Thread: 350 sbc build need advise

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Originally Posted by Travlin55

First I would like to introduce myself I'm John Taylor have been a body and paint guy for about 45 years all over the country and now semi retired I am currently building a '51 Plymouth bussiness coupe and installing a nova front frame stub to make it more road friendly.
So, as I have it tore down I thought I'd go through the motor to see just what is in it as it was already built by the PO. It's a 69 350 4 bolt with flat top Pistons, balanced rotating assy double roller chain, camel hump heads w 202s and 160s been ported and polished, Edelbrock air gap intake and Edelbrock 650 carb and HEI dist.
Currently running a th350 w 2500 stall. 10 bolt rear end w 342 gears. The car weighs in at 2800 lbs and ran out strong as was but I want more! The cam was close to maybe an rv style and just wasn't my cup of tea. The motor has a lot of potential but I believe it could be so much more with the right valvetrain setup and carb/ manifold. So any ideas would be appreciated..... I really am looking for decent drivability but all the power I can get at the strip

Hello John and welcome aboard. My favorite car in all the world was a '50 Plymouth business coupe built as a C/Altered drag car by the Ramchargers car club, a group of Chrysler Corporation engineers. It was a wildly modified car powered by a Gen I 354 Chrysler hemi and held the C/A NHRA record back in the day..

First thing you'll want to do is to identify the pistons and determine the piston deck height (distance from the crown of the piston to the block deck with the piston at top dead center). Some fellows have used pistons that have a reduced compression height (stock is 1.560" for a 350 motor) that puts the piston down in the bore at TDC and prevents using the proper squish/quench (0.035" to 0.045") to eliminate detonation on pump gas. If a motor has the stock piston compression height of 0.025", then a steel shim gasket of 0.015" thickness can be used to generate the proper 0.040" squish/quench. This is just one very small area of engine building, but we might just as well begin there.

Instead of attempting to get more hp from 350/355 cubic inches, I would opt to rebuild the motor into a 383 or even to locate a 400 block and build a 408. The reason I'm saying this is that you can get more power with pump gas from a larger displacement motor and not worry so much about building it high compression and worry about detonating it. A 408 built to 9.5:1 static compression ratio with iron heads and a cam to match will pull like Jack the Bear on good pump gas without detonating. Aluminum heads will rid themselves of heat much faster than iron heads and motors using them can be built to higher static compression ratios, about 10.5:1 on pump gas without detonating.

You can also make a small block think it's a big block by using a blower or turbocharger or nitrous oxide. You just determine what fuel you're going to use and build the motor to operate on it without detonating. Speaking of that, if you have E85 available to you everywhere you drive, you have hit the hot rodders jackpot and can build a 15.0:1 race motor to drive on the street.

The fellow who built the motor may have dialed it in with the proper camshaft for the static compression ratio that he built into the motor. You see, cylinder pressure (horsepower) is determined by the closing of the intake valve. If you have a low static compression ratio, like maybe 8.5:1, then you will want to use a fairly mild cam that closes the intake valve early enough to capture the air/fuel mixture that was just pushed into the cylinder by atmospheric pressure. If you use a wild cam with a low SCR, then you leave the intake valve open too long and the mixture that was just pushed into the cylinder is blown back up the intake tract by the piston, which is now driving up the cylinder to compress the mixture that was just pushed into the cylinder by atmospheric pressure. This creates a condition known as "standoff". If you have a car that has a lopey camshaft, at night you can remove the air filter and shine a strong flashlight across the top of the carburetor and see the fog of air/fuel mixture hanging above the carb. This is mixture that was blown back up the intake tract by the ascending pistons. Such a car will not idle cleanly because every time the mixture passes the venturi in the carb, the venturi adds fuel. Pretty soon, the mixture is so rich that the motor won't idle. This is why fellows with such a combination must keep "blipping" the throttle to keep the motor cleaned out.

Matching up a camshaft with the SCR is a very delicate thing and you must know what you're doing or you can screw the whole mess up, making a turd out of what could have been a good running motor if the correct intake closing the point would have been chosen. What we try to do, with a motor that will run on pump gas and uses iron heads, is to shoot for a Dynamic Compression Ratio of about 8.00:1. Don't confuse DCR with SCR, they are two completely different things, although related. Let me put it this way......YOU CANNOT JUST REACH OUT INTO THIN AIR AND CHOOSE A CAM.

Much of what I say may already be familiar to you.....or it may sound like Swahili. Either way, we have some pretty sharp fellows on this forum and we can help you to build a motor that will do what you want it to.
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