Bypass question

[awr68]

Ok, I'm confused now. Is Troy saying that having it near the TB is really great...but it should go between the SC and IC to help lower heat build of the IC?

[Blown353]

Prior to the TB is the ideal location from a flow/pressure standpoint as you dump all the pressure at the point of restriction (throttle body)... but putting it before the IC in a centrifugally supercharged application is ideal for minimizing heat load through the intercooler in off-boost conditions.

For a centrifugally blown car I would place it prior to the intercooler to minimize the cruising heat load-- that's more important to me. It still works just fine there. That's where I put mine when I added the intercooler.

For the water/air car I helped setup & tune the shop first put the bypass before the TB; I had them move it to before the IC. Moving the bypass made a ~15F difference in the intercooler water tank temperature during a steady freeway cruise as the intercooler was no longer having to cool all the bypassed air with the bypass before the intercooler.

[awr68]

Thanks Troy...I understand now!!!

[Kris Horton]

Thanks for clearing that up Troy. I just kind of assumed 69Bird has a centrifugal blower, didn't think to ask.

Maybe, while we're on the topic of bypass valves, Troy can answer a question of mine.

I'm running the basic Pro-Flow that ProCharger sent to me (the back plastic one with a small filter mounted over the butterfly) and was wondering if other bypass options would work alright with my kit. I know ProCharger also sells the large red one and Paxton sells one similar to it. Is there any benefit to running a larger bypass at only 9psi or do they not become a necessity until I up the boost to 12-14psi or more?

[Blown353]

Kris, depends on the CFM being moved, boost level, and volume that needs to be "dumped" when you close the throttle. The smaller valves can't move as much volume as quickly as a larger valve and you might get compressor surge. Also the poppet-type valves tend to react faster than the butterfly style valves.

Rev the car really high and snap the throttle shut... or take it up to redline in a gear then leave it in gear and just take your foot off the throttle. If you don't get a chattering sound (usually sounds like "cha-cha-cha-cha-cha") then you aren't getting compressor surge and don't need a bigger (or faster acting) bypass.

The smaller black plastic Bosch bypass valves work well for lower volume / lower boost applications. A single Pro-Flo valve seems to work well up until around the 700-750hp level (around 1250 cfm or so) and then you either need to run two of them or step up to a larger poppet style valve.

The poppet style valves (i.e. red Procharger "Race", Vortech Mondo) are fast responding and move a large volume of air... that's good. Unfortunately they are also really loud. On the air/water car I did it used an F1R headunit and a Procharger open-style Race valve... in the car all you really hear is the damn air coming out of the bypass valve. It's really loud and very obnoxious. Probably would have been better off using the hose-connection style Race bypass and running it to a large breather or something similar to help muffle the noise.

I run two of the Procharger "Pro-Flo" valves on my car. I used to run one and it worked fine with my older P1SCH and non-intercooled. Once I changed to a D1SC I started getting very light compressor surge above 5300rpm or so. Then once I added the intercooler it got real bad... the single valve just couldn't dump all the pressurized air fast enough during shifts and when I'd let off at high-RPM it started to get pretty severe compressor surge. The intercooler & added piping meant a much larger volume of air needed to be dumped at every throttle closing so the system needed a larger "vent" to prevent compressor surge. So I added a 2nd Pro Flo bypass. It was cheaper than buying a Procharger Race bypass (or a Vortech Mondo) and 2 of the Pro-Flo's are fairly quiet. A Procharger "Race" bypass is LOUD. No more surge issues.

[69bird]

Ok one last question. I'm thinking about running the discharge of the bypass valve into the intake of the s.c. it was pretty loud before. In this case i'm thinking its going to be better to mount the bypass valve after the i.c.?

[Blown353]

You can recirculate it without issue. Besides quieting things down a bit it keeps the air filter cleaner longer.

I would still put the bypass before the intercooler to keep the heat load off it.

