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Discussion Starter · #1 ·
what factors can cause a pressure drop from the exducer (turbo output) to the intake plenum? And how do these factors affect total air mass flow (CFM)?

What I'm trying to determine is how my stock intercooler is affecting flow (although it generally cools intake temps back down to ambient, as I'm measuring IAT post-intercooler). I'm also trying to estimate loss between plenum and turbos - I'm getting 1.85~1.93 bars in the plenum, so I want to know what I'm pushing at the actual turbo.

It's all roundabout anyways, what I REALLY wish for is a tachometer for each turbo so I can know actual shaft speed. That telemetry compared with charge temps and then I can know for SURE if my turbos are efficient or not.

Any turbo experts care to edumacate me further on this? Takes a fair bit of study to put all these pieces together and then apply all this book-learnin' to my actual car and actual boost conditions.
 

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Hi,

I don't know of ALL of the factors and nor do I class myself as a turbo expert (I'm not even a tuner or mechanic) but air flow rate and intercooler flow resistance are going to be two major factors in the pressure drop between turbo outlet and plenum.

A few random snippets in no particular order:

An intercooler with a smaller cross sectional area would, all other things being equal, have a higher flow resistance (therefore bigger pressure drop) than one with a larger cross sectional area.

An intercooler with more turbulators is most likely going to present a higher pressure drop than one with fewer, though it would be more effective at cooling the charge passing through it.

Pressure drop across a restriction will go up with the square of the flow, so if you double the air flow, you'll quadruple the pressure drop.

If you're getting a big pressure drop between the turbo exducers and the plenum, the turbos are having to work harder than necessary. This will create more heat in the inlet charge, which will invariably result in hotter air in the plenum (no air to air intercooler is 100% efficient unless you do some funky CO2 cooling or whatever), though how much extra plenum temperature you actually get will depend on how good your intercooler is.

What's more (and I'm not overly clear on this), I suspect that a turbo working harder to produce a higher outlet pressure will probably result in a higher exhaust manifold pressure (it would need more energy to drive it so this stands to reason) which in turn means higher combustion temps and a greater chance of det, so you'd have to run a bit less ignition advance, thus losing power because (a) you're running less optimal ignition and (b) there's more spent charge not scavenging properly from the cylinders and getting in the way of new fresh stuff coming in on the next stroke.

I'm impressed that your stock intercooler is producing outlet temps close to ambient! I expect you could gain quite a bit in terms of ease of flow if you put something bigger on there, though quite how much you'd gain is beyond me as I have no practical experience of comparing intercoolers.

Cheers,
Kingsley.
 

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Discussion Starter · #3 ·
I'm going to see if I can borrow a bigger intercooler, try it out, see if it makes any difference. I've just gone by the conventional wisdom that the stock FMIC is good for 600bhp, and that if ain't broke, don't fix it.

But I am pushing a bit past 600, closer to 650bhp, so the FMIC might be becoming a restriction.

As far as turbulators...that's something I wish were in the advertising literature - the number of turbulators and internal design means a hell of a lot more to FMIC performance than core thickness, yet that's all you can research.
 

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Just because people say that the stock IC is good for 600bhp doesn't mean that a bigger one won't be better even if you're producing less than that (I know you're producing more). It's a sliding scale - the more you're making, the more restrictive it will be. It's not perfectly fine at 599bhp and in desperate need of change at 601bhp, it's all a matter of degree.

All things being well you might find you can dial in some more ignition advance with a better IC. You do your own mapping I think?
 

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what factors can cause a pressure drop from the exducer (turbo output) to the intake plenum? And how do these factors affect total air mass flow (CFM)?

What I'm trying to determine is how my stock intercooler is affecting flow (although it generally cools intake temps back down to ambient, as I'm measuring IAT post-intercooler). I'm also trying to estimate loss between plenum and turbos - I'm getting 1.85~1.93 bars in the plenum, so I want to know what I'm pushing at the actual turbo.

It's all roundabout anyways, what I REALLY wish for is a tachometer for each turbo so I can know actual shaft speed. That telemetry compared with charge temps and then I can know for SURE if my turbos are efficient or not.

Any turbo experts care to edumacate me further on this? Takes a fair bit of study to put all these pieces together and then apply all this book-learnin' to my actual car and actual boost conditions.
Why not just buy some cheap boost gauges and tap and fit takeoffs to measure boost at various points in the inlet tract. i.e just after each turbo, at the entry to the IC, exit of the IC and entry to the plenum. You'll then see easily where the restrictions are. I saw about 1.5 to 2 psi drop between the turbo outlets and the plenum on my twin turbo RB25 at 1 bar (at the plenum) on standard GT-R turbos.

Pipework from the turbos was the standard twin turbo pipe and elbow leading into custom 3" stainless steel hard pipes then into a R34 GT-R IC,then into 3" hardpipes, through a Z32 MAF into more stainless 3" pipe and then into a Q45 TB on the GReddy RB25 plenum. Probably the equivalent of five mandrel bent 90 degree bends in the stainless pipework. The IC certainly seems to be pretty efficient at lower power levels.
 

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Discussion Starter · #6 ·
Just because people say that the stock IC is good for 600bhp doesn't mean that a bigger one won't be better even if you're producing less than that (I know you're producing more). It's a sliding scale - the more you're making, the more restrictive it will be. It's not perfectly fine at 599bhp and in desperate need of change at 601bhp, it's all a matter of degree.

All things being well you might find you can dial in some more ignition advance with a better IC. You do your own mapping I think?
that's true - when people say "good for 600bhp", does that mean it's 100% at 600bhp then starts to drop off, or does it drop off before that point and 600bhp is the last acceptable point before it becomes intolerably inefficient?

I get my advance through fueling - 60% toluene. With a bigger FMIC maybe I can bump it up a bit more, but I'm already advanced to the point where with pump gas I'd melt my pistons very quickly.
 

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I wouldn't be able to say for sure but my strong hunch is that it's going to be dropping off long before 600bhp. Before I got my GTR I had a conversation with a tuner about possible first upgrades. One of the things he suggested was an upgraded intercooler, which he explained that at this level was primarily for better flow, yet we were only talking about getting about 450bhp from the car at the time, on stock turbos.

If you're running your turbos near the limit, they're really going to appreciate an easier path to the plenum. You could probably fit bigger turbos instead and drive more air through the stock intercooler that way. You'd still get the power but it would be sub-optimal. It would not be so responsive because of the bigger turbos.

If someone fits massive turbos and wild cams to their car with the stock intercooler and manages to get 700bhp, would that start people saying that the stock intercooler is good for 700bhp? Drivability would probably be bad for a 700bhp car - there would be better ways to get there although you might have to spend extra money on more supporting parts, the result would be better.

In my opinion, of course :)
 
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