I'm planning to rebuild the original 225 in my 1974 Swinger (904 trans, 8.25" Sure-grip 2.71 gear). The goals are (1) learning and fun, (2) better mileage, especially around town, (3) less exhaust stink, and (4) staying "mostly" stock. The main motivator is that I have tons of blowby leading to constant water and fuel in the oil, also I'm getting less than 15 mpg (maybe 17-18 highway). The engine has been smooth, quiet, and reliable for the last 5 years/11k miles and I want to keep it that way. More power would be nice, but not more important to me than mileage and minimizing stink in the garage or when I'm stuck in traffic. I do mostly grandma driving. Efficiency is more exciting to me than acceleration.

A couple years ago I got a lot of great info in this thread when I was planning to have a shop do the rebuild. But now I plan to do the job in my home garage, except for the machine shop jobs. I have Doc's book, the BoxWrench DVD, and a shop manual.

The rough plan is to pull the engine, disassemble, make compression measurements, take the parts to the shop for cleaning and inspection, and then decide where to go from there. Here's what I'm thinking now.
- Cam: if it's reusable I'll keep it; if not, I'll decide whether to stay stock or do something slightly different.
- Block/head: Slight shave for a modest increase in compression. 3-angle grind on valve seats.
- Valves: Back cut?
- Porting: Clean up the bowls and round off some edges. I'll do this myself after getting the head back from the shop.
- Manifold/carb: Keep the stock 1bbl setup. My carb was NOS a few years ago and seems to work well.
- Ignition: Keep the stock electronic ignition and coil for now.
- Timing: recurve distributor if needed.

There's a chance that some day I might want to do further upgrades, such as Dutra Duals, 2bbl carb, GM HEI. f there are any mods I should make now to support those "might could" future upgrades, then I'd consider doing them now if the cost is reasonable and if they won't reduce efficiency right now.

If emissions and economy are your concern, I say keep the stock components except:

(1) HEI ignition
(2) increase your dynamic compression to about 8.1:1 to be able to run on pump gas. You will need to do lots of measurements and math, but it isn't too bad, really.
(3) free flowing exhauast with a free flowing three-way catalytic converter certified for use in California. Three way catalytic converters are the most likely to survive behind a carb, but installing a vacuum gauge and an O2 sensor and tuning your carb to be as close to stoichiometric as possible under all operating conditions will help the catalytic live longer
(4) CarbsOnly electronic choke thermostat conversion. Set it to open as fast as possible
(5) custom timing advance curve in the distributor. Lots of power and economy can be gained dialing in the timing advance for your engine and vehicle.
(6) make sure the trans is adjusted properly and make sure you have a part throttle kickdown in the valve body
(7) cleaning up the ports in the head will help efficiency, but a 2.71 rear gear ratio driven like a grandma won't have an engine turning fast enough for stock ports to be offering much restriction
(8 ) synthetic fluids in the crankcase, trans, annd rear axle
(9) low rolling resistance tires inflated properly
(10) fabricate a true cold air intake

That is all I can think off off of the top of my head. Sounds like a fun project.

Yeah, +1 (or +10) to what Reed said. HEI makes a big enough improvement to be moved to the "now" list. I'd add:

• Dutra RV-10RDP camshaft in strong preference to the original (no matter how lovely it might look), carefully degreed-in

• A careful check of the harmonic balancer and replacement or rebuild if necessary to make sure the timing mark is accurate

• A 3.2 or 2.9 rear axle depending on whether you do more city or highway driving (respectively) -- the 2.7s are great if you mostly don't go slower than 65 mph, but below that (especially in a heavy '74) you've gotta stick your foot in the carburetor's power circuit all the time to get going and keep up with traffic, which means poor MPGs.

