turbine to housing tolerance (TD06H-25G)
 

I'm looking to improve the specs on a recently purchased a GReddy spec Mitsubishi TD06H-25G. I would prefer instead of single entry there was a twin scroll option for slightly better spool so went to looking around and found another Mitsubishi turbo, the TD06H-16M that has a twin scroll housing. Both are available in 16cm2 housings except the 16M is twin scroll.

I looked up the 25Gs' specs because I was hoping for a direct housing swap being they are both TD06H's but the first thing I noticed is the TD06-25G turbine specs for the inducer/exducer are 58.8 / 67mm while the TD06-16M is 58.8 / 68mm.

I'm no expert so wanted to ask if I could and did directly swap the housing from the 16M to the 25G would the 1mm smaller diameter inducer turbine clearance on the 25G cause problems? I'm guessing it would change how the flow of the exhaust gasses hit's the turbine and could cause spooling problems so it wouldn't be worth it given I want twin scroll for better spool and would probably need to find something different or would it be insignificant?

Thanks for your time and any input.

Split pulse with half a mm more radial clearance will likely still spool better.

^^
Agree from experience.
Furthermore, the clearance on the small diameter of the exhaust wheel to the exhaust housing interface is the more critical dimension and both wheels seem to be the same.

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First had experience-
Once when my BNR Stage 4 turbo was out for service I put my stock turbo CHRA with 54mm/64mm exhaust wheel into my exhaust housing bored out for my BNR Stage 4 with 64.6mm/74.2 mm P trim exhaust wheel and to my surprise the turbo spooled fine with decent response and peak boost despite the almost 1/4" clearance around the exhaust wheel.

This was with a free flowing performance exhaust, I imagine it would be a real slug with a stock restrictive exhaust where the there is less difference in pressure between engine side of exhaust wheel and exhaust side of exhaust wheel.

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Word of mouth-
I posted this before and another member changed up exhaust housings for an exhaust housing meant for the next size up exhaust wheel and he reported back it spooled fine as well.

Got ya, Slides. Thanks for the reply.

Hey, Blue TII. I've seen the work you've shown in modifying the stock manifold and BNR stage 4 to external wastegate setup and funnily enough almost sent you a PM with this question being you seem really knowledgeable. Your work really inspires me in my own build.

I figured even with the 25G Inducer turbine fins being 1mm diameter shorter if the exducer fins are the same the exhaust still has to go around them like normal so they should "seal" in the exhaust gasses from going completely around the turbine itself. Sounds like even with a slight gap the twin scroll would preform better though.

I really can't find the specs of the 16M's 58.8/68mm turbine and am wondering if the hot side housing between the 16M and the 25G have the same geometry internally with the exception of the twin scroll and the 16M's inducer fins just stretch a little farther out and narrow down a little more. I'll have to look around and maybe make some calls.

The only other option for twin scroll on a TD06H would be from kinugawa however it's a T3 instead of T4 flange and 12cm2(0.89 A/R) instead of 16cm2(1.13 A/R). Any opinions on if the T3 and 12cm2(0.89 A/R) would become a restriction? I've got a large street ported engine and 3" turbo back exhaust as far as flowing exhaust is concerned and am probably going to get a custom turbo manifold made using a Racing Beat flange. looking to be able to hit high 300's for rwhp. I've heard the stock S5 turbo is around 1.00 A/R and while a lower ratio can mean faster spooling from the higher velocity exhaust it can also become detrimental when going for higher horsepower and noticed the FD/FC kits from Greddy had the 16cm2(1.13 A/R) housings.

Thanks again for your input and experience, Slides and Blue.

Is there a particular reason why you are going with an older design of Mitsu turbo as opposed to something newer from Garrett or Borg Warner?

I was actually planning on a BNR stage 3 as a pragmatic solution to the power I was looking to run but got the TD06H from a friends FD part out so it was pretty cheap and in good working order.