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MG TD TF 1500 - Pressure relief valve

How to make a pressure relief valve in 40 min.

Reducing bush 1/2 - 3/4 brass
Ball dia. 22 mm.
Socket 3/4 with 4 holes dia. 10 mm.
Spring of 2.5 mm. wire
Washers for shimsing if required.
End plug 3/4

Press the ball against the reducing bush and it will form a perfectly tight seat.

Now I just hope it will work.



YS Strom

Sorry

Reducing bush has an internal dia. of 15 mm. i.e. 5/8 I guess. This will give a thru area of 2.3 cm2.
YS Strom

Surely 1.77 cm2.
R WILSON

Yes, you are right, it should of course be 1,7 cm2.
YS Strom

Pressure relief valve installed.

YS Strom

Is a petrol/air mix at the ideal explosive level a good idea under the bonnet.
Wastegates are normally only used to dump air.

Ray TF 2884
Ray Lee

Ray

Pressure relief valves in super charger installations are meant to open only when engines backfires to protect super chargers and if so, most of the fuel has already been burnt.
YS Strom

Makes sense, all my experience is with turbo blown marine propulsion diesels so no fuel in the air.
Ray
Ray Lee

Great idea for blow-off-valve. I wish I had seen that about 43 years ago when I made my contraption!

I once read that the blow-off-valve should be close to the supercharger. I think the theory is that .... if the engine backfires, the flame front and expanding gas will move from the inlet port towards the s/c and push much of the unburnt mixture out of the valve before it ignites, reducing the manifold explosion. If the valve is close to the engine (and the s/c is a long way away) the flame front will ignite past the valve, towards the s/c, igniting the whole contents of the manifold which has to explode back towards the valve resulting in a bigger bang trying to escape through the valve.

Bob Schapel
R L Schapel


Bob

Hmmm, sounds like a good idea. As I have a 1 m. duct, with a considerable volume between the front mounted SC and the engine, it might be worth having an extra near the SC.
YS Strom

Maybe. Check with local supercharger experts. My experience doesn't include your situation. My s/c is mounted on the manifold, close to the inlet ports, so it doesn't apply.

Cheers,
Bob
R L Schapel

Bob

With a SC mounted on the manifold, there is hardly any room for locating a PRV one way or the other relative to the flow. But with a front mounted I think it makes sense to fit it near the SC, as you mentioned.

In my TD I now have an XPAW with a homemade SC kit, but have not yet installed a PRV. I had a number of blow backs before I got everything right, but it is still working well with no visible harm to the SC. With its flat surface on top of the manifold it is much easier to make a PRV. A bolt in the manifold surrounded by say 4 holes and a large washer with two O-rings, a spring, a washer and a nut - very much standard I guess.


YS Strom

I think you'd find all the air and fuel compressed between the head and blower will flash in less than a heartbeat, basically, before the relief valve has a chance to function. The relief valve will attenuate with the resultant pressure buildup at any location.

Backfiring is a stressful situation for the blower and the occupants!
JIM N

Jim

Even with a 1 m. duct I also think it will take less than a heartbeat to burn the whole fuel mix, but the PRV will open almost instantaneously with the increased pressure I believe and one or two valves will also be more or less open at the same time. But how much that will reduce the pressure is another question - probably not very much.

With the engine and SC in a test bench, I have just tightened a hose clamp just enough to keep the hose in place and in case of backfire it has blown off. After a lot of fiddling it doesn't backfire any more, but who knows what will happen in the future. As the hoses will dry over time and stick to the pipe, this method cannot be used as a permanent solution to the problem. The best solution is probably to install a suitable friction clutch and as I have room for that, I will look in to the possibility of doing so.
YS Strom

One of my professors worked for GE. He was to test electrical contacts in a hydrogen environment. They purged with argon 3 days and then purge with hydrogen 3 days. The glass aquarium test box had aluminum foil on the top in case there was a little pop. Dr. McMaster climbed underneath the heavy bench just in case. When he flipped the switch, the explosion rattled the building, the glass turned back into sand instantly and it blew the steel door open to the room. A lot of good that aluminum foil relief valve did.
That was at zero boost; how much quicker would air and gasoline combust mixed at 6psi?

At school, we used to test welders with steel sheets immersed in garbage cans full of salt water. Lowering the sheet into the water created more conduction to plot the voltage/amperage curves up to many hundreds of amps. The finale of the test, when the plates were fully immersed, was to dead short the plates with a hammer. All the while, that current was generating chlorine and hydrogen gas- it sort of smelled like bleach. Sometimes that short circuit would create a harmless but deafening explosion that echoed through the lab!

Dragsters have large blowoff valves. Go to Youtube and enter "blower explosion" and watch the carnage.
JIM N

Attended a demonstration of a box of approx. 1 L, equipped with a rotating steel brush and a spark plug. First it was fed with a small volume of petrol, but the spark didnīt ignite the fuel mix. After adding the same volume it ignited with a small puff flying the lid open. The volume of an inlet pipe is not much more than that. An aquarium is of course a different story. When the same test was made with approx. 3 times the original volume of petrol nothing happened. The test was designed to demonstrate how important the proportions petrol/air is for combustion of a fuel mix.

Tried to find data on how fast a fuel mix burn, but couldnīt find any. May be like gun powder, it burns faster under pressure if I remember correctly.
YS Strom

This thread was discussed between 22/03/2018 and 27/03/2018

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