Just in case anyone might need/find this info useful, am posting for reference.

SSI/RCM GT2 Header outer pipe diameter 43mm, Inner 38mm

Standard supplied up-pipe with GT2 Headers Inner diameter 56mm at 3 bolt side, Inner diameter 54mm turbo side. Note however there is a lip between the pipe and the flange that means airflow transition potentially isn't smooth.

Harvey Trick Up-Pipe 3 bolt supplied by AS Performance
Inner diameter 60mm at 3 bolt side, tapering to inner diameter 46mm turbo side with smooth transition to flange.

Perrin Small Bore Up-Pipe inner diameter 41.15mm top and bottom (2 bolt)
Perrin Big Bore Up-Pipe inner diameter 47.25mm top and bottom (2 bolt)

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Everyone will have different set ups, but the above may help to explain why on power outputs above 350bhp the standard GT2 up-pipe can rob you of spool and push the powerband further up. This is especially significant when you have a FMIC as you need to fill those long pipes with pressure before coming on boost.

Harvey's design approaches tackles the situation but have a large bore at the header side to match that flange diameter and then taper to 46mm to speed up the airflow. These pipes have been proven even at 750bhp on time attack cars.

Perrin offer both a small bore for power outputs to 400bhp and a larger bore for 400bhp+ yet still smaller in diameter than the std GT2 up-pipe to aid spool. The Perrin flanges are also extra thick so won't crack etc.

Hope this info is useful for those looking at header/up-pipe options.

 

That's great info, I'm sure will help many people.

Don't suppose you have any standard headers and uppipes around to measure the size? I'm intrigued more than anything!

 

Great info thanks for sharing.

I had read that gt spec headers can hurt spool and it's nice to see some technical reasons why this might be the case!

For that reason I've decided not to fit them just yet and stick to ported stock headers.

 

Good info on the header/ up pipe sizes - very useful indeed!

 

Standard STI up pipe is 42mm internal diameter.

 

Any back to back data to back up the slower spool times of the GT spec?
Is this for both single & twin scroll?

 

Unfortunately I don't as the standard ported headers went straight on to the car and didn't think to measure those before to be honest.

 

Single scroll in my application.

I can provide an ECUtek road dyno plot before and after with no map mods and you can see the shift in torque.

Green is RCM GT2 manifold with RCM supplied up-pipe.
Red is Stock manifold, mildly ported to match, and a Perrin Small Bore Up-pipe.

There is a sudden tail off on the Red plot at the top end as I ran out of road (Malta is small) and had to stand on the brakes.

 

So there seems to be around 15% torque increase even at 3000rpm,I'm not sure how accurate those virtual dynos are especially back to back but it doesn't prove the rcm are worse for spool&torque.
Do you have boost graphs?
The rcm t/s I've just fitted are a definite improvement over the oem.

 

Here are the boost graphs overlaid over each other. Pre-manifold/up-pipe change and post. Same ROM which is still a works in progress.

Both logs in 4th gear

 

Stock Ported Headers, Perrin Small Tube Up-Pipe, Map Tweak overlaid on to previous runs in 4th gear. Not 100% scientific and have cut off at 5,500rpm but shows the gain low down after a map tweak.