I have a 1962 261 in a Canadian 1962 Pontiac Stratochief that I've recently converted to HEI. It runs great but I forgot to map out the old points timing curve before doing the conversion and the info is hard to find out there.
The HEI is a modified new unit spec'd for a 250 but I did have to go through it and tweak it a bit to make the advance mechanism work nicer. I have an assortment of GM weights and centers, as well as aftermarket combos and various springs. I run 91 octane (no ethanol) and there are no engine mods other than a 2" mandrel bent exhaust I built.
So what kind of curve do these engines like? I'm fine with trial and error but I'm hoping to find some base numbers to start out with. Initial? Total advance? Amount of vac advance?
Initial isn't hard to figure out but total timing is where I'm not sure. I have a feeling the "standard" 36 degrees total may not apply here?
This page on tuneups might be helpful as well, and you might contact this guy. He might have a lot of useful info to provide: http://devestechnet.com/Home/TuneUpGuide
"Dragsix" has better information on stovebolt distributor setup from his work with a Sun distributor tester than all of the above "cut and paste" information from a guy with no hands-on experience at actually making these engines run right. You can dial in a distributor advance curve pretty easily with a delay-flash timing light and a tachometer if you don't happen to have a Sun machine handy. The combination of initial and centrifugal advance should be somewhere in the mid-30's, with an additional 8-10 degrees added by the vacuum advance for light-throttle cruise. Once a load goes onto the engine for hill climbing, passing, trailer towing, etc. the vacuum advance drops off and does not enter into the total until the load goes away again. One degree of over-advance will cause as much of a power loss as 4-5 degrees of retarded spark from the optimum advance point, so always err on the side of "not enough". I've got hundreds of hours of dyno run experience on all sorts of engines, and that fact is pretty much universal, no matter what engine is being tested. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Here is the info you asked about as published in the factory specs for the '61 Chevy 261 Jobmaster, US-spec truck engine (extracted from the Vehicle Information Kit, courtesy of the GM Heritage Center). Not sure it is terribly helpful, since the intended use of the Jobmaster was heavy duty use.
The '62 VIK doesn't include this info, but the distributor PN 1112403 was the same from '55-'62, so I don't think the factory likely changed the advance curves during that time frame. The last image is the similar data from the '61 Chevy passenger car VIK for the Hi-Thrift 235 - same advance curves as for the truck 235 and 261, which is no surprise since it used the same distributor PN.
Initial timing setting for the 261 Jobmaster was spec'ed in '61 as TDC and total advance was only 24-28 deg BTDC at 3500 RPM. Vacuum advance was a heady 22 deg at full vacuum (> 15 in. vacuum), according to this data, but practically, limits to less than about 8-10 degrees for moderate throttle opening (6-8 in. vacuum).
So, by "normal standards," the factory timing shows as significantly retarded, likely due to the intended use of the HD truck engine, which would have seen lots of low RPM, full-throttle use, and the expected use of lower grade fuel. I would expect the Pontiac passenger car 261 may have had a more normal timing curve, initial setting and total advance, probably low to mid-30's total advance and something like a 5 deg BTDC initial setting (similar to the passenger car 235), as Jerry suggests. The timing marks on the flywheel/flexplate may be stamped with the numeric initial setting. Today's fuels will support a bit more total advance, as well.
FWIW, the stock advance curve in the '403 distributor seems to work pretty well, in my experience, both for both (US-spec) 235 and 261, not to say it couldn't be tweaked.
Thanks Jerry. You’re right. The Delco Remy spec for 2403 Is sort of variable at 12-14 Degrees All in at 1750 rpm, so about 3500 rpm. I have put about ten 2403 distributors on my Sun machine. I rebuilt about half of them, one was a nos unit, 4 were not rebuilt. The nos and unrebuilt got to about 11 degrees. The NOS did not come up to specification because the old but original grease had dried up, gummed up, and prevented the weights from working properly. Once I took it apart and cleaned it, it was in at 13 degrees. The rebuilt units did better, most got to 12-13 degrees, one got to 14. So you have anywhere between 24 and 28 degrees of mechanical advance. I have attached a 55 and 59 AEA and Sun tune up cards so you can see the different settings depending on what year vehicle.