Catalytic converter: Most aftermarket cats (meow!), including virtually all of the so-called "performance" cats (meow!), are a waste of money. They are cheaply-made weaklings, warranted only for 25,000 miles and barely able to clean up the exhaust of even a clean-running fuel-injected vehicle...let alone the dirty exhaust of a carbureted Slant-6 engine. If you will be running a cat (meow!), then as Reed says, the trick is to get a California-spec one. They're type-approved to a much stricter durability and performance standard, so while they cost somewhat more up front, you won't have to spend more money later because they won't fail or melt in response to the dirty exhaust being put through them. The one to get would bethis one. Put suitable heat shielding above the cat (meow!) on the underside of the floorpan and/or on the top of the floorpan above the cat (meow!). The cat (meow!) should be as close as reasonably possible to the engine without actually being in the engine bay; you don't want it heating up the air in the engine room.

Rest of the exhaust system: like this (can just use single 2-1/4" headpipe off the stock exhaust manifold if you're not putting in Dutra Duals). Wrap the headpipe from the manifold to the cat (meow!) with this stuff to keep the heat inside the pipe, which will do two good things: keep the engine bay cooler and keep the catalyst hotter for maximum efficacy/minimum stink.

Run a 195° thermostat if you can get away with without running into carburetion issues (see here, and install a clutch fan.

Also, easy freebies: bypass the OSAC valve (run vacuum hose directly from correct port on carburetor to distributor with no stops in between) and try removing the vacuum hose from the EGR valve and plugging the end of the hose to see if you can get away with it without pinging, with an initial timing setting of 3 to 5 degrees BTDC. Make sure the manifold heat control valve and the thermostatic air cleaner are working and intact.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

All of Dan's points are excellent, but I triply emphasize this one. If your vibration damper is original, then there is a very good chance the vulcanized rubber securing the outer ring with the timing marks to the inner core that slips onto the crank shout has degraded and allowed the outer ring to slip in relation to the center core, resulting in inaccurate timing. I have had this happen to a number of slant six engines I have owned over the yers, including the original damper on my 76 D100 that had the outer ring break completely loose from the center core while I was doing 70 mph on the highway. Fortunately, the truck has power steering and the power steering sheave bolted to the damper kept the outer ring trapped against the timing cover. I wouldn't have noticed anything except I was driving at night and my lights got dim and the engine started overheating. Useful lesson learned.

I second the idea of 2.25" exhaust all the way from exh manifold to rear bumper. That will make more power and better MPG, assuming the mixture is tuned right.

If it were me, I would install 1.7/1.44 valves and do some pocket porting, which will make more power over a wider range, and will allow you not to lug the engine so much in different conditions, which will lead to better MPG (if you do not "enjoy" the power too much).

Ignition timing will be critical, as will fuel mixture. Good point about the balancer degree marks. Get a wideband O2 sensor like the following and tune it and drive it to keep your cruise AFR between 14.5 and 16.5.

https://www.summitracing.com/parts/avm-30-0300
(I have used this on 5-7 vehicles and have had ZERO problems. Other brands I hear people complain about frequently.)

Sounds like you need to rebuild the engine if it has blowby and stinks that much with stock components. Good plan. Ask the machinist (find a good one, with recommendation from someone near you who knows about engine rebuilding) to set your bearing clearances to 0.0015 - 0.002" on rod and main bearings.

Lou

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Home of Slant6-powered fun machines since 1988

I should have added that I plan to replace the timing set and balancer, as well as the water pump. Not sure about the oil pump as I know there are concerns about the gears failing in the replacement pumps.

All of your exhaust ideas are welcome. If and when I replace the pipes I might as well install Dutra Duals, right? But that's in the future and for now I want to focus on what needs to happen inside the engine. Cam, valves, etc.

