Nigel Posted June 25, 2007 Share Posted June 25, 2007 So, I decided to roll the dice and try one of those ebay knockoff Greddy Type RS blow-off valves. I bought it a year ago, but I finally got to try it out this past weekend. Quality wise, it “looks†great, and it came as a complete kit with gasket, 2 metal tee’s and silicone vacuum line. The anodizing looks good, but I’m not sure if the piston is hard anodized. Anyway, performance-wise, it’s a mixed bag. It has no problem holding full boost (15psi) but only up to 4th gear. If I floor it in 5th at 3000 rpm, I can’t boost past 10psi! As soon as I get close to 10psi, the BOV starts fluttering, the boost gauge needle bounces around, and I can’t get anymore boost. So it’s obviously leaking. But why only in 5th gear? Full throttle in 4th at 3000 rpm and I get 15psi no problem, even when I try building boost slowly in 4th (although, maybe not quite as slow as it builds in 5th at that rpm). Now, mind you, if I back off the throttle just a bit in the gears other than 5th, but still making boost, the BOV immediately starts fluttering, and it won’t stop until I completely back off. Adjusting the preload on the diaphragm spring doesn’t seem to make much difference, other than muting the effect somewhat. I don’t really know much about pneumatics, but it appears that the BOV requires a very minor pressure differential between the intake manifold and the intake plumbing where the BOV is located for the BOV to open, even with the diaphragm spring preload set really hard. Does anybody know if this has something to do with ratio of the diaphragm diameter to the piston diameter? But why would I get a pressure differential at full throttle in 5th but not 4th? And why doesn’t the spring preload have much effect? I’m going to order a genuine Greddy BOV, but I’d really like to understand what’s going on here, because I’d hate to spend the extra money only to have the same problem because there’s something else I missed. Any insight would be greatly appreciated. Nigel '73 240ZT Quote Link to comment Share on other sites More sharing options...
Clifton Posted June 26, 2007 Share Posted June 26, 2007 Tigthen it up a little and try it again. Are your running a turbo larger than stock? It sounds like compressor surge, low rpm, low flow, medium boost. It woudn't happen with a T3 though. Quote Link to comment Share on other sites More sharing options...
Nigel Posted June 26, 2007 Author Share Posted June 26, 2007 Hmmm... compressor surge? It's a T3/T04 turbo, so it's not stock, but I can't remember the specifics (T04 "b"?), and I can't find the receipt at the moment. I bought it years ago, but up to this point I never ran it at more than 10psi, because I didn't have an intercooler. I never had any issues in the past. I installed an intercooler and the new BOV (I was using a 1G DSM BOV previously) last year, and this past weekend was the first time I actually got to try everything out, so I bumped the boost up to 15psi, and that's when the problems started. It never occured to me that I could get surging at low boost and low rpm, but it makes sense. I tried tightening up the screw on the BOV, and it didn't really seem to help much. I know this turbo always seemed to be really slow at building boost. Back when I was running ~9psi, If I floored it in third gear at say 2500 rpm, I wouldn't hit 9psi until around 37-3800 rpm. So, what does all this tell me? I'm having a hard time differentiating between the sounds that the BOV is making and what could be surging. Is this BOV really the issue, or is my turbo not sized properly? Or do I not have something set up properly? This is new territory for me, so I don't have any experiece to fall back on. I hope I don't end up spending the whole summer trying to sort this out! I didn't get to drive the car at all last year, and I was really looking forward to just enjoying driving it this summer, not spending all my time troubleshooting. Any advice on how to proceed? I guess I could remove the BOV and try boosting in 5th gear again. But if it surges, then what? New turbo? Thanks!!! Nigel Quote Link to comment Share on other sites More sharing options...
Clifton Posted June 26, 2007 Share Posted June 26, 2007 I'm sure your bov is fine. I have a Greddy type R. I had to trim some of the spring to get it to even open at low vac/boost when the throttle closed. Now the screw is only turned in about 4 turns. You can really crank it down and keep it from ever opening if you wanted. If it was leaking you wouldn't here a flutter follwered by the fluctuating needle. It would just vent and you would make boost. If it surges at low rpm, don't boost so much there. You really don't need 10 psi at 3k in 5th. Just drop a gear so the engine is using more air and it won't surge at that pressure or back off a little so it's under the threshold. The turbo (TO4B) on my 71' surges and so does the T66 on my 73' if I try boosting too much at lower rpms. It's easy to avoid. You could go largher on the hotside. It would slow it down and make a little more power but it would be easier to just get used to it and keep it were it needs to be to avoid it. Also surge is hard on thrust washers. They are cheap and easy to change but I would keep doing it on purpose. Quote Link to comment Share on other sites More sharing options...
