compressor surge (not flutter)

[turbo buicks]

guys i have a question about comp surge under light throttle, hi load (IIRC) such as passing a car on the highway. how many of u have experienced this with a large turbo, specifically a BB72Q? i have been plotting out the specs for a stroked 231 (242 cid) on a comp map prog and im coming up good at WOT but at the initial full boost point and lite throttle, hi load (passing scenario) im running into surge. does this happen to you guys and if so what do u do about it? i was thinking about the highway scenario u could use something that would open the WG all the time, no boost, no surge, im not too worried about the initial max boost point b/c its right on the surge line and would only be for a second (maybe if any). i picked this turbo for its capabilities and features and the point im gonna be running it at is right at the most efficient point on the map, plus i got a deal on it. i would also like details on your combos as to what they are (in general) and where you make full boost and how much u wind it out. also i am using around 80-88% for engine VE in my calcs (ported big valve stk heads w/rolr cam) and 80% for PSIC eff. and 78% for the 72's eff. per the rating on the map. and as a side question, would revving to 6500 be unsafe for a girdled 109 with forged stroked rotating assembly and what about a non rolr cam, is this in the valve float range?

thank you for reading

 

[VadersV6]

I had this problem on my cobra when I had the blower. I originally had 1 bypass valve, but plumbed in a second one. (one before and one after the IC). Under vacuum, the bypass valves are open, and are actually letting air come in from the intake tube and bypass around the compressor into the motor. It actually becomes an N/A motor under cruise conditions. When you build boost, the valves close and you start using air from the compressor. After doing this, most of my surge problems went away. What surge remained, was due to running a return fuel system style fuel pump I had in there (Walbro 255) on a returnless style car. The computer was having a hard time maintaining the proper fuel pressure under cruise conditions. You should map out your fuel pressure curve while cruising, and make sure it looks right, and isnt flopping all over the place like mine was.

 

[TurboBob]

what are you using to control boost?

Wastegate type, boost setting, etc.

If you can turn your boost down and the surge stops, there is a cure.

Bob

 

[turbo2nr]

I had experienced the same thing with a BB T-70. I installed a turbosmart BOV, and had to loosen it, and cut the spring. Then I adjusted my throttle blade a little more closed than most people run it, causing IAC to run a little high, BUT, car now runs smooth everywhere. Just after boost, if I back off, no more surging.

Do you use a 3" throttle body also?

 

[turbo buicks]

sounds like youre making a vac leak in the intake and controlling the air flow that way. id imagine the effect on a MAF car would be bad in perf and milage. how is it? i was trying to think of something more to be used to open the WG when not in boost. no wg, no turbo, or have the maf after the vac leak. i hhave a greddy R bov now i could try to mod it but when i had it on when the car was running i had to have it tight to keep it from opening. i like the iac idea. still tho, doesnt messing with the computer make it run weird. i have a stock ported jat jackson TB. bosost will be about 20-25 on street and using a ext. wg. prob a delta gate.

 

[daverr]

howabout a bigger exhaust housing.

 

[turbo2nr]

No, no air ever enters the BOV. Only lets out extra air when turbo is on the left side of compressor map. That's the definition of "surging". Does not mess up MAF. ECM ignores MAF on deceloration (backing off after boost)

 

[turbo2nr]

If you uhave something to open wastegate when no boost, then how the heck are you ever going to build boost? BOVs are to improve drivability. When you get a big turbo and go off to the left of compressor map, you need a BOV, or you will get bucking and surging, simple as that.

 

[AltaGN]

TurboBuicks...the surge you are experiencing is common with larger turbos and intercoolers. The compressor wheel is trying to operate in the surge zone of the compressor map and is pulsing. It wants to make boost, but, the throttleblade is almost closed so the air is bouncing back against the wheel.

Sometimes playing with the fueling will make the problem a bit less, but, the only real cure is trying to drive around it-more pedal or let off...not always possible in traffic. No bov, etc. can help in this case.

 

[turbo buicks]

ill see how it turns out after i get it all together and my bov back on. also daverr i have a .82 housing.

 

[turbo2nr]

TurboBuicks...

No bov, etc. can help in this case.

And just why do you say that? It worked just fine for me and a few others on this board with drivability problems and a big hair dryer .

 

[AltaGN]

because this form of surge usually occurs between zero and up to ten pounds of boost...setting a bov to blow off at that low level is going to not only blow off boost when you want it, but, it is also going to screw up the fueling.

 

[turbo2nr]

Yeah, I know what you are saying. Mine just opens when I let off I guess. I hear it open(little pssst) at shifts when RPMs drop and when I "get out of it", or back off while spooling. That's where I got surging. Not just crusing around at a steady throttle position.

 

[AltaGN]

He was talking about the surge that happens with a big turbo/ic that occurs when the throttle is barely cracked in easing by someone in traffic, or coming up a ramp and merging...

Not much you can do about it other than say bad words, let off, and come back down again.

 

[TurboBob]

there is a cure with some turbos, I have fixed it with Stage-II (old school turbo), TA61, and PT61. It is combination specific.

Here is the scenario:

your wastegate is set to 20 psi,

you are entering the freeway, 2nd gear, 3000 RPM, holding the boost down by just cracking the throttle.

The boost gauge is showing 5 psi, and fluttering horribly.

What is happening is the turbo is pushing ~20 psi in the intercooler and against the throttle body. High boost + low flow = SURGE.

What will fix it in some cases is to lower the wastegate setting at low throttles. The BSTC has a throttle setting to accomplish this.

So, the question (again): what kind of wastagate setup are you running and where is it set? If you turn the boost way down, does the problem lessen or go away?

Bob

 

[shanks]

A BOV can help if its the right kind, i have a BOV off a 1st gen talon on my car. This was orginaly a bi-pass valve, on a talon it sends the air back in after the maf. This type of BOV will work becuse, it uses the differance in pressure between the turbo and throttle plate and throttle plate and intake.

like Turbobob was saying, if you have 20 psi in the pipe and only 5 in the intake the bov will open, but if the pressures are equal the valve will stay shut. This is not set up like a HKS BOV, this has the face of the valve facing the boost pressure, but boost also fills the diaphragm to hold the valve shut.

Lots of Talon people where saying this valve leaks at 20psi but i dont see how it can, esp since the diaphragm is at least 2.5 x bigger than the face of the valve. So the force hollding it closed whould be much grater. I have experinced it helping surge when say i got on it up to say 15 psi but let up but didnt compleatly let off, so i was still geting 5 or so psi but the BOV was venting at the same time, as soon as the pressures some what equalized the valve colsed. I got my BOV for 30.00.

HTH

Evan

PS, i have a TE62 w/ a power stroke ic.