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Classic Engines, Modern Fuel

Comments on Topic: Modern fuel burning more slowly - No .... but

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Submitted by The Author

The apparent effect that modern petrol burns more slowly not only causes damage to the engine, it also can "upset" the operation of the carburettors.

Chapter 8 describes how and its effect.

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Submitted by Tim

Dear Professor Ireland,

I have a question regarding a statment in your book. The statement is made on page 66 under the section "Enrichment Effect".

In this section it starts off by stating that: 

"Air is only drawn through when cyclinders undergo the induction cycle".

This is very clear, and easy to understand, and makes perfect sense to me. 

Here is where it gets a bit difficult for me to "wrap my head around" the idea: 

"With a single carburetor this is twice per revoltion of the crankshaft. Once per revolution for engines with two carburetors" 

1) I don't understand why the intake "air pluse" would occur only once per revolution of the crankshaft on an engine with two carburetors?  In otherwords what causes there to be a difference in the number of air intake pulses between having one carburtor vs having two carburetors on the same engine?  Also, I assume the engine you are referring to contains four cylinders, like the XPAG used as the test bed? 

This leads to my next question: 

2) On a six cyclinder engine, fitted with twin carburetors, is there still only one air intake pulse?

and yet another question I have:  

3) Is there any difference in the number of air intake pulses between, lets say a "variable jet" SU carburetor and a "fixed jet" Weber carburetor and the number of revolutions on a crankshaft?  

I am really enjoying reading your book, as I am in the process of tuning up a Lancia Fulvia engine fitted with twin Solex carburetors. This is not an easy task with the fuel we have available here in California. 

Thank you in advance! 

Tim Mills

Santa Clara, CA


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Submitted by The Author


Firstly, thanks for your comment about reading the book. I am glad you are really enjoying it.

Re the comment "with a single carburettor this is twice per revolution.....". Firstly, I must apologise I should have stated "With a 4 cylinder engine...".  

Two cylinders (normally 1 and 4) are 180 degrees out of phase with the other two cylinders (2 and 3). Imagine #1 is dropping down the cylinder and undergoing the "Suck" cycle, #2 is coming up the cylinder undergoing "Exhaust". #2 will start to "Suck" around the time #1 reaches bottom dead center. i.e. the two "Suck " cycles are 180 degrees apart or you get two suck cycles per 360 degrees of revolution.

With single carburettor, this is providing the fuel to all 4 cylinders - hence will experience 2 "sucks" per revolution.

With twin carburettors, these are normally arranged such that each carburettor feeds 2 cylinders. There is usually a balance tube between the carburettors but the cycles occur so quickly that to all intents and purposes there is no pulsed flow through this tube. Hence one carburettor only provides the fuel for 2 cylinders and will only see 1/2 the number of pulses, i.e. one "suck" per revolution.

On a 6 cylinder engine the suck pulses occur 120 degrees apart (rather than 180 degrees on a 4 cylinder engine). On a 6 cylinder engine there are 3  (rather than 2) "sucks" per revolution or with twin carburettors, 1.5 sucks per carburettor per revolution.

The number of air intake pulses per revolution depends only on the number of cylinders. Not the type of carburettor. The only difference in this case between variable and fix jet carburettors is the way the pulsed airflow affects the mixture.

Hope this helps.


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Submitted by Tim

Thank you for the detailed explanation. Yes this really helps, and it now makes absolute sense to me. Happy Easter weekend motoring!  

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