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Engine knock and fuel pump


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My Pi friends and I had a discussion about the influence of the fuel pressure and flow in relation to engine knock.

 

In other words can a bad performing pump cause engine knocking??

I am happy with an answer from the motherland of my fine running Triumph Pi (cp)

 

Greetings and thanks

Ben de Groot

Edited by Bendirk
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My Pi friends and I had a discussion about the influence of the fuel pressure and flow in realation to engine knock.

 

In other words can a bad performing pump cause engine knocking??

I am happy with an answer from the motherland of my fine running Triumph Pi (cp)

 

Greetings and thanks

Ben de Groot

Hi Ben,

The short answer : its not the pump.

The longer answer :

By knocking I think you might mean 'pinking', a light tinkling sound, that happens when the throttle is opened suddenly from cruise. That sound is indeed knocking, also called detonation. And you can confirm this if it goes away on high octane fuel ( eg 99RON).

The reason it happens is the PI lacks a 'acceleration enrichment' There's no throttle pump (Weber) or carburettor damper ( SU). Transient enrichment is needed to counter the inlet manifold wetting with fuel when the throttle is suddenly opened. This results a in lean mixture - but leaning itself wont cause knock. What happens is the fuel that evaporates rather than wet the walls is of low octane. The high octane aromatics are less volatile and stay as liquid fuel. So brielfy the mixture passing the inlet valves is low octane, and it detonates. This pinking goes away after a second or so, when the wall film has built up to a level that liquid fuel droplets are torn off in the air flow or liquid film reaches the inlet valves.

The cure is to run rich, especially at low throttle openings. This ensures the manifold depression does not dry the walls completely. And to time the metering unit to the camshaft as in the manual. This ensures that as each injector squirts, the spray hits fast-moving air while the inlet valve in open, mimising wall wetting. So.....not the pump, providing the fuel supply pressure is correct.

More here:

https://supertrarged.wordpress.com/2013/06/12/the-lucas-pi-lean-spike/

 

https://www.highpowermedia.com/blog/2900/wall-wetting-the-tau-factor

- he describes wall-wetting but misses the octane fractionation - that can be found in most ic engine textbooks.

 

 

Peter

Edited by Peter Cobbold
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Hi Peter

Thanks for your answer. I had already "studied" your very interesting explanation about lean burning and thought I understood the principle.

But I was thinking that if the pump could not deliver enough fuel ( 16 gal/h at 110 psi) this could lead to a lean mixture.

Reading your answer this is not the case. I gave my car to a good friend for some days. Probably he had used Euro 95 fuel instead of Euro 98 and after that mentioned the pinking.

I now have the opportunity to use even 102 Ron fuel and that wil cure the problem

Thanks for your help and advice

Greeting from Ben

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Hi Peter

Thanks for your answer. I had already "studied" your very interesting explanation about lean burning and thought I understood the principle.

But I was thinking that if the pump could not deliver enough fuel ( 16 gal/h at 110 psi) this could lead to a lean mixture.

Reading your answer this is not the case. I gave my car to a good friend for some days. Probably he had used Euro 95 fuel instead of Euro 98 and after that mentioned the pinking.

I now have the opportunity to use even 102 Ron fuel and that wil cure the problem

Thanks for your help and advice

Greeting from Ben

Ben,

There is a way to run on 95 RON without pinking. But it means recurving the distributor. I have to ask permission of the auto engineer

who told me how he did it.

Peter

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Hallo Peter,

Thanks for your offer but from now on I'll feed the engine with Ron 98 of even Ron 102 so I don't have the need to recurve the

distributor

Moreover I'ill run a check on the fuel pressure and flow.

Greetings from Ben

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