@Jeffrey67dart
Here is some food for thought;
You said
[email protected] and 110LSA. I'm gonna take some liberty and call the exhaust 244, and add typically long roller acceleration ramps to come up with advertised numbers of 292/300/110. For this exercise, it doesn't have to be accurate, it is for illustration purposes only.
Ok, so if you map that out, here is what that cam will sortof look like to your engine;
292/300/110+2/ overlap of 76, and Effective=
76;
the compression duration is 106* for an Ica of 74*
the power duration is
98* for an ex opening at 82* Atdc.
What does it mean?
Well I know you're interested in fuel mileage, so that's all that I'm gonna talk about in this post.
Lets go look at the Power Extraction. With just 98* of extraction, there will be a huge amount of energy still left in the expanding gasses when the exhaust valve begins to open. Immediately the pressure in the cylinder will drop , so all that energy is going straight down the primary header pipe, as wasted. You paid for that energy at the gas-pump, and now there it is whooshing out your tailpipes.
At the other end of the exhaust cycle is overlap. This cam has around 76* of it. Think about this; your compression duration is 106*. Your extraction is 98* and now,overlap is 76*. This is not a small amount of overlap, it is in fact HUGE. This is 72% of the compression and almost 78% of extraction. This is a cycle just like the other 4.
So what is overlap and how does it affect fuel mileage?
Overlap is the period of time (in degrees) that both the intake valve and the exhaust valve are open. The header works to create a low pressure on the back of the slow-closing exhaust valve, as the piston is coming up to the top of the stroke. As soon as the intake valve opens, the plenum sees that low-pressure, and the atmosphere starts rushing to fill it. So now the piston arrives at TDC, and begins it's downward trek, encouraging the plenum and atmosphere, to keep on filling the ever increasing volume in the cylinder as the piston is moving down. Ok but lets back up to the top. When that intake valve opens, and the plenum sees the low pressure area in the chamber created by the header, guess what, the fuel-air charge rushes in there, in an attempt to equalize the pressure ..... but the exhaust valve is still open! So there is nothing stopping that air-fuel charge, at low rpm, from charging straight across the piston and out into the low-pressure area in the header. Guess what happens to your fuel economy? That's right, the gas you just paid for, is ... whooshing down the pipes.
And finally is the compression cycle; yours is just around 106* advertised. (Ima guessing). That means, that the intake valve is not closing until the piston has travelled 74* after bottom dead center. Try to picture how far up the cylinder that might be. I'll guess over 1/3 of the way up. Your engine cannot start to build cylinder pressure until that stinking intake valve finally closes. So even tho you might have a Compression Ratio of 10.7, That ratio starts with the piston at the bottom, not 74 * after passing the bottom! So, in your case, your Effective Stroke might be down to 2.86 inches, and the actual compression ratio that the engine sees , could be down to 7.94, and we call that Dcr or Dynamic Compression Ratio.. I mean I'm just guessing.
To illustrate the point. This 7.94Dcr represents less cylinder pressure than say 8.0 Dcr. And less pressure translates to less power, especially at Part Throttle, and that translates to less energy being transferred to the crank at a given throttle-opening, and once again, this requires MORE throttle opening to maintain a given roadspeed.
These are the three major penalties to fuel economy from choosing a cam of that size and configuration.
The next penalty will be ignition timing.
Cruising at say 2250 rpm , your engine might want hi 40s to mid 50 degrees of timing advance, to cause the peak energy in the fuel being burned, to arrive at the flywheel in it's maximum potential to do work. Your factory-type distributor can only rarely deliver the right amount of advance, at 2250, and only with a lot of work. So there is the fourth penalty to fuel economy.
Food for thought.
My personal discovery, showed power extraction, favorable to fuel economy, starts at about 110*. As to overlap, 50 degrees is already showing itself to be quite a bit. What I discovered is that for fuel economy, with power, I had to slam the valves shut and whack them open to prevent these three penalties from taking over. And I had to let the engine pick the cruise rpm, not me dictate it. And after that I had to figure out how to supply the required amount of ignition advance.
