Ideal Camshaft Lobe Centreline Angle
Discussion
Pumaracing said:
What you are doing here is not quite the same situation as Jenkins was facing though. If you increase capacity but leave the head and cam alone then the engine will not rev so high, the powerband will shrink and the cam is now effectively a milder one than it was on the smaller engine. Jenkins was trying to get the most power out of every engine in every situation.
For example in 354 cubic inch drag engines he used 290 degrees of inlet duration, in the 330 that dropped to 286 degrees and in the 302 to 282 degrees. His findings on LSA were in terms of what worked best after he'd reoptimised the state of tune for the different capacity. The more flow per cc the less cam duration is needed.
In fact he preferred smaller capacity bottom ends for drag use because they revved higher and gave more power. Track engines would not sustain such high rpm for extended periods though so he made the bottom ends bigger to bring the rpm down and increase reliability.
So it's quite possible you won't find the same solution on LSA if you leave cam duration alone as if you changed duration at the same time as the capacity.
It's highly unlikely that I would be changing the rod ratio wihout a change in capacity, like I said before there are other factors to consider. This has happened once where a particular engine went from a long stroke 1800 to a short stroke 1800 and the rod angle improved considerably(higher rod ratio). This engine had marginally wider LSA (1.5 degrees) than it's predecessor. Once again there are other factors, lower bore shrouding being one.For example in 354 cubic inch drag engines he used 290 degrees of inlet duration, in the 330 that dropped to 286 degrees and in the 302 to 282 degrees. His findings on LSA were in terms of what worked best after he'd reoptimised the state of tune for the different capacity. The more flow per cc the less cam duration is needed.
In fact he preferred smaller capacity bottom ends for drag use because they revved higher and gave more power. Track engines would not sustain such high rpm for extended periods though so he made the bottom ends bigger to bring the rpm down and increase reliability.
So it's quite possible you won't find the same solution on LSA if you leave cam duration alone as if you changed duration at the same time as the capacity.
My main focus is on a particular range of engines which has 3 capacity levels so I'm well aware of the relative affects of a particular profile across the range of capacities/head flow. I am not a supporter of overcamming an engine and trading out torque for work done, it produces an engine that is hard work to use I prefer to spend time perfecting the head. The most powerful 1900 to date had 286 degree inlet cam and 276 degree exhaust , the valve acceleration was high as was the overall lift, as well as producing a good wide torque band the specific output was very good at over 147bhp/litre, Lobe centres were 103/105 although these were set as always using lift at TDC.
I have upgraded a couple of engines where both the capacity(lower rod ratio) and cam have changed, but the result is the same, the cams have ended up 2-3 degrees closer on LSA then with the previous incarnation of the engine.
Th idea of any 'rule' to establish LSA derived from rod angle is a good one and necessary if you are working on a single cam engine as the LSA is fixed once the cam is cut. Primarily my work is on various 4 valve engines where the LSA can be varied at will so such a restriction doesn't exist. From that perspective the rule is esoteric, the most important factors being duration, overall lift and valve acceleration. Experience of many hundreds of engines with a clutch of different cams gives me a good start point for LSA/LCA and this is normally pretty much on the money.
Dave
Edited by DVandrews on Saturday 28th January 12:53
Pumaracing said:
Yes. For high duration cams anyway. The Sig Erson 134 grind was a cracking cam for road vehicles with 500 thou lift from 276 degrees on a 110 LSA but it wore out like buggery. Piper and Kent used to do a copy, probably still do but smaller LSAs work better on longer duration cams.
Incidentally, a custom Pinto cam I spec'ed out in "some stupid software that will never work" back in 1998 and had ground for my car against everybody else's opinions - including the camgrinder, who asked me "What are you going to use this for? It's 100% opposite what everybody asks for.." - gave me a result the established Ford crowd over here didn't belive in. Seeing those specs I guess I know why...Edited by Pumaracing on Saturday 28th January 08:35
They called it "The caravanning cam" before they saw it run.. it was 12.6 / 11.4mm lift, 268/260 durations on a 109 LSA. Heaps of power most everywhere in the curve. That took me a bit out of the crowd of "I'll try 4 cams from KentCams and see what I get" to the "I'll run some sims, and buy the one most likely to be what I need" crowd of one. Me. Ended up never buying an off-the-shelf cam again. Of course I was shunned as "The teorethican"... but we kept on winning...
