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Slant six engine - Performance upgrades

Before making any changes that may affect performance or durability, please understand that the information and opinions presented here have not been verified and that you proceed at your own risk. We are not responsible for any damage or other negative consequences that may arise from use or abuse of this page.

Dan Stern recommended, on applicable vehicles, bypassing the OSAC valve and cutting out the EGR if possible and legal; on 1974-75 models, replacing the Holley 1945 with a BBS or Holley 1920; and switching to NGK ZFR5N extended nose plugs (on 1974 and later vehicles, be sure to take off the metal washers first). The carb switch may entail further work due to port and/or linkage differences.

Bob Lincoln’s suggestions

Bob Lincoln is an automotive engineer whose advice has long been respected in the Allpar forums. He recommended:

  1. Install lighter (weaker) advance weight springs in the distributor on the advance weights. You remove the rotor, reluctor and the mounting plate to access the advance weights. In the breaker point ignitions this worked well - kicks in the mechanical advance earlier in the RPM curve. This is good for more power and mpg. Not sure how the electronic ignition will respond. If it pings, try setting the base timing back a couple of degrees, or re-install the original springs; or run 89 octane to compensate.
  2. Rebuild the carb and make certain the float level is absolutely dead-on. The factory sets it a tiny bit high. I try to get it to the nearest 32th of an inch to spec. Measure with carb upside-down and right-side up - I think the kit includes a cardboard scale to check both.
  3. Make certain that both the heat riser valve in the exhaust manifold works, and that the flapper valve in the snorkel works - that the tube down to the heat stove is intact, and that the flapper closes off outside air on a cold start, sucking air up the tube from around the exhaust manifold. This greatly improves cold operation and gas mileage. The heat riser valve needs spraying with a good penetrating oil every 6 months.
  4. The factory spec for the valve clearance is .010 inches intake and .020 inches exhaust. It's quietest there. If you can live with just a little more slant-6 chatter, set it to .011 and .021, and check compression. If below 140, set it to .012 and .022. This boosts compression a little - it should not be too much for the head gasket (should be under 150 psi). It may affect the ability to idle properly at normal speed, so you may have to turn the hot idle up a little, maybe to 800-900 RPM.

    The way I discovered this is that I mis-adjusted it. It's supposed to be set with engine at hot idle. I was afraid of using a socket wrench for fear the vibration of the rocker arm would knock the socket off the wrench and down into the running engine. So I shut the engine off and tried to set it at TDC for cylinder 1 and every 60 degrees for the other cylinders. Can't do it - expansion/contraction messes up the setting. It has to be done at hot idle. So I used a 3/8" box wrench. It's short and it's a wrestling match, and the vibration goes up to your elbow and makes it sing. But it's the only way.

    By adjusting with engine off, I turned the clearance in too much, so the valves wouldn't close enough. The engine started and stalled with a wheeze. So I turned the rocker adjustments all out 1 1/2 turns as a best guess and tried to start again. VROOM. It shot to 3000 RPM with almost no throttle. If I can get this in gear, I've got a rocket. But it wouldn't idle below 1500. I checked the compression and it was 180 psi! Normal is 125 psi. Way too much for the engine. I couldn't get it right, and had the car towed to a mechanic who set it right. The recommendation for .011 and .021 is a compromise between performance and durability.

Factory-performance slant sixes (Bob Scott)

From mid-1961 to 1963, 50,000 225-cid Slant Six engines were produced with an aluminum engine block. These were discontinued because production was slower, more expensive, and not as reliable as with the steel blocks, according to various allpar.com interviews.

The Hyper-Pak was available for the 170 and 225 slant sixes. It had a radical cam, Carter AFB 3083S four-barrel carburetor on a long-ram intake manifold, steel tubing exhaust headers, higher compression pistons, and a special exhaust. Advertised at 195 hp (gross output, for the 170), Hyper-Pak engines were the rulers of the lower stock classes at the drag races, and the Hyper-Pak was available from dealers.

Modern Hyper-Pak type package.

Clifford Performance sells headers, blueprinted heads, revised cams, and other performance slant six parts.

Doug Dutra creates “Dutra Duals,” manifolds that support dual carburetors and are less prone to cracking.

You can quickly and easily bolt on electronic ignition from later slant sixes, or you can order a kit from Mopar Performance.

Fuel injection and forced induction

A former Chrysler engineer wrote: “A 170 CID slant six HyperPak would deliver a comfortable 182 Hp, and turn 8000 RPM if you kept the cam lobes lubricated. I am sure there was more available, but it is anyone's guess what would break first. My suggestion would be start there and progress slowly. When it blows go back a step. Have a few engines handy, a ready wallet, a good dynomometer, and lots of patience.”

