Keep the answers coming, someone will say the magic word.
409 068 truck block compression
- Thread starter Windingout
- Start date
With the W engine having the flat head, the combustion chamber is in the cylinder. The piston to head clearance area is the quench pad on the 09 piston. It's the shape of the piston past that quench/squish pad that provides the combustion chamber volume. That's where the W gets its compression number. That's what makes the 409 the engine it is and makes it different from all other engines with their combustion chamber in the head. So, piston and gasket thickness determine quench, piston shape determines compression. I hope I made that clear, it's a little early in the morning and I haven't had my coffee yet!
It seems to me that anything over a perfectly flat surface is going to add volume/reduce the compression ratio.Consider the compression differance between the 333 truck head and say a 690 car head.I think the same thing would apply,to a lesser degree perhaps,when comparing a Bob Walla head to an Edelbrock head.
I am understanding the lesson here.....so now school me if you will on ideal quench or a spec to shoot for...........I've read .045- or bigger on sbc,BBC.....but the W is different.A quench of .045 in a W motor,I've read your getting excessive and giving up power.On the other hand too small you end up with excessive cylinder pressure and pre-ignition......
From what I've learned,anything over .060 in the quench area eliminates it.As to the lower end of the spectrum,many things come into play.Piston type,piston to wall clearence,rod type [aluminum or steel],engines peak rpm[all rods streach some at high rpm].To small a quench will not cause excessive cylinder pressure,or pre-ignition,but will cause broken engine parts due to piston to head contact.As to the quench area on a w,Ronnie likes to use .000 deck,and Fel-Pro gaskets [.040-.042] on his stroker 409s,and his stuff works and lives.Yes,you might get away with closer,but if something has been proven over time to work,why would you change it?
If your using steel rods and your piston side clearance is not excessive I think .035 quench is perfect for the 409. I know some of the Guys are using custom Pistons with what's been coined the "power slot", in the quench area. I hope one of them chimes in here. I am running diamond pistons and my quench is around .042. Even with aluminum rods unless your spinning to the moon a tighter quench is good but I would increase it to at least .045. The valve recess even with the Eddies is minimal at 15 cc's. Stock 690's are 7 cc's I think. Not enough to change comp by much. Although if you have the Eddies and a truck block better take that in to consideration when ordering Pistons. Bottom line is pick your head gasket with quench in mind not compression ratio, leave that to piston selection.
How do you know what your piston to deck height is going to be before you order pistons?Is this something the manufacturer accounts for when machining the pistons or is this something you measure after receiving the pistons and then modify them after you see how high they come. If they do not come up high enough you would be s.o.l. I see how you can control it with the gasket thickness but the piston height escapes me.
You can work with your machine shop when you prepare the block for your rebuild. The piston manufacture will put the pin where you want it. Bottom line is your machine shop guy should be doing the math for you. Sometimes even the best of us screw up when measuring and ordering pin height and have to do a little fancy dancing to get quench close to what is acceptable. There is a little wiggle room. After all this isn't rocket science! 
You're going to have to have your machine measure and square the block.The blueprint spec.for the distance from the crank centerline is 9.600.Once that's done,you can supply the piston maker with the spec's and they can go from there.These old blocks can vary on the deck surfaces from front to rear,as well as be different from side to side,by quite a bit.The piston maker will need to know the deck height,stroke you're planning to use,and the length of the rod that you're going to use.On a stock 409,the rod length is 6.09,the big block Chevy rod is 6.135[much better rod],and the best choice is a 6.385 length rod for several reasons.
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What is the process for figuring deck height.......my block was pulled from the machine shop under advice by Jack Gibbs.I had given my machinist Jacks # so they could discuss this process,and there was so much confusion,Jack said pull the plug.......
Since you already have the pistons,mock the engine up using the crank,and one piston and rod assembly.Measure the distance from the deck to the piston top at it's highest point at maximum rock.Do this on all four corners of the block.You may have the piston deeper from one side to the other as well as one end of the other.Since you'll want the piston to protrude a little[Jack likes .002,Aubrey likes.004],you can then determine how much to mill from the block.If your machinest can't understand Jack,he doesn't know W engines! Find one who does,regardless of distance.The last thing you need is someone screwing up these hard to find,expensive blocks.
I rock my piston to the highest point then rock it the other way and average the difference. That's the measurement I use to determine how far I think the piston will pop up in the running engine. This is how I set quench.
What about before buying pistons.......?
As I stated before,have the block align honed,then have the block decked to 9.600 on both sides.Then supply the piston supplier with the crank stroke,rod length,and desired deck height not counting the head gasket thickness,and he'll be able to determine necessary piston wrist pin height.
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Dr Richard Kimble
Well Known Member
we have some good solid information coming out right here 
I'd just like to add that I too , try to minimize the machining off the block .
I'd just like to add that I too , try to minimize the machining off the block .
I'm at 28 quench on the high side based on 39 compressed FelPro gasket. FelPro may actually be closer to 42 so mine might be higher. My engine guy wanted postons without the pin hole bored to make it all correct. Think I read somewhere that the factory 62 409 with a single shim gasket was 28?
Oops!!!!!