Which is better for a 69 engine with super six manifolds 2 inch or 2 1/4 inch exhaust pipe and why? Will I really see a difference?

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55 GMC with slant six and aod33 over drive.

Also what would be best spark plug for it, and does it make a difference on pipe size?

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55 GMC with slant six and aod33 over drive.

If making-up a custom pipe, do it in 2 1/4 so there is no chance for mis- alignment "restrictions". (the manifold outlet is 2 inch on all factory SL6 exhaust manifolds)
Extended tip plugs are a pretty good chioce for a stock SL6 used mostly for "around-town" driving.
DD

You will get many answers depending on if your engine is stock vs modified, etc.
I have run both 2" and 2 1/4". My experience has been that mileage went up with both sizes on both motors.

The following set up is not necessary but interesting........I drive very aggressively and both engines are very spunky even though one has over 200,000 on it and the modified engine is very new........

Just for fun I am running a 2 1/4" head pipe into a 2.5" FlowMaster HP-2 that is 18" long and 2.5" to a FlowMaster 70 with a 2.5" tail pipe. This system is quieter than stock and zero drone and gets better mileage with either my completely stock 1979 engine or my mildly modified 1974 engine. The 74 Dart Swinger will go up and over our mountain passes easily while getting excellent mileage.

As an experiment I have tried every muffler and pipe combination over the last 12 years driving 120 mile a day round trips. I log every change and drive it a minimum of 6 month intervals before making any changes.

I use NGk UR4 spark plugs. See pictures on link below my name.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Tune-up parts and technique suggestions including spark plugs are in this thread.

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

Ted,
You use both an HP2and 70? Is there an advantage to that?

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55 GMC with slant six and aod33 over drive.

Why such a hot-running plug? Do you have problems with spark plug fouling?

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

An alternative is the NGK UR45IX. They have a very thin Iridium center electrode, and they come pregapped at .059"! I have run them for a year, great color, never had a miss. I use the stock gap, HEI ignition, extended rotor in premium distributor cap, Magnecor wires.

When I switched from the last batch I had of the old US made Autolites to NGK UR4, there was not really any noticeable differences, but the switch to UR45IX were different. Not scientifically proven, but the engine seem to run - in lack of a good expression - smoother, especially at idle, and the seat-of-the-pants-only-feeling is that the accelleration definitely improved. I don't change my plugs often enough to care about the slightly higher price, but they are a little more expensive than UR4.

Ted, have you tested them?

Olaf.

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Aspenized

Dan, I believe 4 is the stock heat range for NGKs. GR4s are spec'ed as stock replacements for Autolite 66 and RN13YCs for the 1960s Slants. I guess the later years went a bit cooler (65 or 64s or RN12YC or RN11YC), so GR45s or GR5s work well too and I have run those. I don't know if the UR4 for the later heads is a direct swap over for stock plugs on those.

As for me, I have never been able to tell a significant difference in running between different spark plug types, brands, or heat ranges (within the small range I have tried). The only way I have seen a difference is in replacing OLD (30k + miles) or oil fouled plugs.

I put an old set of Autolite 63s in the turbo Dart just because Seymour had them laying around and suggested using them. They have about 13k miles on them (including 100 drag passes and plenty of WOT on the street)and still running fine. I guess I should have pulled one/them to check at Woodburn, but I was too busy...

My 2 cents...

Lou

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Home of Slant6-powered fun machines since 1988

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Dan,

Not fouling but UR5's run dark brown. The current stock 1979 motor in the car is a $100 barn find. It had over 200,000 on it when I installed. I have since success fully flushed it, and there is not much valve guide seals left if any.....so some oil does leak down the stems and shows up on UR5's and UR6's as dark brown. I backed off on the timing to 52 degrees total with the UR4's and they run a light beige to white. The stock steel shim intake gasket is leaking on some cylinders where the plugs read white or little color.
This motor is just temporary until I finish upgrading my modified motor.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Olaf,

Yes, they do run great. I had them in my 9.5 to 1 motor. Since it is out being upgraded, my son swiped them and is running them in his race Camaro.......kids. He bought two more and that is the last time I will probably see them......I didn't know he had them until he showed me how well the Holley 750 was running over the Edelbrock 600.....good color!

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Just to clarify, that was also my experience with the NGK UR4s, they run great. Very nice color, but in my experience over decades, NKGs tend to be a little paler in color than many other spark plugs under the same conditions.

Olaf

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Aspenized

For what it is worth. We installed 2 1/2" from the manifold to the rear bumper with an off the shelf magnaflow muffler on our Duster and picked up 2 tenths in the 1/8. This replaced a well worn OEM style exhaust system. The engine is bone stock with an Autolite 2100 and 2.73 gears.