As to the ramp length from .050 to the advertised;here's the deal; for a given duration @.050 each cam type, has it's own acceleration ramp characteristics. Typically a hydraulic roller might have ramps of 54* from the .050 spec, to the advertized spec. It could be a lot more to a little less but 54* is likely typical.
A solid roller can be a whole bunch faster, And I have heard of one having ~39 degrees; which is really short.
Flat tappets are generally faster; from 44/46 degrees typical for hydros to maybe 38 for solids but I seem to recall seeing 29* one time.
However, the advertised numbers, on hydros are NEVER from .050 to intake valve actually closed and not leaking. That point could be many many more degrees later. Whereas with solids you control that point precisely with adjustment of the valve lash.
And then, after you have chosen the type of cam that best suits your needs, you still have two more choices . Within each type of line, you still have a broad range of acceleration ramp lengths, and they don't always all begin from the same advertized point.
And finally, the LSA plays a large role in the total degrees of compression plus extraction.
In the cam I built at the beginning of this story, namely the assumed to be
238/244/110, which I doctored to look like
292/300/110+2/106/98/76 overlap; compression plus extraction is just 204degrees. You can advance/retard the cam anywhere you want which will trade degrees between compression and extraction, but the total will remain the same. 204 is pretty small for a streeter. More often with a flat tappet hydro, we see 220 to 230 . So right away you can see that the long ramps on your roller, are stealing from compression plus extraction. This cannot easily be compensated for. Nor does it have to be in every case . For instance, for a drag-racer power extraction degrees can be sacrificed because the engine will still make more absolute power at higher rpms, even tho each individual cylinder is less efficient due to the early opening exhaust valve.
But for a streeter that spends most of it's life below say 4000 rpm, IMO, it make no sense to sacrifice that efficiency.
One of the tools for changing the compression plus power( c+p) is by changing the LSA. For each 1 degree change in LSA, there is a 2 degree change in c+p. Therefore reducing the Lsa to 106 from 110, is 4 degrees. And times two will post in increase in c+p of 8 degrees , or from 204 to 212. You can put those new-found degrees to either compression or to power or split it any way you want. If you put it all to power then your power extraction will increase from 98 to 106, but I can tell you flat out that your fuel economy will still suck. For two reasons; 1) changing the Lsa from 110 to 106 is 4* and your overlap will increase by double that so from 76* to 84*,and you already know what overlap does at low rpm. And 2) 106* of power extraction still leaves a lot of energy in the expanding gasses going out the tailpipe.
So where do you draw the line?
IDK, every person has to decide what is most important to him. A bias in any direction always calls for a sacrifice in another.
I gave up the 292/292/110+2/108/104/76 overlap, because the c+p was only 212. It made horrible fuel mileage.
My next cam was a
270/276/110+2/117/110/53 overlap with a c+p of 227. Fantastic mpgs. Why? Three reasons; 1) the overlap lost 23* and 2) the Power extraction gained 6 degrees, and 3) the c+p increased 15*!
When that cam dropped lobes I moved up just one cam size to
276/286/110+2/115/104/61* overlap, with c+p=219. And the mileage dropped into the basement. How could that be? Three reasons; 1)overlap increased 8 degrees and 2) power extraction decreased 6*, and 3) c+p decreased 8*.
As you can see, these numbers sound really small, you know 8*/6* and so on, and I had no clue such small numbers could have such big results. Lemmee tell you; that measly 6* of power extraction cost me hundreds of dollars to compensate for, and I never got the mpgs back. Not even close. But oh-yeah, the top-end rush was worth it! Right? Yano, the rush was closer to the 292/108, but I really missed the inter-provincial traveling. Just 6* of power extraction and 8* of overlap, absolutely killed it. And here I am 14 years later,waiting for that stinking cam to die, so I can finally put a solid lifter cam in it's place,lol.
Food for thought.
