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Old 03-25-2009, 09:01 PM
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The right gear for use with a small block and 700R4 is a 3.73:1. With its .7 overdrive, that puts the 4th gear final drive ratio at 2.61. A 3.08 rear gear will put the final drive ratio at 2.15 and I doubt that anything less than a big block would pull that gear. It's not so much about getting off the line, it's about where the motor will spend most of its time trying to pull the vehicle down the highway with a headwind.

Butch, maybe you should do some study on intake closing points and dynamic compression ratio. If you use a short cam with a 10.2:1 static compression ratio, you'll build excessive cylinder pressure and the motor will rattle like a can of rocks no matter what kind of pump gas you run in it.

In the first place, I would never run 1.6 rockers on an XE cam. The lobes are ground to maximum velocity and any further strain on the lifter/lobe interface over what is introduced with 1.5 rockers will likely result in your disassembling the motor in a short while to clean out all the crud left from the disintegrating cam lobes and lifter faces. This assumes that you are using the correct spring rate recommended by the cam grinder.

In the second place, the intake closing point of the intake valve on the XE268H is 39 degrees after bottom dead center @ 0.050" tappet lift. You won't find this info in Comp's site or literature. You have to know how to figure it yourself. They only give 0.006" closing point on the cam spec card. You have to have the 0.050" number in order to figure the DCR on the KB calculator. So, DCR with the XE268H cam and 10.2 SCR is 8.77:1. You may get this to work, but I'd rather back it down and play it safe using pump gas.

The 286H Magnum cam closes the intake at 45 degrees after bottom dead center, resulting in a DCR of 8.43:1. With a squish of 0.035" to 0.045", I'd feel OK about assembling this motor to operate on pump gas.

shsouthard, I explained squish to another board member, so will copy and paste that explanation.....
"Squish, also called quench, is the measurement from the piston crown to the underside of the cylinder head with the piston at TDC. It is set with the piston deck height (measurement from the piston crown to the block deck surface where the head bolts on) and the thickness of the head gasket. Most professional engine builders will shoot for a squish of 0.035" to 0.045". David Vizard stated that the motor made more power and was more detonation-resistant the tighter he set the squish. I've noticed lately that he prefers 0.035". Any tighter than that could produce problems with the piston crashing into the head at higher engine temperatures and higher engine speeds because everything grows and stretches. There will be a little flex in the crank, a little extension of length in the rods from both stretching and thermal growth and the piston will grow taller from thermal expansion. So, 0.035" between the piston crown and the flat part of the underside of the head adjacent to the chamber might be the tight limit on a small block chevy to give the mixture maximum velocity across the chamber to help make the motor detonation resistant and also to prevent metal to metal contact.

I'll give you a for-instance on a build. You have a block that has never been decked and measures 9.025" block deck height (measurement from the centerline of the main bearing bore to the block deck where the head bolts on). Your stack of parts measures 1.875" (crank radius for a 3.750" crank), 5.700" rod length and a piston compression height (measurement from the centerline of the wrist pin to the crown) of 1.425" for a total stack height of 9.000". This means that when you assemble the motor, the piston will be down in the bore 0.025" (piston deck height) with the piston at TDC. To reach the target 0.035" to 0.045" squish, a very thin head gasket would have to be used, such as a 0.015" steel shim gasket. To use a shim gasket, the block decks and cylinder head mating surfaces must be flat and smooth, but it can and has been done. Chevrolet used to use steel shim gaskets in the factory motors they assembled. Another way to reach the target squish would be to cut the block decks 0.010", leaving a piston deck height of 0.015", then use a slightly thicker composite gasket such as a Victor Reinz #5746 gasket that compresses to 0.025". This would result in a squish of 0.040". If for some reason, a builder wanted to use an even thicker gasket in this build, he might cut the block decks 0.025" so that the piston was exactly even with the block decks at TDC. This would produce a piston deck height of 0.000" and is known as zero decking the motor. You could then use a gasket that compresses to 0.040" and have your 0.040" squish.

While you are learning this stuff, you might also want to google Singh grooves. These are channels that are cut into either the head or the piston crown and allow routes for the squished mixture to be jetted toward the spark plug.