Nigel Posted June 26, 2007 Author Share Posted June 26, 2007 This has been a big help. Thanks! The more I think about this, and the more I read, I realize that I've been mistaking compressor surge for the BOV leaking. Like I said, this is all new to me, and I'm learning as I go. I'm not going to bother ordering another BOV now, and I'm just going to readjust the one I have. I think I have it set too tight, believing it was leaking. I remember trying to push the piston in with my thumbs when I had it off the car, and it was really difficult unless the adjustment screw was backed almost all the way off. Consequently, it never made sense to me that it could be leaking. So, maybe these knockoff BOV's aren't so bad. Mind you, I didn't buy the cheapest one I could find, but it was still a fraction of the cost of the real deal. If it lasts me a couple of years, I'll be happy. I'm sure I can work around the surge limit in 5th. I'm not very happy that it's happening, but I'm not about to replace the turbo. Now if I could only figure out why it's so slow to build boost... Any ideas on that? Thanks again!!! Nigel Quote Link to comment Share on other sites More sharing options...
oinojo Posted June 26, 2007 Share Posted June 26, 2007 Exposed - Knock Off Parts On the outside, the two adjustable GReddy Type-RS blow-off valves look the same, but inside, they are vastly different. The difference in diaphragm seal design and construction is obvious. The fake valve's seal is as thin and fragile as it looks, and has begun to rip already. View Related Article - interesting article i stumbled upon while researching those cheap knockoff blow off valves and other "performance" material. Definitely backs up the saying "you get what you pay for" Quote Link to comment Share on other sites More sharing options...
Tony D Posted June 26, 2007 Share Posted June 26, 2007 Curiously, it is strange they would cut costs on diaphragm used. Doing a flat diaphragm when the rolling hat style is in the original style is ludicrous! I can state for the record, a flat diaphragm is NOT designed for a long-stroke actuator. That means less valve lift. Atlas Copco used a rolling diaphragm of about 6" diameter in their loading valve, and when they redesigned for a flat diaphragm doing the EXACT same job, they went to almost 18" diameter to get the same piston travel. The only thing I would do if I bought a Greddy Valve is disassemble it, and greas the heck out of the rolling diaphragm with the old tube of Atlas Copco Graphite Grease I still have laying around. That stuff greatly increases the lifespan of that particular style of rolling diaphragm. Talcum Powder is another good idea. The diaphragm will roll on itself and expose/fray the reinforcing strands, eventually leading to failure---which is why they went to the flat style like they'd used in their smaller, cheaper machines for years. To fully disclose, I am a pirate. Yes, I left Atlas Copco to work for a competitior who now manufactures their own line of parts for the Atlas Copco machine. I was instrumental in quality control and making sure our stuff in the beginning was up to snuff so we could really do damage to their aftermarket parts sales....not give me a raise, eh? We'll see who's worth what... I digress... Quote Link to comment Share on other sites More sharing options...
Clifton Posted June 26, 2007 Share Posted June 26, 2007 If you are easily making 10 psi by 3000 rpms, it sounds like it's spooling just fine. If it spooled any faster you would just have more problems. How fast do you make full boost at 4500? So you have an idea. I have my BOV set soft enough that it will open just a little on an idle rev (felt with my finger on the valve). Even that soft it has no problem holding 30 psi. Quote Link to comment Share on other sites More sharing options...
Tony D Posted June 27, 2007 Share Posted June 27, 2007 So you have an idea. I have my BOV set soft enough that it will open just a little on an idle rev (felt with my finger on the valve). Even that soft it has no problem holding 30 psi. The old Cartech compressor bypass valve would hold 20psi no problem and lifted fully and completely at any lift of throttle giving a 'sigh' sound and not the current-rage psh-h-h-t-t-t! Like Clifton said, it would hold anything I threw at it, and lifted even when at idle. Matter of fact, at idle, under vacuum, it was OPEN. Let air bypass the turbo directly to the plenum (throttle plates downstream of the plenum on this application) helping spool even more by letting the turbine spin up without all the air load on it in the first place. First hint of boost, and that valve slammed shut, and stayed shut till I lifted my foot. Quote Link to comment Share on other sites More sharing options...
Nigel Posted June 27, 2007 Author Share Posted June 27, 2007 - interesting article i stumbled upon while researching those cheap knockoff blow off valves and other "performance" material. Definitely backs up the saying "you get what you pay for" Yeah, I came across that article yesterday. No great surprise. I figured when I bought this thing that the diaphragm would be the weak link. We'll see how long it lasts. I just wanted something that would last me for a while until I can affort the real deal. Who knows, maybe I can get a few years out of it... Nigel Quote Link to comment Share on other sites More sharing options...
Nigel Posted June 27, 2007 Author Share Posted June 27, 2007 If you are easily making 10 psi by 3000 rpms, it sounds like it's spooling just fine. If it spooled any faster you would just have more problems. How fast do you make full boost at 4500? So you have an idea. I have my BOV set soft enough that it will open just a little on an idle rev (felt with my finger on the valve). Even that soft it has no problem holding 30 psi. I'm reviewing one of my data logs from the drag strip this past weekend, and I'm at 0 psi at 2700rpm and 12 psi by 3700 rpm at full throttle in third gear. That's with me taking it really easy in 1st and 2nd, and pausing before I floored it in third. At full throttle in 4th (with no pause shifting out of 3rd), at 4200 rpm I'm at 0psi and I'm at 12 by 4400 rpm. How do those numbers sound? I'll play with the BOV more this weekend. I think I need to back the adjustment screw right off. Like I said, I probably have it set too tight. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.