ENGINEERMAN

Hi everyone:

I have a question regarding the proper operation of the TURBO SX Blow off valve that I have installed (see photo). As you can see I have it installed with the pressure side from the hard pipe feeding into the bottom of the BOV and then it is relieving to the intake downstream of the MAF to preserve the A/F ratio. I have the control line (top of BOV) running to the Intake Manifold to sense the vacuum/pressure in the manifold and control the behavior of the BOV. I have a K27/8 turbo and have performed the modification to the Intercooler inlet manifold to allow better flow for quicker spool up. I have installed a Lindsey Racing Dual Port Wastegate as well.

What I have noticed is that the BOV is relieving (blowing off) under low to medium boost levels and even a little at high boost. Boost builds quickly and is steady right up through redline (I am running about 16 psig currently). Since I have a K27/8 Turbo so there is plenty of flow.

I recently went for a ride in a friends NOBLE on the track and his BOV stayed shut tight as a drum until he lifted for a shift at which time it relieved the pressure and I would think that that is the way the BOV should function (do not relieve, i.e. "waste" ANY compressed air until it is time to bypass when the throttle plate is closed so as to keep the turbo spinning). I am sure that there are no vacuum leaks in my vacuum tubing system and I have installed Laust's multi-port vacuum port onto my intake manifold so as to get a nice clean supply to the BOV control line. My questions for all of you are:

1) In the 944 Turbo is it normal for the BOV to be bypassing when you are under boost? Shouldn't it be holding pressure tight as a drum whenever there is boost pressure? I can understand it being open when there is vacuum due to the design of the system but I would think that it would be closed when under pressure.

2) Or is this just a "problem" with the TURBO SX BOV? I have inserted some washers under the spring that biases the piston to increase the pressure under which the BOV will start to open without much impact. But based on the design of the BOV and the natural differences in the diametral areas the piston SHOULD stay shut naturally since the top side area of the piston (where the control line connects) is quite a bit larger than the bottom side of the piston (where the hard pipe connects) and since both should be seeing the same pressure (as long as the throttle plate is wide open) there should be a natural bias to stay closed under boost and open under vacuum even without the spring.

I am a little stumped as to why this is happening. Maybe that is the way it was intended to work in the 944 Turbos and would appreciate your telling me so if that is the case (but if it is would be an area for improvement since it IS wasting energy and reducing the amount of air getting into the combustion chamber I would think). I would appreciate any comments or suggestions from those of you who are familiar with this situation.

Thanks in advance for your help!

Tom

xsboost90

1. you dont still have a cycling valve do you?

2. maybe this vac. line should just be tee'd into the line going to the wastegate/IC line as if you had bypassed the CV. - thats how mine is and does not relieve pressure until you lift- friend of mine runs a large blowoff valve and his does not until he lifts.

3. not sure how you have you vac. port installed, but some things have check valves to hold only vacuum-no positive pressure- and others fluctuate between positive and neg pressures. You need both on the BOV line.

ENGINEERMAN

Hi Dan:

The Control Valve was removed many moons ago by the guy I bought the car from . Actually he installed the MAF and the BOV as well and he had the BOV control line plumbed to the Intake Manifold, i.e. DOWNSTREAM of the throttle plate which seemed to make sense to me since that way as long as the throttle plate was open the control line would see the same pressure as was present in the hard pipes and thus hold the BOV piston closed but when the throttle plate closed the control line pressure would go into vacuum while the bottom side of the piston that is connected to the hard pipe would remain under boost pressure and thus the BOV piston would open immediately and relieve the pressure (and allow the turbo to stay spooled up) due to the significant pressure differential (at least in theory).

So if I understand you correctly you say that your BOV valve control line is plumbed into the turbo plumbing (hard pipe) UPSTREAM of the throttle plate? Please confirm. I would think that this would make it so that the control line ALWAYS saw the same pressure as is present in the harp pipe. How then could a pressure differential be generated to open the BOV piston? I'm not saying it doesn't work but it doesn't seem to be intuitively correct on the surface at least. Please confirm that I understand you correctly and if I do it is not very difficult for me to re-plumb the control line to the tap into the hard pipe to test it out and see if it makes a difference.