Workshop said:
What software were you using?
Hardly any GT-Power... it was Performance Trend's Engine Analyzer, basic version. Cost just a bit more than a HappyMeal at the local McD. But it still showed me what I had felt in the car using an RL30 and others... all the bottom end just disappeared with a long duration cam. So I just played around with the generic profile made from specs I took from the top of my head at the time.. and ended with a cam like the one mentioned. Workshop said:
Yeah, I've got that.. Have played around with it and it was 10HP off what the engine dyno showed on a friends engine. Will be interesting to see if the same happens with the engine I'm building for my next car.
The most important to remember with sim's are: GIGO. The more precise you are, the more precise it will most likely be. Most likely, because like all reasonably priced sims it's made with a ton of assumptions that's not always correct, and by God I have found and corrected a few in my 10+ years of beta-testing the EA Pro for Kevin (Gertgen, of PT).And, if you ignore temps, pressures and dimensions going haywire you can make a 600cc inline 4 making 1400 bhp at 25 000 rpm on gas in the sim. (You really can, I did it in EAP.) Only thing is that as far as now, it's rather impossible to have twin 38 mm intake valves in a 65mm bore, 17:1 compression is rather hard to get in said 150cc per cylinder and make a 4-valver burn efficiently at 18º TDC at 24 500 rpm... so it's a tool, nothing more, and it's no better than the user of it.
Geez - I take a couple off days off to do some serious dyno testing and what happens. This thread takes off like a Saturn 5 and leaves me with a whole bunch of catching up to do. Really - some interesting side issues to the original subject. One thing I feel I need to address soon is the terminology as I see as many people confusing this as understanding it. As a result we are not, as a group, communicating that well. Maybe I should take the blame for this as I assumed (bad thing to do) that we were all on the same page here.
Guys I am still carbon monoxided out here after thrashing all day trying to get 800 hp from a totally street drivable 87 octane fueled 525 inch BB Chevy. Would you believe 797.5!! How damnably frustrating. Maybe next week after some exhaust changes(fingers crossed). Still all was not lost though - the plasma ignition we tested came through with great results. Needs to be a big PH article though. We might have to see what the PH admin says to that!!
DV
Guys I am still carbon monoxided out here after thrashing all day trying to get 800 hp from a totally street drivable 87 octane fueled 525 inch BB Chevy. Would you believe 797.5!! How damnably frustrating. Maybe next week after some exhaust changes(fingers crossed). Still all was not lost though - the plasma ignition we tested came through with great results. Needs to be a big PH article though. We might have to see what the PH admin says to that!!
DV
David Vizard said:
AS usual Dave there is no fast answer to the wealth of comments you have thrown out there for, not just me but us all to contemplate. I have answers that will be seen to fit and explain some of the apparent anomalies you raise.
We are hoping you will raise these then so we can discuss the issue in more depth on here. I'm sure the readers will appreciate a more detailed understanding of LSAs and which engine factors really affect it. I have a vast amount of calculation and analysis on this subject but I'm not just going to put it all out there for no return unless we get into a mutual discussion of the topic.Guys been reading with with great interest and finding the input invaluable. David I carried your aseries boook everywhere in my Mini days, a legendary piece of literature! Currently rebuilding my ff2000 pinto and thinking of trying to change the standard LCA of 113 degrees, I figured this could be altered by playing around with valve seat height, valve legnths and rockers although min valve lengthhas to be 110.55mm. Obviously there will be a trade off with lift in one direction or LCA in the other. I thought 110 LCA and sacrifice some lift? Other alternative is go to 115 to get max lift with min legnth valves? Compression is 9.9-1. Jim
I'm finding this thread very interesting, however I'm having trouble understanding what's happening to the piston in this graph with a short rod just before BDC. The graph implies it starts accelerating upwards before BDC (and down again after)
If someone could explain I would be gratefull
Prepares himself for "you thicky" comments
If someone could explain I would be gratefull
Prepares himself for "you thicky" comments
DVandrews said:
The graph is plotting piston acceleration rate, not piston movement, so the raised portion is not the piston rising in the bore, but the piston acceleration increasing a little (or rather the deceleration rate falling off a little).
