E28 Turbo FAQ

Discussion pertaining to positive pressure E28s.
turbodan
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Post by turbodan »

-Forced induction: By far the most effective means of increasing power output, with tremendous potential available for far less than the cost of a NA stroker engine. A stock engine with head studs will reliably support over 20 psi with intercooling, a good tune and a sufficiently large turbine. Standalone injection should be considered mandatory for boost pressures over 1 bar. The bottom ends are very robust and will be the last parts to sustain damage if detonation occurs. The head gasket will fail first, which can crack the cylinder head if the engine is run for very long in that condition. They tolerate a considerable amount of detonation at lower boost pressures but become more sensitive as boost increases. The driver must listen carefully to the engine to avoid damage if detonation occurs. Aftermarket MLS head gaskets resist detonation slightly better which can result in damage to the piston ring lands in the event of severe, sustained detonation. Even slight detonation is audible with a full exhaust system while moderate detonation can usually be heard over an "open dump" wastegate.
Some awesome cool guy just wrote that up on wikipedia in the M20 section. Seems relevant...
Gunni
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Post by Gunni »

psi is irrelevant as we know and it´s not really a guideline either for the torque to be made.

standalone shouldn´t be considered mandatory. It´s ideal, but shouldn´t be mandatory..

It just sounds to general and not specific enough.

I can attempt to venture that a stock M20 8.8:1 compression (ARP) will be able to hold about 400lbs of torque as a guideline.
That´s more along the lines of actual guideline rather then 20psi or 10psi.
turbodan
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Post by turbodan »

It should be general, its just a blurb on wikipedia. And as you know, torque and boost are related to each other. The limit as I know it isn't the strength of the motor but the threshold of detonation, which isn't a factor of torque output. I've never blown a head gasket because I was making too much torque.
Gunni
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Post by Gunni »

The threshold of detonation comes from cylinder pressure/temp.
Which is directly related to torque output.
turbodan
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Post by turbodan »

Gunni wrote:The threshold of detonation comes from cylinder pressure/temp.
Which is directly related to torque output.
Correct, and we increase combustion chamber pressures by increasing boost.

Torque will vary depending on stroke/rod length as well, whereas the onset of detonation should be more consistent for the same compression ratio and boost pressure. Feel free to look at it completely backwards though if you prefer.
Canuck YYC
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Post by Canuck YYC »

turbodan wrote: Torque will vary depending on stroke/rod length as well,
Are you talking stroke to rod ratio (or rod to stroke ratio if you like), the stroke length and the rod length independently affecting torque or something else all together?
turbodan
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Post by turbodan »

I'm talking about bottom end geometry. What else would I be talking about?
Canuck YYC
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Post by Canuck YYC »

Are you saying that stroke dictates torque production (I.E. more stroke = more torque)?
turbodan
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Post by turbodan »

Canuck YYC wrote:more stroke = more torque?
That is my understanding.
Scottinva
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Post by Scottinva »

I was following what you guys were saying, and I looked up what you guys were discussing. So maybe this finding will help. I believe this is what Dan was saying.

"In the simplest sense, torque is related to the stroke of the piston and the stroke is determined by the crankshaft. The longer the stroke, the more torque. The stroke is determined by how far the connecting rod journal is offset from the centerline of the crankshaft. If the journal is close to the centerline, it makes a smaller circle in relation to that centerline and thus a short stroke. If it is further out, it makes a larger circle resulting in a longer stroke.
The further away the connecting rod journal is from the centerline, the more torque is applied. Just as a longer lever can lift more weight, a longer stroke applies more torque."
turbodan
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Post by turbodan »

That is what I'm talking about, generally speaking. I think the Canuck is probably quite familiar with these basics, so I'm wondering where he's going with this...
Canuck YYC
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Post by Canuck YYC »

That’s wrong. Full stop.

A crankshaft’s stroke is not a lever in this instance though I’ll admit I don’t completely understand why. The inevitable defence of this is “low speed, long stroke engines make more torque” but that’s misleading as it’s not causal, it’s resultant.

The mathematical formula for power – hp = (Ap x BMEP x Sp) / 132,000
where Ap = total piston AREA in sq. inches
BMEP = Break Mean Effective Pressure in PSI
Sp = Piston speed in feet per minute

Nowhere in that, or any other mathematical formula for the production of power, is there a link to crankshaft stroke. None. It’s not a “determiner” in torque production. Not only that, the long stroke engine generally has more friction losses.

