Thread: 350 sbc build need advise

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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

By the way can someone please tell me how to post pics ? I thought it would be good to show my progress as I build my 51 ply

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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Welcome John, sounds like a nice project! I believe you will have to have like 10 or so posts before posting pictures, it was done to keep the scammers from posting their trash on here. Richard has tons of good advice and technical information on engines along with others on here who have been building engines for years, hard to go wrong following their advice!

Great information! Thank you so much for your replies. I will check the deck height as well as other possibilities you have mentioned and get back to you. Thanks, JT

Welcome Travlin55, good to see you here. On the motor - go 383 or 408.. there's no replacement for displacement! And then everything else that Tech1 said.

To post pictures ( with NO links) from the bottom of the page, press "Go Advanced", type a few words about what we might see, then scroll down until you see "Manage Attachments", that'll take you to a page that lets you select the pictures ( navigate to their location) then upload to this site , lower right press "Done"... takes you back to this page and you can preview or submit the post.

That's the quick and dirty way.. until you get the 10 or more post listing, you cannot use links with your posts. So you can't use a hosting service yet.

Ok so the deck seems to be stock at .045

Originally Posted by Travlin55

Ok so the deck seems to be stock at .045

So, that would be 0.025" piston deck height and a 0.020" gasket, added together to make a 0.045" Squish/Quench, is that correct? Did you find a part number on the pistons?
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So did you check all four corners of block to see if it is square????????

Originally Posted by techinspector1

So, that would be 0.025" piston deck height and a 0.020" gasket, added together to make a 0.045" Squish/Quench, is that correct? Did you find a part number on the pistons?
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Well no, I'm getting 45 thousands from the top of piston to block deck

Originally Posted by jerry clayton

So did you check all four corners of block to see if it is square????????

Sorry Jerry you lost me with that question . A machinist I'm not.

Originally Posted by techinspector1

So, that would be 0.025" piston deck height and a 0.020" gasket, added together to make a 0.045" Squish/Quench, is that correct? Did you find a part number on the pistons?
.

There is what looks like a casting clock on the inside of one skirt with the number 1436 and 89 in it . also a W or M on the inside of the casting on top, and a 4 on the piston in the side near the wrist pin. Other than that there is the number 40 stamped on top of the piston

To be accurate you need to check the distance in the 4 corner cylinders to see if the block is even height on both sides---------and also if its 45* from crank which equals 90* from rt to lt side

Chev sb typically are tilted a little from front to back in opposite directions------It takes quite a bit of high dollar tooling to correct a block to the center lines of the crankshaft bore and the camshaft core hole

Originally Posted by jerry clayton

To be accurate you need to check the distance in the 4 corner cylinders to see if the block is even height on both sides---------and also if its 45* from crank which equals 90* from rt to lt side

Chev sb typically are tilted a little from front to back in opposite directions------It takes quite a bit of high dollar tooling to correct a block to the center lines of the crankshaft bore and the camshaft core hole

Ok, got it, will check it and get back. Thanks , JT

I think I know what has happened.
Stock from the factory, the block measures 9.025" from the centerline of the main bearing bore to the top of the block where the heads bolt on.
When we put together the "Stack" of parts to fill the block, we use 1/2 the crank stroke (1.740" for a 350 crank), 5.700" for the rod length and 1.560" for the compression height of the piston (the measurement from the centerline of the wrist pin to the crown of the piston just above the top compression ring.) If you add these 3 values together, 1.740", 5.700" and 1.560", you will find a stack dimension of 9.000". Now, if we fit that 9.000" stack of parts into a block deck height that measures 9.025", then we find that with the piston at top dead center, we have a space of 0.025" from the top of the piston to the top of the block where the heads bolt on. These are all standard factory dimensions, so when you add a 0.020" thickness head gasket, the distance from the top of the piston (including the gasket thickness) to the underside of the cylinder head is 0.045" (0.025" piston deck height added to the 0.020" thickness of the head gasket. This will normally work fine on a pump gas motor with iron heads that is 9.5:1 static compression ratio or less.

Problem arises when Jimmy-Jack decides he wants more cubic inches and takes the block to the machine shop to be bored and honed for larger pistons. The machine shop doesn't give 2 hoots in hell for setting the squish/quench at between 0.035" and 0.045" like Jimmy-Jack told them to. They are in business to make money and so they order up the cheapest cast pistons they can find, even though the compression height is shorter than stock (1.540" instead of 1.560"), then bore and hone the block and fit the pistons into the block. It doesn't really matter anyway, because they have built the motor to 8.5:1 static compression ratio and it would practically run on kerosene without detonating. Jimmy-Jack has no idea that the piston compression height is 0.045", which puts the squish/quench at a worthless 0.065" with the gasket thickness added to the 0.045", because he never measures it before he bolts the head on and fires the beast off.

Fast forward a few years and Mr. John Taylor comes into possession of the motor, tears it down and finds the short rebuilder piston down in the bore at 0.045".

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