by dr20t » Thu Sep 29, 2016 11:01 am
With stock headers and uppipe, your restriction is in the headers and uppipe. So by going to an 8cm, you still have a backpressure source in the stock headers and uppipe
Let me give you this example -
2 inch pipe taped to the end of a straw. Your flow and pressure limitaion is in the straw. So changing the 2" pipe to 3" wont introduce or alleviate additional pressure pre turbo.
Turbos spool off a combination of heat, flow and pressure (which are not mutually exclusive and dependent on one another).
Pressure is one of the most important factors, as the lower the backpressure, the more your engine can operate as close to a naturally aspirated engine as possible. However, in simple terms of a relatively mild setup, without optimising the engine dynamics to improve its aspirated VE (that is for most people on this forum), the best way to regulate spool is by sizing your turbo exhaust housing accordingly.
By keeping stock headers and uppipe, the 8cm won't achieve a difference in power that would warrant losing some of the spool or backpressure.
You're also knock limited on 98 which does play into this.
Also (beyond the scope of this thread discussion), there is a very strong correlation between pressure, pulses, harmonics and heat that are always dynamically at play to impact performance.
Happy to discuss further.
Mick