Thread: Changing compression ona built motor

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Originally Posted by HotRodN

Tech,
If I were to decide to go with 5.7" Rods,,, what changes,,, compression length?? I assume I'd go from 1.130" to 1.430" given the .300" difference. I also assume that torque would be slightly lower than with 6" rods.

Yes, piston compression height for the 5.7" rod will be from 1.425" to about 1.433", depending on manufacturer. Choose the piston you want to use and add the compression height of the piston to the rod length and stroke radius, just like above. That will tell you how much to cut the block decks.
My choice might be the Keith Black KB253+30. Same piston crown design as I selected for the 6" rod except the compression height is for a 5.7" rod.
https://www.uempistons.com/index.php...26e4bc2877d034
I did not choose the 6" rods for any torque advantage because I doubt there is any, I chose them for compatibility with the internal balance crankshaft. If you go with 5.7" rods, you cannot use an internal balance crank, you must use the external balance crank and balance it with a SBC 400 damper and flexplate/flywheel. The 400 damper and flexplate have an eccentric weight built into them. Again, budget a little extra cash for finish-balancing at the machine shop. It might be spot-on from the maker, but maybe not. 1.433" + 5.7" + 1.875" = 9.008", so you could cut the block decks to 9.008" for a zero deck and 0.041" squish.

Originally Posted by HotRodN

I noticed you have Hypereutectic pistons chosen,,, would forged pistons be a slightly better choice or,,, do the hyper's perform well enough to not worry about the more expensive forged. Keep in mind this is not going to be a drag car but I will step on it occasionally for the adrenaline rush LOL.

If it were my money going into this motor, I'd choose hypers. Now, if I were going to use a blower or spray, then I'd step up to forged. If you keep the motor out of detonation, even cast pistons will work well in most cases, but I like to use hypers for a little insurance.

Originally Posted by HotRodN

The thought was if it was right from the factory it should be right now.

You've already figured out that the block might not be dead nuts from the factory or maybe someone took a cut on the decks who didn't know what they were doing, so I think you're a little smarter than the guys at the machine shop.
NEVER ASSUME ANYTHING. CHECK EVERYTHING.
Doesn't your machine shop have a precision square? Use it with feeler gauge blades to check for 90 degree square between the banks, front, middle and rear.

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The use of 6 inch rods will give you a much lighter piston weigh because you will remove .300 section of the piston and the additional weight of the longer rod isn't as much. With the 6 inch rod the crankshaft counterweights don't have to be relieved so much allowing for a much simpler internal balance, many times without the addition of heavy metal

Tech,
We don't have a precision square at the shop and the one I have won't work for checking the block. It's not big enough, I think it's only like a 6" square. We do however have a large granite surface plate but I doubt they will let me put the block up there to check square with a height gage. But if so,, I could also check cam bore to crank bore center line. While I can still get that dimension, it's just a tad tougher using mic's and calipers. But I can get real close using just those tools.
HotRodN

I don't understand why replies I make don't always get posted,,,, must be something I'm doing.
Okay,,, I'll stick with 6" rods. Just makes sense and is easier to keep the build going as planned and save guess work. For now I'm trying to get that warm feeling about the Summit block but given the quote from a different shop even if I had to have some work done to it I'd probably still come out cheaper. Feels like the shops today are gouging prices to stay afloat with the downturn in the economy,,,, just MHO. Still don't see how Summit can sell the blocks at these prices. But hey I'm happy with it. Once I have pricing on all the parts and compare to going the LS route I'll pull the trigger.

Tech, one option to get this built cheaper for the moment could be to use the 487X heads I have on another engine until I'm ready to get the Pro-Filer heads. Things are getting tight with hours reduced so just looking at all my options. The 487X (last 3 digits) heads were used on the SBC 350' back in 70 thru 72 and I know on the Corvette motor to produce 350 HP from the factory engine. So the thought is I could at least get the 350 on a 383 build with cam until I can swing the other heads.

