Dump valves

[Rees4]

Like many of us S4 owners i sometimes feel that my car is not performing as good as its been on other occasions.I usually put this down to atmospherics and fuel and from one day to the next my car does feel slightly different.Without testing them,could my dump valves be on their way out?. I am concidering upgrading them to Forge or Bailey types, wots the best and do they make a difference?

[Dippy]

FYI they are not dump valves, they are recirculating valves or 'diverter valves'.

Failing DVs could indeed cause a loss of performance and risk early turbo failure, if they are leaking. On the other hand sticking DVs can cause hesitation. If you have the old OEM types then yes you should replace them.

I favour piston valves, and am a fan of Forge. That's not to suggest that they are technically better than their competitors (Bailey, Hyperboost etc.) but they are a UK company and have EXCELLENT customer service. You can service the DVs (I just did a 2-year service so it's not that much of a pain) and Forge give you the kit FOC.

The other option which is a very good one (but not to my taste), is to use the new OEM Bosch 710N DV. It is diaphragm, so still risks failure, but it appears to be a LOT better than the old type. It is also cheap, so you could replace them every year for 5 years for the same price as some piston valves (that's a guess - someone else may correct that duration).

Either way you should still test your valves periodically. Piston valves are not prone to tearing, so you maybe only need to test every major service. IMO if you have the 710Ns maybe you should test every 3 months. All you need to do is take them off, use a pen to depress the plunger, and hold a finger over the vaccum nipple for a few seconds. Then let go - if you hear the plunger snap back they're OK. If not they need replacing/servicing.

Having said all that, THE NUMBER 1 CAUSE OF PERFORMANCE PROBLEMS IS THE MAF. If you haven't done so already, do a search on "MAF" and have a good read.

Other items to check (search) are "TBB" and "F-hose"

[Rees4]

nice one dippy.

[JonnyX]

Whilst a good quality piston valve is unquestionably more durable you have to make sure it
is serviced regularly as they can stick or can get full of crap which may affect the sealing ability.
You also need to have the right spring - Forge are very good with advice in this area.

The major difference with piston valves is the response time (slower). However, I am not sure this
makes that much of difference (even on a race track).

There was an article, which I can no longer find, where some guy did an analysis of the various
different types of valves (response time, cracking pressure etc.) - anybody else still have
a link to this?

[Wak]

I dont agree that piston valve are slower, this may be of interest to you.
just an amateur opinion.....
http://www.tt-forum.co.uk/ttforumbbs/vi ... ve+testing

[Dippy]

DVs are a compromise: You want them to open fast to avoid boost spikes and turbo stall, but then you want them to close quickly to avoid hesitation.
There's no doubt that diaphragm valves open more quickly than piston types, however diaphragm valves in turn are more prone to sticking open because of air pressure. With a chipped car, this becomes a problem which is why many people, myself included, have gone for stronger springs in their DVs. Of course this means that there is less hesitation/better response, at the risk of increasing boost spikes: Compromise.

As I stated, I have gone for a strong spring piston valve. There is no chance of sticking and I get a good response: The DVs seem to open quickly enough to keep my turbos spinning. Most importantly there is a VERY low chance of a leak (compared to diagphragm types). My only issue is boost spikes, but at least I have an APR bipipe (I wouldn't trust a TBB).

[Wak]

There's no doubt that diaphragm valves open more quickly than piston types,.

Did you have a look at my link? Thers every doubt in my mind !

Diaphragm valves do not open more quickly, depending on spring they open later but always take longer vacuum range to open fully than a piston valve!

in reality we are talking milliseconds but from my tests if a Dia. takes 0.3 the piston take 0.1 relatively speaking.

[JonnyX]

http://www.s4biturbo.com/art-dvtests.php

For some contrarian evidence....

[Dippy]

Wak,

For your comparative test to be valid, you'd have to have matched springs in each valve.

Forget tests for a moment and just thing about the mechanics. A diaphragm is lighter so for identical spring resistance it will move more quickly than solid piston. Also the piston types tend to have two o-rings so have twice the frictional surface. Also I'm not sure that 'open fully' is a valid comparison. Air pressure will equalise before the valve is fully open. Beats me why some people have tested max flow rate with water!

[KayGee]

re: Beats me why...water

Like most flow tests, they are far easier to measure when done with water than with air. Most people don't have access to a flow bench, so tend to make their own.

I have to agree with Dippy, Pistons are slower, want proof go look at Forges website, diaphram valves for those who want fast operation times....

http://www.forgemotorsport.co.uk/conten ... duct=FMDVR

[Wak]

Wak,

For your comparative test to be valid, you'd have to have matched springs in each valve.

I do need to understand this more so any help is appreciated.

From what I understand the difference in the mechanical operation is what we are measuring hence why would you want matched springs? We are measuring the behavior of each DV setup against 1 constant. That Constant being the vacuum applied at the top of the DV be it a vacuum gun or a cars induction vacuum.

