I'm doing some reading in an attempt to get up to speed on a distributor recurve on my 3/4 ton truck slant six transplant.

What are your opinions regarding the following article?

https://www.performancetechnician.com/p ... Engine.pdf

I see some major, giant problems with this article.

Citation needed. Maybe it's true, maybe it's not. If it's true, maybe it's to an extent that matters the way these guys claim, and maybe not. I don't insist on reliable support for obvious statements like "It's usually hot in Phoenix, Arizona at noon on June 15", but if they're going to base detailed advice on a claim like "today's fuel burns faster", then I'm going to need something better than "Trust us, we've been tuning engines for years".



There is flatly no way this advice can possibly be correct, because there's a giant diversity of engines with enormous variance in factors that play into what the initial timing should be. There's also huge diversity of stock camshaft characteristics, and there's no official definition of "mild", "performance", or "radical" with respect to camshafts, even if we assume every engine on the planet is a Chevrolet 350.

And even if we do that, this advice directly contradicts the claim about today's gasoline burning faster. The whole reason for spark advance in the first place is so to give the fuel/air charge time to burn so it exerts maximum shove once the piston is just past TDC. If you were to have no spark advance, and fire the spark plug at TDC, the piston would be well down towards the bottom before the fuel's maximum bang happened, and the engine's output would be feeble.

But if today's gasoline burns faster, then that implies we need less spark advance, not more. So right off the bat, these guys are contradicting themselves.

Uh…there are idle issues that can be traced to initial timing, but there are a lot of much bigger issues that are higher up on the list of problems caused by sub-optimal timing. Strictly in terms of idle, there's usually a whole lot of leeway for initial timing without causing even minor problems, let alone major ones.

Yes, there are a lot of them, but no evidence is provided that they differ significantly in ways that matter to ignition timing.

The subsequent how-to-tweak stuff, the show-and-tell about vacuum advances and centrifugal weights, is less problematic. But they build on a structurally-unsound foundation.

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Jase,

Dan makes some valid points and these guys may not be a great source, especially for our oddball 225 engines. The best thing you can do, IMHO, is to post questions about your dist recurve on this site and get specific advice about your engine combo and desired usage.

Briefly, on a stock or near-stock engine (I have done many of these) I use a bit more advance than stock at idle (about 10 deg) and I close up the mechanical advance slots to get max advance of 30-32 deg at around 2600-2800 RPM (no vac advance connected). I use a vac advance can that is 8.5 - 11 depending on what is available and adjust so it just does not ping when it the engine is fully hot and going up a decent hill. There are many more factors, but this is the basic advice. This is closer to what the factory did with the mid-70s to early 80s engines, although I use a bit more advance at idle.

Lou

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Thanks Dan, that is all I needed to hear, and I enjoyed your explanation of their shortcomings...



Thanks Lou, I will follow your general advice and confirm that what you suggest is what I have.. before coming back with more questions... for context this is what I have done so far, and what the dream contains at this moment:

The engine is a 76 out of a D200, and the distributor had 15* slots which I shortened last summer.. so IF I remember correctly I now have 8* initial, and 32* total, and have not changed the advance springs from what ever was in it stock.. Also I limited the vacuum advance to about 10* at the crankshaft by limiting the size of the slot on the advance can. I need to verify that the above numbers are correct before asking for advice.

The'54 Dodge M37 (basically a military version of the original Power Wagon) truck is about 6000 lbs, with 5.38 gears in the differentials, and 35" tall tires... my desire is to improve /fuel efficiency/ power... Not expecting great economy, but at present it is somewhat less than expected... single numbers for sure.. but then a lot of my driving is in 2nd, and 3rd gears... The present carburetor is a 1945, and I look forward to giving it to someone who wants it...

The mid to long term goal is to fabricate-cobble a 4bbl intake and maybe more butchery to re-work some jeep tubular exhaust manifolds to fit the engine room.. THEN on to head work.. I realize that the intake and exhaust are not really the problem.. but wanting to play with them first, before spending money at a machine shop.. all of the above is me learning how to braze, Playing with a welder, and having fun... and maybe learn something in the process.

Sounds like great fun, Jase. The 1945 is probably a great first bet for power and economy on that application, if you have a good carb. My first thought for a carb for that truck is the Carter BBD. I would probably not do a 4bbl unless you really build up the engine internally and want to rev it. People make good power and excellent economy with the BBD (assuming well tuned and throttle shaft not worn out). I like the timing settings. Why did you limit vac advance to 10 deg at crank? Just curious...

Lou

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Home of Slant6-powered fun machines

Yes a BBD would be great, but for the work involved in converting a cast iron intake to a BBD, why not put a Q Jet on sideways, divide the intake front and rear? No expectations that the secondary's will be involved very often if ever... I would not choose a 4bbl unless, unless it is a spread bore, and specifically a Q Jet.

As for limiting the Vac Advance, D. Idiot suggested a mid 60's truck advance curve as a starting point,( years ago) and my FSM from 67 seems to suggest something like 8* of V.A. in the vacuum can... I ordered the wrong one, so I made due with limiting the can I had... I have not played with adjusting the spring tension within the can yet.

I'm still in the deep end of the learning pool here, and not sure I know what I am doing.. but having fun along the way as long as I learn something....

Throughout the years they seemed to mix and match governors within there (the part that some people weld up to limit travel) and vac advance cans. I've seen VA cans that say 8.5, or 9 or 11.
I forgot what that adjustment in the hose nipple actually adjusts... I don't think it changes the amount stamped in the arm but changes when it applies, aka how much vacuum needs to "be there" for it to activate.
With any of them I never adhere to what the under hood sticker or some manual says the base timing needs to be "at", only as a starting point and tweak from there for "that" engine in "that" body to where they run the best.
And that idea doesn't change for "today's gas" , just may have to play with things once in a while as seasons change, or if you start getting your gas somewhere else.

On the 1945 carbs it's been a crap shoot getting a good/bad one. I remember getting a few of them from a junkyard and scrounging parts from all to get a good carb. I've had a few with worn out throttle bodies and had to get them rebushed.
I miss the days when I had a buddy that worked in the tool room at American Lock who had full access to all the mills, lathes and such and could do me "favors" like that....

Yes I believe so, I've not ever adjusted one yet... I plan on testing mine to see when it activates, and then install a vacuum gauge just to see when it comes into play..