#1 Ported vacuum advance vs manifold vacuum advance
Posted: Mon Jul 30, 2018 1:59 pm
I have decided to "take on the world" on this subject and cleared the idea of this thread with admin before posting this.
I want to show that :
a) ported vacuum is an effective method for rapid warm-up BEFORE the engine is fully warmed up ... and WHY;
b) manifold sourced vacuum its the most EFFICIENT solution for vacuum advance for the engine AFTER the engine is warmed up ... and WHY;
c) that the best solution for our classic cars is therefore to have BOTH ported vacuum (during warm-up) AND manifold vacuum (at operating temp), but,
d) in cases where the practical choice is one or the other, that manifold vacuum would be the preferred choice for vacuum advance ... and WHY.
This is a subject that relates to ALL petrol cars that has a vacuum advance diaphragm on its distributor. Eg, I know of at least one car, a 1987 Honda Civic 1500 (Carb), that has manifold vacuum as the vacuum source for its vacuum advance diaphragm on its distributor, so I KNOW that the case I am presenting has been implemented at least once, by a reputable manufacturer, on a car sold in vast numbers.
PLEASE NOTE: My aim is NOT to prove anyone wrong or to MAKE anyone believe or implement what I do. All I ask is an opportunity to state what I believe, but more importantly, WHY I believe so.
It saddens me to see how unsuspecting folk can be misled by
a) the sheer number of voices on a forum (the "pack mentality");
b) the "this is how its always been done" argument ;
c) the "this was good enough for the manufacturer, therefore it is good" argument;
d) the unwillingness of the above to ask WHY?
e) how much opinion is presented without base;
f) that a reference to web sites (which may, or may not, also be in error), somehow lends credibility to their argument.
My credentials:
I have a BSc degree (Electronic Engineering) from the University of Pretoria 1987 - but I will not claim to know it all. I will claim to have no idea how much I have yet to learn and how much of my degree is now forgotten!
I have extensive EFI experience gained since I started playing with Megasquirt in 2005: I have fitted megasquirt (myself, with my own hands, programmed and tuned by myself) to more cars than I can remember but Jaguar V12 in particular:
1) My own '80 pre-HE XJS (which I still have and is still running);
2) My ex S3 OTS E-type (which had the pre-HE engine when I fitted EFI before I upgraded it to the HE engine)
3) An HE-engine based Cobra replica I built and sold.
Non Jag includes more than 10 Rover V8's both 3.5 and 4.6, one my own. Lost count. Most recently I EFI-ed my 1974 Jensen Interceptor with the Chrysler 440ci engine, first low compression, now high compression.
It is this background which has led me to the conclusion I'm now at with regard to vacuum advance.
Right, shall the games commence??
I invite robust debate but be ready to provide measureable evidence of any statements you make.
I'll start by posing a few questions hoping you will participate with enthusiasm:
1) If we could build an internal combustion engine with 100% efficiency, it would not need a cooling system. Correct or false?
2) the less efficient the internal combustion engine, the bigger/better the cooling system needs to be. Correct or false?
3) Define the condition: "Idle";
4) Let's pretend "the book" states that the timing for an engine must be 12ºBTDC at idle and that idle speed must be 800rpm. What exactly does that 12ºBTDC apply to?
I want to show that :
a) ported vacuum is an effective method for rapid warm-up BEFORE the engine is fully warmed up ... and WHY;
b) manifold sourced vacuum its the most EFFICIENT solution for vacuum advance for the engine AFTER the engine is warmed up ... and WHY;
c) that the best solution for our classic cars is therefore to have BOTH ported vacuum (during warm-up) AND manifold vacuum (at operating temp), but,
d) in cases where the practical choice is one or the other, that manifold vacuum would be the preferred choice for vacuum advance ... and WHY.
This is a subject that relates to ALL petrol cars that has a vacuum advance diaphragm on its distributor. Eg, I know of at least one car, a 1987 Honda Civic 1500 (Carb), that has manifold vacuum as the vacuum source for its vacuum advance diaphragm on its distributor, so I KNOW that the case I am presenting has been implemented at least once, by a reputable manufacturer, on a car sold in vast numbers.
PLEASE NOTE: My aim is NOT to prove anyone wrong or to MAKE anyone believe or implement what I do. All I ask is an opportunity to state what I believe, but more importantly, WHY I believe so.
It saddens me to see how unsuspecting folk can be misled by
a) the sheer number of voices on a forum (the "pack mentality");
b) the "this is how its always been done" argument ;
c) the "this was good enough for the manufacturer, therefore it is good" argument;
d) the unwillingness of the above to ask WHY?
e) how much opinion is presented without base;
f) that a reference to web sites (which may, or may not, also be in error), somehow lends credibility to their argument.
My credentials:
I have a BSc degree (Electronic Engineering) from the University of Pretoria 1987 - but I will not claim to know it all. I will claim to have no idea how much I have yet to learn and how much of my degree is now forgotten!
I have extensive EFI experience gained since I started playing with Megasquirt in 2005: I have fitted megasquirt (myself, with my own hands, programmed and tuned by myself) to more cars than I can remember but Jaguar V12 in particular:
1) My own '80 pre-HE XJS (which I still have and is still running);
2) My ex S3 OTS E-type (which had the pre-HE engine when I fitted EFI before I upgraded it to the HE engine)
3) An HE-engine based Cobra replica I built and sold.
Non Jag includes more than 10 Rover V8's both 3.5 and 4.6, one my own. Lost count. Most recently I EFI-ed my 1974 Jensen Interceptor with the Chrysler 440ci engine, first low compression, now high compression.
It is this background which has led me to the conclusion I'm now at with regard to vacuum advance.
Right, shall the games commence??
I invite robust debate but be ready to provide measureable evidence of any statements you make.
I'll start by posing a few questions hoping you will participate with enthusiasm:
1) If we could build an internal combustion engine with 100% efficiency, it would not need a cooling system. Correct or false?
2) the less efficient the internal combustion engine, the bigger/better the cooling system needs to be. Correct or false?
3) Define the condition: "Idle";
4) Let's pretend "the book" states that the timing for an engine must be 12ºBTDC at idle and that idle speed must be 800rpm. What exactly does that 12ºBTDC apply to?