Calculating full spool rpm's

[aves911]

In my spare time I like to dabble in the math behind successfully boosting the power in our cars. While I do not have the same level of hands on experience that many of the members of this forum do, (Just recently finished my first engine swap) I do like to try and plan out my upgrade path so that I can build a fast, streetable daily driver that maximizes all the mods I install.


So that aside..my question is:

Has anyone had any exposure to the physics, thermo, dynamics, etc behind how rapidly a turbo spools? I would like to play around with some equations and come up with some relations connecting lag with displacement, compression ratio, cam lift, air flow, turbo specs, etc. Pretty much all the variables you can adjust when building up a car.

The goal of this of course would be so that I could take a given turbo and then derive the set of supporting mods that would get the turbo to spool by a certain rpm.

Since I can't go out right now and play around with trial and error, nor would I want too. Even though it is part of the tuning fun I've got a couple years to think about this probably and so any info any one has that could get me started would be great.

Thanks,
Sorry the post is so long,
-Jacob

 

[NosLaser]

You ain't gonna do it through a mathematical equation. These were some yoyo's some time back debating for years that a 16G is all a DSM motor will ever need since it flows more CFM than the motor. Hmmm...that theory seemed to have flown right out the window huh? There are so many other variables that determine spoolup time; supporting mods actually being a smaller variable. Things like timing, and when that timing occurs is a larger factor. For instance, retarding ignition timing and dumping fuel will cause an explosion later in the cycle which will heat things up in the exhaust manifold thus lighting the turbo up and making it spool hella quick. Case in point, some big equation and chart really wouldn't do anything to assist you in coming to real world conclusions.

Regards,

 

[aves911]

Well again, I said that you could draw some correlations based on just about everything and tie it together.

For timing, for the most part from what ive read, you want around 23ish degrees at the top so you know what your timing is and for fuel you could estimate what your fuel usage would be in cc/min. Practically you wouldnt want to yank all your timing and dump fuel because the car wouldnt run well. So going along with those assumtions you could plug in just about all the variables you would come across.

Also, from what I gather, turbos are spooled just like any turbine, energy transfer from high temperature high velocity gasses. Therefore you should be able to calculate rate of spool based on that.

But you would really need some raw flow data for that.
So I guess in retrospect this wouldnt be real practical to work on

Thanks anyways
-Jacob

 

[NosLaser]

...Practically you wouldnt want to yank all your timing and dump fuel because the car wouldnt run well...

This is true, but this is where engine management systems come in. You only need a split second for this effect to take place. By retarding timing and adding fuel, you are going to produce extremely hot explosions that make their way past open exhaust valves and into the exhaust manifold lighting the turbo up something fierce. But of course this doesn't make for efficient combustion so you would need your engine management system to change those parameters as rpm increases. I really don't think any mathematical equations would be able to offer anything substantial for what you are looking to do. Yes, of course you can figure out BSFC, combustion chamber volume, cylinder volume, etc. but those are merely tuning 'tools' that have to be applied and put into practical use.

Regards,

 

[Keaka26]

By retarding timing and adding fuel, you are going to produce extremely hot explosions that make their way past open exhaust valves and into the exhaust manifold lighting the turbo up something fierce.

sounds like Toyotas Anti-Lag, no? Would there be any practical way of implementing this on a DSM? I think i also heard that the anti-lag system is bad for turbos...But those big name rally guys really dont care because they have tons of turbos to throw around.

 

[NosLaser]

Yes, it's similar to an anti-lag system. And yes, it's hard on turbo parts which is why a lot of people will use parts made of inconel which can withstand the extra heat and pressure.

Regards,

 

[P8ntBalla]

I think the "anti lag" system you are talking about is referred to as the "bang bang"(not positive on that) system. It injects fuel directly into the manifold, the fuel ignites, and therefore spools the turbo, and keeps it spooled inbetween shifts. The thing I don't know is how they would be able to time the pulse of the injector on the manifold in conjunction with the exhaust valve timing, because it sounds like the explosion in the manifold could back up into the combustion chamber. It wouldnt actually back up into the combustion chamber as it would go for the path of least resistance(which would be through the turbine), but I would imagine that the explosion in the manifold would restrict the exhaust gases coming from the combustion chamber.


I could be completely wrong though, I only heard some rally guy talking about it on speed vision(yes, way back when they called it that, thats why I don't know much about it.)

 

[bigfoot]

Big Bang is the way some cars like Toyota and I think Subaru run anti-lag. O yeah, I think they also use a system called LMRT (Lots of Money for Replacment Turbos). I once heard Ben from AGP talk about a guy running anti-lag on an autocross car with a L2R, bet that was a fun ride.

 

[Keaka26]

Good lord, L2R with anti-lag would be INSANE....and costly..