I apologize right now for the long, confusing post. I always say it takes more words to correct an error than it does the post the error. This is an example of that.
It is a long read but IMHO it’s worth it.
I love how the first paragraph says he has the FACTS and not opinion.
That right there makes it suspect. But you do love your confirmation bias. That’s a fact.
What IS the fact is that not all engines need all that timing at idle. In fact, most do not need it.
I’ll use the OP as an example.
If his compression ratio is what he says it is, and if his cam timing is what he says it is, then at the very most it will need is 20 degrees at idle. And I give that about a 10% chance.
More likely it will need to be in the 16-18 degree range. Any more than that will do nothing but jack up the timing curve.
Let’s break it down both ways and see what it looks like. I have to make some assumptions here because the manifold vacuum always crowd never explain it in detail. Here we go.
Assuming the above engine needs the 30 initial claimed, you must use manifold vacuum advance because you can’t have that much initial mechanically and get the curve correct later in the rpm range. But wait…why am I agreeing with Bewy? Because if the compression ratio is low for the cam timing you either need to raise the compression ratio (expensive and labor intensive) or you need to reduce the cam timing which is also expensive and labor intensive. Or if you are way wrong on your build you may need to do both. Double expensive and labor intensive.
The quick and dirty way to deal with what I’ll call compromised build is to hook up manifold vacuum advance and jack a ton of idle timing in it.
It’s the cheapest and least labor intensive way to help correct a bad build.
It has serious drawbacks backs. For one we are dealing with a mechanical system. It takes time for the vacuum to drop enough to keep from getting tip in rattle. I hate tip in rattle.
We are not talking about several seconds of slow movement. But it’s slow enough to cause issues. Think it through. Let’s say it takes .5 seconds to get the MVA off. How long does it take for tip in rattle? Much less than that.
You have the surface area of the diaphragm to activate the mechanism. The bigger the surface area the slower the response. Then you have the actuating arm which rotates the cam plate and you have the cam plate.
All that **** has mass. All of it is moving. And it takes time to move it.
With a ton of timing to reduce at a tip in cruise you get the rattle. What else does it affect?
Well, once you give the distributor full time vacuum it’s always on. If your idle is 1000 rpm and you need the 30 claimed above, and your vacuum can starts adding timing at 11 inches and you have 10 inches of vacuum you either need to adjust the can to pull timing at less vacuum OR you need to increase the mechanical initial to cover that up.
So let’s say to make all this **** work somewhat together we decide we need 20 mechanical initial and 10 from the can. We go to the distributor machine and make it all happen.
We put it in the car and it’s rattling its brains out. Why is that? It’s because with that much mechanical initial you can’t shorten the curve enough to take enough timing out by peak torque rpm with the vacuum cam and it rattles. Again, why is that?
It’s because almost all engines (and I argue that ALL engines like we are discussing) want a timing curve. And that curve is based (basically) on peak torque and peak power.
All engines want MORE timing at peak power and LESS timing at peak torque.
We have now made the timing curve with enough initial that even when the vacuum comes off it has too much timing and it rattles.
You can continue to go around and around trying to tune the rattle out of it by varying initial timing, trying to slow the curve down (which in turn when you get to full power/low vacuum affects the curve) or worse yet limiting the total advance, which in turn kills peak power.
If you use MVA rather than timed (ported) VA you can not change the point where the VA starts because you’ve already started it.
Does this sound confusing and circular? It should because it is both of those.
Of course if I’m wrong and the OP’s build is as screwed up as some think it is, then MVA will not cause a tip in rattle nor will it show a power loss on the dyno.
But Turk, you can’t tune and test for this on the dyno. You surely must know that.
I beg to differ. You can do it if you want to and you know how to do it. It’s not hard. It is time consuming.
What you will see in the power numbers on the dyno is the inability to get the timing curve correct for peak torque and peak power. You’ll kill power at one or the other. Sometimes both.
If you have to increase the mechanical initial because you don’t have enough idle timing with MVA then you screw the curve for the time when the MVA comes off. Too much initial relative to MVA means without MVA the curve will have too much timing too soon.
We can continue to try and clean it up but all that happens is you shift the crappy part of the curve to a different rpm.
And what about the claims of engines needing as much as 50 (FIFTY) degrees of idle timing? How in the world do you get a curve correct for the times when the MVA is off? You can’t.
Let’s look at the other end of the curve.
Chrysler small blocks generally need 35 total or less. If the quench is tight you might only need 30 total. The BB stuff with OEM heads might need as high as 40 total. If you have aftermarket heads you’ll be in the 30-35 total range.
Saying that and agreeing that it a fact (it is) then how in THE hell do you deal with 30 degrees of initial timing when the engine at full song only needs at the maximum 5 more degrees of timing at that point? Or if it needs ZERO added timing when the MVA comes off. You don’t.
You may be able to do it if a computer controls your timing but no way can you deal with that type of curve mechanically.
What if the claim is 50 at idle with a total timing need of 30??? Or 35?? Or even 40??? Again, it’s an inverted curve and getting that in shape mechanically is near impossible. I can’t say it’s impossible because someone somewhere wrote an article on how they did it, but of course there is no verifying testing to prove it.
Of course if the engine really needs all that initial and you don’t want to fix it the correct (hard) way then use MVA.
TL;DR it is very complicated to make MVA work if the engine doesn’t need it. Most don’t. The moral of the story is build the engine with the cam timing that fits your compression ratio and you won’t need crap like MVA at idle.