View Full Version : fmic questions

Adam M
21st June 2001, 10:53
Seems to be a topic of the moment, so I was just wondering if I could gather some information.

Just wondering who is interested in going down this route, or has one of the various incarnations on order.

Plus how many of you are concerned about issues related to

a) lag
b) bodywork cutting
c) oil/water temps

These are the things that concern me.

Most importantly lag.

It has been said to me many times that lag shouldn't be an issue, and stef has coonfirmed this, but these seem to be with respect to smaller trubos. I am considering trying a larger turbo on my car, and dont want to suffer more.

I have a few thoughts. Would the intercooler change the points at which max torque and power are achieved. I can understand a lag time being present in that it is the amount of time required to pressurise a larger than standard system, but once pressurised, the volum of the system should make no difference, right?

One other thing, will a bigger turbo suffer a bigger increase in lag time when shifting to a larger intercooler system as suggested in another post?

I am not sure on this one. consider two turbos, eg vf 22 (big) and td04 (small). the smaller turbo has a smaller exhaust housing, lower inertia and a smaller compressor so spins up faster. Since it has a smaller compressor, it flows much less air than the vf 22.

Once connected to the fmic, it is the volume of air they can flow at their own spool up point which will determine how fast the fmic is pressurised. I know that it is rpm dependent, but surely the bigger turbo flowing more air once on song will be able to pressurise the system faster than the small turbo.

There are other factors to consider that I am overlooking I am sure. Such as the shape of the output graphs for the turbos. If turbos roughly behave in the same way, then once on boost, the bigger turbo will be flowing more air at any point but I suspect this is not true.


What I mean is, at the point a t which the big turbo comes in (range of rpm) is it flowing more air than the smaller turbo would be (because it may be beyond its sweet spot)?

If it is, then I would have thought bigger turbo would generate less lag when connected to an fmic, than a smaller turbo.

I would really welcome anyones comments on this one.

21st June 2001, 13:54
I'm thinking out loud here, and as you know have no practical experience... Feel free to pick holes; it'll catalyse the thread at least. I separate the statements for people to rip apart easily.

A bigger turbo flows more air once spinning.

However, it takes longer to get spinning.

FMIC gives a larger volume of air to pressurise.

Therefore pressure increase per "revolution" of the compressor is less

Therefore less resistance on compressor

Therefore should spin up at least at same rate, probably faster, than with TMIC

This will mean to reach target pressure will take more revolutions of the compresser, but the compresser will reach optimum speed faster. With a larger turbo, flowing more air per revolution, this should move the balance in your favour.

I would think that it will be a balancing act between the two: faster spool up (?) vs larger volume of air to pressurise...

Please, someone, pick this to bits but let's get the debate started.
but once there I would imagine that i

[This message has been edited by chiark (edited 21 June 2001).]

Adam M
21st June 2001, 14:05
Dont think so Nick.

There will be some influence, but I think it is fair to say that the spool up of the turbo will be pretty much the same with top mount and front mount as it is dependent on the inertia and drag of the turbo. Since these are not changing the turbos (big or small) should spool up at about the same point as before. Pat has explained tehre may be some difference causing them to spool up faster but I will let him explain that.

I think that once the turbo is spinning teh bigger one will fill the intercooler faster than the smaller one would once it starts spinning. Therefore having a big turbo will mean less pressurisation lag with a front mount than having a small turbo with a front mounted intercooler.

Does this make sense to anyone?

21st June 2001, 14:40
Right, I'd buy that theory... I am starting to wonder if the best way to confirm this is to do it, which isn't really what you want to hear.

Firefox? Pat?

Adam M
21st June 2001, 15:29
Nick dont worry, I will be doing it soon, and potentially with a bigger turbo than I have now.

Of course Pat has the biggest turbo!

NITO might argue, but he'd be wrong! http://bbs.22b.com/ubb/wink.gif