Ideal Camshaft Lobe Centreline Angle
Discussion
Rwdfords said:
Hello Carlt5
You quoted one of my posts above but did not comment on it, is there a specific question you have?
there are lots of questions I have , but I've had my bottom smacked already for thread hijackingYou quoted one of my posts above but did not comment on it, is there a specific question you have?
I quoted your post more in reference to the fact it clarified a lot of terminology
Thanks for the new info above , as has been plainly obvious in this thread , my maths and data recording abilities are seriously crap
DVandrews said:
I have done some emprical testing of my own and it seems to contradict what you have posted.
However it isn't something that is easy to test.
My own experiences on identically specced engines that have gone from a high rod ratio(long rod) to low rod ratio (short rod) show that to achieve the largest area under the torque curve, the lobe separation angle has to go down by around 3 degrees. Admittedly the capacity and stroke have increased, but all other factors have remained the same. No doubt there are other factors to consider.
Of course there is a finite amount of draw on the cylinder, however the purpose of the cam is to make sure that there is an air supply while the piston is descending.
You wouldn't expect the valve events to exactly match the piston events as air takes time to accelerate, things dont happen instantaneously, if you move the inlet cam LCA around you can easily see the affect on torque, too far retarded and the torque will drop off rapidly in the mid range.
The thing about any theory is that it should fit the data and in fact predict it.
I'd agree that the rod angle has an affect on LCA, but having seen the opposite affect on a number of engines in controlled circumstances I am a little sceptical.
At this moment I am working on another engine where the rod ratio will be changing from 1.78 to 1.5 (a stroke increase) all other factors will remain the same. The cam timing was optimised on the dyno and gave the best curve at 106 ATDC, 108 BTDC. When the engine is converted I would normally advance the inlet cam by 2-3 degrees or so and retard the exhaust cam by a similar amount, this would bring the LSA in by around 2-3 degrees, normally this will give the best results.
I will ensure that when this engine is mapped I will first try widening the LSA to see what sort of result it gives.
Dave
The engine mentioned has now been mapped and the cam timing adjusted in order to give the largest area under the torque curve. As expected the best results were when the LSA was narrowed by 1.5-2 degrees or so over the previous position , a wider LSA was tried but power and torque dropped off everywhere.However it isn't something that is easy to test.
My own experiences on identically specced engines that have gone from a high rod ratio(long rod) to low rod ratio (short rod) show that to achieve the largest area under the torque curve, the lobe separation angle has to go down by around 3 degrees. Admittedly the capacity and stroke have increased, but all other factors have remained the same. No doubt there are other factors to consider.
Of course there is a finite amount of draw on the cylinder, however the purpose of the cam is to make sure that there is an air supply while the piston is descending.
You wouldn't expect the valve events to exactly match the piston events as air takes time to accelerate, things dont happen instantaneously, if you move the inlet cam LCA around you can easily see the affect on torque, too far retarded and the torque will drop off rapidly in the mid range.
The thing about any theory is that it should fit the data and in fact predict it.
I'd agree that the rod angle has an affect on LCA, but having seen the opposite affect on a number of engines in controlled circumstances I am a little sceptical.
At this moment I am working on another engine where the rod ratio will be changing from 1.78 to 1.5 (a stroke increase) all other factors will remain the same. The cam timing was optimised on the dyno and gave the best curve at 106 ATDC, 108 BTDC. When the engine is converted I would normally advance the inlet cam by 2-3 degrees or so and retard the exhaust cam by a similar amount, this would bring the LSA in by around 2-3 degrees, normally this will give the best results.
I will ensure that when this engine is mapped I will first try widening the LSA to see what sort of result it gives.
Dave
Dave
An interesting thread,
This is just my 5p worth.
I follow David Vizard's theory that the ideal LCA is proportional to valve size and the displacement under the valve.
Bigger cylinder = more demand on the inlet valve requiring a tighter LCA with more overlap to get best power if i understand correctly.
I have no doubt Mr Vizard's graph tells an important story and I wouldn't like to dismiss his theory out of hand. There must be a better way than saying 'this particular engine needs 107 degrees and that 110'.
Maybe there is a way of identifying commonalities amongst engines and formulating a theory. Scientists and engineers commonly describe very complex systems through formula and look up charts so i tip my hat to Mr V for attempting this with his work on cam speccing. He's just doing what DV always does best - targeting the heresay and BS that surrounds engine tuning by taking a more scientific approach.
Puma Racing engines makes an important point about Rod length to stroke ratio's and how they effect the opening of the valve in relation to piston position . That is somewhat contradicted by David Vizard's tests that utilised different rod lengths.
But maybe on smaller capacity motors, rod length effects become more significant ?.
