[06meanstreak]

I'm trying to decide what cubic inches to run. I've got a sbc dart aluminum block that is 4.155 bore, but it is standard deck height. I plan on running 2 stages, 1 plate and 1 fogger. I'd like to run a 421 or 434, but am worried about pistons with less than 1.100" of compression height holding up under 2 good size stages. Any input on how reliable these combo's would or would not be?

 

[Power Mad Racing]

421 JMO

 

[bykr]

Depends on the ring package.You can get a .043/.043 3mm in there.I've got 1.075ch and the top ring is still .275 down.

 

[06meanstreak]

Is that with a 3.875 or 4.0 stroke? Any downsides to using that ring package? And, if you don't mind, how much nitrous do you run?

 

[TubbedTruck67]

Id stick with the 3.875 stroke in a stock deck hieght block. Im curious what the min safe compresion hieght is for a 3.875 stroke stock deck hieght motor though. If you use a 6" rod with a 3.875 stroke I believe the CH is 1.063 is that not enough? I have plans on a 5.850 rod to get a better ring package am I thinking right?

 

[bykr]

Is that with a 3.875 or 4.0 stroke? Any downsides to using that ring package? And, if you don't mind, how much nitrous do you run?

4.25 stroke.Plenty of people have used the same package,but some prefer the 3/16 oil ring.The piston was made for a 350 shot-it's a new build I'm working on.

 

[06meanstreak]

Id stick with the 3.875 stroke in a stock deck hieght block. Im curious what the min safe compresion hieght is for a 3.875 stroke stock deck hieght motor though. If you use a 6" rod with a 3.875 stroke I believe the CH is 1.063 is that not enough? I have plans on a 5.850 rod to get a better ring package am I thinking right?

That's my problem or question really, is what would be a safe compression height to run when running 500+hp worth of nitrous. Shafiroff says that without going to a tall deck block, a 3.75 stroke is the safe max giving you a 1.125 compression height, but i'm wanting some real world experience from those running 421 and 434 cubic inches. I would have no problem running a 5.850 rod, but all crankshafts over 3.75 are made for 6" rods and if you can cut the counterweight to run the 5.850 rod then you'd pay a helluva price in adding heavy metal to balance it.

 

[06meanstreak]

4.25 stroke.Plenty of people have used the same package,but some prefer the 3/16 oil ring.The piston was made for a 350 shot-it's a new build I'm working on.

You must be running a taller deck block. I can't run a 4.25 stroke with standard deck height and end up anywhere near a 1.075" CH.

 

[Monte Smith]

There is no law, that says you must run a 6.00 rod. Put a 5.850 rod in it and build a 434, works fine. You may have to cut the counterweights a little, but that is no big deal. Just may require some mallory on the balance.

Personally, I would not run a 3mm oil ring. I like a 20lb oil ring and you can't get one that heavy in a 3mm config.

Monte

 

[06meanstreak]

There is no law, that says you must run a 6.00 rod. Put a 5.850 rod in it and build a 434, works fine. You may have to cut the counterweights a little, but that is no big deal. Just may require some mallory on the balance.

Personally, I would not run a 3mm oil ring. I like a 20lb oil ring and you can't get one that heavy in a 3mm config.

Monte

I might not have done all my homework on this, but from what i've found so far, to run a 5.85 rod i'd not only have the cost of machining counterweights and adding mallory, but would also have to order custom pistons. Not a huge deal, but definitely a factor.

With your experience Monte, what would you advise for my predicament knowing that I am planning on putting about 500hp of nitrous to it with a 9.025 deck height? I would like to run the most cubic inches I can, but I don't want to sacrifice reliability on the piston side of things.

 

[R Hollingsworth]

My motor is 436ci with a 4" stroke and 5.850 rod, it has 1.150ch and did very well. Had the throws on the crank turned down and I think it was about 500 to balance and turn it down, with the cost of the heavy slugs. We saved a little on that with a balance plate that bolts on with the flywheel.

 

[06meanstreak]

Who did you get your pistons from?

