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Hey Mike, I have got a recipe for you that will inject some serious power into that wheezer 403 you have WITHOUT HAVING TO DO PISTONS (!) well, unless you really need to that is... Read on:
Oldsmobile 6 casting heads have a 64cc combustion chamber versus your cavernous 83cc 4A heads. I have a set of good 6 heads in my possession that I was originally going to use on the 403 build I recently brought into existence. They have had hardened exhaust seats installed already by the previous owner for unleaded / ethanol blend junk pump gas. To make them run on the 403, the head bolt holes will have to be upsized from 7/16" to the 403's larger 1/2" - no big deal on that, can even be done at home carefully on a decent drill press. These highly sought after "6" heads could also be milled just a bit to increase the compression just a tick more, but that's splitting hairs. The reason why I ended up not using them is because I would have wanted to install larger valves in them. The intake I just measured is showing 1.877" - definitely could use a larger intake valve, especially on a 403. I just measured a 403 intake valve finding it at 1.990". It wouldn't take any particularly exotic machine work to install some 2" intakes in these heads (which is what I was going to have done). The exhaust valve out of the 6 head is measuring in at 1.561", while the 403 exhaust valve is measuring in at 1.503", so you could leave the exhaust valves alone, but do suggest a slight upgrade to help it exhaust better. These 1970 6 castings have the cast in bosses for smog identical to the 4A heads I have here also. The 4A heads I pulled out of the '79 Firebird Formula donor car did not have A.I.R, so the side by side comparison is identical. Any reasonably competent machine shop could machine in the A.I.R. ports into the castings if you really really need to have it in place. What state are you in? Do they actually do a stringent visual VIN code checklist inspection? My dad used to have to pass very strict smog testing in Phoeniz, AZ, but I yanked his complete A.I.R. system out of his '76 Seville eons ago and they never noticed or cared that it had port plugs in the cylinder heads (that we painted flat dirty black to make them visually disappear). Here in TN, they make you pass tail pipe sniff, visual inspection for the catalytic converter(s), check your gas cap, but do not crawl around under your hood to see if every last bit of your smog crap is in place. Since many Firebirds / Trans Ams swung either way, with or without A.I.R. from the factory, I'd be inclined to remove it entirely, pump, lines and all, as they may be hard pressed to nail you on a GM "maybe equipped federal / maybe California only" type of scenario with the A.I.R. system anyway...
So here is your current absolutely DISMAL compression ratio: 8.09:1 = downright terrible...
Now here is what happens if you bolt the 1970 "6" castings on with their much smaller combustion chamber volumes:
Almost a full point and a half of compression increase. A vastly more respectable 9.46:1
Now *IF* your cylinder block deck surfaces can be cleaned up decently (takes some work), you can use some .027" Cometic MLS head gaskets, bumping up your compression even better yet with the 6 heads bolted on:
9.77:1 - now we're talking!
Ok, so we cut a deal of some reasonable nature for my 6 heads - they are literally sitting here doing nothing, and I do not have any plans for them that I can even imagine at this point. You run them over to your machine shop, have them bowl hog them out for use with the larger valves, do a 3-5 angle machining on the seats, some very basic pocket porting / blending (valve guides check good on these heads), drill the head bolt holes upsize to 1/2", and maybe even machine open the cast in bosses to accommodate the A.I.R. only if absolutely needed. Have them upsize the rocker mounting holes from the dinky 5/16" to the much needed 7/16" so that you can install some real strong adjustable rocker studs in along with some Speedmaster Stainless Steel roller rockers (that will all fit under your stock valve covers with a few tricks I have up my sleeve), install some good springs and retainers, and now you have solved your compression problem without having to chase exotic $$$ pistons around. 9.5:1 from the dismal 7.9:1 that some say they really had is a respectable jump, and nobody could tell anything was different save for the "6" number cast into the head versus the 4a.
Next, install a mellow hydraulic roller cam. People will say this is way too small, but I've run it and it breathes very well and will definitely pass smog:
This cam idles smoothly, but pulls like a freight train. More importantly, it will definitely pass smog sniff tests. Remember, the magic here is the "area under the curve", meaning look at the shape of the cam lobes as compared to an old school flat tappet and it becomes immediately obvious that the valves stay open WAY longer in the usable lift / flow ranges = BIG increases in power without pissing off the smog cops, and will idle dead smooth with just a tiny hint of a different cool distinctive roller cam exhaust note.
Tweak the secondaries on the Q-jet with a different performance hanger and rod set to provide more fuel. Tweak the spring tension on the air doors just right. BUT, I'd suggest you resist the temptation to richen up the primaries (the front smaller two barrels). You need some decent mileage, and do have to pass smog, so richen up the big rear secondaries, but leave the small front primaries alone.
Do yourself a BIG favor and obtain and install this distributor:
Do note that you WILL have to remove the ignition control module out of this distributor and literally throw it away - that is a known issue with this distributor. Install a quality Delphi module in it:
Once you install a quality module into this distributor, you will find that it has a pretty darned good vacuum advance and also a mechanical advance curve already in it! The vacuum advance is also adjustable with the little Allen wrench they provide with the distributor (goes into the vacuum hose port where you can increase / decrease the vacuum advance activity). Price just a replacement adjustable vacuum advance for your stock distributor - it will shock you as compared to the cost of this complete distributor. Now I can feel the flame guns heating up on this already, but many, including myself have run this distributor perfectly on a billet roller cam as per this distributor having a 4340 Chromoly drive gear. I approached with caution on this, but after running one on a billet cam gear for thousands and thousands of miles, yanking the distributor out occasionally to inspect for wear / troubles = nothing, zip, zero, zilch - perfect wear patterns on the (expensive) billet roller cam and also this distributor gear, while also running a high volume / high pressure oil pump that places more load on the cam gear and the distributor gear. I have had one of the new $$$ composite distributor drive gears here ready to go in case I started noticing any bad wear patterns, but nope - nothing at all. I know I'm going to get flamed for this, but that's OK, as this is a proven scenario by myself personally along with a few others I have worked with in my area. Of important note, you ALWAYS have to spin the distributor shaft occasionally with a drill motor to build oil pressure with the distributor out to visually confirm that the tiny oil jet in the oil gallery plug right near the distributor gear is properly squirting oil. This little jet drilled into the oil gallery right there is the key to distributor / cam gear survival in an Olds. If this gets plugged up, your gears barely get any oil and buh bye camshaft and distributor gear with a quickness. Of sad note, if this jet in the oil gallery *IS* found to be plugged, it is next to near impossible to access it with a pick tool to try to clean it out. I have only successfully managed to do it about 1 out of 15 attempts. If it is plugged, and you can't get it unplugged, you unbelievably can only access this to repair / replace this plug by pulling the transmission out (DOH!) and accessing it from the backside of the engine block. Definitely no fun, so fingers crossed that your oil gallery plug jet is NOT plugged (!).
Have I made your head hurt yet? It shouldn't, you should be smiling from ear to ear not having to spend major coin on pistons to get your compression up. Never mind that replacement pistons can upset the balance of your rotating mass, which now also means you would have to get everything balanced to accommodate the lighter weight aftermarket pistons.
April 5th, 2023, 06:39 PM
Good news for you :)
