This sounds like a sensible idea. Many modern engines use drive by wire and intake cam timing in order to maximize throttle opening in order to reduce pumping losses, so I would say that this makes a bit of sense in the same way.
Yes, they vary timing. I'd be changing lift, and it would be static, not variable. But maybe it is similar in reducing pumping losses. The big questions are (1) can this work and (2) if it does, does it make a measurable improvement and (3) does it hurt something else (timing? driveability?).
I'm not sure that I follow you. You would have to run with a larger throttle opening to produce the same power. Your statement about filling the cylinders with air seems to neglect the effect of the throttle plate. Or am I not understanding you in some other way?
What I'm saying is that it doesn't matter if the throttle or the valve limits air flow into the cylinder, either way the engine will see pumping losses of the big kind. By that I mean the ones associated with having the cylinder not fill with gases all the way. EGR in modern gasoline engines allows for pretty decent efficiency above 30-50% throttle, but below that, since the cylinder can't fill up with air, BSFC gets progressively worse. Here's a fairly new gasoline engine, and here's a heavy duty diesel. Notice how the diesel is fairly efficient over the entire range of power that it makes, which the gasser isn't until the torque it's making/air it's pulling in is sufficiently large. Those low load losses with gassers are what I refer to when I'm talking about big pumping losses. Little pumping losses are volumetric efficiency at different engine speeds. So, while changing the lift may help flow at certain engine speeds, for a few percent increase in efficiency, changing load via gearing, or other methods, can change efficiency much more than a few percent.
That being said, changing the lift after changing the gearing may result in way more benefit since changes in VE are magnified at high load, at least that's what I've read.
What I'm saying is that it doesn't matter if the throttle or the valve limits air flow into the cylinder, either way the engine will see pumping losses of the big kind. By that I mean the ones associated with having the cylinder not fill with gases all the way. EGR in modern gasoline engines allows for pretty decent efficiency above 30-50% throttle, but below that, since the cylinder can't fill up with air, BSFC gets progressively worse. Here's a fairly new gasoline engine, and here's a heavy duty diesel. Notice how the diesel is fairly efficient over the entire range of power that it makes, which the gasser isn't until the torque it's making/air it's pulling in is sufficiently large. Those low load losses with gassers are what I refer to when I'm talking about big pumping losses. Little pumping losses are volumetric efficiency at different engine speeds. So, while changing the lift may help flow at certain engine speeds, for a few percent increase in efficiency, changing load via gearing, or other methods, can change efficiency much more than a few percent.
That being said, changing the lift after changing the gearing may result in way more benefit since changes in VE are magnified at high load, at least that's what I've read.
OK, I've got you now. One of the reasons that a diesel has better volumetric efficiency is that it runs all the time at WOT. So there are much fewer pumping losses caused by a throttle plate, and as you point out it is all from the valve in that case. In a gas engine, we have both the throttle plate(s) and the valves. So, would this change make any difference? I suspect it will, but not that much. (Obviously, there is some relation flow-wise between the two, and if we have any engineers on this thread maybe they can teach us the equations.) BTW I have already taken care of low-hanging fruit like gear ratios, etc. so I am at the point where I can try tweaking a few things, or else I just yank out the entire motor and either rebuild it to high-mpgs standards, or replace it with something else. This is more time and money and I am not going there now. This mod is also very easy to implement and totally reversible, so no harm done and we learn something -- if I do it.
Is there anything in common with this mod, and the Miller cycle? In the Miller, IIRC, the intake is kept open way longer so some reversion occurs.
What's your gearing at right now? Anything above ~1300-1500rpm@55mph in top gear isn't ideal IMO. If the intake valve stays open long enough for the cylinder to travel upward and push some of the air it sucked in out, then it would be similar to the four stroke Atkinson cycle. You can go ahead and try it, but just make sure to test it thoroughly as per the whole A-B-A regime with reasonably consistent conditions in order to get enough good data.
What's your gearing at right now? Anything above ~1300-1500rpm@55mph in top gear isn't ideal IMO. If the intake valve stays open long enough for the cylinder to travel upward and push some of the air it sucked in out, then it would be similar to the four stroke Atkinson cycle. You can go ahead and try it, but just make sure to test it thoroughly as per the whole A-B-A regime with reasonably consistent conditions in order to get enough good data.
Gearing is 3.73 with very tall tires (oversized, aftermarket). Revs at 55 are about 1200. (Top gear, lockup and OD all engaged of course.)
Well then you're as set as set can be IMO and I think you may see some benefit from less lift. Maybe not. It'll certainly be interesting to try. Just outa curiosity, what kinda mileage do you see round trip cruising at ~60mph?
Porthos - '96 Chevrolet Cavalier 90 day: 31.3 mpg (US)
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Quote:
Originally Posted by plym49
WHy don't you swap in the powertrain from a Prius into that Cavvy?
Why didn't I think of that?!
Quote:
Originally Posted by plym49
If you are running the GM V6, I believe that you have a cut-down SBC and you should be able to try the same rocker trick.
My Cav has the 2.2L I4 OHV. They say it has the same lobe profiles as the SBC, so anyone who grinds SBC cams could regrind my cam. But since they can only remove material from the existing lobes, I have question how much of a difference they could make from the stock lobes. As you said, cam swaps take a lot of time and money, and I'm not sure I want to put that effort into a project with unknown results.
Advancing my cam will still take a lot of labor; Probably half that of a full swap. But the cost will be minimal, and I know it will move give the FE tuning I'm seeking. Moving the power band into a lower rpm range (via the cam advance) and going with higher gearing will probably be the best thing I can to with this engine/trannie combo.
But enough about my car. I can't wait to see the results if you do the rocker swap. Make sure you get us some good before/after numbers. With tables, and charts, and grapshs!
you'd prolly gain much more in all directions switching out to roller rockers, or even just roller-tip rockers
any 2, non-canted valve engine is pretty much undervalved...so reducing the lift & duration of the intake valve will just multiply that, plus now your potentially dropping power, which can make the engine work harder, thus using the more fuel, to move the car the same as it would with a stock ratio rocker
any 2, non-canted valve engine is pretty much undervalved...
I disagree. There are plenty of two-valve engines that have the room for larger valves and yet they don't come equipped that way. This is because their induction needs are met by what they came with.
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