So I opened up my GT4 to see if the previous owner had installed an upgraded plenum. For some reason I thought the 981 GT4 plenum was plastic but this one is cast and has the 9A1 engine code on it. Is that right?

 

Believe that’s off a 911…

 

Looks like an X51 plenum. Lucky you.

 

Will that intake bolt directly onto the 3.4 heads or are the 3.8 heads different castings? Not sure if there would be any performance advantages of just that but trying to better understand the 9A1 engine architecture more fully. That intake was original equipment on the 3.8 991.1 GTS, right?

 

It will bolt onto 3.4 heads. But I've been told with the smaller displacement engines the gains with the x51 parts are minimal and you lose torque lower down the rpm range.

Source:
@BGB Motorsports



EDIT: I am bad at reading and only saw your other questions afterwards. The 3.4L and 3.8L cylinder heads appear to share the same castings (https://parts.porschechandler.com/oe...bC1oNi1nYXM%3D). And yes the X51 was standard on the 991.1 GTS and an optional extra on the 991.1 S.

 

Guess we need the 3.8 underneath it all then.

 

the intake runners are not stock either - so looks like who ever had it before did some solid work for you

 

I wonder how much the X51 intake system alone helps the 3.8 GT4 engine, especially towards the upper end of the powerband? Does the throttle body end up in the exact same location as the original GT4? I ask because the intake has to be turned 180 degrees to work in the Cayman vs the 911. Is there an easy way to make both sets of flaps work on the 981 GT4 as designed for the 911?

 

It's been well-documented. Check out post #42 for dyno comparisons:
https://rennlist.com/forums/gt4-spyd...install-3.html

 

Thanks for the link. I did some digging but for whatever reason didn't end up on that exact thread. Lots of good info there. I love that intake design.

 

The link is very interesting, but it seems to me that it deals with the complete X51 package (including the cams), not only the intake.

 

Please send me photos of this car. Was this car owned by Rick L.? If so and it's the car i think it is, have you any idea what you have in your hands? IF that is the car that i think it is, it had almost $175K into it. I have the dyno runs - if i am not mistaken, you have one of my Stage II cars. You have the full x51 Power Kit upgrade, clutch too possibly. It's hard to say because when they change hands, parts come off. I am in St. Pete for the weekend. Call AJ in the shop if you get antsy!

John

P.S. i cannot read any threads about X51 Power Kits - half the world thinks i am making up how it works and the results we got. They all think it's smoke and mirrors. No one ever asks though about my ECU file that opens and closes the flaps 3 different times.

Good luck and congrats - if what we see is true, you have a BGB car!

 

Sounds like someone hit the jackpot!

Just curious because I don't know but aren't the flaps vacuum actuated? Are there any issues with using oem Porsche parts to connect to the throttle body inlet on the X51 intake?

 

No. Back in the day we used to use the IPD silicone y pipe i believe beause the factory one is for the 74mm throtle body on the GT4. I think with the Clubsport, that went away and you can use OEM.

The flaps are vacuum actuated. I got Cobb to give me access to the Sound Symposer tables back in the day because a 991 Power Kit manifold has one middle flap and two side flaps; one for resonance and the other for intake runner accumulation. Most 981 / 991 road cars have one single resonance flap.

 

Hi John! Glad to see you still on here. Your build threads were some of my favorite reads. Do you plan to make a build thread for your 991.2 turbo engine swap?