KE4DGI pointed us to a team that ran 9.85 @ 132 mph in the quarter mile at Bristol with a turbo slant six using fuel injection.

Al Toews pointed to his own supercharged-slant-six project, using multiple-point fuel injection.

Slant six performance issues and suggestions (Paul Clark)

Upgrading to a 'super six' intake, with a 2bbl carb, should give better performance and gas mileage. (Dan Stern:) It was released for marine/export only in 1967, and was found in North American cars and trucks starting with its release on the "Super Six" Volare/Aspen.

Get both the intake and exhaust manifold together, as it's a headache separating these and putting them together again; also the throttle linkage is different, so be sure to get everything between the gas pedal and the cylinder head. The Carter BBD is a good 2-bbl carb, easy to rebuild and tune, and will give great performance. This is also how to spot the super six; the air horn is bigger than a 1bbl, so the old air cleaner won't fit. If you're not sure what it looks like, go pop the hood on any 2bbl V8 mopar, where it's the standard item. (The carburetor was somewhat different from the one used on V8s).

If it's performance you're after, another thing you should seriously consider is swapping the rear end to a shorter ratio- 3.23 was standard, though 2.79 was an option for highway mileage (unlikely with a 170) and 3.55 was an option for performance. Both these two are not uncommon in the 7 1/4" rear ends which are standard behind /6 cars. My 1965 B'cuda had a 3.55 rear end, and while the tradeoff is a lower top speed, the reason it was used was to leverage the slant six's torque. It's amazing how many people think right away of hot rodding their engines when a rear end ratio swap will make such a greater effect on their seat-of-the-pants power.

[Dan Stern noted:]

As far as an 8 3/4" rear, it's unneccesary behind a slant 6 unless you are going to compete in drag racing. A V8, with greater torque, can break a 7 1/4" rear, but it was standard behind 273 cars. The only other advantage the 8 3/4 gives you is the ability to swap gear pumpkins to 'quick change' the ratio from street to strip. Other than that, my advice is to run your 7 1/4 until you break it- if you manage to do that, then consider upgrading- or maybe just get another 7 1/4. A shorter (3.55) ratio will give you more durability anyway.

San Diego's Fred O'Hara added:

A fatter exhaust system helps performance and reduces heat build up that hastens the demise of the exhaust manifold. I have replaced mine at least a dozen times for gasket leaks, cracks, etc. 2 1/4" seems good with a turbo type muffler.

The next best thing I did was buy a high output coil. For $35 I eliminated the miss that most people said never existed. It idles steadier and runs better. Of course the new ballast resistor was a good buy as well.

The stock distributor was pricey at $175 but definitely worth the long term investment. I have been getting by on cheap rebuilt parts as long as I can but they tend to fail often. The carbs fall apart after two years but one day I will shell out for a new Holley for $300. This week I am going to rearc and add a leaf to boost the rear springs (another $300). The sway bar did not come on my car so I added them front and back. Buying the original shop service manual has also been a move I will never regret.

Ed Campbell contributed:

Blocks made before they started the thin castings are most desirable. I think the break point is 1973, but not sure. Headers are a waste of money on a slant 6 intended for street use and take away the heat riser, making cold engine drivability pretty bad.

If you go with a 4-barrel setup, you want the smallest Carter carb for it. Anything bigger will give you problems unless you are very careful about opening the throttle very slowly at low speeds. A Holley carb may make the car a little faster, but they require nearly as much tinkering as an English motorcycle.

Fancy, expensive ignition systems are a waste on these engines because, with their long stroke, it is not safe to run them fast enough for them to make any difference. A modified cam will give a lot of power increase per dollar, but don't get crazy with this because you quickly start losing low end performance as the cam gets more radical.

Other slant six performance notes

The 170's relatively short stroke allows it to rev high. Both Russ Shreve and Steve Remington - a Valiant engineer and expert - agree that the 170 is the engine to use if you will turn over 6,000 rpm frequently. According to Russ, a 170 CID slant six HyperPak would deliver a comfortable 182 hp, and turn 8000 RPM if you kept the cam lobes lubricated. He also said that:

It had a shorter stroke than the 225, but the same head and valves. Low end torque was less, but ultimate power was the same. In addition, with the lower block height it was a sturdier engine, and with the shorter stroke could rev higher with less chance of an rpm failure. The Achilles Heel was the cam lobes.

If he [someone upgrading their six] is going for higher power and RPM he is going to need the HyperPak clutch and special clutch bolts. Otherwise he is most likely to have a clutch explosion. I also recommend an explosion shield. A clutch explosion can cut a car in half.

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