The construction differences are needed to examine the behaviour.

JohnnyX's link seems to verify what I have found, for a given vacuum a level of displacement occurs. Piston DV's displace more for less vacuum It doesnt seem to be contrdictory to my findings although his measured ranges are wider.


Surely this is saying the same as I am? what am I missing here?

From my results
Piston Valve starts to open at 4.5inhg and the piston is fully open at 5.5ingh
only take 1inhg to open

Diaphgram valve starts to open at 7inhg and is fully open at 11inhg
therefore it needs a higher vacuum to open and takes 4inhg to travel its range to open.

I didnt measure flow through rates this was just a pure vacuum test with perhaps some inaccuracies from the blow through check method but I am fairly confident that this shows a diaphragm DV opens later in the vacuum range and takes longer to open across its displacement.

I'd be happy to hear views on why this elementary level of checking may be wrong.

The boost pressure pushing may be a factor, but if you are using boost pressure to push then you are still creating a stalling effect on the turbo.

someone explain to me please???? I dont see a fault in my logic???

[Wak]

I have to agree with Dippy, Pistons are slower, want proof go look at Forges website, diaphram valves for those who want fast operation times....

http://www.forgemotorsport.co.uk/conten ... duct=FMDVR

KAygee, I'm happy to be proved wrong, but Forges website is going to be full of all the marketing that sells their products, and they do have good products.

But even they make mistakes, how many of you bought a DVR with its diaphragm valve on the fast promise / reliability premise.
How many dont realise the diagphrams were tearing regularly and they now have the polymer version that comes in the Optimus because of that problem.



Not trying to sway people from the marketing that has built their beliefs, just want to get down to a little bit of science of things to offer a comparative point of view and it may make you think about what you are buying.

p.s. if anyone wants to visit to try a valve out they are welcome to , trouble is I only have one of each so you need a single DV car to try them out on.... and it aint going to be mine!

[JonnyX]

Wak, what we are missing is rate of change of vacuum. It can't be argued that a diaphragm requires a smaller displacement to open
(i.e. less air has to move in the vaccum pipe to cause a certain travel, regardless of pressure). Now, I have no observations on this
(no boost guage or engineering experience) but we could imagine two polar scenarios.:

The rate of change of vaccum is (relatively) slow therefore the valve response to pressure is critical (the valves reponse to
pressure change dominates :- this is entirely dependant on spring rates and sealing)

The rate of change of vaccum is (relatively) fast (can be ignored) therefore the time taken for the valve to 'open' dominates
(this will be some compund of the distance the 'barrier' has to move to open and the velocity of the 'barrier')

Of course, purely hypothetical. However, some information on the rate of change of vaccum would maybe enable you to
characterize your observations more.

[KayGee]

Wak,

I think you are measuring spring rate+frictional losses. In terms of sensitivity and speed of response I believe you need to measure amount of vacuum in volume required to move the diaphram from one state to another. The inference is because you are moving a larger object it takes more volume to move it than the smaller lighter diaphram. This is how I understand it.

However I'm no DV designer either.

There are several issues regarding the DV as I see it.

1. Speed of response. The diaphram type is generally thought to be more responsive because it has much less mass to move. These offer better speed of response thus offering better throttle response.

2. Longevity. Bosch units are sealed units, diaphram tears, buy a new complete unit. At least partially answered by 710N as these appear to be vastly superior to the early units. Again DVR unit offers this ability as you can replace the diaphrams.

3. Tunability. Bosch diaphram units are fixed spring (except Forge DVR) so aren't as suitable for tuned engines being designed to work at @ 1bar.

4. Seviceability. Bosch unit is sealed, it stick or tears throw it away. Forge style, take it apart, clean it grease it, re fit it.

5. Cost. Bosch units are really cheap, Forge units aren't.

I have referred to Bosch and Forge units purely for ease of use, OEM style = Bosch, Forge = aftermarket DV. I also used DV to mean dump valve, our engines run re-circulating dump valves but they are still dump valves.

[Wak]

I see what you are saying... in essence the rate of change of vacuum from 0psi to 20psi vacuum is very fast. the opening
difference is too fast to notice. and the Piston valve may open quicker on the bench but it may be slower because of its mass against the diaphragm in the fast change over of vacuum, this I think I follow and can appreciate.

However it doesnt get away from the opening points.... so lets get rid of the changeover time and ignore that and assume the opening vacuums are the points they are fully open as the changeover is so fast.

I found that the 4-5inhg opening DV's felt better, the stiffer springs just made them worse.... once you hit 20+ ingh for an opening vacuum...... your car isnt developing 20inhg so the valve is not going to open.

4-7 is probably optimum, but I found 4.5 to feel the best on the road.

by the way Forge Optimus Diaphragm and DVRS Piston are both the adjustable valves they do.