300zxls1

iTrader: (13)

Hey I am new to the forum and just have a quick question about the advantage or disadvantages of Atmoshpheric dumps. I see it alot in the import crowd. I was curious if it was only done for sound or actually helps performance. I am hoping to in the rear future do a DIY truck manifold single turbo kit. I am just gathering all the info I can. I just thought if it would help perofrmance in some way I might do it. Also it would be easier to have it vent to atmosphere, sense you wont have to touch the downpipe to route the exhaust gas back in. Thanks for the help and sorry if this is a dumb question.

smokinHawk

iTrader: (48)

it does help performance some as it will have less backpressure and for a turbo thats always a good thing. less restriction on the wastegate so you may not have boost spikes, or not as bad.
its easier to route you dont have to worry about tieing it back into the down pipe.

cons
louder (unless you like louder)
depending on where you route it you might smell exhaust gasses.

300bhp/ton

Simple answer - NO advantage.

Basically ALL modern turbo cars (petrol NOT diesel) have a dump valve, you guys tend to call them BOV (Blow off valve).

However there are two types:
1. Recirculating
2. To atmosphere

They both do the same job.

When you have the throttle open and boost is being produced, a lot of high pressure air is in the air intake. As soon as you lift off of the throttle you close the throttle blade in the TB.

This traps the high pressure air between the turbo and the throttle body. This is not exhaust gas, but fresh air from outside which has already been compressed.

This trapped air helps to stall the turbo, i.e. stops it spinning. This is BAD. Because when you re-open the throttle there is a LAG time before the exhaust gases can spin the turbo fast enough to once again produce boost. This is TURBO LAG.

Lag is often confused with where in the rpm range boost is achieved, this is not LAG but something else. - A common mis-conception.

A dump valve releases the trapped high pressure air between the throttle body and the turbo, when the throttle blade is closed. This helps to prevent turbo stall and thusly will reduce turbo LAG and improve throttle response.

A recirculating dump valve vents the high pressure air back into the air intake, and thus recirculates it once the throttle is re-opened.

A to atmosphere simply dumps the high pressure air to the outside with a very audiable 'psssshhht' sound.

If anything the recirculating type is probably better, but only becuase it creates a higher pressure area on the other side of the turbo, which should again help improve turbo spool time. This however is very marginal.

It's worth noting that a dump valve DOES NOT make any more power. And DOES NOT eliminate turbo LAG. The biggest contributor to turbo LAG is the reduction in exhaust gas flow. This is proven because ALL anti-lag systems (ALS), such as the 'bang bang' system, directly address exhaust gas flow. ALS systems are not a sensible option for road cars. Too expensive and too damaging to the engine/turbo.

In addition, turbo diesels also suffer turbo LAG, yet they do not have a throttle body and throttle blade, so there is never any high pressure trapped air to worry about. A dump valve would have no function on a diesel engine, and would have to be engineered to even get it to make a sound (such setups exist, they are expensive and just make noise. For posers and nothing more).

As for preference between the two. Well that depends whether you like the sound of a dump valve or not. Personally I don't. Here in the UK the only people with dump valves are CHAVS (very similar to Ricers). They are basically scum, who take a mass produced hatch back such as a 134bhp Ford Escort RS, with a 1.6 litre turbo engine or a 1.4 litre Renault 5 Turbo, they slam it too the ground and add 19" wheels so it not only has a terrible ride, grounds out constantly but also corners far worse than stock. Add a 4-5" exhaust, paint it in some garstly colour, add some subs and a wing and get a dump valve. They then think it's the best thing in the world and is faster than every Ferrari ever made.

I admit US culture is different. But in the UK, the only people who think to atmosphere dump valves are cool, look like this and drive cars like this:

DarkblueTA

I have a DIY rearmount kit like Zombie's and my waste gate is dumped fwd towards the exhaust pipe(like a 1.5" gap between the two exhaust parts). I was thinking about making it dump into the pipe but after some testing, its not loud at all for some reason and just sounds like the car has a flowmaster muffler on it..so I think I'll let it alone for now. Just my $.02

300zxls1

iTrader: (13)

Thanks smokinHawk and DarkblueTa. You cleared it up for me. As for 300bhp/ton I think you misunderstood what I was asking about. I am not talking about Blow off Valves that recirc or vta. I was talking about wastegates that dump to the atmospher instead of runing it back into the downpipe. From what smokinHawk said it will give less restriction and less restriction is always good for turbos. I think I will end up doing a atmospheric dump with the wastegate on my DIY turbo setup cause it will save some welding and time and will help exhaust restriction. Everyone thanks for the help.

300bhp/ton

my bad. gess I should:

a) read the post better
b) have Captin Subtext activated

JZ 97 SS 1500

iTrader: (2)

Venting to atmos. will help very little since you are running a T4 and sounds like poo.

JZ