SBC Rocker Arm Ratio
 

I was wondering what effects I might see if I change the intake valve ratio on a SBC. Here's my reasoniong: the stock rocker arm ratio is 1.5:1. If I change the intake rockers to 1.3:1 (using cam break-in rockers), then I will see less intake valve lift. Extraction (exhaust valve) will be unchanged. Since the intake valve is opening less, less fuel/air mixture can enter cylinder at any given rpm. The ECU will adjust mixture to stoich as long as I am running in closed-loop. In order for the engine to produce the same power, given the smaller valve opening, then I will need a larger throttle opening. This reduces pumping losses, slightly improving volumetric efficiency. This in turn might translate into slightly higher mpg. Peak hp of the engine would be reduced, of course, and that is the trade-off.

I welcome informed comments as to whether or not the above might be true.

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.

The guys that could really answer that would be the cam company techs.

It may improve volumetric eff at certain loads, but in terms of the pumping losses that result in the largest efficiency drop in gassers, it won't do anything because those are a result of limiting the amount of air to enter the cylinder. This can be done via a throttle, or valve, but as long as the cylinder can't completely fill up with gases efficiency will suffer. Honestly, the only improvement I think you could count on from something like this would be what you could see from limiting your right foot.

Well I did just about the same thing to my sbc, not 1.3 rockers, but I did put antipump up lifters under the intake pushrods. Obviously this lowered the valve lift at the lower rpm, and I have a running dispute with another motorhead as to whether the effective duration was shortened.

The compression at the low rpm increased, quite similiar to advancing the cam. If you look at the xfi cam thread it speaks of lower lift, peak torque at a lower rpm, and smoother idle. I do know that going with higher lift and more duration( more overlap) lowers low rpm torque and raises the rpm that the power range and peak hp occurs.

Instead of swapping the rockers, I'd swap the cam. With the SBC you have a world of choices, including economy cams. An econo cam would not only change the lift, but also the timing and duration. That way, your whole valve train would be tuned for economy, instead of just changing the intake lift.

I wish I had as many cam choices for my Cavalier. My only options seem to be stock, two racing profiles, or a custom grind. :( I think I'll just advance the stock cam 5 or 6 degrees, and call it good.

To roflwaffle:

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?

To oldschool:

Hmmmm can't you use a dial indicator at the pushrod end to compare duration?

DO you see any change in mileage when driving economically?

Yes this is true, but the difference is I can swap out all 8 intake rockers in 30 minutes and be done. A cam swap takes longer and costs a bunch more. If/when I go deeper into the motor I would of course swap cams. I could still experiment with the 1.3 rockers. Which of course raises the issue of how to design the most economical conventional SBC. That would be the real home run here, and to do it right would have to incorporate all-electric accessories and stop/start capability.

WHy don't you swap in the powertrain from a Prius into that Cavvy? :) 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.

In my experience they only know what makes power. Mainly because that's how they stay in business. <smile> But it is worth a try. I have a friend who runs the dyno at a race engine shop that only builds Chevy's. Maybe I can get him to run some baselines. Then we would have actual numbers!!!! Including fuel consumption in pounds per hour per HP, over the rpm range. But time is money for them and I don't know if the boss there would allow a science project. LOL

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?).

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.

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?

Why didn't I think of that?!

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

if you are really despirate...look here www.hotrocker.com

to Plym49, #8:

my truck with the antipumpup lifters got around 20 -21 mpg at highway speeds driving for economy, that with a 3 speed manual.

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.

I have made a lot of changes over the years for various reasons, so it is tough to select the right baseline. But for purposes of this thread, I'd say 19 plus.

I don't know your engine. But GM likes to not reinvent the wheel. Are you sure that you don't have the same stamped steel rockers as on a SBC?

If so I can buy a set of 16 1.3 rockers and sell you four.

The whole idea is to reduce power (reference point same rpm and same throttle opening). So to generate the same power (IOW what is needed at that point), you have to open the throttle more so volumetric efficiency goes up. Maybe.

There is no such thing as working harder. Power is power.

Sounds right. Mine is a 4x4 so it carries more weight, plus the A/C is usually on, etc etc.

I'll have to disagree too, malibuguy. While that blanket statement may be true when searching for maximum HP, the FE goal is to increase torque at lower rpm's. The link you supplied to hotrocker.com (nice link BTW) shows they got max torque up to 2000rpm using only a 1.1:1 rocker ratio. That would drop a stock lift of 0.414 to a mere 0.304. :eek: And all the economy SBC cams which I can find specs for, have reduced lift and duration. So 'undervavleing' seems to be the way to go for FE.

plym49 - another trip back to j-body.org, for spec's on my rockers, shows that not only does my 2.2L have a SBC profile cam, it uses the SBC rockers too! Sweet. If your experiment works out, perhaps you'll sell me four of your unused rockers ;)

oldschool - any clacking at low rpm with your anti pump up lifters?

OK. If I do this I will let you know. I think that a set of 1.3 break-in rockers is about US $100, and I believe that they are only sold in sets of 16.

you guys are re-inventing muller cycle industrial motors. they have work scene the 1920. the hot gas still have expansion when the valve opens. muller cut back on the intake charge. this cut down on the hot waste gas going out. if really big motors were used with muller motors you would have slower reving motors with more tork and more efficacy. come to think of it a high line mazda used this with superchargers instead of a bigger motor.