Anyone experience with closed butterfly valves? (without the poppets valves)

[dmtrmp6]

I have been running my 7 with the over run valves soldered closed for past 8+ yrs. Not experienced any problems tuning or running, and no visible oil smoke or consumption problems during that time. DM

Following info from Burlen fuel systems website

"Overrun Limiting Valve
A precisely set spring-loaded plate valve is located in the throttle disc. The valve opens under high manifold depression conditions, i.e. overrun at closed throttle, reducing the depression and supplying a quantity of correct fuel/air mixture through the throttle disc. The correct combustion achieved reduces the value of hydrocarbon emission which would otherwise be emitted under these conditions."

[jeffremj]

Following info from Burlen fuel systems website

"Overrun Limiting Valve
A precisely set spring-loaded plate valve is located in the throttle disc. The valve opens under high manifold depression conditions, i.e. overrun at closed throttle, reducing the depression and supplying a quantity of correct fuel/air mixture through the throttle disc. The correct combustion achieved reduces the value of hydrocarbon emission which would otherwise be emitted under these conditions."

I have this info in my SU book as well., but I still don't understand how burning more fuel makes for less emissions. With the throttle closed there is still metered air/fuel flowing, just less of it.

As mentioned, in my experience, over-run emissions seems to be from worn valve guides. I would imagine that the early emission standards had to last for 50,000 miles and at these higher mileages (on 1960's designed engines) they saw the affect of such wear.

[dursleyman]

[/quote]I have this info in my SU book as well., but I still don't understand how burning more fuel makes for less emissions. With the throttle closed there is still metered air/fuel flowing, just less of it.[/quote]

The way I understand it is that when the throttle closes at high engine speeds it causes a very high vacuum and so draws excess fuel from the carb jets. The additional air bleeding through the butterfly valves counters this to correct the mixture going into the engine and so meets emmission requirements. It doesn't reduce the amount of fuel used but does allow it comply with the legislation.

[jeffremj]

The way I understand it is that when the throttle closes at high engine speeds it causes a very high vacuum and so draws excess fuel from the carb jets. The additional air bleeding through the butterfly valves counters this to correct the mixture going into the engine and so meets emission requirements. It doesn't reduce the amount of fuel used but does allow it comply with the legislation.

This still makes no sense. As I have already mentioned, the butterfly is after the carb's bridge, which is where the fuel is mixed with air so there is no 'additional air', only additional air/fuel mixture entering the inlet manifold.

[Beans]

... This still makes no sense. As I have already mentioned, the butterfly is after the carb's bridge ...

Ask yourself what happens when you suddenly close of the air flow in a tube, using a valve;
Upstream of the valve you'll get a sudden pressure built up whereas downstream of the valve you'll get a low pressure area.

[dursleyman]

I always hated fluid dynamics as a subject at college now I remember why!

[jeffremj]

... This still makes no sense. As I have already mentioned, the butterfly is after the carb's bridge ...

Ask yourself what happens when you suddenly close of the air flow in a tube, using a valve;
Upstream of the valve you'll get a sudden pressure built up whereas downstream of the valve you'll get a low pressure area.

But after the bridge is metered air/fuel flow, not just air flow. My experience with a TR7 16V undergoing such a deceleration is lots of 'smoke' which I believe came from oil via the 8 inlet valve stems.

[Beans]

I always hated fluid dynamics as a subject at college now I remember why!

It's quite good fun actually and makes understanding why some things work as they do much easier

[jeffremj]

I always hated fluid dynamics as a subject at college now I remember why!

It's quite good fun actually and makes understanding why some things work as they do much easier

Still no explanation, only subtle denigration of someone who is trying to understand what seems to be a very complex interaction (if what you say is the real reason).

My explanation is much easier to understand and, by my 'experiment', would seem to be the better answer.

[Beans]

...only subtle denigration of someone who is trying to understand ...

If that's your point of view I'll rest my case. I am not a teacher

[jeffremj]

...only subtle denigration of someone who is trying to understand ...

If that's your point of view I'll rest my case. I am not a teacher

I am also not a teacher, but I try to help others get things right on this forum.

Anyway, back to the reason for the article.

If I remember my science/maths stuff, look at the extremes. If the throttle fully closed, the engine would probably lock up! If there were any 'leaks' in this situation they would be used. The main leak areas would all give oil borne air - via the valve guides and possibly crankcase oil/air mixture via (worn) piston rings. As the throttle is never fully closed, the leak areas still exist and have an affect on emissions.

In summary, the poppet valves have main 2 effects:

1) reduce the deceleration effect on the over-run
2) reduce oil based emissions by reducing the inlet vacuum and allowing more fuel mixture through to help burn up extra oil vapour.

They also have few demerits:

1) they can fail open
2) they probably restrict air flow - the reason I went to solid butterflies - I needed that extra hp
3) they can fall apart and drop into the engine.

[Stag76]

The opening of the poppet valves will only cause the slides to rise a very small amount due to the low manifold pressure,
resulting in a very lean mixture introduced to the manifold to complete the combustion and reduce hydrocarbon
(un-burnt fuel) emission, like the book says it does.
The other method used was the smog pump which pumped air into the exhaust ports after the valves to
complete the combustion.
The Rover Federal Fuel Only EFI used an over-run valve that opened under low plenum pressure, bypassing the throttle plate.
Engine Management Systems that control spark advance the timing in conjunction with injector pulse-width control on over-run
to complete the combustion.
The Vacuum advance mechanism fitted to distributors on carburetted engines also advances the timing when manifold
pressure is low.

[john 215]

Hi,

Solder them up, done this to dozens of poppet valves over the years never had an issue.

Later SU's had a electrically valve on the float chamber that closed on over run to stop fuel being drawn into the inlet manifold, they never gave any trouble .....NOT. Modern systems turn the injectors off on over run for emission and fuel consumption reasons.

Cheers John