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408 TT sizing

 
Old 01-12-2019, 10:59 PM
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Originally Posted by tblentrprz View Post
Please explain
I think he is referring to what happened to the OP with his previous set up..even if he had a donkey dick size cam if the exhaust wheel ran out of breath at 5800rpm it's not gonna carry power any further and would probably start a sharp drop in power including high *** iats and high back pressure.
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Old 01-13-2019, 12:12 AM
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I was going to give a more detailed explanation but GT griller summarized it pretty good. I've done hundreds of turbo builds, turbine size dictates peak RPM/power every single time.
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Old 01-13-2019, 05:18 AM
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Originally Posted by GT Griller View Post
I think he is referring to what happened to the OP with his previous set up..even if he had a donkey dick size cam if the exhaust wheel ran out of breath at 5800rpm it's not gonna carry power any further and would probably start a sharp drop in power including high *** iats and high back pressure.

Obviously nobody would ever be that stupid to run such a huge mismatch of a setup though. Because that would be beyond stupid.

And to say turbine size dictates peak power, is equally stupid. Because if you are saying if a T3 peaked at say 5000rpm...that a T6 double the size would peak at 10,000rpm ?
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Old 01-13-2019, 01:44 PM
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I never said it was a linear [relationship] increase though . You are assuming that.
This is not about t3 or t6 or t5 or vband housings, this is about turbine size.

The fact that you can have a donkey dick cam, short runner intake with a big plenum, but have it peak early due to a turbo with a small turbine validates my statement.

You can have a small cam, truck style intake with a ~12" runner, but have it peak late if you have a big turbine for the engine displacement.

The fact that you can have a big camshaft, small runner, but have it peak early due to the turbine size, and on the flip side, have stock cam, stock intake, and have it peak late only tells you that the turbo has a bigger weight in terms of dictates peak power
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Old 01-13-2019, 03:48 PM
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lol.....of course it is about T3,4,6 whatever.

Have you ever seen a 100mm T3 turbine ? or likewise a 40mm T6 ? Those numbers do generally reference a range of turbine sizes believe it or not.

Having a moronic combination may validate your statement....but as said, only an utter idiot would build such a combination in the first place. And a big camshaft with a small turbine if you've ever built one will not peak early, in fact it will be a piece of **** everywhere because both aspects will never work well together.

So again, only an idiot would actually try.
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Old 01-14-2019, 02:46 AM
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i've seen a 96mm turbine on a t6, t4, t5 and vband housings, just to give you an example. Of course there's no way you can have a 96mm turbine on a t3 housing, I am referring to the turbine size spectrum for whatever housing we talk about.

All I am saying is the following:

You can have a moronic combination in which the end user uses a donkey dick cam, and a short runner intake, with a small turbine for the displacement. The result? A curve that plateaus early, say, 5600 rpm.

You can have a moronic combination in which the end user uses a factory cam, a factory truck intake with long runners, with a big turbine for the displacement. The result? A curve that plateaus "late" say 6700 rpm.

The difference is that in factory form, the factory cam and factory truck intake peak much earlier than 6700 rpm. The combination peaks at approx 5.7k rpm.

Why is it that the engine is now peaking at 6700 if the cam and intake dictate peak power, which should be around 5.7k rpm?

I have done plenty of these comparisons on the dyno, in which at first, I was expecting an early peak due to a long runner and small camshaft, but yet the power peaked much much later than factory, approximately 1000 rpm later.

My only answer is turbine size. I'm not here to argue tbh, I'm sharing what I have found after doing tons of these engines.

Maybe there is another explanation as to why the peak changed. I would assume it would have something to do with the work being done by the turbocharger (mass flow rate increase) a, but that is a subject that I am not fully knowledgeable in (thermodynamics).

Last edited by subeone; 01-14-2019 at 02:54 AM.
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Old 01-14-2019, 10:14 AM
  #27  
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For arguments sake. It was a very similar cam to the ljms stg 3 (almost identical valve events, I'll try and find the cam card) that was in the 370/pt88 combo..that same cam was used in the 408. I understand it was on the small side for the 408 but I still beleive the restriction was more turbo than cam. The high charge temps once in high gear and the decline in power leads me to think it was turbine inefficiency. I dont mind recamming it for the twin set up but I also dont wanna end up too small of a turbine and I'm not well informed on sizing twins.
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Old 01-14-2019, 10:17 AM
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Originally Posted by turboed370 View Post
I dont mind recamming it for the twin set up but I also dont wanna end up too small of a turbine and I'm not well informed on sizing twins.
If you go a bit too big, that's ok, because once it comes in.... it will be a near spiritual experience.
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Old 01-15-2019, 01:26 PM
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For your "Vague" goals mentioned in your first post, the second post pretty much summed it up IMO.
If you're not trying to break records and looking for a nice street setup with a peak RPM in the mid 6k, You can't go wrong with mid 60ish mm compressors and turbines. Like I said, my .68 AR T4 housing lights off those 66/65 turbonitecs real nice.
Moving up to an .81 A/R housing will push my RPM's a little higher if I wanted too. Plus, I have all the other supporting mods to do this as I know some day I'll want to tighten the screws on it lol.

Good luck. It's hard to go wrong with the goals you're looking for.
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