Bad Rat said:
I have a 69 Pontiac Bonneville with a 428 -360 H/P
I think the stock compression ratio is 11/1 and doesn't like anything to drink but Premium fuel , even using the premium fuel that's available today, it still pings
I have been adding some octane booster to the premium fuel, it helps, but really doesn't cure the problem, I did find a additive called "lead substute," that worked really good, but havn't been able to find it in the last few years,,
My Question is,, what would be a good tune-up as far as timing, carb jetting, etc, be , eventually if I keep the car, I will probably either change the rod legnth or go with lower compression pistons, because fuel prices are never going down much ever again, most likely much higher,,
The motor is all original with 38 K actual miles on it, runs perfect, except for the fuel mileage , which I would guess to be maybe 12 -14 mpg if that good, I havn't driven it for over 5 years, because of a restoration.
any ideas Guys?
Sweet car! One of my all-time favorite Pontiacs was a '61 Bonneville w/a 421 and 4-speed. My Dad had one and we loved that car. Next to that was a '58 Chieftan w/a Tri Power/4-speed, black on black. My brother in law owned it.
Some good ideas so far. I'd like to add my thoughts.
It's a shame the gas has gotten so bad- in 1970 I was hampered by not having
enough CR in my 428 to run the cam I wanted to run, and I had 72 cc chambers, too! IIRC the small valve 428 had an advertised CR of 10:1. The chambers and production tolerances generally gave a CR a bit lower than what's advertised. But even so, the CR is still going to be on the high side. Anything over about 9.5:1 is going to be a problem w/pump gas- as you've found out.
That said, I would suggest you first make sure there were absolutely no vacuum leaks, no bad guides or blowby allowing contaminants into the cylinders to dilute the fuel/air charge. Checking the plug condition will help determine this. Also be sure the plug heat range is correct, and that there's not different plug heat ranges mixed in the engine. Depending on the heat range you are now using, a colder plug may help.
It would be good to know how the carb is set up. If there was any work done to the carb in the past in an effort to increase the mileage, there's a chance it's now too lean- especially since the swill we now have for gasoline requires a bit more jet than gasoline of the past.
Another thing you can do is to rid the chambers and piston tops of any built up carbon deposits. If left unchecked, there can be a fairly significant increase in detonation just from this alone. Deposits can also create white hot spots in the combustion chamber that will act as a spark plug that is always firing, this will cause detonation, too. There have been recent
threads that talked about ways to rid an engine of built up carbon, a search will give you more info on this rather than me repeating it all here. BTW, this doesn't require taking anything apart.
I'm not a big fan of reducing the timing too much. This just
kills power, and at some point won't help detonation if the CR is too high. But you
can tailor the ignition curve to be as good as it can be under the circumstances. Depending on the curve that's now in the distributor, one way to do this is to use a very conservative curve that comes in later than usual (may require stronger springs for the mechanical advance), along w/a fairly conservative initial and total timing setting along w/a fairly large amount of vacuum advance that used manifold vacuum. The above suggestions can all be done easily over a weekend, are invisible to onlookers and will cost less than $20.
Adding a points replacement module under the stock cap will give you added fire at the plugs, but if the points are in good shape, they'll suffice.
Should removing the deposits and recurving the distributor not work well enough, another possible solution if you wanted to keep the casting number 46 heads on the engine for originality's sake would be to use a set of
Keith Black p/n 892 pistons (shown below). They have a D-shaped cup instead of a round dish. The cup is only 10 cc so the deal would be to enlarge the cup to give the dish more volume.
I wouldn't worry about longer rods at this point. Using a thicker head gasket- while lowering the CR- is a less-than-ideal solution because it kills the quench/turbulence that's necessary to promote good flame travel and reduce detonation. But w/o knowing the exact measurements inside the engine, like the piston deck clearance and the thickness of the gasket that's on there now, this can't be entirely ruled out as a possibility. It will require you to disassemble and measure things to know for sure, but you can use a compression tester to get a rough idea of what you're looking at.
As has been said, a set of later model D-port heads like the 6X-4 can also be used, they'd drop the CR and give you bigger valves than you now have. I wouldn't bother adding 1.77" exhaust valves- IMO they're much ado about nothing in the 428 and especially the 455 engines unless they're built stout. The later production heads you'd be looking to use already have hardened exhaust seats and guide plates and screw in studs, With a stud change you'd have an adjustable valve train. Guys have used the bottleneck studs w/lock nuts to get adjustability, but the cost of good straight studs is a minor consideration for the added strength IMHO.
If you were wanting more performance from your 428, I'd have to recommend a set of Edelbrock aluminum Pontiac D-port heads w/87cc chambers. The D-ports allow you to retain the original manifolds and while they're certainly not original they can obviously be painted to look more at home under the hood of an otherwise correct car.
Good luck.
