stupid BOV Q's?

[the_anti_honda]

ok so i have a stock 03 EVO VIII. just purchased it and decided to swap my Greddy type-s on from my GSX i figured my gsx runs about as much boost as the EVO does stock so i should be able to swap it right on and it will work near perfect.... Wrong! put it on and no matter how tight i make it or how loose the damn thing flutters or make no noise at all. thats not the biggest problem actually i could care less if it makes noise or not although its disapointing. the car seems slower and will not idle right. it also wants to die when i push in the clutch at a stop sign. WTF is going on and why does this BOV suck ass all of a sudden.

[DTunedEvoX]

Have you done the "Single Spring" mod to your Greddy Type-S? Might wanna run a search on how to do it .. then try ...

[kimletrim]

Yeah, do the single spring mod and make sure you recirculate and not VTA. You can experiment with VTA but make sure you have both springs in the their. But the Type S will work.

[Blaze]

First let me say that I am not now, nor due I have any future plans to sell any performance car parts, this info is just FYI gleaned from my own garage tinkering. When I initially read posts here about the blow off valve possibly leaking and costing HP it sounded like BS to me, but after doing my own testing it seems to be true!

Before explaining my testing methods and results, I will give you my take on blow off valve operation and function, (if you’re already up on this you may want to skip this paragraph). A blow off valve basically has a spring-loaded valve that is controlled by a piston type actuator. The actuator is driven by the intake manifold pressure/vacuum located after the throttle body. In this way under acceleration the manifold pressure combined with the actuator spring tension keep the valve closed. When you lift off the gas and the throttle plates close, essentially removing the turbo pressurized air from the motor intake, a vacuum quickly develops after the throttle body. This vacuum is routed to the actuator piston causing the valve to open providing an alternate path for the turbo pressurized air. The blow off valve prevents a pressure surge in your turbo system by giving the pressurized air a place to go when the throttle plates are closed. As I understand it this valve function has two benefits; for one, surging could potentially damage your turbo and two the surge could slow the turbine causing more turbo lag.
So the ideal blow off valve would have a large enough opening to properly vent the turbo pressurized air, and it would open immediately after the throttle plates closed (you lift off the gas) and would close utterly and completely the nanosecond that you step on the gas.