Also, I'm not married to my 2.71 differential gear. It's what I got from the salvage yard when I wanted to upsize my wheel lug pattern as I was converting the front brakes to disc. I can definitely see changing to, say, a 3.21. I imagine 1600 rpm instead of 1350 at 35-40 mph could make a big difference in drivability and efficiency. Here are my calculations of rpm vs speed, for my later reference more than anything else....
tire circumference 80 inches (205/70R14)
5280 * 12 / 80 = 800 axle revolutions per mile
800 / 60 = 13.3 axle rpm per mph
13.3 * 2.71 = 36.0 engine rpm per mph
13.3 * 3.21 = 42.7 engine rpm per mph

My opinion? You'll never get rid of the exhaust stink until you have EFI.

Some people have a better sense of smell than others. I can hardly see.... but I can smell...and I can tell a carb'd car a mile away. It's the #1 reason I dislike driving my old cars, by far. I don't mind holes in the floor, leaking window seals, no radio, no AC, none of it. But smelling like the exhaust pipe - I could do without.

Some cards are better than others but they all smell if they're carbureted. On the other hand, an EFI car - once fully warmed up won't leave your clothes smelly unless you intentionally stand in the exhaust stream.

Who firsted it? My recommendation is 2-1/4" from the engine to the muffler, 2" tailpipe with resonator. That way you get your more-than-ample flow with your less-than-annoying noise levels. Exhaust cools down as it moves through the system, which means it contracts in volume, which means it needs less pipe size for any given flow. If you run 2-1/4" all the way back, you'll effectively create a megaphone/trumpet type of effect, in exchange for nothing (no practical improvement over 2" tailpipe).

Also: thoughtful selection of engine oil. New engine carefully built with tight clearances? 0W30 or 5W30.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Having used 2-1/2" pipe front to back on a stock motor, I can assure you that exhaust cooling is real. You end up not really making any more power, and you have an exhaust that fills up with water from condensation.

Back in the dark ages when I was in the GS club even they were using 2" tailpipes on 455 cars with 2-1/2" headpipes because it increased torque.

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Official Cookie and Mater Tormentor.

Agreed you will probably not see much or any gain with 2.25" tailpipe vs. 2" tailpipe. I would run a 2.25" muffler and pipe myself, if any significant mods are done to the head or compression.

I also agree with Greg that you will never get rid of some residual stink with a carb, but given that your motor sounds worn out, you should have a lot less stink on a warmed up engine with it all rebuilt and tuned properly. EFI is a major advance, but is wayyyy more complicated to build and install.

Lou

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Home of Slant6-powered fun machines since 1988

If you are running Dutra Dual exhaust manifolds, run dual 2 inch pipes to the muffler than a single 2 inch pipe after. Or two inch pipes to a "Y" where the pipes meet and go into a single 2 1/4 inch pipe before the muffler and then a 2 inch pipe after the muffler.

I thought you were still in the Great Slant club!!

Here is some good advice for those of you smelling your carbureted engine.

Start driving!!

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Exhaust from a car with no cat (meow!) is going to reek, no matter whether the engine is carbureted or fuel-injected.

Engine in good condition + proper tune + fuel injection + feedback control + excellent ignition + cat (meow!) = least possible reek.

Reek-potential goes up as any of these factors is subtracted from the equation, but good motor + good tune + carb + cat (meow!) is going to be a whole lot less reeky than whipped or good motor without a cat (meow!).

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I am the second owner/driver of my 82 Ramcharger. The first owner had it maintained by a dealership that actually did good work. With the new CA certified catalytic converter I installed the exhaust is barely stinky even though it still has the original Carter BBD under the hood. I installed a Walker quiet flow muffler and an exhaust tip that curves down and points at the ground. It is probably the quietest vehicle I have ever owned. Going down the road the tire noise is greater than the exhaust noise (and I have street tires on it, not knobby off-road tires).

As already mentioned, the HEI conversion should be the first thing you do. If you can find an HEI module in the junkyard, grab it in favor of a new module made in China. Better yet if you can find a real GM NOS module. That alone will make a difference in cold weather starting, idle, and who knows, may reduce the tailpipe stink a bit.

And if you really want to get rid of the exhaust smell, switch to LPG.

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If it was easy, everybody would be doing it.