anyone else ever have this problem? even with my wastegate staying shut, at about 7000 rpm my boost goes down to about 15psi.... gt40, stock ports, huge air filter... gotta be my exhaust =[

 

i think thats the cause of my issue =[

 

You're probably right, I'd just give it to me and be done with it.

seroiusly though -- Plenty of people are running the full RB exhaust and having great experiences. I can't say from personal experience, so I'd do a bit of searching, but I think the full 3" (slightly larger on RB actually) should support well over 400hp to the wheels. Stock ports maybe the issue??

 

Stock ports will yeild a higher pressure ratio, making it EASIER for it to hold boost. This is a gt-40 after all. I dont mind the tradeoff of a couple psi for the silence the rb exhaust provides =]

 

"Stock ports will yeild a higher pressure ratio, making it EASIER for it to hold boost."

true in theory only if you look at the induction side of the power plant. those puny stock ports however make it HARDER to maintain higher boost. one reason is it wont evacuate the exhaust fully so there will be more dilution on the next intake stroke effectively making less power do to less space for fresh air/fuel. Another reason is pumping loss. harder to pump out that high volume of boosted burnt fuel/air through a small hole. And lastley the small opening and short duration isn't allowing gas flow and or velocity as a street ported exhasut would... therefore less energy to spin a turbine.

I'm not saying its stock ports thats your problem, but its a possibity. I'm sure theres people who use stock ports and have no trouble maintaining boost with a turbo of that size. Perhaps its a combination of the RB exhaust, stock ports and manifold design???

~Mike...............

 

I would say its the combo of the 2. My friend has the RB exhaust and hes making over 400whp. He does not have stock ports an holds 15-16psi all the way to 7.5rpm. So if anything its your ports. The stock exhaust ports are very restrive IMO.


Also, the RB exhaust is quieter because of its solid design in the cb(heavy as ****), not because theres not enough flow.

 

I hold 15-16 psi without problem. its holding 18-20 thats the issue =]

 

So the best porting for holding high boost would be to leave the intake small and port the exhaust hugely?

 

The more overlap there is, the more boost will flow through the motor without actually getting trapped in the cylinder. This makes for better fill and keeping air veloicty higher, but you need a turbo that will flow more CFM. Porting a motor adds overlap. Porting shouldnt really have anything to do with holding boost, but lots of huge bridge ports and p-ports have to run huge turbos just to keep the compressor in the efficiency range due to a crazy pressure ratio

 

so even with a 1 bar spring the problem persists.