38 replies to this topic

[Minigman]

Dragging this thread up again, what would be the max CR on a 276 cam (which is what is currently fitted)?

1330, twin HS4s, 98RON with booster additive.

[DeadSquare]

All the time that the engine is "de-coked", you can probably run at 10.5:1 with the timing set at a compromise, which with a big engine in a road car, won't be that noticeable.......but, unlike a race engine which doesn't do many miles, carbon will build up and one of the triggers for pinking is a 'red hot coal' in the combustion chamber.

[Spider]

On the same assumptions as per my earlier post here, 10.28:1.

[Minigman]

On the same assumptions as per my earlier post here, 10.28:1.

I thought you said 9.69 earlier but that was with a 266.

(Edited- you’re referring to the mk1 forum post I think where I posed the same question - thanks for the reply on there too).

Quick calc then;

Running 8.3cc dished pistons flushed with the deck and allowing 3.3cc for the head gasket, so 11.6cc UV total.

To get a 10.2:1 ratio; SV = 1330/4 = 332.5cc / (10.2-1) = 36.14cc - 11.6cc = head chamber volume of 24.5cc?

Edited by Minigman, 07 December 2019 - 10:55 AM.

[Minigman]

All the time that the engine is "de-coked", you can probably run at 10.5:1 with the timing set at a compromise, which with a big engine in a road car, won't be that noticeable.......but, unlike a race engine which doesn't do many miles, carbon will build up and one of the triggers for pinking is a 'red hot coal' in the combustion chamber.

Ok makes sense. Sounds best to reduce the CR in that case. I’ll keep the 276 currently fitted instead of swapping to a 266, fit an Aldon Yellow dizzy and reduce the CR to 10.2:1 (as per Spider’s suggestion). I hope then the fuelling issue below 2500rpm and above 4500rpm is resolvable.

Edited by Minigman, 07 December 2019 - 10:59 AM.

[DeadSquare]

 

On the same assumptions as per my earlier post here, 10.28:1.

I thought you said 9.69 earlier but that was with a 266.

Quick calf then;

Running 8.3cc dished pistons flushed with the deck and allowing 3.3cc for the head gasket, so 11.6cc total.

To get a 10.2:1 ratio; 1330/4 = 332.5cc / (10.2-1) = 36.14cc - 11.6cc = head chamber volume of 24.5cc?

 

 

Now, where did my wife put my slide rule ?

 

All these calculations are all very well, but for most of the time they bear no relation to what is really happening.

 

Even if there was no valve in the way, yanking a piston rapidly down the bore does mean that the air pressure above the piston is the same as under the bonnet.

 

For almost 70 years, I have wondered if anyone has ever actually known how much short measure their pint of mixture is in their 568.25cc engine.

 

In a full race engine, it is believed that like a 2 stroke, the plug of exhaust gas charges off like a sonic piston down the tail pipe, leaving a partial vacuum behind it, and this vacuum effect helps to keep pulling in mixture past the still unclosed exhaust valve as the piston slows down, stops and begins to return to TDC;  furthermore, it is hoped that this effect keeps enough mixture rushing in before the inlet closes, so that there is actually more than a pint of mixture in the 568.25cc pint pot when both valves close.

 

See what I mean ?

[Minigman]

On the same assumptions as per my earlier post here, 10.28:1.

I thought you said 9.69 earlier but that was with a 266.

Quick calf then;

Running 8.3cc dished pistons flushed with the deck and allowing 3.3cc for the head gasket, so 11.6cc total.

To get a 10.2:1 ratio; 1330/4 = 332.5cc / (10.2-1) = 36.14cc - 11.6cc = head chamber volume of 24.5cc?

Now, where did my wife put my slide rule ?

All these calculations are all very well, but for most of the time they bear no relation to what is really happening.

Even if there was no valve in the way, yanking a piston rapidly down the bore does mean that the air pressure above the piston is the same as under the bonnet.

For almost 70 years, I have wondered if anyone has ever actually known how much short measure their pint of mixture is in their 568.25cc engine.

In a full race engine, it is believed that like a 2 stroke, the plug of exhaust gas charges off like a sonic piston down the tail pipe, leaving a partial vacuum behind it, and this vacuum effect helps to keep pulling in mixture past the still unclosed exhaust valve as the piston slows down, stops and begins to return to TDC; furthermore, it is hoped that this effect keeps enough mixture rushing in before the inlet closes, so that there is actually more than a pint of mixture in the 568.25cc pint pot when both valves close.

See what I mean ?

Haha yes I do get the gist of what your saying.

I’m very confused as you no doubt can tell!

[Ethel]

The calculations can still be valid if it's the same engine with the same purging and ramming characteristics.

I'm intrigued by Spider's calculations though. To my mind the relationship between crank angle and swept volume would be constant across all bore/stroke ratios. So what else is being factored in?

[DeadSquare]

The calculations can still be valid if it's the same engine with the same purging and ramming characteristics.

I'm intrigued by Spider's calculations though. To my mind the relationship between crank angle and swept volume would be constant across all bore/stroke ratios. So what else is being factored in?

 

"To my mind the relationship between crank angle and swept volume would be constant across all bore/stroke ratios"

 

To my mind, there is no relationship between a crank angle and what ever the swept volume is.

 

 

I

[Ethel]

I didn't express that clearly enough.  I was on the mobile theme, it's a feeble excuse, but I'm sticking to it 

 

The cross sectional area of a cylinder is constant so its volume is directly proportional to its height. The cylinder we're interested in is the one above the piston that's tallest at bdc and has zero height/volume at tdc. The transition from one to t'other is governed by the crank angle: when the crank's at 90⁰ the volume is half. The transition from max, via half, to min will be 1/2 a sine wave as the piston goes from rest to rest with max velocity half way down the bore. As long as your crank's big ends rotate in a circle they'll have this relationship regardless of bore/stroke ratio.

