R34 Gtt RB25det neo Issue After Modifications

[ChrisGtt]

Hi All,

New member here, I've joined to help my son with an issue that is truly doing our heads in.

I'll start with a bit of an overview. Chris carried out some modifications to the engine to try and extract just a little extra power out of the motor and make it a bit more efficient. Previous to this work, the car already had an upgraded exhaust, increased boost up to 14psi, modified standard turbo to metal impellers and slightly oversized, upgraded fuel pump, been dyno-tuned at JEM, and some other bits n bobs like coil packs etc. Before the current mods it was making 236rwkw at 14psi but after the new mods it now makes 209rwkw at 14psi.

The work he carried out is as follows, timing belt service, water pump replacement, return to sump radium catch can, had oem head rebuilt and machined 0.2mm, fitted Athena cut ring head gasket 1.2mm thick, arp 2000 head studs, added rear prp head drain, added an external wastegate 40mm turbosmart welded off the stock manifold and plumbed back into a 3in dump.

The car has been on the dyno and apparently the ignition is working, fuel flow and fuel pressure is fine, injectors are fine, spark plugs are working fine, Maf is working, O2 sensor is working, TPs sensor is working, VCT is working, CAS is working, boost leak test showed no boost leaks, compression test first crank cold 135psi and got to 150psi across all cylinders once cranking, timing is confirmed at tdc with a dial gauge and the cam dots line up, no blockages in intercoolerm throttle body or pre-turbo intake, no blockages in exhaust, cat conveter or down pipe, turbo looks fine and spins and wastegate looks fine and is new.

Soz for all the detail but after all this, it has been in 3 times for dyno and each time comes back stating it's not going to make power. After the first (or second, can't remember now) trip to dyno we did find the timing out by one tooth and this improved things, but it is still well down. Does anyone have any ideas? 

Any replies much appreciated as we're just not sure of which direction to take this.

Regards

Rob (Chris's Dad).

[robbo_rb180]

What ecu?
Same shop every time?
Same ambient temperature?

If timing is correct then pull plugs and see if lean or rich.
Dyno can also read different if they have changed ramp rate or not set tacho/speed properly
 

[ChrisGtt]

Hi Rob

It's a nistune ecu.

Yes same shop, good reputation.

Apparently the computer on the dyno is compensated for temperature. 

 

Chris was saying that the lean or richness of the mixture would be covered in the actual tune, but he's already picking up in advance that it's under what it should be.

Regards the dyno, it's also noticeable on the road.

But I just want to add something that has just come in, it appears there might be something in the setup of the external wastegate. In the exhaust manifold there is a centre divider between the front three and rear three cylinders. The external wastegate has a pipe from each side that join at the wastegate. Theory is those two pipes just might be pypassing that divider and introducing a problem somehow. Is anyone familiar with the divider in the exhaust manifold and the effects if the two wastegate pipes were to create a bypass path?

Thanks for your questions Rob, interested in your thoughts on this external wastegate bypass theory.

Regards

Rob

[GTSBoy]

Well, I was going to say, "Is the wastegate set up so that it is not leaking?"

That's actually a different thing to what you're asking about. In theory, if you have allowed the two halves of the manifold to commmunicate (more than the stock manifold does - which is at least a little bit) then the negative effect should be apparent in delayed spool, not in outrigth power.

OK, maybe just maybe, a badly set up twin scroll wastegate "crosstalk" might kill the top end, although it's hard to see how.

What is happening with the original wastegate in the turbo? is it sealed off properly. If it's left flapping in the breeze, it will f**k everything up. If it is still there, and can be returned to service, I'd be capping off the external (presuming the two halves can and will be isolated from each other after doing so) and have a go with just the stock wastegate.

I have an internal wastegate in my highflowed rear housing. The bigger opening and flapper should be fine to >250 rwkW. So there's not exactly a pressing reason to have the external. Done right, an external will give better results. Done wrong, it might just be possible to have worse results.

Report back!

[ChrisGtt]

Hi

Yes we're a bit optimistic we might get something out of sealing up the two pipes of the external wastegate and reactivating the stock one.

I believe the orginal stock wastegate was tack welded shut. It doesn't budge and visually looks closed and sealed.

The car has noticeably more lag so it could be at least part of the problem, maybe all. It's definately one of the key changes made since last tune. And it's one that's more difficult to fully understand and measure, not like for instance timing or cylinder compression. 

Thanks for that feedback, we'll let you know how this progresses. Fingers crossed.

 

Regards

Rob