I have experienced knock in the past fom the TXS RFL blow off valve when it was sprung lightly with a custom spring. Why it knocked at part throttle while venting I cant say. So more recently I went back to a stock Evo IX BPV with a restrictor in the bottom port to provide boost spike-blow open resistance. How well that worked I really cant say, I didnt do enough testing. Even more recently I went from those HKS 272 cams on m EVO IX, to something a bit more nutty- GSC S2. I went to a Greddy Type-s BOV{single spring mod per evomods} on a STM UICP the same time I up rated my cams and intake manifold. Ever since tuning the S2's Ive had a lot of low end spool knock. light throttle acceleration and every thing you could have in a nightmare of being a rookie self tuner. I recently had a thread on a problem of hesitation/breakup i was having. I had one sentence in that thread that said Id try the secod spring in the blow off valve, specifically the smaller one in addition to the bigger one.

Well, coincedently the knock has subsided ALMOST 100% in transient states of throttle. The only downside is BOV surge is a little easier to induce at low rpm/load because most likely the difference between pressures is not enough to cause a big opening moment to the BOV. I still get a full vent from WOT greater than 3500/4000.

To me this sums up that cams are a give take setup. You want them but there is some inherent supporting sacrifices to be known, and having a blow off valve work with low vacuum and high boost is a good step from stock cams and decent boost levels.

Keep in mind YMMV.


PS. Clearly a situation where dejon tools setup would be helpful. Now that I have two springs in my TYPE-S I will feel more comfortable using the TYPE S bottom port with a possible valve.

the bottom port of a blow off valve or bypass valve is clearly necessary to me- 94awdcoupe, 240ztwinturbo among others have noted in my forum lurking the necessity for this. I do intend to create a local reference port on my UICP for this reason. And add a posibly add a control method.

 

good luck with the knock. i feel lucky that it hasn't been a problem for me with my cams.

i've tried the dejon tool stop leak kit. great for wot, nasty stutter at on/off transition to change bottom port pressure. bite the bullet and get a synapse dv or cobb dv.

 

Is the car simply getting a rich misfire and the knock sensor picking it up? Then you pull timing and add fuel thinking it is typical knock, accentuating the problem?

Cams can dramatically change low load tuning due to the added overlap. This can be a big problem on MAP based systems as the low vacuum signal can also be very rough and depending on when the ECU measures MAP relative to crank position, it can have a significant impact on fuel and timing.

A change to a larger cam usually means it needs more timing and less fuel under low loads to prevent poor drivability.

 

Mrfred maybe knowledge has allowed you to avoid what 03whitegsr has suggested. My low load low rpm has been tricky and inconsistent with my fueling. I might try extending the cl transition out a little farther for light accel.

I was definitely pulling timing beyond reasonable amounts and scratching my head.

I will get better results and redo the single spring mod to see..

One thing I did do was use a vibrant 5/16 id heavy wall with the brake booster barb on the type s and the intake mani.