So depending on the year, the initial is either 0 or can go to say 5 degrees for a total advance (with out the vacuum advance taken into consideration) 24 to 33 degrees depending on what you dial in for the initial. there were no real changes to the motors, except for 59 oil system changes, so it is somewhat clear to me at least that any 235/261 can take a few degrees of initial even if the specification is for TDC. And frankly, that has been my experience over the years.
Those are factory settings for the 2403. Pat Smith, former president of inliners international, and the long time crew chief for Dyson Racing IMSA cars, had a 261 that think ended up running its best at like 42 total degrees, so 28 at the distributor (14 degrees distributor advance X2 for what the motor actually sees) and 14 initial degrees for a total of 42. That was with decent gas. Pat's method was to keep advancing a degree or two until the motor pinged, and then backed off a couple of degrees.
So really what you end up with depends on what the motor likes. But there is a caveat, because total timing is impacted by the vacuum advance. So with a 2403 distributor you have to balance the mechanical and initial so that the vacuum advance does not create a pinging event. It’s not adjustable so it can be a little trial and error.
Your HEI is going to be different. You can add an adjustable Vacuum can to the HEI to help dial in the timing curve. But first you need to know what the HEI’s mechanical curve and total mechanical advance is, and then make changes with Weights and bushings and sort out what timing the motor likes. Tom Langdon wrote a little article on vacuum advance.
Just one more comment, Mike- - - -some people don't realize that those Sun specifications are published in "distributor degrees"- - - -half what the crankshaft and pistons see because the distributor turns at 1/2 crankshaft speed. One of my earliest memories around the shop was watching Dad set up dual coil V12 Lincoln Zephyr distributors that had been modified to run flathead Ford V8 race engines- - - -actually two distributors in a single housing, each powering 4 cylinders. The 8-lobe cam from a V8 distributor was installed in the V12 housing and was reground to 4 lobes, and the coil saturation was high enough to throw a hot blue spark 3/4" long- - - -on 6 volts! An 8 volt battery would really light things up! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Right. It’s one of those things, I have known that for a long time and just forget Sometimes that not everyone else will know that so I made a couple of corrections. If anyone is confused, let me know and I will try to do a better job at explaining.
Great info, exactly the sort of stuff I was looking for.
I have played around with this stuff on a small block chev, but wasn't sure what kind of numbers the 261 likes. It is sounding like not a lot of difference between the two so that's good.
The distributor is also a new cheap chinese made unit, the advance mechanism needed tweaking and the weights, etc. leave a lot to be desired. I'd like to set it up with better parts anyways.
I had a long post written about dyno testing and timing advance, and my web server dropped offline and ate it! Basically, I duplicated the findings my father got with chassis dyno runs of a dirt track racer with a flathead V8 and alcohol in the early 1950's with a series of water brake bellhousing dyno pulls on small block Chevy race engines 50-something years later. Both series of tests proved that one degree too much timing advance at full throttle robs as much power as 4-5 degrees of late timing at the same engine RPM and load. Once the "sweet spot" of maximum spark advance is found, any further advanced is trying to run the engine backwards.
"Run it uphill and advance the timing until it pings" and maybe timing the run with a stopwatch is simply a very inaccurate way of simulating a dyno pull without having sufficient control over a bunch of variables such as air temperature, relative humidity, atmospheric pressure, and a host of other fudge factors that can have a huge influence on performance. Like the computer nerds like to say- - - -"Garbage in- - - garbage out!" Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
I had a long post written about dyno testing and timing advance, and my web server dropped offline and ate it! Basically, I duplicated the findings my father got with chassis dyno runs of a dirt track racer with a flathead V8 and alcohol in the early 1950's with a series of water brake bellhousing dyno pulls on small block Chevy race engines 50-something years later. Both series of tests proved that one degree too much timing advance at full throttle robs as much power as 4-5 degrees of late timing at the same engine RPM and load. Once the "sweet spot" of maximum spark advance is found, any further advanced is trying to run the engine backwards.
Very cool that you have data (and history!) like that.