Thanks for the comments and I look forward to your response.

Tom

95ONE

Yes. it is suppose to stay shut the whole time until there is Vacuum. If your BOV is not doing this, something is not right.

Vacuum from manifold is correct.

Check spring and adjustments on BOV. It may just be a bad unit.

ENGINEERMAN

Hi Dan:

I ran out quickly and tried your suggestion regarding the revised control line plumbing, i.e. plumb the control line in to the Hard Pipe line UPSTREAM of the throttle plate. The BOV now works as it should, i.e tight as a drum under boost and then sh-sh-sh-sh when I lift!

Thanks for the advise!

Best regards!

Tom

Naplesguy06

Is that an old Huntley Racing MAF I see on there? How does it work for you? Do you know who manufactured it for Huntley?

ENGINEERMAN

Hi Naplesguy06:

Yes, it is a Huntley MAF. It came with the car and I have had no problems with it. I use it in conjunction with a MAFTERBURNER piggyback system and wideband O2 with temperature correction. Unfortunately I know very little about Huntley except that they are out of business (luckily my MAF seems to work very well). I bought the car from a guy in Florida who may be able to give you more info. If you want I can try to get you his number but you are probably better off asking on this site.

Good luck!

eniac

Yeah Huntley is out of business...if anyone can find that **self-edited**, Derrick still owes me $1700 and 2 turbos that were paid for but never shipped :/

ENGINEERMAN

Hi Eniac:

I noticed that you said that you had run boost levels of up to 27 psig in another post. I have been running about 18 psig on the street but feel that I have too much back pressure due to the restriction in the exhaust outlet of the K27/8 turbo to go any higher. What mods did you have to perform to go to such high levels? How much boost is safe with a stock head that has not been o-ringed? Your comments would be appreciated.

Thanks!

jgporsche

I'm almost positive the Huntley MAF is a Split Second MAF.

Keithr726

How is your AFR?

eniac

At 27psi open exhaust 3" pipe and yes restriction is a concern. When I upgrade my turbo to something larger I will have to go to 4" pipe. I normally run 23psi on the street with a muffler but I have a button on my steering wheel to instantly switch to 27 if I need that extra. Its like nitrous but the bottle never empties. hehe. I have had many turbos on this car and enough failures that I could have probably bought another 951. One of the turbos had the center cone spin off. I never figured out what exactly caused it but one of the theories was high boost with too much exhaust restriction. For the next turbo I redid everything on the intake and the exhaust. I've had the same turbo on now for 2 years problem free.

As far as how much boost you can safely run really depends on how much fuel you can put into the car, stock injectors won't cut it, mine are 83# but I would like to go even larger on these. You would need a wideband and some datalogging to be sure your AFR stays where it should. Too much boost and you will go lean which results in a blown headgasket...if your lucky. The stock computer does a good job at retarding the timing for more boost however higher octane is still a must. Really I don't think the stock head, studs, or anything else internally needs too many mods besides freeing up the air coming in and out of the turbo. The biggest mod I have done is the countless hours upon hours of fine tuning the fuel curve and it's still a work in progress.

toddk911

Has anyone tried different spring rates to see if there are any differences?

ENGINEERMAN

Hi guys:

I am running a Innovate Wide Band and the Mafterburner piggyback system and so can not only monitor the A/F but also map & adjust the A/F. The car has a stock head and has about 134,000 miles on it. The head was completely rebuilt about 5000 miles ago.

I think that the biggest issue at this time is that I have almost 34 psig in the cross over pipe running 18 psig of boost. According to Dave Lindsey he says that he would rather see a ratio of less than 1.5 : 1 and I am already pushing close to 2.0 : 1 and therefore can't see much if any benefit in going higher than 18 psig. Would you agree?

eniac

If your running stock injectors then no, definately do not go over 18psi. I have the stock x-over as well and can tell you there is a whole world of difference between 18psi and 27psi. Sure a ratio of less than 1.5:1 would be more effecient, lower EGT temps, and in the end allow for more power, but that doesn't mean raising boost above 18psi will do nothing in stock form.