Originally Posted by HotRodN

Tech, one option to get this built cheaper for the moment could be to use the 487X heads I have on another engine until I'm ready to get the Pro-Filer heads. Things are getting tight with hours reduced so just looking at all my options. The 487X (last 3 digits) heads were used on the SBC 350' back in 70 thru 72 and I know on the Corvette motor to produce 350 HP from the factory engine. So the thought is I could at least get the 350 on a 383 build with cam until I can swing the other heads.

Makes perfect sense. Go ahead and build the 383 short block with zero deck, then pop the iron heads on until you can do better. Those 487X heads have a 75cc combustion chamber, so static compression ratio would be 9.6:1. Use the same Fel-Pro 1003 head gaskets that you would use with aluminum heads, for a 0.041" squish/quench.

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just to let you know how it works---blocks are set up with a fixture that has a 2' solid bar throw adapter rings that size it to the main bore-then a center bar with centering cones is installed in the cam bore and block is centered on those two axis-there is a plate at the end that has the indicating surfaces on to measure to deck at the 45* each way------then there is a triangular piece that you can put in any cylinder against the wall and indicate to main diminsion established from the front plate.



http://www.bhjproducts.com/bhj_conte.../bt1_intro.php

http://www.bhjproducts.com/bhj_conte...tfixt/dhbe.php

http://www.bhjproducts.com/bhj_conte...ntfixt/btk.php

Jerry, you are the man with those links, thanks very much. The verbage in those links just verifies what I've been preaching for years....

Twisted Blocks: On a twisted block, it is necessary to choose a point on the deck surface to use as a reference
point for set-up. As a result, you may be compounding the out-of-square condition.

Uneven Deck Clearance: Deck clearance often varies between the top and bottom edges of the piston due to the
deck surface not being at 90 degrees to the bore. On race engines where deck clearances are held on the verge
of piston to head contact, any discrepancy in deck angle means lost compression.

Bores Not At 90 Degrees: Since most production lines use the pan rails as the reference point, it is common to
find the bores at angles other than the intended 45 degrees when referenced from the cam-crank centerline.

Cam and Ignition Timing Variations: If you have run into cam timing variations between cylinders on opposite
banks, it may not be the fault of the camshaft. Again, since most blocks are machined from the pan rails, the
cylinder “V” may not be at 45 degrees when referenced from each side of the cam-crank centerline.

O-Ring Groove Variations: If you are cutting O-Ring grooves on a boring stand which references from the pan
rails, it is not uncommon to have drastic depth variations.

Poor Intake Manifold Fit: This condition is usually blamed on the intake manifold. However, if the included angle
of the deck surfaces is not 90 degrees or if the top of the block has the incorrect angle, the manifold may not be
at fault. This problem is often manifested with the motor sucking oily vapors from the crankcase because the intake manifold/cylinder head sealing surface is open to the crankcase on the bottoms of the ports. If the sealing surface is open on the top, the leak can be found with ether or other combustible substance, but an open leak on the bottom of the manifold/head cannot be found with ether.

Block Deck-Height Measurements: The Blok-Tru is marked with the height from the crankshaft centerline to
index surface. A simple measurement from the block deck surface to the index surface of the Blok-Tru plate,
added to the Plate’s marked dimension gives you the deck height. This measurement can be easily made using
our Deck Height Micrometer, shown on page 8.

.

Thanks Tech----
as for the twisted deck motors-I have never found a chev block that wasn't twisted-I think it came from the blocks being broached thru a machine pushed from one end-results are consistent with the offser in the clylinders side to side where both sides surfacing doesn't start at same time-differance of the rod width offset.

The block tru plate is 7.500 inches from the crankshaft/main line center-----
those triangular bricks for measuring individual holes will set against the wall and on the center bar-just add 7.5 to your depth mike tool and you have it to prove results in all cylinders