Engines like this rev low, and maybe rod ratio isn't that critical, because the piston speed stays low.

I think this relates to the old sbc 400. They had bad rod ratio i believe and suffered from piston skirt wear and ovaling out of cylinders near bdc

 

If I remember right Jon Kasse build a ford 400mod. type engine for EMC one year that used pistons that had long skirts that had a "tall" compression height. But I'm sure that he tailored the camshaft for whatever rod ratio he had.

 

If you are building an engine why don't you tune everyone in on what you want to do as each application might require a different answer.

 

Sure

Low revs, high annual usage, towing 26,000 lbs everyday

 

Here is my take on Compression Heights aka CH. I've never designed an engine yet where I started with CH but in saying that I do take CH into consideration. If I'm building an engine with no any real modifications, I'd use a factory CH where the piston will be more available and much cheaper than something custom. I'd say most engines are built on what has been established as "common" or most used for the application.

Now, turning the page and talking about a specific built race engine. The CH is initially determined by establishing a "parts stack". You first determine how far down the top ring must be due to valve pocket depth or application like using NOS. Once you determine how far down the top ring must be, you add the thickness of the ring land that is desired. You then need to determine how much accumulator area you need or also referred to as the 2nd belt of the piston. The 1st belt is the part of the piston above the 1st ring. After determining that area, you then add the 2nd land. You then determine your 3rd belt which is pretty much always right around .075". You then add the the oil land thickness, the amount of material in between the oil land and wrist pin which at minimum is right at .025" and then half the diameter of the wrist pin. Now you have determined your optimal/minimal CH!

Now, here's where the CH will need to be altered and or adjusted. If you are needing to put a longer rod in and you don't have enough deck height on the block. You will then look at using a support rail under the oil ring and decrease your CH. You will still be limited to how much cause you can only go so high. The other scenario is that you already have a set and or ideal rod/stroke combination but you can't get the deck height of the block short enough. You then are left with no choice but to increase the CH of the piston to connect the dots.

Back in late 90's when I worked at Patterson Racing in Augusta, KS. Our signature engine back then in Comp was the 304". We didn't have the luxury of short deck block like they have available today. We already had a determined bore and stroke combination. Our rod ratio was already 2:1 and we didn't want to go any longer. Our only choice was to add CH to the engine and back then, the CH was 1.800", which is a very tall CH. Was it optimal, no but we weren't left with any choice.

Overall, I have not found there to be any torque and or power advantage to running a certain CH. Personally, I feel the shorter the better but without having to run a support ring. Nothing at all wrong with having to run one and there are many applications that are left with no choice but to run one. So, when I design an engine, I first determine what bore I want based off the cylinder head. I add stroke for CI and then determine a rod length based off what ratio that I'm looking for based off the cylinder heads potential. I'll add the shortest CH I can for the application and then determine my ideal block deck height. Now, if I can't get the block to my ideal height, I will first make adjustments to the CH to try and compensate...

Hopefully, that gives you a better idea about and around what CH is about... But to answer the question, can you have to much CH? I don't really know if you can have to much CH but if you look at the math and physics of it all. You are definitely losing mechanical efficiency when the CH increases.

Nick

 

Move up to this.

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The 7.3 is a solid engine, I will ammend my previous post and say, I would like to push the limits of weight capacity, perhaps as high as 50 or 60 k pounds. I realize this will need a ultra tall deck and a 4.75-5.25 stroker crank.

 

Great insight Nick.

Is there a general rule one would use regarding compression height and wrist pin/oil ring intrusion? Assuming a naturally aspirated bbc build. At what compression height would you typically see the need for a oil support rail?

I realize not all the pictures on jegs/summit are of the actual product but it looks like the GMPP 572 avoids the support rail just barely and that has a compression height of 1.465, would this be a general ballpark of where you would want to be to avoid a support rail??

