I've read several articles on setting ignition timing on a SBC and I am somewhat confused on the best course of action to take. My 406 SBC has a stock vacuum advance on a stock HEI distributor. The ignition timing set at 4 degrees BTDC, ( The engine is just built for low rpm torque: Small cam, quadrajet carb, longer rods, balanced, blueprinted, stock heads, 9.5:1 compression...not too modified.) It runs quite strong, but does have a dead spot right when I stomp on it (bad accelerator pump?) Questions, 1. If I replace the stock vacuum advance on the distributor and put on an adjustable one, and push the initial timing up to 14 degrees or so, will that help or hurt fuel mileage? 2. Is it worth the effort and cost on a low RPM engine? Will l see any performance gain? 3. Should I replace the mechanical advance springs and module as well? 3. If it is all worth it, what are the best numbers to set the initial and total timing at? Thanks.
Its hard to say exactly what your motor will like, but more likely than not its not ideal with the set up you have now. In general they will USUALLY like a little more initial and a bit quicker mechanical curve for performance. You will just need to experiment and see what it likes. The vacuum advance will help the economy most times over not running vacuum advance. Even on a low RPM motor the timing can make a pretty big difference.
Just a observation here,but you do know that performance & mileage is a oxymoron don't you. You can have one or the other but I don't think your gonna have both. HRP
Some good reading. Here is a link to an article that does a very good job on explaining ignition timing and vacuum advance. Probably the least understood area of tuning an engine. http://www.lbfun.com/warehouse/tech_info/timing & vacuum advance/Vaacuum_Advance_Specs.pdf
Rear gear as well as rear tire diameter should also be considered when tuning for mileage and/or performance.
Indeed. It all has to work together. I have super broad and long spreadsheets which take into account just about everything you can think of. My Falcon gets 33MPG on the freeway, at 75, with a hopped-up six. My '29A can roast the tires in 1-4 (of 6), with a hot V8, and gets 28.5MPG on the freeway, at 75. Both have carburetors. True performance equals zero waste.
Yup... but with limitations on the distributer, or any piece of an internal combustion engine, it only works most efficently at certian rpms.... I personally am interested in the textbook science of timing, especially since i got an allen distributer machine
C9 had an EXCELLENT post YEARS ago on here .... the BEST tutorial on timing I have ever seen ... I don't know if there is still an archive for old posts on this new version so it might be hard to find -- maybe somebody can post it here .....
I agree....the better the car runs.....the better the gas mileage . I does sound like the carb needs an accelerator pump....take the air cleaner off, look in the front section of the carb and quickly give it gas...you should see two streams of gas going into the carb from close to the top of the carb in the front barrel section....if no gas streams shoot out....then fix the carbs pump. One way to set timing on a chevy is to let the engine go to normal heating....advance the dist a little, when it barely turns over, retard the timing until it starts without going "runt..hesitate...runt" ( turns over like the battery is really weak). My vette 327 is set at about 12 plus btdc...and my 283 in my 49 is set at about the same...both run great and start great.
This is also why it is so critical that every component that you select is a good match for what it is expected to do. You can't just grab a pile of stuff, throw it all together, and expect it to work. It does take some figurin', but it can be done. My best to-date tune has been 7MPG to 21MPG. Customer thought it ran pretty good at 7. His only complaint was the mileage. Now it is quicker, faster, and has triple the mileage. Up to the extent of the maximum efficiency of all involved parts, and the constraints of the internal combustion engine, what is not turned into power, is waste. Those who tell you high performance must come at the expense of efficiency do not understand the laws of physics. High performance IS high efficiency.
I'll check the accelerator pump as suggested. The tailpipes are pretty black, so it is running rich. The plugs are not as black, but do seem to be rich as well (checked after a 200 mile highway trip). My truck currently gets 12 mph highway and I want to improve that as much as possible. I want to get this COE running reliably on the road asap so I've been working it out every day. If I want to lean it out, how do I tell what parts are in the carb (rods and jets) and which do I change first? Any good and affordable Quadrajet tuning guys in KC?
I just read this and so wished I had this knowledge 40 years ago. It all makes sense now. I have always used manifold vacuum but did not understand till now the different requirements needed for performance engines. So I guess now I know what an epiphany is.
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