Turbo surge limit - what's the real 411

[huguetpj]

Ok. Been a while since I posted but I searched and found nothing conclusive. Still searching the net but want to see if anybody here has any better insight.

When I started my project my goal was to reach 10-12PSI tops. Once I blew my first motor and had to rebuild I upped the stakes to 18-20PSI. Unfortunately I just came to realize I made a bad turbo selection back then.

When I decided to run 10-12PSI, the TO4E 60 trim seemed to be the better choice because of it's efficiency at 10-12PSI compared to the 50 trim. The other day, when doing some compressor maps for a friend I decided to plot some new lines to see how much boost I could run with my turbo and at what efficiency. I was sure it could run 20PSI without problem, I was more worried on the efficiency part. But when plotting I came to realize that if I try to run 18PSI with the 60 trim the first point in my map (3.5krpm) is to the left of the surge limit line.

My question is... can this cause something catastrophic or should I not worry that much? Cause I've been running 14-15PSI for a few days now and I have not felt the turbo spool slower or hesitate or nothing along those lines (the boost curve in the SBC spikes quite nicely) even though even at that boost level the point on the map falls right to the left of the surge line.

I know most of the calculations are aproximate values (Ve, CFMs, etc)... but just want to play it safe here. I want to see if my car can stay out of the shop for more than a month this time

This is the 6trim map. Notice the blue circle on the left of the surge line:

And this is the 50trim... I should have gotten the 50 trim...

[Nismo_Freak]

Your line should not vary like that, the line will be on the same path as the 10PSI line until the boost pressure exceeds the 10PSI.

VE, flow, etc. does not change until the turbocharger pushes more pressure.

[huguetpj]

But that would be if you suppose that the turbo spools linearly, which it does not since it spools faster the more PSI it's pushing. I was just calculating that I could have full boost by 3.5krpms... a more exact way of doing it is by reading the precise value were I reach 18PSI... which I will do once I get my wastegate to stay shut.

But even though if we suppose the line should continue along the 10PSI line, which by the way I drew by estimating to hit full boost by 3.5krpms also, the 18PSI would stretch right next to the surge line. Anyway all this is more theoretical than practical.

My concern is how does one know if the turbo ir surging or not. Thanks for the inputs by the way.

[nissanfanatic]

The turbo will stop making boost until you get back into an acceptable airflow range(RPM). S14tat has that exact same problem. We run identical turbos, he runs 15psi, I run 10.5. I can hit full boost by 4k while he doesn't make 15psi until 5000-5500rpm.

BTW, I looked at this calculator and I found the 50trim TO4E to be the best choice for up to about 430whp. I will be switching to a straight T04E 50trim. The 60trim will make better peak power tho. You'll just have to deal with the surging.

Pressure is a direct result of flow. You can have flow without pressure, but not pressure without flow. So if the compressor stops flowing(as it does when surging), pressure discontinues. You may be able to lessen the surging with a really good exhaust system(tubular mani, ect) and some sort of an upgraded intake manifold.

[Nismo_Freak]

But that would be if you suppose that the turbo spools linearly, which it does not since it spools faster the more PSI it's pushing.

The turbo does not spool faster for higher boost pressures, LOL.

The turbo utilizes 100% of the exhaust energy until the wastegate opens, so it doesn't magically spool faster once you run a higher boost target.

[nissanfanatic]

Quote »The turbo does not spool faster for higher boost pressures, LOL.

The turbo utilizes 100% of the exhaust energy until the wastegate opens, so it doesn't magically spool faster once you run a higher boost target.[/quote]Exactly. Unless you run straight spring where the wastegate will open sooner, spooling will look exactly the same. Check it out on that calculator I posted.

aaccckkkk. Whats up with those AF ratios? Torque curve suffered really badly from the AF ratio it looks. Peak torque would prolly have been a lil higher, but not much as it looks both AF curves went down for peak torque point at 4500 then shot back up.

[C-Kwik]

But that would be if you suppose that the turbo spools linearly, which it does not since it spools faster the more PSI it's pushing. I was just calculating that I could have full boost by 3.5krpms... a more exact way of doing it is by reading the precise value were I reach 18PSI... which I will do once I get my wastegate to stay shut.

I will concur with everyone else that the higher boost curve will follow the same path up to the moment the wastgate opens.

I think part of the problem is you may be calculating that you will reach full boost at 3.5K RPM for both boost curves. This is very wrong. The higher boost curve will reach full boost at a later RPM. To see what I mean, take a look at nismo Freak's posted Dyno. Follow the torque curves as they will show you right about where each boost level was hit. 7 psi was at about 2500, 10 psi was just below 3000, and 15 psi was just below 3500. The spacing will vary from turbo to turbo on a given motor, but it does clearly show that full boost is not reached at the same RPM at different boost levels.

[huguetpj]

Great inputs guys. Thanks a bunch. I still need to go to the store to get the T and everything to move my SBC signal to reach 18PSI. Once I do that, what I will do is measure where am I hitting different boost levels... 10-12-15-18 and graph that to see how the curve looks.

Now... one question remains.... should I worry about the health of my turbo because of the surging (if it surges at all)? I don't like reading about "catastrophic failures" on the net

[nissanfanatic]

Yes you should. Operating in the surge area for extened time can cause bearing failure.

[toki]

60-1 nukka. 60-1.

or a gt40 and a P trim

[Nismo_Freak]

60-1 nukka. 60-1.

or a gt40 and a P trim

God no.

Compressor surge can actually cause the wheel nut to back spin and come loose. Happened on my friends turbocharger, as the wheel then decided to manually port his housing at 50,000 RPM.

[huguetpj]

Now I'm getting worried. I'll plot the boost curve asap. Should I do it in 4th gear as in the dyno? I'm guessing yes cause it's loads the engine more so I could get boost the earliest, worst case scenario.

[Nismo_Freak]

Now I'm getting worried. I'll plot the boost curve asap. Should I do it in 4th gear as in the dyno? I'm guessing yes cause it's loads the engine more so I could get boost the earliest, worst case scenario.

Put it in 4th on the road, and floor it.

With a 60 trim you shouldn't see 7 PSI until about 3300 or so I'd imagine, with 14 PSI being around 3700, 19-20 around 4000.