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Old 02-11-10, 01:27PM   #1
marks73ta
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Default Compression height vs piston stability

I'm building a 370 and the piston compression height is pretty tall. I have a great set of Ross pistons meant for a 400 build that I may use on the 370 instead. Using a 6.7 rod and stock stroke it puts me about .019 in the hole on stock block specs of 10.2 deck height. Which is fine for a boosted application. My question is. If I am doing custom mix and matching of parts what would be the optimum compression height to address any piston stability? The 400 CH is 1.7 but is that too tall? If I get custom pistons instead of using the 400 set I can mix and match to the closest sizes. This 370 will likely rpm higher than usual but I don't know how high it will need. Especially with boost, it may not need to rpm too high. Anywhy. What is a good CH height? If the 400 CH is good then I'm set. Mark L
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Old 02-11-10, 10:57PM   #2
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You would be in shock if you ever seen pistons from a winston cup engine, we are talking a piston with a ring pack and skirt thats like a total of an inch maybe a lille more. If they can be stable then i dont see how any claims that other pistons cant be stable. just B.S i think
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Old 02-12-10, 02:11AM   #3
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I've always felt too short of a compression height and too tall of a compression height would make the a piston unstable. That's why I'm wondering if anyone, with all the combinations that are being built now in the Pontiac world, has opinins on th ebest comp height vs piston stability. Just asking in case I have to do custom parts I cna go for the best dimensions right off the bat. But like your example about he nascar piston skirt length. Everything would depend on the next thing just like any combinaion. I guess I'll ask then---is the 400 pin height with a 6.7 rod length and a 3.56 stroke going to make for a stable piston? That's an easier question to ask. At least the 6.7 rod gives that stroke a nice 1.88 r/s ratio. MArkl
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Old 02-12-10, 03:01AM   #4
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Quote:
Originally Posted by marks73ta View Post
... Using a 6.7 rod and stock stroke it puts me about .019 in the hole on stock block specs of 10.2 deck height...
"Stock" deck height ranges between 10.230" and 10.240", which would make your .019" figure be .049" - .059". Unless you know that your '58 block has had the decks cut down significantly?

As for your "piston instability" concerns- I don't personally think there's reason for any worries. I'm not aware of any potential for problems with "large" compression heights on pistons, other than the obvious mass (weight) of pistons situation. On the contrary, I believe that such pistons benefit stability and longevity of parts.

As for real-world examples- I ran stock-dimensioned '58 shortblocks for close to three full seasons in mini-rod pulling. In its final form (before I went to the blown class), it ran alky through a Hilborn 4-port on top of a Doug Nash tunnel-ram, with bowl-ported -16 heads (and very heavy domed pistons). It turned 7,200 throughout each pull (up to twenty seconds) and held up very well. This was with no harmonic balancer, just a crank hub- gotta' love those '58 forged cranks!
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Old 02-12-10, 01:29PM   #5
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Thank you Jack, that's great information. Yes, after I wrote that I was thinking about the actual deck height being alittle taller. If I use 6.7 rods they are usually chevy rods with the BBC .990 pin so I would have to bush them down to .980. I can do 2 things. Offset the rod journals which also need cutting to BBC size or ofset bush the rod small end. If I have to offset grind the the crank anyway, I might as well offset/destroke enough to use a 6.8 rod and bush the small end centered. I would rather destroke than longer. Just me. I want a shorter stroke. Using 10.23, a 400 CH of 1.73 (not sure if that is correct) a 6.8 rod leaves me using a 3.4 stroke. If the crankpins can be cut down to 2.200 and reach the 3.4 stroke I might do that. That makes a higher 2.0 r/s ratio but I can deal with that. Gotta start finding a machinest. Mark L
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Old 02-12-10, 01:52PM   #6
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Jack, did you ever have problems with the Pontiac rod bearing size at 2.250? I may just build it to Pontiac specs and save the money. Stock stroke, stock journal sizes, stock 6.625 rod etc and spend it on better heads etc. I'll just need pistons that way. Thanks Jack. Mark L
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Old 02-12-10, 04:19PM   #7
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Rod to stroke ratio when looking at power output, piston stability, etc. is WAY overrated at the rpm levels we run in. You are wasting your money and efforts worrying about it and offset grinding, destroking. Use off the shelf parts and Pontiac bearing journals and spend the money on better boost components, heads, fuel system, oiling system or something that really matters to power or longevity.
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Old 02-12-10, 08:16PM   #8
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Hey Robin, I'm coming to that conclusion. I lke to get crazy with unusual or the last minute lates trend, it looks great on paper but once I get my head out of my arse I'll probably go factory specs.The 370 will be reved higher than the 400 or 455 but the stock dimensions will do just fine. As soon as my TA is gone and I have some money I'll start the building process. Robin, where do you go for your machining work? I just drove down to see where Best Machining is located in Glendale as many have pointed me in their direction. Mark L
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Old 02-12-10, 08:19PM   #9
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Jack, is the front snout on the 370 crank the shorter version compared to the later 400/455s? If so do you do anything specal for a dampner/balancer? Mark L
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Old 02-12-10, 08:23PM   #10
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Mark,forget about the short snout!Just bolt the balancer on.Tom
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Old 02-13-10, 03:29AM   #11
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Ditto to Robin and Tom. And no, stock 2.250" rod journal diameter is not a problem (even in my hemi @ 9,000 RPM), nor is the crank snout length.
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Old 02-13-10, 01:58PM   #12
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This is sounding better all the time. By the way i just scored this '63 Catalina. Will be bringing it home next week.
http://phoenix.craigslist.org/evl/cto/1583452756.html

The sheet metal is almost factory fresh. No rust thrus anywhere. Hardly any surface rust. Interior trashed however.
Gotta get it home and look it over underneath then decide on a plan. I want to build the 370 for this, probably twin turbo/intercooled and some kind of strong 5 or 6 speed. Docile off boost for tractability and gas mileage but boost it up for power. I want to drive the wheels off of this thing. Anywho, I'm going to make a post asking for pictures and thoughts/ideas for the final configuration. Still can't get that Gasser thought out of my head. MArk L
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Old 02-13-10, 02:03PM   #13
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That's pretty cool. I wish you could find stuff like that around here, in that good odf shape. That will be a fun project.
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Old 02-19-10, 02:33AM   #14
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Mark, As far as piston stability is concerned, the higher you can put the pin[within reason] the better. In our Pontiac's, you can't really go too high with that short of a stroke. If you can get the pin height near the center of the piston, as opposed to more piston above the pin than below it, it will be more stable. It's the tall compression heights that make a piston unstable. Look at the 440 Chrysler piston, the compression height is well over 2". They are stupid tall. More piston above the pin = more piston rock when piston changes direction. I like moving the pin up into the piston for two reasons: 1. It makes the piston more stable, and 2. It makes the piston lighter, which is always a good thing.
In your case, what you want to do is fine. With boosted apps, you don't want too short of a piston because you want the ring grooves as low as you can get them.
Good luck on your project!
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Old 02-19-10, 04:15PM   #15
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Thanks Paul. I'm probably going stock specs, 3.56 stroke, stock journals, lt wt ACE PPR 6.625 rods and light pistons. Make it light as reasonable and enjoy the rpms. MArk L.
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