Alright. I'm getting the Merlin 3 block tall deck. But i don't know what length rods i need. Alot of the rotating assemblies i saw had the 6.535 rod with a 45ish dome piston. I want the longest rods i can get. How do i figure if the piston will hit the valve or not. Am i just trying to get the deck of the piston to be as close to flush with the block deck as i can get it. What is the math for the quench. I haven't decided on a cam yet. Any help would be nice

Josh

 

4.00" stroke=6.135 rod
4.25" stroke=6.385
4.50" stroke=6.535

Get the 4.50" stroke crank. Price is the same regardless of the stroke. Be sure to match the piston to the rod length...

 

Stock big block conn-rod length is 6.135". Your tall deck block is 0.4" inches taller than a low deck block. 9.8" for low deck and 10.2" for a tall deck. To figure the height of the piston relative to the deck take 1/2 stroke (454 stroke is 4") + rod length + compression height of piston (stock 454 piston is 1.645"). Then subtract that value from the deck height. This gives you how far down in the bore the piston will be. Then add the compressed width of the head gasket and that will give you the amount of "squish" clearance. That is the distance between the flat area of the head to the squish area of the piston.

The combination that you describe with the 6.535" rod is correct for a "tall deck" 454.

It is really difficult to predict how close the valves will come to the pistons without a trial fit of the set up you plan to use. That would include the head with valves and light springs (cam manufacturers sell light springs just for mock up purposes), head gasket, piston, rod, crank, bearings, and very importantly, the cam you plan to use. Put a piece of modeling clay in the valve relief area of the piston and rotate the motor BY HAND over for at least two revolutions. Then take the head off and by measuring the thickness of impression left in the clay you can determine the piston to valve clearance. You should have at least 0.1" of clearance otherwise you should have the pistons flycut. In general, long duration cams or high lift cams will have closer piston to valve clearance. With a combination of the two you should be aware that it is very possible for the valves to come close to the piston.

Given the choice between a longer conn-rod or longer stroke, I would go for the longer stroke. I think the benefits of l/r ratio are way over hyped. I would rather have the bigger displacement from the long stroke. Your tall deck block has plenty of room for more stroke.

Hope this helps.

 

Given the choice between a longer conn-rod or longer stroke, I would go for the longer stroke. I think the benefits of l/r ratio are way over hyped. I would rather have the bigger displacement from the long stroke. Your tall deck block has plenty of room for more stroke.



There are lots of article that say long rods don't make any more power. I would rather increase displacement through a longer stroke then worry about using the longest rods posible.

 

Well the motor is already a 540 so i think that the stroke is long enough so by george i should go with the 6.385. I'm gonna see what pistons i'm getting so i can get their measurment. And why doe the rod length get longer with longer stroke i would have thout it would have been just the opposite.

 

Anything?

 

When you start getting into motors this big, you are talking about hi performance stuff. So you try to get the lightest piston possible so the big sucker can rev. The piston compression height is reduced as much as possible to get it lighter. This also makes it a shorter skirt piston usually so you also don't always end up with OEM type noise/oil control. So with the lighter/shorter piston you can use a longer rod.

Why...because with the increased stroke the rod ratio, even though some don't consider it a big deal.....there is a general range you need to stay in to get reliabilty, decent noise and oil control. So with increased stoke it's nice to get longer rods if possible to keep rod ratio in the ballpark.

A short deck 540 with a 6.385 rod ends up with a 1.50 ratio. A stock 454 is a 1.53. A 427 is 1.63 as is a 350. Some mega cube deals in IHRA are down to 1.2 range....but that's an extreme. If you used stock rods in a 540, you get 1.44.

So the general plan is to use the stroke you want, the rods you have to to keep a decent R/S ratio and then whatever piston you have to to get it all in there. Even with a 6.385 rod, the piston pin is up into the oil ring groove and requires spacers. There is just not room to move out to a 6.535 long rod unless you move to the tall deck block.

With the tall deck block, you could use 6.535 rods in a 540 and come up with a 1.53 ratio just like a stock 454. A 632 is down at 1.37.

All that said, the 1.50 ratio 540 works fine. Oil control is OK and noise is no problem. Mine never even makes piston rattle when cold even with .0045 clearance and oil usage is OK. I know of another 540 in a Vette that has tighter wall clearance that is literally driven daily (and cross country to Forum Cruise) and never burns a drop.

So if done right, you can make one of these things anything you want.


JIM

 

With the 4.25" stroke for the 540 and the 6.385" long rods, you'll be able to use pistons that don't require the pin bore to be up in the oil ring groove. I think for your application the 6.385" rod will work well.