Anyone have any experience from recurving the distributor advance? Results? Most people know that for good performance and economy you need the right ignition point, but it may be less obvious that the ideal point changes with the rpm and load and centrifugal advance and vacuum advance is used to (hopefully) get the total advance close to ideal all the time. I've seen a few clips on youtube lately where people modifying distributors state that with modern fuels we want some changes as the fuel works better with more initial (low rpm) advance, while we don't want more at high rpm, so just moving the distributor doesn't work, that would lead to too much advance at high rpm. A high performance engine will need a different curve than a low performance stock engine, so that's also an example when this may be suitable. I'm just curious, don't need to work on any distributor now but you never know what projects turn up tomorrow...
I recurve all my distributors in an effort to maximize fuel economy. Every engine has different needs and you have to find out what has worked for others and then try to replicate it and go from there. The most aggravating part is pulling and reinstalling the distributor multiple times. In general, you want to get your mechanical advance set and usually "all in" by around 2500 rpm. You also might have to limit the amount of mechanical advance, depending on how much initial advance your engine requires at idle. After you have that done, you begin experimenting with the vacuum canister and usually limiting its advance since you have the mechanical coming in quicker. It's a process of "try it and see". To give you an idea of how different engines have different requirements, a typical SBC will usually want around 34 BTDC with mechanical advance "all in" at 2500 and the vacuum disconnected, whereas my Nailhead wants about 30 BTDC. The SBC will want more advance at idle and usually the vacuum is connected to the manifold port which adds the vacuum advance to the initial advance at idle. This gives the SBC mid 20's BTDC advance at idle. The Nailhead, on the other hand, doesn't want but about 12 degrees BTDC at idle, so you use ported vacuum. (This means the vacuum doesn't come in until you open the throttle.) As you can see, it's not a one size fits all but it is worth the effort to optimize your advance curve if you drive you car on the street. If you just drag race it, just work on the mechanical and go with it. Head design, cam selection, compression ratio and other factors determine what the engine "wants". That's probably more than you wanted to know.
I have always run sbc gm point distributors and use the lightest springs in the curve kits. I want the advance all-in between 2000-2500 rpm. Like was said above, every motor/combo is different.
We had a local genius with distributors when I started playing with hotrods, around 1975. You would bring in your distrib., give him info about your car, weight, gearing, type of trans, engine mods, etc. and he would set it up on his machine. Every car I had him do, felt like he gave it a HP increase. You could feel it, in the seat of your pants.
We revisit this with new insights due to the gasoline we have today. There's indeed credence to changing the timing curves. While today's fuels are better than the days of old they also lack the resistance to higher RPM detonation tendencies in standard blends. Old combustion chamber designs don't extract the low RPM energy that new designs do, and that's a result of chasing efficiency and fuel mileage in modern times. We get more work at lower RPMs than before, but raise em and we need more octane or a different timing curve to avoid detonation. Good place to remind us that detonation (different from pre-ignition or pinging) is a silent killer.
I know we all hate spending money on something we can do ourselves. However, this is an area where having the engine "Dyno tuned" really pays off. The engine shop I go to, Boyd's Motorwerks, likes to break in and then find the near optimal settings on his engine dyno. And no it's not cheap. However if you have him do the ***embly as well, any oops are on him. So you get a properly broken in engine, tuned for the conditions of the day. And this, in my experience, gets you to where most of us would never have gotten an engine to. The final couple of percent only brings your engine closer to boom! And ch***is dyno tuning while also not inexpensive, works. I have many years of experience with both ch***is dynos and engine dynos, performing emissions research. And I can tell you that either way is an excellent way to sus out issues with calibrations. Going back to the expense, how much do you have in the engine? Balance that against the cost of the dyno time. Ask your engine shop about it on your next build. It seems like a good expenditure to me
Right. You didn't say what you're after, if its total all out performance then an outside source for recurve that knows how do it is the way. Otherwise, if its just a street motor or warmed up stocker, "all in" by 2500-3000 is just fine, total 34-36 (sbc)(depending upon the motor). You can do this by just getting a set of springs or kit that gives the right number. Oh, and a good timing light-dial back preferable.
SBF's are the same way ^^^^^^^^^^^ Most like 36* all-in around 2000 RPM or so.( stick shift cars maybe 1500RPM) Most DO require a limiter on the total advance . When tuning for total advance of 36* all-in by 2000...the initial advance setting is irrelevant. 6sally6
I guess I'm just old school. I don't trust those dial back lights. All you need is a degree marked balancer. It is easy to mark a balancer yourself. The last few I have bought were already marked. As a matter of fact, now that I think about it, the last new timing light I bought has a dial back feature, but I never use the feature.
I didn't trust 'em either until I got a top shelf one and learned how to use it properly. I discovered I wasn't nearly as good as I thought. --and my old ears, which have certainly seen better days, and eyes have also let me know that some ***istance is definitely needed.
"It's easy to mark a balancer your self". Yep. Straight eight Buick, perhaps others, have the timing mark on the FLYWHEEL! So I marked the balancer and installed a pointer. Not rocket science. Ben
This thread takes me back to the 60s when I had an Olds F85 that I was gradually buying and bolting on various speed parts as I could afford them. One of the first things I bolted on was a set of ****** Headers but I noticed that the power seemed down and it showed signs of being lean. I was living in the Chicago suburbs back then and with a short drive to the city, Mr. Norm's dealership was selling "Power Tunes" on what was the first or one of the first dynos available to the public. It was a first come, first serve drive-in deal so I drove my happy *** in there to get me some. I waited while 2 or 3 cars went through ahead of me and when they opened my hood and saw the Quadrajet, they said, "We don't know nothing 'bout 'dat". Turns out, they were ****ers on anything but Holleys, and even with Holley carbs they had a general cheat sheet for what jets, distributor weights, and springs to use, mostly for Mopars but a few others I suppose. The customer got one 'before run' and one 'after run' and as long as the peak numbers were improved, that's all you got. I can't say that they attempted to maximize to find every hidden horsepower or pound of torque. These are the reasons why I labeled them "****ers". I did get the numbers for my car, as it was in its ultra-lean condition. I vaguely recollect it was something around 180hp at the rear wheels. I was somewhat crestfallen because I had been bragging to another customer about the numbers I was expecting to see. That was embarr***ing and a lesson to keep my bragger shut. There was no internet, no chat groups, etc., in those days so I did the best I could with magazine articles and asking lots of questions of the big boys. I bought a distributor advance kit and pulled the factory metering rods and took them to the shop where my dad worked and explained to him how I wanted the rods re-tapered 'richer'. So 15 minutes later I had my own "Power Tune Kit" based on my best guesses. I went back to Mr. Norms for my 'after run'. They ran it like I brung it and I think I recall, my peak HP bumped up to 280-ish at the rear wheels. I felt good about that. (With the advent of the new-fangled dynos, we began to open our eyes to the difference between "advertised horsepower" and "actual power available at the wheels". It was years later, with the aid of computers and printed power curve read-outs, etc., before we could get a quantum leap in info out of those 'dynos'.)
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