My mechanic recently transplanted the entire new JDM / Spec C turbo, manifold, uppipe, sump etc setup into a STI7. Its running an EBC for boost control.

Today, while he was just trying to squeeze the last bit of power from the engine, we realised that the boost just would not go above 1.45bar. We even disconnected the waste gate acutator pressure hose (!) and the turbo still would not produce more than 1.45bar. Car in question is decat.

As a comparison, quite a few STI7s around have spiked to 1.7bar with only a decat. We were extremely puzzled as to why the twin scroll setup is so conservative in max boost, even with the wastegate not opening.

Ideas anyone? Cheers.

 

The wastegate could be being blown open by exhaust gas back pressure. A stronger actuator might keep it closed but at the expense of higher EGBP, with risk of possibly overspeeding the turbo.

 

doesn't the spec C have the VF 35 with the P15 exhuast housing? That's going to affect the boost charactristics compared to a standard STI7. Causing higher back pressure if anything, allowing the wastegate flap to be blown open more readily

Paul

 

The new STi 8 comes with a twin scroll turbo, they no longer use the single feed units like the VF30/34/35 ..... that is only used for the euro STi and the common JDM WRX cars.

Carlos H.

 

 

 

 

take a throttle return spring from any car (reasonably sized) and hook it to the arm of the turbo and then over to your IC or any other part. This will definetly let u run more boost.

Most IHI VF turbos have a weak 8 psi wastegate spring. The wastegate blows open easy. And some Japaneese tuning shops like Power Enterprises (PE) offer different springs for their actuator, IIRC there are three available: 8,13 and 19psi springs.

Here is the latest PE 1825F turbo, it is the twin scroll version, and will only work with the entire manifold and downpipe assembly of the JDM STi ver 8.



Carlos H.

 

RT, how was the response of the engine with the new turbo/manifold/downpipe ?. Especially interested on the low and midrange response .....

 

aaah, the new new Spec C!

Could just be a weak actuator

Paul

 

looking at the WG, it seems like it now opens vertically and not horizontally (hope it makes sense .....) like the older VF 22/23/24/28/29/30/34/35

 

Carlos,

It should open as you say, due to having to vent from the 2 gas streams, so should have 2 flappers

Paul

 

nice ..... would be better to have a detailed pic of this unit

Carlos H.

 

 

Thanks for the input guys. I will mention to my mechanic about the possibility that the wastegate was blown back open.

Unfortunately, I have not driven the car in question. But the owner says that the boost comes in significantly lower. Of course, the decat and EBC all add to this as well. Several corners can now be taken in 1 gear higher on a gymkana(?)-like course without falling out of the boost zone.

 

RT

How are you measuring boost, mechanical gauge or reading out of ECU?

Mike

 

Boost is measured off a Defi gauge. With the old setup (VF-30 / VF-34) turbocharger and unequal length manifold, 1.6-1.7bar was never a problem.

(Not that its good to run such high boost, but point is - that level was achieveable should you want it.)