How much horsepower would a cam headers and 4 barrel add to my 1978 305?

Answer 1

This is a combination of lots of factors but somewhere between 50 and 70 horsepower should be achievable. A 4 barrel is usually worth 20 hp on a stock motor, headers 15 to 25 hp, a cam should get you another 10 to 30 hp. You need to do some homework and figure out what your compression ratio is, figure out what manifold you want to run and narrow down your cam choice. Exhaust pipes rear gears and converter all play in as well. That being said none of your bolt on stuff is worth a dang if you don't have your carburetor jetting and power circuit at least close, distributor advance is also VERY important!!! I have been playing this game for a while I run into this all the time, cams and heads won't make any power without a proper advance curve. If you are running a stock HEI than you will need to look at the mechanical advance as it is probably stuck, rusted, has the weight hinge pins half cut through or is sprung so tight it won't work for anything performance oriented. Also look at the vacuum advance canister it is probably leaking, and if it does still hold vacuum it is probably giving too much advance. Most 305's had EGR valves wich run a ton of vacuum advance, like 25* in some cases, way too much for anything sporty. Go to schucks and get the B.S. advance kit form China use the lightest and the next lightest spring on the mechanacle advance after you clean inspect and grease all the contact points on the weights and cam. Get a 10* vacuum advance from a 454 motor home, install and make sure the magnetic pickup isn't hanging up on the reluctor wheel, if you can feel any drag at all you need to file the reluctor, as this will actually retard the timing as RPM goes up. It will probably need a shim or two under the gear, most HEI's are really sloppy this way. Get it running and rev it up to three grand use a dial back timing light and check you timing, set it at 35* or so, let it idle and check the inital timing. It will probably be at 16* or there abouts, now hook up your vacuum advance and repeat, you will see about 45* this should work pretty good. Just don't go over about 50* with the motor reved up and the vacuum advance plugged in and you shoud be okay. If you try this with the stock vacuum can you will be up over 60*, too much for hotroding!!! My advice is put a Q-jet on it, yes a Q-jet, they are the best don't listen to your loudmouth buddies who tell you to run a Holley or an Edellbrock. Holley's are a pain, demons are junk, the quality control is TERRRIBLE!!! Both have chronicly leaky power valve gaskets, it does no good to replace as the new parts start leaking in about an hour, the vacuum secondaires never give good perfomance even after lots of tweekin' on smaller motors. Edellbrocks are okay but the fuel metering in the booster venturiis is too rich for most applications and there is almost nothing you can do about it, if you do go this way get the electric choke model, the manual chokers are like .007 richer on the primaries. Not what you need, if you do go with a Q-jet you will probably have the best luck, that is if your not retarted and can figure out how to make one work. They are not that complicated. Give me a call if you need more help, I actually have a 78 305 and I drive it every day. I am in the process of doing similar modifications, pluss i run a carb and distributor service so I know of what I speak. Let me know how it goes.

Andy Stafford

509-521-2910