[rwhite692]

Troy thanks for the "real-world" info regarding the noise issues on the various types of bypass valves, I will be running two (twin turbo application) and I will definately consider the hose-connect type and some sort of breather arrangement...-Rob

[Blown353]

Originally Posted by rwhite692

Troy thanks for the "real-world" info regarding the noise issues on the various types of bypass valves, I will be running two (twin turbo application) and I will definately consider the hose-connect type and some sort of breather arrangement...-Rob

If it's turbo they're blow-off valves and not bypass valves. They are similar in form & function but the opening/closing conditions are different.

For a bypass valve (supercharger) they are open as long as there is some manifold vacuum, usually 2-3 in/hg. That way as long as you are at light load it is open, dumping the air moved by the supercharger and taking a load off the supercharger and saving a few HP. When you step on the throttle and vacuum drops, manifold vacuum (running to the diaphragm on the valve actuator) is no longer enough to hold the valve open against the spring and it closes and boost starts building. The boost is also fed to the diaphragm and gives it additional holding power. Once you let off the throttle and manifold vacuum returns the valve re-opens and stays open as long as vacuum is sufficient to overcome the spring pressure.

For a turbo, the blowoff valves aren't open constantly under cruise like a centrifugal. They are normally closed. They only open when they see a pressure differential between the turbo plumbing and manifold vacuum so they only open when you close the throttle under boost (preventing compressor surge) and then they close back up. They're normally poppet-type with an adjustable spring and a line from the diaphragm that runs back to the manifold. The spring holds them closed under manifold vacuum only as manifold vacuum alone isn't enough to overcome the force of the spring. Step on the throttle and build boost and you now have pressure in the intake piping trying to push the valve open against the spring, however you also have manifold pressure now fed to the diaphragm trying to push the valve closed plus the pressure of the spring. Thus the valve stays closed and it builds boost. However, when you're on boost and let off the throttle the combination of boost in the piping pressing against the poppet plus manifold vacuum now on the diaphragm side (from closing the throttle) is enough to open it to vent the intake pipes to prevent compressor surge. Once the differential across the valve drops (because the boost in the intake piping has decreased back to a cruise level) the valve closes back up again. Adjusting the spring on the blowoff changes how early and how much it will open, adjust it too much to the "weak" side and the spring pressure is no longer enough to hold it closed under manifold vacuum and it will stay open all the time like a bypass valve.

Basically, a turbo blowoff will only be noisy when you transition from boost to vacuum (i.e. letting off the throttle when under boost, either to slow down or shift.) They won't be loud like a centrifugal bypass because they aren't letting air out all the time, only during transitions from boost to vacuum... and turbos don't flow enough air under cruise conditions to need a bypass or even then make a lot of noise out of them (unlike a centrifugal supercharger.) The more CFM you are flowing and more boost you're making, the louder they get during shifts/letoffs. If you want to quiet them down some you can recirculate them back into the airfilter/inlet side of the turbo.

You can recirculate blowoff valves too, it makes them quieter. It's also pretty much mandatory on a MAF based computer as the computer has already accounted for all that air. Dump it to atmosphere and the car will go really rich on shifts... to the point you can get fireballs out the back. Pretty commonly seen on turbo imports where guys have "upgraded' to a dump-to-atmosphere blowoff valve, removed the catalytic converters, and didn't retune for it. The car goes pig-rich on the shifts because of the blowoff dumping to atmosphere and the unburned fuel ignites once it gets more oxygen when it leaves the tailpipe.

[Kris Horton]

So Troy, you're basically saying that if I toss an HKS blowoff valve on my blower, I can expect pyrotechnics out the tailpipes? *calls JC Whitney*

All kidding aside though, I have seen a HKS Blowoff Valve used on a ProCharger (it was a V6 late model Mustang) and it seemed like the compressor was surging. It sounded kinda cool, but definitely not worth ruining your blower or your engine for a simple novelty.