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

[ChrisGtt]

So Thanks for the comments etc. To follow up on this, we went down the path of fitting a divider down the middle of the external pipe that was added to the exhaust manifold and the divider went from very close to the external wastegate all the way up to the "V" part where the pipes from each side of the manifold joined.

After this modification it was finally in a position to do the dyno-tune with some degree of success. Top end power was down about 10kw (250rwkw down to 240rwkw) I believe from previous but it seems to be more responsive lower down and at least it is now driveable and fun and back on the road to be enjoyed. Apparently the timing couldn't be run the same as it was running into knock and boost was down about 1psi. For all we know this could have been from the fuel being a bit older, or perhaps some slight complication from the new head gasket as we didn't have compression figures from before that mod to compare. I'm no mechanic and this is second hand info but I just wanted to follow-up to those that commented or read the original post with interest.

After so many months of stuffing around this is a big win. The interesting part was most of the info around this was gained from information around Barra motors and not GTR as the manifold setup on the Barra with single turbo was more similar. 

Thanks for those that helped with info.

Regards

Rob