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Discussion Starter #1
Hi Guys,

Well I have a technical question,

At the moment I have custom straight exhaust Down pipe, Y pipe with test pipe (no cats), just end straight muffler. Car goes nice, and sound nice to.( 15g turbo). But this weekend I am replacing turbos with DR 750 (finally). What I was wondering is, is it really good to have straight pipe from turbo to the back . Because all mechanics in my country ( also rally drivers) is telling me, that turbo also need some back pressure, because of bearings in the turbo. Without cat or (in diesel DPF), Turbo bearing have preasure just in one direction, and this is prone to failure.... With little back pressure, you have almost even pressure on bearing. At some point this theory looks understandable, but from forum reading I see no cat is better than with cat.

So I have also hi flow cat, which I did not install it yet, but thinking to install it because of "to"fast dr750 spool and stock short block and also because of turbo pulsation (round 3000rpm).

What I see from dyno charts is that hi flow cat can rob you cca 10-15awhp...

Any experience with that? And leave emisinon thing.....
 

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I ran 15gs for a long time with a turbo back exhaust, you will make more power and the car sounds better. There is no negative effect toward the turbo by having no catalytic converters. If you run a chrome ecu you should have no problem bypassing emissions as well.
 

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Discussion Starter #3
I thought so too to drive with no cat, but all service mechanics will tell me different..... I am talking with Porsche service, Audi service, BMW service guys (all my friends), and with guys who repair turbos, and rally driver ....

I really dont know....
 

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Just a quick Google found that...

Hope it helps.

Bob.
 

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Lol, another "backpressure" discussion. As mentioned above, you want to minimize backpressure on a turbo car. There really is no benefit. I have never heard the bearing argument...

FWIW i gained about +30 AWHP going from a extremely restrictive main cat (might have even necked to 2.5'') to a 3'' test pipe (275 to 305 AWHP, little over 10% increase). Dyno not performed on the same day so take it with a grain of salt, and that was not a high flow cat. You will lose a little power with the cat, but not a huge amount (maybe more with bigger turbos and higher boost).

The other consideration is the smell, even if you dont have to pass emissions. I know on my daily i opted for a high flow catted DP, but my 3000gt is catless now that it is not my daily. Another note is that you might smoke more, if you have VSS issues for example.
 

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Discussion Starter #7
My understanding is also less backpressure better for turbo, but they are car mechanic specialist, and I am just an architect, lool.

Thanks for help guys
 

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It should also be noted that not all HFC's are created equally. There are ceramic and metallic cores as well as different "densities" or ratings so to speak (100, 200, 300+ CPSI). The "loss" in power will vary wildly among cars and setups, but the least restrictive and proven for power will probably be a Mil.Spec 100-cell unit (IMO anyway)
 

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I bought my HFC from 3sx years back,, whish I knew which type they sell.
Ah, you probably have a Random Tech one then.

-sent from my Galaxy Note 9
 

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I could not find any specifics on the RT's since they are not really offered or used for these cars anymore lol (aside from maybe a universal weld-in type).

But with that said, I have only tuned one 3/S where we added a RT 3" HFC to an otherwise catless setup. Digging back a few years... it only added maybe 2-300RPM of lag at 18-20psi using IPS TD04L-19T's, however, the before and after also removed the factory MAFS (GReddy e-Manage Ultimate in MAFless/SD configuration) to offset that. The car ended up making 470 and eventually 500AWHP VD SAE, but I imagine backpressure must have been absolute hell haha. Stock rebuilt engine sans forged pistons, BTW, so this is somewhat relevant to you.

For me personally and more recently, I logged 4-500RPM quicker spool by removing my AMS 300-cell HFC (metal substrate). This is with a TD05 setup though...

Anyway, hopefully that helps a bit for my two cents' worth!
 

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Discussion Starter #13
Thank you very much for this information. Appreciate it :)

Now I have to decide it if it is better to have some more lag on dr750 on stock internals, because from what I read is that dr750 is killing stock internals because of fast spool and low end torque....
 

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You are most welcome! Yes, that is correct especially with the stock connecting rods. The HFC will help, but another option is to do the opposite or reverse of the commonly suggested cam setting of +1 INT / -3 EXH... by doing -1 INT / +3 EXH instead, you simultaneously kill off the quick spool and notorious surge. I believe Matt at DR originally mentioned it, and Dmitri applied it with success before upgrading from the V1 DR750's. You can see his boost curve below from 2015:

 

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Discussion Starter #15
Man you are legend..... thank you very much for this info. Will do cam -1 INT / +3 EXH and hope my internals will be good. But does this preset of cams kills also top Hp?
 

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mmm not exactly sure how to answer that - correctly anyway. Like anything else, I think YMMV applies here... depends on the setup and tuning.
 
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