If you don’t believe me, check out the following: link If you don’t believe that, download the latest free version of Engine Analyzer Pro or your other favourite engine sim. Build 2 engines that are identical with the exception of the bottom end geometry – one with a long stroke/small bore, the other with a short stroke/big bore of the same displacement. Things get even worse for the long-stroke engine when you increase valve size as a percentage of bore size (which you’d do if you were building an engine).

Incidentally, with this formula and some statistics, you can easily determine who’s pulling power numbers out of their ass and the realm of fantasy. You can determine what is within the realm of reason for say a naturally aspirated street engine based on typical average and typical maximum values for piston speed and BMEP. Utilizing your piston speed constraints, the engine’s # of cylinders (rarely a guarded secret), and some bore/stroke ratio normals / constraints (B/S ratio as a means to determine Ap as constrained by Sp) you can quickly determine if the package falls into the realm of reasonable, possible but unlikely or fantasy. The same can be applied to turbo/supercharged engines, you just need a reasonable collection of data as to what’s “normal” there too.
mooseheadm5
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Post by mooseheadm5 »

Piston speed is related to stroke.
Canuck YYC
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Post by Canuck YYC »

Yes. And?
mooseheadm5
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Post by mooseheadm5 »

Canuck YYC wrote:Yes. And?
Nowhere in that, or any other mathematical formula for the production of power, is there a link to crankshaft stroke. None.
That mathematical formula contains piston speed. Piston speed is related to stroke, therefore that formula includes a link to stroke. QED.
Jeremy
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Post by Jeremy »

Canuck YYC wrote:Yes. And?
Stroke determines piston speed at a given rpm and piston speed is right there in the middle of your power formula.

If higher piston speed = more torque

THEN

longer stroke = more torque
Canuck YYC
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Post by Canuck YYC »

Now you have the cart pushing the horse, but don't take my word for it. Do as I suggested with the engine sims (they are free after all). Torque and stroke are related, just not the way you've been taught to believe.
Scottinva
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Post by Scottinva »

What does taking this so scientifically help me do with a turbo car?
Jeremy
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Post by Jeremy »

Now you're changing your story. First you claimed they weren't related. Now you admit that they are but "not the way we've been taught to believe"? What, exactly, do you know about any of our educations and backgrounds to feel qualified to make such a statement?

And could you please come to a point? So far, you seem to be holding back whatever it is that your entire argument is about. It's difficult to give much credence to a person who says "no, you're wrong", gets proven otherwise, then backs up and changes their story with no reason or explanation.

Jeremy
Canuck YYC
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Post by Canuck YYC »

Actually I never said they weren't related, I said stroke doesn't determine torque - however your point is well taken. When I'm back in front of my library and not at work, I'll do my best to lay everything out. If you're so inclined I'd be happy to continue this in it's own thread as my point (intention) was to address and correct the mistaken, albeit common, belief that stroke makes torque.

Quick note as posting from an iPhone is rather tedious...
You stated earlier that if increased stroke = increased piston speed, and increased piston speed = increased power then stroke = power. The mistake in that logic is you're forgetting aboutt he rest of the equation. If you increase stroke and change nothing else, then yes, increased piston speed and increased power, but also increased displacement - now we're not comparing apples to apples anymore. If however you decrease your bore size with your increased stroke to maintain the same displacement (apples to apples), you'll find your net gain is gone.

Forget the sim-just work it out on paper if you prefer.
Canuck YYC
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Post by Canuck YYC »

slow5
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exhaust

Post by slow5 »

i was wondrering it when i put my turbo on if i should do 3'' or 2 1/2'' someone said that 3'' would be too much it will give me more top end as 2 1/2'' would give me ecceleration? opinions?
alijonny
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Post by alijonny »

I've got single 1.75" exhaust. It makes so much back pressure that I need 4 wastegates.
slow5
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turbo

Post by slow5 »

well i was thinking about goin with 3" do you think that would be too much?
turbodan
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Re: exhaust

Post by turbodan »

slow5 wrote:i was wondrering it when i put my turbo on if i should do 3'' or 2 1/2'' someone said that 3'' would be too much it will give me more top end as 2 1/2'' would give me ecceleration? opinions?
2.5 is fine up to a certain power level, but you have nothing to lose with 3". Forget all the bullshit about different power delivery between the two.

Might as well go 3" so you dont have to do it again down the road.
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