Using two air compressors, a hand actuated vacuum pump(with a gauge), and a bunch of hoses and fittings, I began testing the characteristics of the stock blow off valve. Simply putting air pressure on the valve without applying similar pressure to the valve actuator will not give you an accurate indication of what the valve is doing in the car under acceleration. The stock blow off valve does have a fairly heavy spring loading and the actuator piston is larger than the valve area, giving it an appropriate mechanical advantage, so far so good. So I apply 19 PSI to the valve and the same to the actuator, now with the actuator pressure and added spring tension this valve is as closed as it is ever going to get, but it still leaks. The problem is with the valve/valve seat, it is just plastic on plastic and it simply does not make a good seal. While the leak is not huge, it is more than you might think (I would say comparable to a3/8” hole in your intercooler). After seeing this I decided to get an aftermarket valve and do more testing.
Most after market blow off valves have are larger valve area (good so far) and generally use a much heavier spring loading, this last part is good in that it should let the valve close faster when you step on the gas, but it also means the valve will not open as fast when you lift off the gas. This last part could cause some surging/drivability problems at part throttle, especially in a system like the Evo using an air flow sensor. From my own engine tuning experience, most engine hesitation and bucking is cause by the engine running to lean.
Consider this scenario, you’ve got your after market valve with a very heavy spring tension so it won’t open easily especially at lower engine rpm’s where the vacuum level may not be enough to overcome the spring tension. Under part throttle you lift closing the throttle plates but the blow off valve does not open (or opens but not enough) so you get a minor pressure surge. This surge causes the airflow meter to see even less air flow than it normally would using the stock blow off valve, even though the engine is still getting the same amount of air the computer starts delivering less fuel than is really needed in this situation and you run lean. This is what causes the engine to stumble and hesitate. My thinking at this point is if you get a valve where the actuator piston surface area is larger than the valve area you should not need to use a heavy spring tension to get the valve to close quickly and completely.
So now I go to my local after market supplier buddy and tell him that I am looking for a blow off valve, but it needs to have dimensions similar to the stock Evo valve, I want the piston actuator to be bigger than the valve area, oh and I would like it to have some sort of rubber o-ring seal for the valve. He tells me that I just described the Greddy type-s, I look at it, it’s kind of pricey but it did meet all of my criteria, so now it’s back to the garage for more testing.
I had to make my own hose adaptor but I am sure someone out there makes the right adapter for this car. Anyway the valve is bigger than the stock unit, good so far, accept that the actuator fittings were leaking a bit, I put some Teflon tape on and tightened them, no more leaks. This valve seals completely even with as little as 6 PSI on the actuator and 20 PSI on the valve, absolutely no leaking (excellent!). Next is the vacuum opening test, the stock valve starts opening at around 16” of vacuum, even with the spring tension adjustment all the way out the Greddy needed almost 22” of vacuum (this could be a problem). I install it as is and sure enough there is increased hesitation, in fact coming to a stop the engine almost wanted to stall a couple of times. The nice part about after market valves is that you can take them apart and easily rebuild them, so I decide to take it apart and see about getting a softer spring for the actuator. I get it apart and low and behold there are actually two springs (one slightly smaller sitting inside of the bigger one) pushing on the actuator, and it looks like you could easily use just one. I did testing using each spring by itself, one is heavier than the other, with the smaller lighter spring the valve seals with 20PSI on the valve and 14 PSI on the actuator (that works) and now the valve starts opening with only 8” of vacuum on the actuator(sweet!). With the larger individual spring the valve remained sealed with 20PSI on the valve and 12PSI on the actuator, under vacuum the valve starts opening at 12” of vacuum (still better than stock). I decided to go with the heavier of the two springs since even it opened sooner than the stock unit.
I install my slightly modified valve in the car and went for a drive and all I can say is wow! Talk about smooth, the car drives even better at part throttle than with the stock valve, very cool. Even stock I noticed the part throttle drivability was a little jerky once and a awhile, this new valve seems to smooth that out quite a bit. By putting this valve in I essentially richened up the part throttle fuel a little.

On the Greddy valve there is another fitting underneath the actuator, all I can figure is that you could connect it to the manifold before the throttle body so that the pressure differential would allow the valve to open sooner even with the heavy spring loading. I have to strongly advise against this though because with that configuration under acceleration/boost you would have the same pressure on both sides of the actuator piston, then the only thing keeping the valve closed would be the spring tension. Even with both springs installed it is not strong enough to keep the valve sealed at 19 PSI. This fitting does see the same air as the valve when it is open so I put a rubber vacuum cap on it for good measure. EDIT: better to leave the cap off, it works either way but...

I could not tell from my butt dyno if the car was actually making any more power with this valve but it certainly drove better. A few day’s later I went back to my local dyno were I had already done some initial testing (see initial testing thread)

http://forums.evolutionm.net/showth...ht=MUSTANG+DYNO .

I had a few items to do some testing on (I will post separately on those later), one of course was the blow off valve, and now here is the best part! When compared back to back runs with the stock blow off valve the dyno actually showed an increase of about 3 HP with my modified Greddy valve! So here’s a modification that improves drivability, power, and possibly reliability since the turbo does not have to work as hard to produce the same boost. Like I said I am not selling anything here, I am sure there are probably several valves that would work just as well, given they meet the above mentioned criteria and you get the spring tension right.

One other note regarding the valve modification, when you take the actuator cap off it is spring loaded, so when you get to the last one or two screws make sure you hold it good so that everything doesn’t go flying, it’s really not that hard to do assuming you have at least some mechanical ability.

I have gotten some good info off of this site, I hope this helps you guy’s out.

Eric

[the_anti_honda]

thank you very much for the helpful information. i was not aware there was a spring mod for the Greddy Type-s and the symptoms explained my car exactly.

this forum has been nothing but help and everyone has been cool as hell. not like other DSM related forums that im not going to mention or bash. i guess us EVO driver are a diffrent breed.

[Blaze]

thank you very much for the helpful information. i was not aware there was a spring mod for the Greddy Type-s and the symptoms explained my car exactly.

this forum has been nothing but help..

That's what we're here for.
There's no shortage of cool peeps here.