Tips on modifying a 350-4
Posted by Matt_battousai@reddit | projectcar | View on Reddit | 16 comments
Just bought a 1969 Buick lesabre. It has the 350-4 (5.7) with the th400 transmission.
The engine was rebuild 3 years ago and it will need some changes. Spark plugs need changed, front serpentine belts need changed etc.
I’m looking into replacing the carb, intake manifold and open air intake.
I already have long tube headers.
What carb you be suited?
350 (cid) x 6000rpm works out to a 516 carb.
Should I go to 600 four barrel?
Also do I need an intake manifold and upgraded fuel system to run a carb like that?
Change fuel filters, pumps etc.
Any other mods that you guys recommend?
Thanks!
carburetor47@reddit
For a low cost, mild performance improvement, you could switch to a stock cast iron intake, a Buick QJet, and a dual exhaust. You'll need to change the throttle cable because the pivot point on the 2 bbl is different than the 4 bbl (which needs a longer cable). Add an HEI distributor to replace the old points, and you're all set.
Matt_battousai@reddit (OP)
Saw a HEI is around 500 so will wait a bit till I get that but good shout. Already have the long tube header!
name4231@reddit
Honestly the motor will never be a powerhouse. The bore is very limiting on valve size and therefore power. I’d look into saving up for 4.10 3.73 posi gears and an OD transmission. Or look into a gear vendor OD for your th400 and turn it into a 6speed. Car will maintain its reliability, feel much quicker and be more fuel efficient with either a 4 speed auto or the gear vendor th400
Albino_Echidna@reddit
Nobody will be able to help with a carb or intake manifold without more information on the heads and the rest of the motor, those will be key to making effective recommendations.
Matt_battousai@reddit (OP)
What else info would you need? I can get those
Albino_Echidna@reddit
The specific heads, the valve size in them, compression ratio, and ideally even the stall converter in the car.
4linosa@reddit
Buick bottom ends aren’t known for high rpm. Being under square they make torque early so they don’t need to spin fast to get you moving. Is it a stock or aftermarket crank? I wouldn’t spin it to 6k unless you are sure about the crank and having the oiling sorted out.
Matt_battousai@reddit (OP)
Think it’s a stock crank. The rebuild was too clear. I think just internals cleaned and seals done.
4linosa@reddit
I’d look further into the recommend max rom on that crank then. Also, a great resource online is v8buick.com. I learned the hard way that a Buick 350 was not the same as a Chevrolet 350 from that era. This site helped me learn a lot.
rudbri93@reddit
The 455 is actually quite oversquare, 4.3 bore and 3.9 stroke. But yea, they arent fans of high rpm with those big crank pins. Op should probably put a new oil pump and timing cover on their list.
v8packard@reddit
The OEM carb was a Quadrajet, and they are very good when set up properly. They were 750 cfm in 1969, and that works well because of the sophisticated boosters in the Quadrajet. The formula you see for carb size doesn't represent the ability of down leg booster, annular booster, and hybrids to properly atomize fuel with different plenum designs and venturi sizes. The common Holley straight boosters are not well suited to larger plenums and venturis, and often need heated intake air for best atomization. That old formula represents these straight booster carbs best.
What intake manifold will you use?
As for mufflers, Dynomax makes mufflers that work well, and actually muffle. If you want a muffler that is a bit louder look at Magnaflow.
Matt_battousai@reddit (OP)
I was looking at the TA stage 1 manifold. Seemed to be made for the older 350 Buick engines and is lighter? I’m not 100% sure and info online is all over the place. I need someone with good experience to help me figure out a good system.
Magnaflow might be the one! Thanks dude
v8packard@reddit
What is wring with your existing carb?
I honestly prefer Dynomax. But Magnaflow is good.
Matt_battousai@reddit (OP)
Nothing is wrong I don’t think. Just thought a new carb would be a good upgrade?
v8packard@reddit
Well, that's not always the case. If you have a stock engine, with a properly functioning and properly calibrated stock carb you will not realize much if any improvement with a new carb. You might get better results doing things like converting to electronic ignition and dialing in your ignition advance curve.
Matt_battousai@reddit (OP)
This is all the info on the engine