surge line

[zguy36]

Has anyone on here run their turbo up to the surge line? I know this is a bad thing and I think that my turbo is reaching that point. I always thought that the turbo would not operate there, as in not be able to reach that point, but am beginning to think otherwise. I am running a T3/T4 hybrid with the stock T3 ehaust side and V trim T4 compressor. After upgrade to a megasquirt and larger injectors, I am able to turn up the boost. This tiny turbo builds full boost by 3000rpm. When the boost is turned up to 20psi and the rpm is 3000, my boost flucutates rapidly until about 3600rpm then smooths out. I am thinking this is the surge line of that compressor. Has anyone actually had this happen?

[thehelix112]

http://www.bsmotor.com/turbo/kalkuler.html

 

Plug in the values there, select the TO4-V map, and you can see that at 2.36 PR and 20lb/min (3000rpm) you are well into surge. I don't know what electronics/boost controller you have but you need to set a rpm-dependent map that gradually feeds boost in as rpm rises. I am in the same situation with my GT35, 20psi is the limit until past 4500.

 

Dave

[Clifton]

I ran the same compressor but with an O trim .69 T4 hotside. I too could get it to surge at a little over 3k at lower boost levels(5-9psi) with around 1/2-3/4 throttle. It was on my roadrace car and I didn't have a choice but live with it, only happened coming off a long turn. Same fluctuating needle. Once I could give full throttle the revs would come up quick and all would be good. Turbo never came apart and I ran it for years on the street before it became a track only car. I did have to replace the thrust bearing once but those are cheap. On the street you probably aren't at full boost at lower rpms enough or if you are it won't be for long. This is another reason I dislike T3/4's with small turbines/ar's on L6 motors.

[zguy36]

Thanks for the help guys. I did a quick calc with my airflow calculator and yes, I am following the surge line of the V trim compressor all the way from 5lbs/min to 15lbs/min where it then goes back into a somewhat efficient range. I'm in the midst of a job search right now, so no cash to spread out to the car. I do have my tubular manifold done so that I can strap on any turbo with a T4 exhaust flange. I've got a few bugs to work out with this turbo setup anyway. Maybe you guys can shed some light onto this bug:

 

At high rpm, 5000-6000, I have a bad miss. Sometimes it is bad enough that the car won't accelerate past that point, others it just doesnt' make much power. Sometimes, it pulls really strong clear to redline.

 

Specs, megasquirt II, GM hei ignition coil, new plugs, new wires, 24 deg advance, 16psi boost, 11:1 AFR.

[Clifton]

Why so rich? Try leaning it out a little. 11:1 will definetly keep it from making power and if it was a little richer at times could cause a miss.

 

At high rpm' date=' 5000-6000, I have a bad miss. Sometimes it is bad enough that the car won't accelerate past that point, others it just doesnt' make much power. Sometimes, it pulls really strong clear to redline.

 

Specs, megasquirt II, GM hei ignition coil, new plugs, new wires, 24 deg advance, 16psi boost, 11:1 AFR.[/quote']

[zguy36]

I originally was going for lean best torque and had it tuned to 13:1 I tried going a bit richer and it kept feeling better. I would like to keep it at 12:1, but it seemed to miss less at high rpm with the richer 11:1 It is hard to tell what makes the miss go away, or if the car is just running better at that time.