Tuned a rx7 not long ago that made good power but had a few small issues biggest issue was mixtures going almost 1.0 to richer over 7500rpm their was around 1 psi drop off and power was dropping off earlier then I normally expected want to know other peoples experiences on whether a 3 inch exhaust is too small or any ideas

 

without the appropriate porting the engine is going to lose efficiency up there and thus it will require a lot less fuel. Most street ports I have seen need to drop injector pulsewidth (or adjust the VE table or however the engine management works) once you get much over 7000rpm. Otherwise it will get super rich.

What hotside is it?

 

I should have said porting is not the problem my engine has the same ports and does not have this issue it has a 3.5 inch exhaust and a GT4202 though and a lot of other cars I have done with same porting have not had this problem

 

wastegate setup?

 

i think there is too much it could be and i havent got enough money to change the entire setup lol, we'll check the exhaust back pressure thats easy enough, if thats no good ill put a massive exhaust system on it.

its got a 60mm wastegate , 3" intercooler and piping, its got the biggest exhaust housing available for the gt3788r.
the t04e was similar wasnt it? except more extreme and earlier rpm with the stock ports
though i dont understand why at lower boost the problem is still there

 

Fighting this same problem as we speak, but I am making some progress.



As I hit 7000 and over the car goes richer 1.5 -2.0 points richer and begins to stumble. What I noticed is that it did not matter what my boost level was, 7psi or 14psi, it goes richer at 7000, stumble and lose power. I thought it was ignition related, since I thought 10.2:1 was not a problem to light off, but after adding an ign amp and varifing integrity of the other ignition components, it appears to be the drop in the engines Volumetric Efficiency causing the stumble. Fuel values in my map at 6500 - 7000 - 7500 -8000were almost the same at first. I begun leaning from 7000 up by 1 point and made pulls each time while logging. I noticed after removing about 8- 10 points of fuel in this area the wideband (Innovate LC1)still did not read lean WTF? However, the car felt better each time.

Does the VE fall of so greatly above 7000 (10 points less fuel)? Sure seems that way.

I am running a running a tube manifold made of 2" pipe and a GT4094R with 3" exhaust without cats or muffler. The engine has a Large Street port.

To the OP I think it may be related more to VE than the 3" pipe.

I am also interested to hear other peoples expereinces with fall of of VE at high rpms, and what your maps look like in this area.

 

Is the wastegate discharge plumbed back into the exhaust or does it go to atmosphere?

 

Well let's compare the turbine maps between a GT3788 and a GT4202. This stuff is right on their website. GT37:



a max of about 31 lb/min mass flow. Now the GT42:



The smallest GT42 hotside flows a max of about 37 lb/min, so 19% more mass flow through the smallest available GT42 hotside vs the largest available GT37 hotside. Now that's a quick 'n' dirty calculation but without discovering some other problem I would say that the turbine housing is the most likely restriction.

 

If memory serves it runs a smaller wheel than a P-trim. So a TO4B - TO4R with 1.0 A/r would have better flow on the turbine side. I don't feel like looking it up but there was a reason why I didn't like this turbo and I think that was it.

~S~

 

So you guys think its turbo then well if it is then I guess it will just have to be do the back pressure test lev and rule it out and will go from their

 

as i said in lev's members rides thread on ausrotary, as it chokes and boost starts to fall off the compressor does not require the power of the full exhaust flow feeding the turbine, if you can map gate duty or target MAP to a bit below what it can achieve with the gate shut (after whatever engine speed it first gets to or within a few % of peak power) you will still get the power the compressor needs from the remaining turbine flow, but let the excess through the gate & reduce back pressure. Assuming the exhaust is not a restriction.

The back pressure will reduce power due to extra effort on the exhaust stroke & reduce the amount of air it will breathe in choke conditions due to carry over into the intake stroke. I bet if you can get the gate to open in the top end you will make more power (flat lined from where it chokes) at less boost in choke as the engine won't be recycling combustion gasses as much.

You really have to do a pull and log manifold pressure vs rpm with the boost control as it is now. Then run target pressure vs rpm a bit below what it is making after peak power is first reached so that it is cracking the gate. From what Lev posted you were using an eboost? can it run target pressure vs rpm? Might have to run it through the ecu to get the most out of this setup otherwise.

 

Manifold pressure will be high Levs tube is very small that could be an issue well do back pressure test on dump pipe and go from their

 

this is actually a very "normal" boost control strategy on modern turbo piston engines. Depending on how the boost controller is set up, another thing you can do is turn the base duty cycle value down but turn the gain up. This will give you a gain-induced spike right in the mid range and allow boost to taper off a bit, with the wastegate opening to relieve backpressure. I literally just did this the other night on a Subaru. Subaru's OEM boost control system is more sophisticated than anything you can buy or use on a reasonably-priced standalone. It's very very powerful if you know what you're doing.

Long term solution still seems to be a different turbo though.

 

im pretty happy with the turbo in terms of the power it makes. almost 400rwkw in a street car is heaps.
im more concerned whether or not im putting unnecessary stress on the engine somewhere

 

If you can control the gate properly past peak it will have a little bit more power in the "overrev" and hold a lot less heat.

 

Yeh we can work the gate how ever we want I can turn the boost up more no worries their gate works flawlessly unlike the turbosmart garbage that was on their to begin with