I haven't any data but maybe a 4 valve engine of similar curtain area to a 2 valve will require similar LCA ? .
Maybe both arguments are correct and Mr Vizard's graph just needs some tweaking for a broad range of European engines. Maybe a correction factor for 4 valve engines that takes into account the larger specific curtain area of a mult valve engine could be used .....and maybe a few more lines for different rod/stroke ratios.
This is just my 5p worth.
I follow David Vizard's theory that the ideal LCA is proportional to valve size and the displacement under the valve.
Bigger cylinder = more demand on the inlet valve requiring a tighter LCA with more overlap to get best power if i understand correctly.
I have no doubt Mr Vizard's graph tells an important story and I wouldn't like to dismiss his theory out of hand. There must be a better way than saying 'this particular engine needs 107 degrees and that 110'.
Maybe there is a way of identifying commonalities amongst engines and formulating a theory. Scientists and engineers commonly describe very complex systems through formula and look up charts so i tip my hat to Mr V for attempting this with his work on cam speccing. He's just doing what DV always does best - targeting the heresay and BS that surrounds engine tuning by taking a more scientific approach.
Puma Racing engines makes an important point about Rod length to stroke ratio's and how they effect the opening of the valve in relation to piston position . That is somewhat contradicted by David Vizard's tests that utilised different rod lengths.
But maybe on smaller capacity motors, rod length effects become more significant ?.
I haven't any data but maybe a 4 valve engine of similar curtain area to a 2 valve will require similar LCA ? .
Maybe both arguments are correct and Mr Vizard's graph just needs some tweaking for a broad range of European engines. Maybe a correction factor for 4 valve engines that takes into account the larger specific curtain area of a mult valve engine could be used .....and maybe a few more lines for different rod/stroke ratios.
Dear David,
I think I can help you to go further ahead in your quest of predicting the LCA for a wide range of atmospheric engine, based on your very large dyno research on the subject.
I read your 3 books I bought you after your presentation during the PRI last December in Orlando over SB & BB on a budget & How to build horsepower were I was very interested by your graph page 107 with the Curve giving LCA in function of CID/inch of intake valve, but also conclude that if it works for 1 inlet/cylinder engine with unit volume proximate of the SB Chevy(witch you indicate to be the main base of your dyno research), this simplified Graph approach did not work quite well for CID larger (LCA too small) or far smaller (LCA too big) neither for 4 valve engines.
When I try to analyze and put some mathematical approach, I always try to go back to the basics: What are the basics involved in this case? I think that in the quote hereunder you quite well define it for your Cam Master, and this graph should be a simplified approach of it:
-To achieve that, we are speaking of a through-flow during overlap, and this one depends of the surface available during overlap. This surface depends of the circumference of the intake valve (also of the exhaust, but let assume that it is within a given % of intake, so it can remain relate to intake) and also of the valve lift during overlap. This surface has for basic units: lenght²
-You speak then of velocity at the seat who has for unit: Length/time
What I have done is to divide the CID by the (intake diameter)²
This leave us with length and as we are talking here about optimizing the scavenging at overlap against optimal exhaust opening point and intake closing point, both subject to the same speed, the factor time cancel there and our length is most probably a length per unit of time = Your velocity at the valve)
What I have done David to obtain a new graph of LOBE CENTERLIN ANGLE in function of CID/the square of Inch of valve diameter is to start from your graph page 107 (in how to build horsepower) for a 350 Chevy with various intake valve that would enable me to report your value of your graph of LCA function of CID/Inch.
The result is hereunder:
http://thumbsnap.com/YqqIEoVH
And bingo, we end up with a nearly straight line (at least from 104 to 118 °)
This is a good indication that we find a relation that only leave constant, and that it would be more useful on a larger scale.
This straight line respond to the formula for US unit (Unit cylinder volume in CID and intake valve diameter in Inch:
LCA= 120.5°- (CID/0.846/Intake valve dia²/number of intake valve)
Example in your book BB/budget page 94 a 581CID with a 2.3 Intake gives:
LCA= 120.5°- (581/8/0.846/2.3²/1)= 104° witch is what you advise (and with your graph we have to extrapolate to around 98° = 6° too low as per your book)
For Metric system it becomes if we use the unit cylinder volume: cm³ and Intake valve Diameter: millimeter:
LCA =120,5° - (Unit cylinder volume(cm³)/0,0215/(Intake valve diameter)²/number of intake valve)
Example for a 2lt 4 valve/cylinder Opel/Vauxhall with 33 mm intake valve:
LCA=120,5°- (500/0.0215/33²/2)= 109.8°
I though about a few correcting factor: Your volume at TDC varies with compression – 1.
You can correct it with a rule of three on the CID in the formula with your present compression -1 against the compression (-1) mainly used to obtain your graph.
