Looking at the compressor map for the stock turbo, it appears to be pretty hard to get the stock turbo to surge. Either my data is off or something else is going on.

I recently switched to E85 and am in the process of tuning it. To properly tune the entire RPM range, I do third gear pulls from 2k to redline. At my stock pump gas 22-23 psi setting, everything goes pretty smoothly, as usual. When I turn the boost up to 28 psi, I get what I believe to be surge starting at about the 2900RPM/10 psi mark. The load will jump up and down for a couple hundred RPM and the boost will stutter or flatline (I had my psi rounded to the nearest 1 psi, so the logs don't show much) until about 3700RPM, where it shoots right up to 28-29 psi, nearly instantly.

I was shocked to experience this, seeing that the compressor map shows it nearly impossible to surge the stock turbo, especially in third gear. I am running through the stock cat back and high-flow cat as well, which may part of the issue, but I can test with my cut-out open...just haven't yet.

Since I didn't think it could be surge, I went over every single vacuum and boost line, looking for a leak, etc, and even checked the MAF, and everything is fine. Also, my load of 280 confirms that no boost leaks are present.

So, am I really surging the stock turbo here? Anyone else have any experience maxing out the stock turbo (or high boost in the 30 psi peak range) that has experienced something similar? When I get a chance I will do a run with my cut-out open to see if the effect is less or more, which should confirm for me whether it's surge or not.

Here is a quick log screenshot (crappy log as I was still configuring DMA logging for the V7 final ROM), but I have highlighted the area where the 'surging' happens. You can see the load (red line), boost (blue line), IPW (light blue), and Maf Hz (bottom line-dark blue) all change inflection to a flatter gradient and then even dip down momentarily, before everything shoots back up. Also, don't pay attention to the time scales...they are off in the log.




Eric

 

I've been surging on my stock turbo ever since I put on the Forge WGA it does it for a couple hundred rpms then it goes away I have stock IX bov as well

 

Mine used to surge at 15 PSI at 2200 RPM in 6th gear on a cold day with boost braking. Couldn't get it to do it otherwise running up to 28 PSI.

Airbox or induction cone?

 

Cone filter.

Plotting points on the compressor map tells me that it shouldn't be surging, which is why I am surprised to see this effect.

I wonder if the E85 and exhaust gases being constricted in my stock exhaust is having an effect on the turbine side of things?

 

I surge on E85 and 91oct

 

it doesn't make sense that you would get surging starting at 10psi at 2900rpm with a 28psi tune versus a 23 psi tune. Both tunes should make the same PSI at 2900rpm. Very odd.

 

Well, I agree to a point. The ramp rate of boost is going to be a bit steeper on the 28psi tune, since the ebc solenoid may be triggering at a later time. I don't know if this is what is happening or not. I can change the reaction time for the EBC to a different setting as well to see if it changes the effect.

 

Sorry to quote myself, but I looked at the combustion of ethanol and gasoline (octane) to determine how much more exhaust gas we are getting when running E85 as opposed to gasoline.

ethanol:
1 CH3CH2OH + 3 O2 ----> 2 CO2 + 3 H2O

gasoline (octane):
2 C8H18 + 25 O2 ----> 16 CO2 + 18 H2O


So, if we standardized to the same unit airflow (O2), we get:

ethanol:
1/3 CH3CH2OH + O2 ----> 2/3 CO2 + H2O

gasoline:
2/25 C8H18 + O2 ----> 16/25 CO2 + 18/25 H2O


So, for the same amount of mass airflow, we get roughly 4% more CO2 and 39% more water vapor in our exhaust. That helps explain why E85 helps spool our turbos sooner, burn cooler, but it also points the fact that exhaust restrictions will come into play sooner. Again, I don't know if that's my problem here, but just throwing this out there.


Eric

 

OK, actually I did the math and used the compressor map and actually, I am right at the surge border. So, this may actually be real surge. Opening my cutout to increase VE should give me some good confirmation.

Just for fun, I can post the compressor map and my calculations if anyone would like to see. Maybe I will just for people to double check my calculations.


Eric

 

OK here's the stock turbo compressor map:


My log's resolution wasn't that great, but rougth estimates show a 1.5 pressure ratio (about 7 psi) at 2700 RPM and a 1.68 PR (10 psi) at 2900 RPM. Assuming VE to be 85% here (I would imagine it would be a little higher, but let's go with this, since it helps the argument ), the follow equations can be used to plot the points on the map:

A(cfm)=[Cid*VE*RPM/3456]PR
PR=pressure ratio=[boost+14.7]/14.7

1.68 PR (10 psi):
A(cfm)=[122*.85*2900/3456]1.68=146 CFM

1.5 PR (7 psi):
A(cfm)=[122*.85*2700/3456]1.5=121 CFM


So, if you plot the following two points, you can see where they fall on the map: (121, 1.5) and (146, 1.68). These two points are barely to the right of the line. With the assumptions in the equations, the resolution of my logs, etc, this shows that I am very close to this line, so maybe this is real surge?


Eric

 

Yup I've seen surge depending on the setup.

I can get the 10's to surge as well totally stock.

 

OK, well at least that makes me feel better that nothing else is wrong...hopefully. But, it still sucks. Now I have to figure out the right combination of parts and setting to relieve the surge...if it truly is surge. I'm not taking off my stock cat-back, so hopefully I can get it to work with it still on.

I have a new 4" intake pipe and filter for SD, so maybe installing that may help relieve the symptoms.

 

On IX with fast spool setup I have surge in 5th and 6th gear with pump gasoline

 

Thanks for posting that Eric. Since I have started tuning my car on E85 I have logged several occurrences where the load drops off then comes back. I thought it was my IX BOV.

 

Ive seen this also running 30 psi on a ported 9 turbo with meth. My load will spike then drop off and pick back up again. My computer has been down for a while so I havent gotten to log it lately. Im going to try backing it down to 28 psi and see what it will hold. At 30 psi right now it holds usually 24 to a high 23 psi.