malfunctioning Wastegate Controller that also causes a 5psi cap in any of the other gears?

I don't think you're experiencing a glitch. It's not documented anywhere that the ECU limits boost in first gear, so getting full boost wouldn't be a malfunction. On my car it seems like I sometimes do and sometimes don't get full boost in first gear.

It's probably just limiting boost at low speed or something, like I said before. And the boost control duty cycle map is calibrated for a system that's always using that map, which would mean weird things happen when the ECU switches from not using it to using it.

By load, I mean strain on the engine.

The more difficult it is to turn the crank, the more load there is. Brake torquing an autpmatic puts a load on the engine (and builds boost). Free-revving with the clutch in = no (well, very little) load. Accelerating in 1st gear, the engine torque is multiplied to the wheels, but it's very easy for the engine to spin (the exact point of having low gears). Less stress on the engine while producing fast acceleration and torque. The downside would be low top-end (as evident by our 32mph top speed in 1st gear).

But how do you define "strain?" How does accelerating in 1st gear cause less "stress" than accelerating in 2nd?

I want to understand this in concrete terms.

I say the load is the same at 10 psi manifold pressure at 6000 RPM in first gear as it is at 10 psi manifold pressure at 6000 RPM in third gear. The only difference the engine can possibly see is that crankshaft acceleration differs, and I don't see how less crankshaft acceleration means more strain.

Hmm, I guess I see your point.

I think what we're trying to explain is that when in 1st gear it would be like holding a piece of string in your hand and twirling it around. Lets say you twirl it at 100 rpms. Then you take a piece of rope and twirl it at 100 rpms. It will be harder for you to twirl the rope since it's heavier, right. So to spin it at 100rpms you'd need something to help you do that let say you have someone push the string to start it, well obviously they're going to need more force to push the rope than the string. Of course at some point you'd get a balance from centrifugal force or something, but you get the point.

What is centrifugal force?

http://www.miata.net/sport/Physics/04-N ... fugal.html

"The more you know"



Note: Read all of the "Physics of racing" articles. Extremely informative.