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Just bought a 69 Galaxie XL convertible with a 429 4 sp. It requires 99.9 octane which is impossible to find. Any thoughts on installing decompression plates between the block and heads? It would be really nice to use pump gas without adding bottles of octane booster.
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If that engine runs good, i wouldn't touch it. Leave as is and go with octane boost. Keep some bottles in the trunk handy. Unless your driving that car daily, you may want to consider some modification. However, if you bought that car for occasional cruising, keep the compression and enjoy the power....go with octane boost!
Last edited by dan_6776 (June 9, 2016 5:35 pm)
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how many miles or on the engine?? unless it is 25K or less I would not remove the heads unless you absolutely had to. Removing a retorqueing the heads will slightly change the shape of the cylinder walls and cause the rings to have to re seat. not a big deal in a low milage engine but not good in a high milage one. Have you tried running pump gas?? if not I would try it, as long as it doesn't ping than you should be good to go with what you can get out of premium.
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If it doesn't ping then no worries as Daze said. If it does ping, you may be able to retard the timing a bit to stop it. Just back the timing down a degree or two at a time until the pinging stops. You may lose some power, but you would lose power if you lowered the compression as well.
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Roosterb1 wrote:
Just bought a 69 Galaxie XL convertible with a 429 4 sp. It requires 99.9 octane which is impossible to find. Any thoughts on installing decompression plates between the block and heads? It would be really nice to use pump gas without adding bottles of octane booster.
Wow! What kind of super, hopped up race engine did the previous owner build for that car? 
Do you have the specifics of the build, like what heads, pistons, cam, etc.? It would seem to be a narrow purpose built engine, which seems odd to me for a car like this. The engine sounds more suited to the drag strip than the street, but if you are a pilot, or have access to one, you could run 100LL Av gas, or buy the new concentrate to boost pump gas. Post up some more details, it sounds pretty wild.
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how many miles or on the engine?? unless it is 25K or less I would not remove the heads unless you absolutely had to. Removing a retorqueing the heads will slightly change the shape of the cylinder walls and cause the rings to have to re seat. not a big deal in a low milage engine but not good in a high milage one. Have you tried running pump gas?? if not I would try it, as long as it doesn't ping than you should be good to go with what you can get out of premium.
Thanks,
I don't know how many miles are on the engine. It appears to have been "worked on" by looking at the newer gaskets and clean and painted block.
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Just bought a 69 Galaxie XL convertible with a 429 4 sp. It requires 99.9 octane which is impossible to find. Any thoughts on installing decompression plates between the block and heads? It would be really nice to use pump gas without adding bottles of octane booster.
Wow! What kind of super, hopped up race engine did the previous owner build for that car?
I have no information on the engine. It appears to have been "worked on" due to the new looking gaskets and paint on the block, new radiator etc. I'm just reading the owner's manual recommending 99 octane gas. Gas here in Washington has ethanol added. I do have a station without ethanol and add booster. Apparently the tranny was changed from a wide ratio to close ratio. I just rebuilt it back to what the Marti report said it came with. (Wide ratio). I don't need a hot rod. First Mpg check was 5.8 mpg. It looks cool sucking gas though.
What's the "New concentrate" you mention? I use off the shelf stuff from Napa.
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5.8 mpg is pretty bad even for a big block... I suspect much of it is due to an exuberant right foot? 
Perhaps do a compression check to see just how high your compression really is. Also many engine builders tend to overkill on the size carburetor (cfm) and/or jet size, so you might check for a rich mixture.
Seriously, at least check your timing. Pre-detonation (pinging) can do serious harm to your pistons, valves and combustion chambers. Retarding the timing to reduce pinging is common practice, sometimes just a few degrees can make all the difference.
On fresh hi-performance builds we used to set timing while running the car on the dyno, retarding it just to the point it stopped pinging... which was usually only a few degrees back from stock specs.
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Just bought a 69 Galaxie XL convertible with a 429 4 sp. It requires 99.9 octane which is impossible to find. Any thoughts on installing decompression plates between the block and heads? It would be really nice to use pump gas without adding bottles of octane booster.
Wow! What kind of super, hopped up race engine did the previous owner build for that car?
I have no information on the engine. It appears to have been "worked on" due to the new looking gaskets and paint on the block, new radiator etc. I'm just reading the owner's manual recommending 99 octane gas. Gas here in Washington has ethanol added. I do have a station without ethanol and add booster. Apparently the tranny was changed from a wide ratio to close ratio. I just rebuilt it back to what the Marti report said it came with. (Wide ratio). I don't need a hot rod. First Mpg check was 5.8 mpg. It looks cool sucking gas though.
What's the "New concentrate" you mention? I use off the shelf stuff from Napa.
OK, just reading the owner's manual. I think you must have misread the 99, because I don't recall there ever being 99 Octane pump gas and no factory engine would have been issued in these cars that would require 99 Octane fuel. The factory fuel requirement recommendation would have been for Premium fuel, which is 91 - 93 Octane. I cannot see, from the new info provided, that there would be any reason to feel you had to run anything other than widely available pump fuel. There is really nothing wrong with running ethanol blended fuel, I know that I've never had any problems with it. You may need a tune up to help the engine run more efficiently, and lighter right foot, but there should be no need for special fuel. Premium and 5.8 MPG is going to be expensive enough.
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Wow! What kind of super, hopped up race engine did the previous owner build for that car?I have no information on the engine. It appears to have been "worked on" due to the new looking gaskets and paint on the block, new radiator etc. I'm just reading the owner's manual recommending 99 octane gas. Gas here in Washington has ethanol added. I do have a station without ethanol and add booster. Apparently the tranny was changed from a wide ratio to close ratio. I just rebuilt it back to what the Marti report said it came with. (Wide ratio). I don't need a hot rod. First Mpg check was 5.8 mpg. It looks cool sucking gas though.
What's the "New concentrate" you mention? I use off the shelf stuff from Napa.
OK, just reading the owner's manual. I think you must have misread the 99, because I don't recall there ever being 99 Octane pump gas and no factory engine would have been issued in these cars that would require 99 Octane fuel. The factory fuel requirement recommendation would have been for Premium fuel, which is 91 - 93 Octane. I cannot see, from the new info provided, that there would be any reason to feel you had to run anything other than widely available pump fuel. There is really nothing wrong with running ethanol blended fuel, I know that I've never had any problems with it. You may need a tune up to help the engine run more efficiently, and lighter right foot, but there should be no need for special fuel. Premium and 5.8 MPG is going to be expensive enough.
I rechecked the owner's manual. It does say 99.8 octane. Anyway, the pinging is under control. Just rebuilt the transmission. The original clutch plate was soaked in gear oil. Maybe it was slipping, though I couldn't tell. We'll see. Thanks all for the input. I appreciate it.
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