BOV/Dump Valve on Carbbed
Discussion
A fellow carbbed TE owner and I were having a discussion about blow off valves. I told him I didn't think the Esprit had one, but he was sure it exists.
I double checked, and indeed there is a dump valve in the diagram. He also copied an earlier response on this forum (from Jim) regarding the whole BOV vs. dump valve. What confuses me is that at least one person added a BOV to his carbbed car.
A typical BOV (not dump valve) works partially by the spring (which can be much weaker than the one in the dump valve), but more importantly by the differential pressure between the front and back of the butterfly on the throttle body. The pressure from the turbo (the front side of the throttle) pushes against the valve door. On the back side the pressure from behind the throttle and the spring keep it closed (so the effective pressure is null, and the spring can easily keep the valve closed.) When the butterfly closes, there is a large pressure differential between the front and the rear which snaps the valve open, and keeps it open until the pressure between the front and back of the butterfly become almost equal. This is all goodness saving your turbo and limiting lag.
Is there a way to get the pressure from behind the butterflies on a carburetor? The fact is that there are four butterflies, and whatever is behind the butterflies is a mixture, sounds like a mess that would need some clever engineering to solve.
I was thinking of maybe creating a switch on the throttle that would operate a solenoid that would either feed the BOV pressure or atmosphere (set it up so that the BOV would see atmosphere at 0 throttle.) Is this how others have set it up?
As a secondary note. What is the dump valve's pressure set for? Is it 8psi?
I double checked, and indeed there is a dump valve in the diagram. He also copied an earlier response on this forum (from Jim) regarding the whole BOV vs. dump valve. What confuses me is that at least one person added a BOV to his carbbed car.
A typical BOV (not dump valve) works partially by the spring (which can be much weaker than the one in the dump valve), but more importantly by the differential pressure between the front and back of the butterfly on the throttle body. The pressure from the turbo (the front side of the throttle) pushes against the valve door. On the back side the pressure from behind the throttle and the spring keep it closed (so the effective pressure is null, and the spring can easily keep the valve closed.) When the butterfly closes, there is a large pressure differential between the front and the rear which snaps the valve open, and keeps it open until the pressure between the front and back of the butterfly become almost equal. This is all goodness saving your turbo and limiting lag.
Is there a way to get the pressure from behind the butterflies on a carburetor? The fact is that there are four butterflies, and whatever is behind the butterflies is a mixture, sounds like a mess that would need some clever engineering to solve.
I was thinking of maybe creating a switch on the throttle that would operate a solenoid that would either feed the BOV pressure or atmosphere (set it up so that the BOV would see atmosphere at 0 throttle.) Is this how others have set it up?
As a secondary note. What is the dump valve's pressure set for? Is it 8psi?
DrieStone said:
A fellow carbbed TE owner and I were having a discussion about blow off valves. I told him I didn't think the Esprit had one, but he was sure it exists.
I double checked, and indeed there is a dump valve in the diagram. He also copied an earlier response on this forum (from Jim) regarding the whole BOV vs. dump valve. What confuses me is that at least one person added a BOV to his carbbed car.
A typical BOV (not dump valve) works partially by the spring (which can be much weaker than the one in the dump valve), but more importantly by the differential pressure between the front and back of the butterfly on the throttle body. The pressure from the turbo (the front side of the throttle) pushes against the valve door. On the back side the pressure from behind the throttle and the spring keep it closed (so the effective pressure is null, and the spring can easily keep the valve closed.) When the butterfly closes, there is a large pressure differential between the front and the rear which snaps the valve open, and keeps it open until the pressure between the front and back of the butterfly become almost equal. This is all goodness saving your turbo and limiting lag.
Is there a way to get the pressure from behind the butterflies on a carburetor? The fact is that there are four butterflies, and whatever is behind the butterflies is a mixture, sounds like a mess that would need some clever engineering to solve.
I was thinking of maybe creating a switch on the throttle that would operate a solenoid that would either feed the BOV pressure or atmosphere (set it up so that the BOV would see atmosphere at 0 throttle.) Is this how others have set it up?
As a secondary note. What is the dump valve's pressure set for? Is it 8psi?
Hi,
The dump valve which is standard to the Lotus is just a dumb PRV (Pressure Relief Valve). It's purpose is to vent the pressure spike created when the throttle plates are closed while the turbine is still spinning. This allows the throttle plates to close naturally without a pressure imbalance and prevent a shock wave back to the turbine. It's opening pressure is determined by a calibrated spring. I do not know what pressure it is calibrated to, except to say that I know mine does not open at less than 12PSI (My boost is increased to 10PSI and I have seen 12 on sudden shifts at WOT without the valve opening). If I had to guess, I would think it triggers between 14-15PSI.
A true BOV is throttle dependent activated either by vacuum pressure or an electrical solenoid prior to any pressure spike being created. It is proactive as opposed to the reactive dump valve.
There are a couple ways to rig one on a carb'd car. Easiest would be to add a throttle position switch and solenoid as you suggest. Another means would be to drill taps into the intake runners behind the throttle plates where vacuum pressure could activate it, effective, but much more complex. Hope this helps. Happy Motoring! ...Jim'85TE
Hi there.
I think some people get confused with a DV and a BOV. In most other markets, a dump valve is the one described previous that is triggered by vacuum to relieve back pressure on the turbo. You can get these as either atmospheric or recirculating.