 

There will be a small difference because the ratio affects change in the vectors the conrod angle imparts on the piston, but that's of the same magnitude even if acting in the opposite direction to the engine rotation after the piston passes dead centre until it reaches maximum (halfway up/down the stroke) and starts to add to the piston's acceleration towards dead centre again.

 

...Maybe I'm edging toward answering my own question & Spider is factoring in bore stroke ratio, but if our sine wave was plotted against time (piston speed) rather than piston travel we'd just have to alter the rpm to get a long stroke engine to be the same as a shorter stroke one.

[DeadSquare]

I didn't express that clearly enough.  I was on the mobile theme, it's a feeble excuse, but I'm sticking to it 

 

The cross sectional area of a cylinder is constant so its volume is directly proportional to its height. The cylinder we're interested in is the one above the piston that's tallest at bdc and has zero height/volume at tdc. The transition from one to t'other is governed by the crank angle: when the crank's at 90⁰ the volume is half. The transition from max, via half, to min will be 1/2 a sine wave as the piston goes from rest to rest with max velocity half way down the bore. As long as your crank's big ends rotate in a circle they'll have this relationship regardless of bore/stroke ratio.

 

There will be a small difference because the ratio affects change in the vectors the conrod angle imparts on the piston, but that's of the same magnitude even if acting in the opposite direction to the engine rotation after the piston passes dead centre until it reaches maximum (halfway up/down the stroke) and starts to add to the piston's acceleration towards dead centre again.

 

...Maybe I'm edging toward answering my own question & Spider is factoring in bore stroke ratio, but if our sine wave was plotted against time (piston speed) rather than piston travel we'd just have to alter the rpm to get a long stroke engine to be the same as a shorter stroke one.

 

Oh,           Right.           Well     if you put it like that,                      of course!

 

 

 

 

 

 

Don't tell Ethel, but he lost me somewhere around "mobile"  Lol.

[Spider]

The calculations can still be valid if it's the same engine with the same purging and ramming characteristics.

I'm intrigued by Spider's calculations though. To my mind the relationship between crank angle and swept volume would be constant across all bore/stroke ratios. So what else is being factored in?

 

The method of calculation is Dynamic Compression Ratios. To get us all on the same page, I've expressed them in this thread as Static Ratios. I've discussed it at length before on the forum. I don't know if it comes up in a search ?

 

 

I hope you don't mind, but it might be easier if I embed some responses in Blue to your post here;-

 

I didn't express that clearly enough.  I was on the mobile theme, it's a feeble excuse, but I'm sticking to it      Gotta luve it

 

The cross sectional area of a cylinder is constant so its volume is directly proportional to its height. The cylinder we're interested in is the one above the piston that's tallest at bdc and has zero height/volume at tdc.  Yes

The transition from one to t'other is governed by the crank angle: when the crank's at 90⁰ the volume is half.  Sorry, but I disagree. This occurs at an angle closer to TDC and it's because when the Crank Angle is at 90⁰, the Angle between the Crank Big End to Mains to the Con Rod in not 90⁰ because the Cylinder is not over the Crankpin when the Crank is at 90⁰ but off set to it. 

 

The transition from max, via half, to min will be 1/2 a sine wave as the piston goes from rest to rest with max velocity half way down the bore. As long as your crank's big ends rotate in a circle they'll have this relationship regardless of bore/stroke ratio. Refer above

 

There will be a small difference because the ratio affects change in the vectors the conrod angle imparts on the piston, but that's of the same magnitude even if acting in the opposite direction to the engine rotation after the piston passes dead centre until it reaches maximum (halfway up/down the stroke) and starts to add to the piston's acceleration towards dead centre again.

 

...Maybe I'm edging toward answering my own question & Spider is factoring in bore stroke ratio, but if our sine wave was plotted against time (piston speed) rather than piston travel we'd just have to alter the rpm to get a long stroke engine to be the same as a shorter stroke one. OK, these are all Stoke to Rod Length Ratios and while they have a bearing and are factor of the calculation I use, the main thing is in fact the Inlet Closing Angle.

[Ethel]

"This occurs at an angle closer to TDC and it's because when the Crank Angle is at 900, the Angle between the Crank Big End to Mains to the Con Rod in not 900 because the Cylinder is not over the Crankpin when the Crank is at 900 but off set to it."

 

Good point, 'n yet another intricacy of engine design, best set aside for now 'because I'm already confusing myself 

 

..but is that not already accounted for if we're referencing TDC from the piston position, as you would timing in a cam? 

 

edit:

 

unless "There will be a small difference because the ratio affects change in the vectors the conrod angle imparts on the piston, but that's of the same magnitude even if acting in the opposite direction to the engine rotation after the piston passes dead centre until it reaches maximum (halfway up/down the stroke) and starts to add to the piston's acceleration towards dead centre again." was me making the same point much more clumsily.

 

Totally agree it's the inlet valve that does by far the most to set the dynamic CR. Wasn't it inertial ramming that set off this headache though? Leading to piston speed, crank angle...

[Spider]

 was me making the same point much more clumsily.

 

Totally agree it's the inlet valve that does by far the most to set the dynamic CR. Wasn't it inertial ramming that set off this headache though? Leading to piston speed, crank angle...

 

I apologise if I seem at times 'pedantic' but angles as seemingly small as 2 degrees make a difference and can be the difference between something that GOES, doesn't go as well as it should or fails in an expensive way.

 

As for the rest of this, I'll leave that for others to mull over !
 

[Ethel]

Not at all, I for one learn a lot from the group brainstorming sessions TMF chucks up every now 'n then.