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Hey, so just thought I would share my experience from today... Ill go the long story route so you have all the details
Had a R33 on the dyno... RB25DET with a unknown modified turbo with a relatively large rear housing (to stock) So I started tuning and I always pull 5 to 10 degree's of timing out from say about 3000rpm onwards while I dial the fuel in for safety...
Anyway the boost response was very lazy until about 4000rpm where it would just skyrocket near on straight up (on dyno graph) to about 16psi where the run ended but his injectors were not big enough to supply enough fuel and almost at 13:1 maxed out...
Now I had already removed the boost controller and just had the internal wastegate actuator connected directly to a boost source (to setup the tune on the lowest boost first for safety) but the customer said it was the stock turbo actuator which I know should open at about 7psi so I thought maybe it was stuck shut and causing the massive boost spike... bench Tested the actuator and was working perfectly. Did a run while filming the actuator and it was indeed opening.
So I put it down to the size of the turbo, been modified with a much larger wheel and was abit too big/laggy and using the original wastegate flap couldnt handle the flow to control boost... Since his injectors were maxing out decided to just set the rev limiter lower and tune what I could until he got bigger injectors..
So got the fueling good up to 5500rpm and went onto the timing tuning... power was increasing in leaps and bounds and then I noticed that max was actually dropping and also coming on earlier...
So my theory is that the retarded timing was just burning too late and putting too much energy into the exhaust stroke, making low power but powering the turbo beyond what the internal gate could control is that right?
Yes, when you retard the timing it has an affect on boost response and pressure as you generate more heat in the exhaust which drives the turbine wheel.
Porsche uses this technique in the factory tunes on their turbos. You see the timing climb with rpm, then a pretty steep dip to help spool up the turbo and then it raises again.
I've been through this with a few cars, the last one was running 1.4 bar of boost, once I had the timing tuned for MTBT it was running only 0.9 bar of boost. It's not uncommon to pull timing back in areas to allow faster spooling of the turbo, you just need to be aware of the heat generated as a result
It's a catch 22 of starting your on-boost tuning with a safely retarded ignition map unfortunately. If your timing is too retarded you will often see the boost start spiralling out of control. I would say that I normally see this as a more pronounced effect when the boost control/wastegate is marginal in the first place.
It's always confusing when you first start tuning a turbocharged car and find that adding timing results in less boost. You will however see the boost start stabilising as you get closer to MBT.