 

[Monte Smith]

I might not have done all my homework on this, but from what i've found so far, to run a 5.85 rod i'd not only have the cost of machining counterweights and adding mallory, but would also have to order custom pistons. Not a huge deal, but definitely a factor.

With your experience Monte, what would you advise for my predicament knowing that I am planning on putting about 500hp of nitrous to it with a 9.025 deck height? I would like to run the most cubic inches I can, but I don't want to sacrifice reliability on the piston side of things.

If you plan to spray 500, a "custom" piston is a given anyway, so that is a moot point. No "shelf" piston, regardless of crank and rod combo, is going to be what you need.

Robert ^^^^^^^gave his 434 combo and I would say it runs pretty good. He has Diamond pistons.

Monte

 

[06meanstreak]

I guess what i'm really asking is if I plan on spraying 500+hp worth of nitrous, what target of compression height should I shoot for in any combination of stroke/rod to have a strong enough piston with a 1/16" 1/16" 3/16" ring package to handle the nitrous? Most say 1" is too low, so what is the minimum you want?

 

[bykr]

I think it more about how far down the top ring is than any given number on comp. height.Talk to whoever is building your pistons,they can work out the ring package for what you need.

 

[Monte Smith]

How far the top ring is down, really does not matter. It is the thickness between the pocket and the top ring that counts. On a heavy nitrous motor, I like to see .100 as that number. Shallow valve angle head, less pocket depth, the easier it is to get some thickness there. With the combination of parts you are going to have, I suggest a .043, .043, 3/16 ring package. If you go 5.850 rod, 4.00 crank, put the piston .040 in the hole and truly have a 9.025 deck, I get 1.135 for your piston CD. That will work, but it will take a custom piston, with somebody making it, who knows whats up. I suggest Mike Pannetta at Diamond. You could also go closer to zero deck, add some meat on the piston and run a thicker gasket. Several things to look at. You did not mention heads. So the valve angle heads, cam size and how much pocket you need, also come into play. Either way, short CD numbers and a lot of nitrous, means the tune needs to be right.

Monte

 

[06meanstreak]

The heads are older Dart raised runner castings based on the 220 port head, but has a 17 degree valve angle. I don't know much more about them right now, but i'm getting them checked right now and flowed. I'm also trying to identify the cam that came out of the engine, but I will probably sell it and get a custom grind once I know for sure what cubes i'm running and how the heads turn out. I'm not too far away here in Georgia, so when I get to the point where i'm ready to start building the nitrous system, i'll come see you.

 

[Foggerman]

How far the top ring is down, really does not matter. It is the thickness between the pocket and the top ring that counts. On a heavy nitrous motor, I like to see .100 as that number. Shallow valve angle head, less pocket depth, the easier it is to get some thickness there. With the combination of parts you are going to have, I suggest a .043, .043, 3/16 ring package. If you go 5.850 rod, 4.00 crank, put the piston .040 in the hole and truly have a 9.025 deck, I get 1.135 for your piston CD. That will work, but it will take a custom piston, with somebody making it, who knows whats up. I suggest Mike Pannetta at Diamond. You could also go closer to zero deck, add some meat on the piston and run a thicker gasket. Several things to look at. You did not mention heads. So the valve angle heads, cam size and how much pocket you need, also come into play. Either way, short CD numbers and a lot of nitrous, means the tune needs to be right.

Monte

Hey Monte, do you like using gas ported pistons in a nitrous combination? Mine just came in today from diamond and are gas ported is why i'm asking, my will see 400+, with a fogger and a plate, 23* headed motor, 3.875 stroke 5.850 grp rod, and custom diamond pistons with a ch of 2.212. I'm just a litte concerned about the gas porting, i wasn't aware they would be, the man at diamond knew i was planning to spray this motor pretty hard. Just wondering what you think. Thanks.

 

[Monte Smith]

A piston in a nitrous motor should definitely be gas ported. I have tried lateral and vertical, no preference there.

Monte

 

[Foggerman]

Thanks.