Hi guys Trying to find out what Dwell. I have a 57 3100 the engine is I believe a 60’s replacement. My distributor is a Delco-Remy 1112403-7E16. Engine runs fine but trying to improve mpg . So would like to get it spot on . Currently set at 31 From what I’ve read on hear I think I should be closer to 40 . If someone has the spec for my distributor that would be great or point me in the right direction. Regards Rick
12 volt Delco distributors (the ones with the tall cap and rotor) should be set at 28-32 degrees of dwell. The short cap models designed for 6V need 38-40 degrees for better magnetic saturation of the coil windings at the lower voltage. Changing dwell will have one of two possible effects on MPG- - - -"slim" or "none"!
Centrifugal and vacuum timing advance changes will make a huge difference. I've proved on dozens of dyno runs that one degree too much timing advance loses as much torque as 4 to 5 degrees of retarded timing from the "sweet spot" of maximum torque, so always err on the side of "not enough advance". My father discovered the same thing in the late 1940's when chassis dyno testing a 1934 Ford 3 window coupe dirt track race car. 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!
It’s a 12 volt. Can I check if the 28- 32 you mention is set with points at 0.019. My points are worn so set so set at 0.016 how does that affect the dwell figure 34? Many thanks
Point gap and dwell Is an either/or situation. Changing one will alter the other.
Definition time :
"Dwell" means "The number of degrees of distributor rotation the points are closed"- - - - -literally speaking, the amount of time they "dwell together". A 6 cylinder engine has a separation between the high points of the distributor cam of 60 degrees- - - - -60 X 6 = 360 degrees of distributor rotation, and since the distributor turns at camshaft speed, it takes two turns of the crankshaft to make the distributor turn once. All six spark plugs fire during one rotation of the distributor. An 8 cylinder engine has distributor lobes 45 degrees apart, and a 4 cylinder distributor cam has a 90 degree separation. Typically, a 66% "points closed" interval is ideal for getting maximum coil magnetic saturation, which is where the 40 degree dwell figure comes from.
There's a disadvantage to the 66% dwell time, however- - - -the point gap tends to be very narrow, and the 100+ volts that gets induced in the primary coil winding as the spark plug fires, and the short time available for the condenser to discharge before the points close again can cause burned points, especially with the much more efficient operation of a 12 volt coil over the former 6 volt system. In the interest of increased point life, GM shortened the dwell time (and increased the point gap) when they switched to a 12 volt electrical system.
With about another 3 paragraphs, I could explain how a dwell meter is actually nothing but a voltmeter with a specially calibrated scale, but that's a discussion for another day. If your points are worn, replace them. It's impossible to get an accurate gap setting with anything but perfectly smooth points. Use a dwell meter, adjust the distributor to the specifications Mike provided, and forget about the point gap! 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!
One of the best ways to attack gas mileage is via your carburetor, Rick. You mentioned 1957 and what you believe is a 1960s replacement engine. What make of carburetor do you have? Rochester? Carter? Other? And do you have to use ethanol-laced gas where you live? (most friends in the UK tell me it isn't an option today) Please let us know.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
Hi guys many thanks for the advice . It’s got a Rochester carb . Ethanol at 5% or 10. I am trying to use the Esso super + as supposedly it is ethanol free for now . Only in some parts of the country apparently. I add a fuel additive for what it’s worth . I converted the air box to run a kn filter which maybe wasn’t the right thing to do .Being freer flowing than the oil bath will be running leaner than is was.
Good points about the air filter. You need as much air as you can get.
The Rochester B and BC models are notorious for having two problems not associated with air filters, however...warped castings (which can and do cause vacuum loss) and the power piston becoming stuck in the bore by virtue of ethanol laced gasoline. Actually some will say any unleaded gas will contribute to sticking of the piston, but the main thing to understand is when that power piston fails due to vacuum loss or because it is stuck, it is always stuck in the "down" position...meaning it leaves the power valve open. And this causes the carburetor to run too rich all the time. Hopefully you don't have that problem, but it is possible. If you look at the very top of this section, you'll see a tutorial I wrote (Straightening the Rochester B and BC Castings) about how to straighten the castings using a common gas bbq grill and a simple piece of flat metal and 4 nuts and bolts. I also wrote a companion report which I need to re-write detailing the common Rochester B problems...the image hosting service for those images sort of had some trouble so there are no images now.
Your fuel additive (depending on what it is) may help. Probably will help, actually. As I said, friends over there complain to me about not having ethanol-free alternatives.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
You might want to remove the carburetor and make sure passage in the bottom of the throttle body aligns with slot or hole in gasket or spacer to intake manifold. This is the manifold vacuum source for the metering rod piston, if blocked it wil run rich.
BC
BC 1960 Chevy C10 driver 261 T5 4.10 dana 44 power loc 1949 GMC 250 project in waiting 1960 C60 pasture art Retired GM dealer tech. 1980 - 2022
The k&N filter is the biggest I could fit in the oil bath filter housing after I gutted it and sits up about 1/2 “ taller than before .so I believe should flow more than before. I did rebuild the carb a few years ago . But I will take it apart over winter and straighten things out as per the article. Sadly the old girl only gets used every couple of months so I’m sure carb must be gummed up . However she always fires up idles and runs ok Thanks for the advice . Regards Rick
The dense material the K&N filter is made with adds a lot of restriction. Oiling the media as they suggest makes it even more likely to hinder the flow. The K&N filter on my Harley Soft Tail got so clogged the bike wouldn't run more than 40 MPH, and washing it in solvent didn't help. I discarded it and switched to a paper filter. and performance and fuel economy increased dramatically. A pleated paper filter at a fraction of the cost does not restrict airflow nearly as much, and it can be discarded when it gets dirty. K&N uses the "P.T. Barnum" business model, and lots of people fall victim to their advertising claims. 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’d like to see some actual numbers on the air flow between k&N filters vs paper filters. Maybe you can supply us with the flow bench results
Let me see if I can find them. I saw a set of tests done on larger ones (for the 03-06 Duramax) and the K&N had both the worst flow and let the most dirt through. It was 3-4 years ago.
