E28 Turbo FAQ

[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]

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]

Piston speed is related to stroke.

[Canuck YYC]

Yes. And?

[mooseheadm5]

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]

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]

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]

What does taking this so scientifically help me do with a turbo car?

[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]

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]

Answered

[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]

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

[slow5]

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

[turbodan]

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.

[Brad D.]

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

x2

[slow5]

yeah thats what i thought i wanted to do 3" cuz i have 2 1/2 now and its made a huge difference.... so i cant imagine the difference with a 3" an turbo. Im using a toyota ct26, does anyone know much about these... someone said it was a BIG turbo but it doesnt seem like it... its out of a supra

[Scottinva]

It's certainly not a "huge" turbo. I have a few in my garage, I have never seen a map for this turbo. It was apparently actually made by Toyota.

[Brad D.]

Ya, the CT26 is definitely not what I would consider a big turbo at all. Maps for this are scarce to non-existent but from what I've read it is similar to a T04E 40 trim wheel which, IMO is too small for an M30 powered car. I would also think that the turbine housing is way to small for decent breathing on a 3.5L engine.

[turbodan]

Looks like the Supra CT26 uses a 1.81" inducer, while the 40 trim T04E uses a 1.87" inducer, so the CT26 is even a bit smaller.

[slow5]

hmm well i picked it up for 40 bucks n its really quite clean, i dont have an m powered one either its just a 535is but it rips... n i dont wanna run a lot of boost prob just 8-10psi so you think that will be efficient enough?

[turbodan]

Maybe 6-8 would be alright. The compressor side wont be efficient but it'll work. The turbine could be too small, but since it was designed for a 24v 3 liter motor, I think it could be alright for a 12v 3.4l. If you really want to use it, go ahead.

[Scottinva]

I think it uses some weird format for the flange, I may go take a look later. I think I remember hearing something about buying the right size flange was somewhat difficult.

[George]

CNC a flange.


That bitch should spool pretty damn quickly.

[slow5]

i have a full machine shop and everything i just needed to know if it would be decent or if i should invest in a different turbo

[Brad D.]

My advice is always what most don't usually want to hear. If you are going to the trouble to fab up a manifold/adapter/etc. then do it right and make it for the best turbo you can afford. It truly is the "heart" of the system and really isn't where you want to skimp. With that being said, the turbo would probably be fine for a low boost setup, but power is addicting and at some point you are going to find that you want to upgrade and it will require a lot of additional fab work.

[slow5]

yeah thats true... hmmm now i have to ponder this.......... thanks for the advice!

[Scottinva]

The way I look at it, the turbo is the last place you would want to skimp. I mean it is the heart of the system. You are "turboing" a car.

[nirajb_uk]

Hi all,

I was just wondering is it possible to fit a turbo to a 528i, and if it is, where can i get parts from etc..?

Thanks,

Niraj

[Xenocide]

Hi all,

I was just wondering is it possible to fit a turbo to a 528i, and if it is, where can i get parts from etc..?

Thanks,

Niraj

read the turbo FAQ, not post the same question a couple threads higher. TCD is probably the best place to start.