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Old 10-18-2004, 09:13 AM   #1
TheMadScientist
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Default I haven't posted a stupid ? in a while

So here it is:

Why is a lot of boost at low RPM bad?

I can see if you are surging the compressor that high boost a low RPM is bad. Not that it is possible but why is running say 20psi at 2000rpm or lowwer bad.
I know you will run into torque limits on the rods. Other than those two things(surge and torque limits) what is so wrong in getting high boost as low in the rpm range as possible.

What caused me to ask is that a subie guru just said that it was bad. I thought that one of the greatest chalenges with a turbo is to get boost as low as you can. I could just be very confused.

TMS
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Old 10-18-2004, 11:41 AM   #2
mick_the_ginge
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IMHO boost at low RPM is not bad, actually it's good if you can get it.

Typically the above statement is made in respect to the part throttle full boost issue. this is where the car stays in closed loop fueling mode and you are running 14.7:1 AFR at lower rpms and full boost. Which as you know is very bad.

IMO if you have correct fueling and timing then having boost at lower rpms is a good thing.

I would like to see other comments on this issue as their maybe aspects of lower rpm boost that I have not thought about.
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Old 10-18-2004, 11:42 AM   #3
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Quote:
Originally Posted by mick_the_ginge
I would like to see other comments on this issue as their maybe aspects of lower rpm boost that I have not thought about.
The reason I posted.

TMS
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Old 10-18-2004, 12:29 PM   #4
big_adventure
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The Subaru WRC car hits something ridiculous like 3.5bar of boost at less than 2500 rpm. Lots of diesels max out boost (which might well be 4 or 5bar+) below 2000rpm (hell, 1200 in the case of Semis) and fall off from there.

I think what the Scooby Guru must have meant is that the stock EJ205 is likely to have breathing issues down there, leading to det, pre-ignition or something else ranging from mildly to massively catastrophic.

The EJ205 also has cute, cast, pencil-thin rods that will happily snap if cylinder pressures are too extreme. If you got 3.5bar of boost at 2500rpm in an EJ205, you would no longer have moving pistons. You would have a box of snapped aluminum.

Now, if it was a specific situation, it's very possible that the poster was referring to surge at a very high PR but low flow situation, or the added det risk caused by running way, way out of the efficiency range of a given turbo. But unless the poster you refer to elaborates, we're just guessing.

If you have sufficient force on the turbine and a sufficiently efficient compressor, combined with a strong engine, I don't think low boost would ever be bad.

-Sean
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Old 10-18-2004, 01:35 PM   #5
mbiker97
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Quote:
Originally Posted by mick_the_ginge
IMHO boost at low RPM is not bad, actually it's good if you can get it.

IMO if you have correct fueling and timing then having boost at lower rpms is a good thing.
This sounds right to me. There is no such thing as boost too early as long as you are fueling appropriately.
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Old 10-18-2004, 01:44 PM   #6
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It's not bad as long as nothing breaks (rings, rods, gaskets). Might make for a pretty touchy throttle if it's real low. I saw a old Cobra once that was basically undrivable on the road because the car lurched forward if you touched the throttle.

That WRC article was amazing, the car must snap your head head back.
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Old 10-18-2004, 01:49 PM   #7
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The really funny thing about the WRC cars for Subie and Peugeot this year is that, while restrictor-plate limited to 300hp, both somehow made something like 450lb/ft of torque at 4K-4.5K rpm. Do the math - that's a good bit more than 300hp. Hmm...
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Old 10-18-2004, 02:42 PM   #8
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Quote:
Originally Posted by TheMadScientist
So here it is:

Why is a lot of boost at low RPM bad?...
Ask a stupid question.... . I think it's cause our engines(like most engines) make more power at higher rpms(to an extent). An engine could make more power at lower rpms by getting a smaller turbo(then stock lets say). So ya end up getting more power in a rpm range where the engine makes less power(lets say 1-3000rpm), and get less power where the engine could be puting out like a champ(4-6500rpm). If you're lookin to accelerate harder then part throttle at low rpms, why not just downshift?

peace
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Old 10-18-2004, 03:16 PM   #9
ride5000
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well, along those lines... it is essentially the same question as to why it is "bad" to "lug" the engine in a too-tall gear at too low of an RPM.

and the answer to that question has been, in my experience, that it puts a LOT of cylinder pressure in the chamber.

now, why does that occur? i think it has to do with the speed of the receeding piston during the power stroke. if the cylinder pressure peaks quickly and the piston does not have time to absorb that pressure and convert it to downward movement, then very little of that pressure will be used to perform actual work. instead it will be dissipated in the piston/rods/bearings/headgasket/rings, etc.

in other words, it's a matter of matching the rate of chamber volume expansion due to downward piston movement to the rate of the combustion event.

jm2c
ken
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Old 10-18-2004, 03:48 PM   #10
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Quote:
Originally Posted by ride5000
now, why does that occur? i think it has to do with the speed of the receeding piston during the power stroke. if the cylinder pressure peaks quickly and the piston does not have time to absorb that pressure and convert it to downward movement, then very little of that pressure will be used to perform actual work. instead it will be dissipated in the piston/rods/bearings/headgasket/rings, etc.


jm2c
ken
I was thinking along those lines on my way home.

My train of thought was like this:
At low RPM the pressure and heat generated have more time to do damage. It takes time for the heat to get into the metal parts so at lowwer RPM you have more time for the heat to get into those parts. Also the pressure or force has more time to do damage at lowwer rpm.

So it is not really BAD. You just have limits on the strength of the components. That is why it is bad. Same thing with RPM limits on rods they will only take so much and then boom.

Am I way off here?

TMS
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Old 10-18-2004, 08:17 PM   #11
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i get a lot of knock at low rpm in high gear. in 6th gear, at highway speeds, the turbo is ready to go at anytime...i can get 10psi by 2500...then i see the CEL flash.

...so ive just learned to not floor it <3k in 6th. haha. ive taken out a good amount of timing from those load sites, but i figure i might as well just drive around it...since the car doesnt like that apparently. i had the same problem with my stage 2 setup, but i tuned it out. i wish my laptop harddrive didnt crap out, so i can see how much timing i actually pulled out. ah well.
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