mousecatcher

not weird at all. OEM vs aftermarket tuning have different objectives. ultimately, power is limited by what the heads can flow.

NoGaBiker

He's not saying it's weird they drop at different rpms; he's saying it's weird they all have a big drop-off in HP and Torque, be it at 5500, 6000, or 7000 rpms. As compared, say, to C7 Corvette dynos:

BGB Motorsports

Yes. I was able to move the intake flap to where I wanted it to open and get better control of the lambda curve but make no mistake, the power falloff at 6700 is inevitable with the stock GT4 intake manifold. The X51 manifold is massive and the fact that I can change the runner lengths over 5K RPM is the reason why this curve looks like it does. This is a Boxster Spyder running 91 octane with our 4.0L X51 upgrade vs a stock Cayman GT4 Clubsport MR. The car came back for cylinder head porting and valve work and we picked up another 15rwhp.

You can see that at the tippy top of the curve it's still struggling for flow but you can get that awesome curve with no drop off thanks to the X51 manifold.

ducsfly

Will just a tune and larger plenum/throttle body add the extra rpm and power?

d00d

4L/X51 and GTgears short ratio/fixed third/LSD would be killer.

mousecatcher

That chart tho is for a 4.0L/X51. I am wondering, is is possible to do *just* the manifold, to help with the dip? ie, no cams, etc. Or does the X51 manifold not mate up to the 3.8 heads.

Voyager6

X51 heads come with matching (different) intake cams from the stock 3.8L.. Exhaust cams are the same as the 3.8L engine.

To be complete, the 3.4 intake and exhaust cams are different and the 2.7L cams are unique from the 3.4L, too. The latter differing in lift and duration.

Voyager6

venom51

I don't see much issue around 5k on mine but I'd call it a sloppy mess between 3500 and 4800 under certain throttle conditions. I can feel some slight burbles in power under partial throttle conditions. Everything after 5k is perfect for my daily use. Oddly everything under 3500 is always perfect. I'd be happy to just to get the trouble area to work a little better.