Also the giugiaro cars had a BOV as standard which was to prevent overboosting. This was changed on the Stevens or even very late G shaped cars to a cut out switch on the fuel pump. The reason for this was it was more reliable because the BOV used to seize due to lack of use. They did on UK cars anyway.
As for adding a dump valve to a carbed turbo, I have a 1990 carbed turbo and have added an atmospheric dump valve between the turbo and plenum. These are the ones that make the whoosing sound. If you want any pics then e-mail me and I'll forward them to you.
Cheers,
Dave Walters
I think some people get confused with a DV and a BOV. In most other markets, a dump valve is the one described previous that is triggered by vacuum to relieve back pressure on the turbo. You can get these as either atmospheric or recirculating.
Also the giugiaro cars had a BOV as standard which was to prevent overboosting. This was changed on the Stevens or even very late G shaped cars to a cut out switch on the fuel pump. The reason for this was it was more reliable because the BOV used to seize due to lack of use. They did on UK cars anyway.
As for adding a dump valve to a carbed turbo, I have a 1990 carbed turbo and have added an atmospheric dump valve between the turbo and plenum. These are the ones that make the whoosing sound. If you want any pics then e-mail me and I'll forward them to you.
Cheers,
Dave Walters
Dave I wonder if there is confusion in terms here (as you mentioned.) The Lotus Manual does describe the dump valve on the "G" Esprits as a Dump Valve (not a blow off valve), which leads me to believe that it's called a Dump Valve in the UK and the US. There is no mention of a BOV. I could be wrong about the terms though.
Anyway, regardless of the name, I am curious how you set up the second vacuum line on your valve before the plenum (the line that would normally come from between the throttle and the head.) Did you do the solenoid idea that I suggested or something else? I would really like to see pics of that part of the setup.
>> Edited by DrieStone on Friday 26th March 01:12
Anyway, regardless of the name, I am curious how you set up the second vacuum line on your valve before the plenum (the line that would normally come from between the throttle and the head.) Did you do the solenoid idea that I suggested or something else? I would really like to see pics of that part of the setup.
>> Edited by DrieStone on Friday 26th March 01:12
So what would the setup be on a US car with Bosch K-jetronic, would I have a pressure relief valve, or not? I'm still a bit confused... isn't the wastegate what keeps the car from overboosting (as opposed to the BOV, as Dave says) ? Would the setup that Kevin uses with the Greddy BOV work in my car, I've heard something about needing to use a recirculating BOV (or dump valve, whatever) in my car. Thanks.
Dan
Dan
86turbo said:
So what would the setup be on a US car with Bosch K-jetronic, would I have a pressure relief valve, or not? I'm still a bit confused... isn't the wastegate what keeps the car from overboosting (as opposed to the BOV, as Dave says) ? Would the setup that Kevin uses with the Greddy BOV work in my car, I've heard something about needing to use a recirculating BOV (or dump valve, whatever) in my car. Thanks.
Dan
Hi Dan
I dont think your model has a BOV standard like mine. The BOV only indirectly affects the wastegate as it flushes pressure from the pipe and plenum so the wastegate closes again.
I think you can use the Greddy in the same way I have, if you put the output to atmoshpere it makes the noise, if you put it back into the intake its called recirc, however this just dumps HOT air back into the intake and nobody wants that!
A BOV should improve boost response as you change gear, BUT it also puts a bit of extra pressure on the clutch and trans which is maybe why Lotus removed it when they went to the Renault G/BOX(i think thats when the BOV's were deinstalled)
Hard to know whether its worth it on a street car, especially if you dont want the noise!
Cheers
Kevin
Hi there, the wastegate does a different job. If it gets stuck the old Lotus (pressure relief valve as we'll called it for now) will prevent the car from blowing up and will open at a set pressure to atmosphere. Later cars didn't have this and instead had a fuel cut out which too was set to a predetermined setting and would cut fuel until the pressure went down.
A dump valve or BOV in other markets is mainly to protect the turbo from when you take your foot off the gas or change gear (which is the same thing). Back pressure is relieved through the dump valve which is opened by vacuum. Some vent to atmosphere giving the whoosing sound and others including all OEM standard manufacturers dump valves come as recirculating which vents back into the air system thus no noise. This was mainly to the selling point of view and also to reuse what would noramlly be wasted air.
I have a atmospheric Sytec dump valve and I simply jut teed off an existing vacuum line around the carbs. IT works perfectly so I won't complain.
Hope this helps,
Dave walters
A dump valve or BOV in other markets is mainly to protect the turbo from when you take your foot off the gas or change gear (which is the same thing). Back pressure is relieved through the dump valve which is opened by vacuum. Some vent to atmosphere giving the whoosing sound and others including all OEM standard manufacturers dump valves come as recirculating which vents back into the air system thus no noise. This was mainly to the selling point of view and also to reuse what would noramlly be wasted air.
I have a atmospheric Sytec dump valve and I simply jut teed off an existing vacuum line around the carbs. IT works perfectly so I won't complain.
Hope this helps,
Dave walters
Hi Folks, I also once had an early eighties Daihatsu Charade De-tomaso turbo, it had the same sort of dump valve attached to the side of the plenum and worked in the same way as the Lotus one but recirced it back into the intake. If you study the configuration of the first 911 Turbos you will see they did the same.
cheers
Kevin
cheers
Kevin
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