Dave
Whaat I'm struggling to visualise is that for an inline four cylinder enigine, each cylinder is on one phase of the cycle, so for any piston travelling towards BDC, there are 2 more more travelling towards TDC. How can their accelearations be different when they are joined by the crank? Is something flexing elastically?Dave
The Excession said:
haat I'm struggling to visualise is that for an inline four cylinder enigine, each cylinder is on one phase of the cycle, so for any piston travelling towards BDC, there are 2 more more travelling towards TDC. How can their accelearations be different when they are joined by the crank? Is something flexing elastically?
If the piston end was also a crankshaft I would agree, but it isnt, the crankshaft big end rotates around a large diameter circle whereas the small end merely pivots about it's axis. This means that the motion of the pistons is a lot more complex than you might think. If the rod were always perpendicular to the crank plane (infinitely long rod) then again TDC and BDC accelerations would be the same.Dave
Yeah I get that, but as the crank moves through BDC I would have thought there would be no Acceleration (as at TDC actually)
>> I understand the movement isn't completely sinusoidal (which is I think what the graph is saying), however the plot makes more sense if it's plotting movement (which it clearly isn't), acceleration would be a first derivative with the 0 at TDC and BDC
Unless the graph is plotting rates of change of acceleration, in which case it makes perfect sense
to me at least
>> I understand the movement isn't completely sinusoidal (which is I think what the graph is saying), however the plot makes more sense if it's plotting movement (which it clearly isn't), acceleration would be a first derivative with the 0 at TDC and BDC
Unless the graph is plotting rates of change of acceleration, in which case it makes perfect sense
to me at least
Edited by Incorrigible on Monday 30th January 21:23
Incorrigible said:
Yeah I get that, but as the crank moves through BDC I would have thought there would be no Acceleration (as at TDC actually)
As it moves through BDC there is a reduction in acceleration to zero piston speed, the graph plots the *rate of acceleration*, not piston speed.Dave
Thanks Dave.
A bit of background on the rate of change of acceleration.
(It's years and years since I did A level physica/mathematics, so a quick refresher was in order)
You edited your post as I was typing!
So ETA:
My question was where does this come from as I thought engine parts were made not to strech and flex too much.
Incorrigible said:
Yeah I get that, but as the crank moves through BDC I would have thought there would be no Acceleration (as at TDC actually)
BenA bit of background on the rate of change of acceleration.
(It's years and years since I did A level physica/mathematics, so a quick refresher was in order)
You edited your post as I was typing!
So ETA:
Incorrigible said:
Unless the graph is plotting rates of change of acceleration, in which case it makes perfect sense
Now you've got it.My question was where does this come from as I thought engine parts were made not to strech and flex too much.
Edited by The Excession on Monday 30th January 21:30
The Excession said:
(It's years and years since I did A level physica/mathematics, so a quick refresher was in order)
Years since my Physics / welding cars up when I should have been in lectures degree too 
Basically with each successive derivative you'll build in more deviation to the sinusoidal action, you need to map the piston movement and do the maths.
>> as usual I was thinking about it nearly hard enough and missing the picture, obviously (now) the speed is zero at TDC BDC not the acceleration
Anyway onwards, back to the original discussion
Edited by Incorrigible on Monday 30th January 23:00
The first thing you need to look at is the velocity, and then acceleration.
Stan
Stan
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[url][img]http://thumbsnap.com/sc/1wKQnoq0.gif[/img]|http://thumbsnap.com/1wKQnoq0[/url]
Edited by Stan Weiss on Wednesday 1st February 18:13
Then we can look at the difference in rod angle and how it effects piston position.
Stan
Stan
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Edited by Stan Weiss on Wednesday 1st February 18:21
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