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To first answer you initial question. You would probably need a CH of right around 1.300"-ish to avoid having to use a support rail. So a CH of 1.465" shouldn't be even close to needing one... But like I said in my original post, it's all application based. I'm not going to use a support rail in my PS and or Comp engines but using one is better then being stuck with a rod that I might feel is way to short. IMO, there are more BBC engine out there in drag racing that have support rails than there are the don't have them. I feel as well as most others, getting as much rod length as you can in a BBC out weighs the use of a support rail...

It depends on the pin size. But you can go down a lot farther than 1.465 , I just did a sbc with 1.425 and the pin wasn't in the oil ring. Probably go down to 1.2 on 927 pin

Keep in mind when you go down on pin size. You will only gain half of what the difference is in pin diameter for the parts stack to be apples to apples. So going from a .990" pin down to a .927" pin will gain you a net of .0315" in CH with everything else being the same...

Nick

 

It depends on the pin size. But you can go down a lot farther than 1.465 , I just did a sbc with 1.425 and the pin wasn't in the oil ring. Probably go down to 1.2 on 927 pin

 

On a .990 pin size.

 

I found and read your post about finding an engine builder and found the information I was trying to ferret out, seems you are wanting to build a long stroke big cuin BBC for truck pulling (?DK), I would talk to a couple of piston makers and see what they suggest as far as piston/pin design for what you want to do. Blower/NOS/NA?

 

Not truck pulling, Hot Shot style, think long haul trucking but with a slightly smaller rig 😉

Like a kodiak/F650/M2-106

 

 

Any chance u have the top part, I can barely read it, I see it says something about inboard pin boss

 

BBC 9.800" deck, 4.250" stroke, 6.385" rod, 1.5/1.5/3 MM ring pack, Wiseco Quick 16 piston,1.120" CH, no rail support. But Wiseco sent them anyway. When i asked them why, said the earlier version needed them and they had not updated the the specs, so they got included.

 

 

Thanks BBF

 

Mark, so your saying your wiseco pistons are 1.120 CH and don't have a support rail?

 

NGN.

I missed your earlier post, so with the 540/6.385 rod I'm sitting at 1.29.

I saw your message about drag racing using the longer rods but would that really be applicable for me? Cruising down the road at 65 with a 15 ton load.

What would you do if it was your rig and your livelihood on the line? Shorter rod (6.135rod- 1.54 c/h) or longer rod (6.385 rod- 1.29 c/h).

 

That's a bit of a tough question... Is this a diesel engine? If so, most diesel engines I've seen have a very big pin and a tall CH to get the proper ring stack in the piston. I'm not saying it can't be done but I don't know of anyone who is looking to do what you application requires that's using a support ring. A support ring will work fine in a racing application but I don't know about your application. "IF" your piston provider can make a piston with a short CH so you can use the longer rod, that's what I'd do...

Nick

 

^^^^^^ DIESEL

Jim

 

The piston in the picture has about a 1.375"CH.

 

These race type of pistons are not for running 150K miles pulling 15 tons, you need to talk to some piston makers.

Having said that, a longer rod is not as hard on the piston pin when the piston stops and starts.

 

Here's a 705 from 2000, 800ftlbs at idle.

 

I've spoken with Bill about this build as well as Sonny Leonard before he passed, both gentlemen steered me away from aluminum blocks and towards cast iron blocks but the engineering in them is certainly impressive.

 

Good luck.

 

there is certainly a minimum CH that you want to be able to package everything in a manner that is durable for the application so correct ring and land thicknesses, suitable pin diameter and and not unnecessarily complicated like oil ring rails. You also don't want anything too tall with C.of.G too far above the pin to minimise the tendency to rock and generate additional friction.

take the deck height, the stroke which are often written in stone and then choose the rod and CH to suit. Sometimes the CH ends up where it ends up on long stroke engines and you do what it takes to make it work even if less than ideal.

Don't' compromise the piston for the rod because as far as swings and roundabouts go the rod is almost all roundabouts.....