Also for the available surface to “scavenge” at the overlap: it is influenced by the ratio of max lift/Intake valve dia and you could probably find a correction factor relate to your present ratio for your specific engine.
David try this type of approach with all your dyno test and let me know what it gives.
I think I can help you to go further ahead in your quest of predicting the LCA for a wide range of atmospheric engine, based on your very large dyno research on the subject.
I read your 3 books I bought you after your presentation during the PRI last December in Orlando over SB & BB on a budget & How to build horsepower were I was very interested by your graph page 107 with the Curve giving LCA in function of CID/inch of intake valve, but also conclude that if it works for 1 inlet/cylinder engine with unit volume proximate of the SB Chevy(witch you indicate to be the main base of your dyno research), this simplified Graph approach did not work quite well for CID larger (LCA too small) or far smaller (LCA too big) neither for 4 valve engines.
When I try to analyze and put some mathematical approach, I always try to go back to the basics: What are the basics involved in this case? I think that in the quote hereunder you quite well define it for your Cam Master, and this graph should be a simplified approach of it:
David Vizard said:
The bulk of the LCA computations that I have developed for my Cam Master program relates to flow Vs cubic inches with the valve size only in the equation for the determination of velocity both at the seats and in the ports and it’s effect on through-flow during overlap.
-We are speaking about cubic inch that relate to unit of lenght³ = a volume (I think we want to maximize scavenging the exhaust gas at TDC with fresh intake charge, without going with over scavenging). This volume at TDC is given by: CID(per cylinder)/(Compression-1) -To achieve that, we are speaking of a through-flow during overlap, and this one depends of the surface available during overlap. This surface depends of the circumference of the intake valve (also of the exhaust, but let assume that it is within a given % of intake, so it can remain relate to intake) and also of the valve lift during overlap. This surface has for basic units: lenght²
-You speak then of velocity at the seat who has for unit: Length/time
What I have done is to divide the CID by the (intake diameter)²
This leave us with length and as we are talking here about optimizing the scavenging at overlap against optimal exhaust opening point and intake closing point, both subject to the same speed, the factor time cancel there and our length is most probably a length per unit of time = Your velocity at the valve)
What I have done David to obtain a new graph of LOBE CENTERLIN ANGLE in function of CID/the square of Inch of valve diameter is to start from your graph page 107 (in how to build horsepower) for a 350 Chevy with various intake valve that would enable me to report your value of your graph of LCA function of CID/Inch.
The result is hereunder:
http://thumbsnap.com/YqqIEoVH
And bingo, we end up with a nearly straight line (at least from 104 to 118 °)
This is a good indication that we find a relation that only leave constant, and that it would be more useful on a larger scale.
This straight line respond to the formula for US unit (Unit cylinder volume in CID and intake valve diameter in Inch:
LCA= 120.5°- (CID/0.846/Intake valve dia²/number of intake valve)
Example in your book BB/budget page 94 a 581CID with a 2.3 Intake gives:
LCA= 120.5°- (581/8/0.846/2.3²/1)= 104° witch is what you advise (and with your graph we have to extrapolate to around 98° = 6° too low as per your book)
For Metric system it becomes if we use the unit cylinder volume: cm³ and Intake valve Diameter: millimeter:
LCA =120,5° - (Unit cylinder volume(cm³)/0,0215/(Intake valve diameter)²/number of intake valve)
Example for a 2lt 4 valve/cylinder Opel/Vauxhall with 33 mm intake valve:
LCA=120,5°- (500/0.0215/33²/2)= 109.8°
I though about a few correcting factor: Your volume at TDC varies with compression – 1.
You can correct it with a rule of three on the CID in the formula with your present compression -1 against the compression (-1) mainly used to obtain your graph.
Also for the available surface to “scavenge” at the overlap: it is influenced by the ratio of max lift/Intake valve dia and you could probably find a correction factor relate to your present ratio for your specific engine.
David try this type of approach with all your dyno test and let me know what it gives.
Edited by Jean De Gheest on Thursday 19th April 07:02
I'm not sure if you will hear from David, he has not posted here for a couple of months. Even the original poster has disappeared.
Thanks for the post anyway, it's very interesting.
I think as far as the formula in the book goes, it's really only good for 2 valve small blocks. I don't really have any faith in using it for any other engine like bikes etc.
I think it's just as easy to start with a figure of
108 and then make additions or subtractions to this number depending on other factors in your engine.
Thanks for the post anyway, it's very interesting.
I think as far as the formula in the book goes, it's really only good for 2 valve small blocks. I don't really have any faith in using it for any other engine like bikes etc.
I think it's just as easy to start with a figure of
108 and then make additions or subtractions to this number depending on other factors in your engine.
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