All of what I'm writing about here is aimed at the elimination of detonation on pump gas with a moderate static compression ratio. There are many instances of iron head motors being run successfully with 11.0:1 static compression ratios if using the correct squish. Of course, a builder must also pay attention to dynamic compression ratio. This is the factor that uses rod length and camshaft intake closing point to build to a certain result.

Let's say for instance that a builder puts together a 9.0:1 motor and chooses a very long cam that closes the intake valve at around 70 degrees (@ 0.050" tappet lift) after bottom dead center. The motor is fairly low compression to begin with. Closing the intake valve so late allows the intake mixture that was just drawn into the cylinder on the intake stroke to be blown back up the intake tract. The intake valve is still open a little while the piston is travelling back up the bore on its compression stroke, so the piston pushes the mixture back up toward the carburetor, reducing the amount of mixture that is trapped in the cylinder when the intake valve finally closes and making a weak explosion when the spark plug fires. This is what happens to poser newbies who use a long cam in an otherwise stock motor. The motor won't make enough power to pull the hat off your head, but boy it sure sounds great to the geeks down at the drive-in with its rumpety-rump.

The flip side is building a higher static compression ratio motor and using a cam that is too short for the scr. Its a high-compression motor to begin with and let's say a cam with an intake closing point of 30 degrees
ABDC is used. The valve closes early enough so that most all of the mixture is trapped and you get a cylinder pressure that is too high for the available fuel. This results in pre-ignition and/or detonation and/or cracked pistons and/or bent rods and/or a broken crank and/or a failed head gasket or two.

So, as you can see, all these things have to be taken into consideration before you can finalize your choice of parts that go into a build. I wince every time a newbie comes on and knows nothing about the scr, squish, piston deck height, block deck height, chamber size, piston crown configuration, piston compression height or practically anything else about the motor, but says he has chosen such and such cam. I usually just click to another thread and smile.

There is another scenario that is possible, although no builder in his right mind would do because it leaves no material on the block decks to take another cut on the decks if you have to clean them up in a later build. That is cutting the block decks to pop the piston out of the block. Let's say that you have a piston deck height (piston crown to block deck) of 0.010" and you, for some unknown reason want to use a 0.060" copper gasket. Maybe you're thinking of adding a blower later, so a copper gasket would be a good choice. To arrive at your preffered squish of 0.040", you would have to cut the block decks 0.030" to do it. This would pop the piston out of its bore by 0.020". This can and has been done, but it would not be the avenue you would want to take on an unblown application.

The other things you want to be cautious of is the configuration of the piston crown and the piston compression height when you are choosing the parts for a build. Rebuilder pistons, although cheap, are normally manufactured with a reduced compression height (centerline of wrist pin to crown) of about 0.020". They do this figuring that the machine shop is going to deck the block 0.020" to clean it up and the piston will be at the same piston deck height (piston crown to deck) as the old original piston was. This is fine for a low-buck crate motor, but for a street rod motor builder, it is unacceptable. Always note the compression height of the pistons you are considering buying and add up your stack. If it is less than 9.000", you're going to have to do some considerable decking on the block to get the piston deck height (piston crown to deck) where you want it to set the squish. If you're doing the building and not paying attention to the piston compression height, you could end up with a squish (or non-squish dimension of 0.065" to 0.070" and then wonder why the motor will not operate on pump gas of any kind without detonating.

The piston crown configuration is another place to pay attention. If the piston does not have a large, generous area on the crown to mate with the underside of the cylinder head, you will not generate enough squish to make the motor detonation-resistant. I'm talking about if you have to use a dish piston to make your target static compression ratio. Some pistons have the dish machined out over the entire top of the crown and leave only a thin ring of actual crown material maybe 3/8" wide around the perimeter of the crown. This is unacceptable. You want to choose a piston with a D-cup configuration. These will have a generous pad of material to mate with the head and will generate the proper squish to quell detonation. Here's an example. Note the generous squish pad on the right side of the photo....
And by the way shsouthard, that's the piston you should have used to give you a 9.6:1 SCR instead of your 10.2:1. Then you could have used a shorter cam and the motor would have been more street friendly.

Last edited by techinspector1; 03-25-2009 at 09:07 PM.
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