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2276 or larger - Arrghhh!

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n3480h1

Well-Known Member
Joined
Apr 24, 2009
Messages
1,522
Location
Iowa
Just when I think I've resolved to build it to 2276, I notice there are 84 and 86mm cranks out there. Common sense and some experience tells me that there is a line which should not be crossed, at least not in an aircraft. I wonder about increased cooling needs, and increased case and crank stress. More power is attractive, but not at the cost of reliability. So, can any of you offer the pros and cons of going with the longer stroke over 82mm?

Thanks.

Tom
 
One really important reason not to go longer than 82mm, is the crankpin to main journal overlap starts diminishing to a dangerous point causing a weak point no matter how quality the materials and forging is. Bill
 
I believe the 911 aircooled motors also hit a wall at 2.7 liters on the mag case, the first generation engine, due to heat problems, stress on the case and the expansion of dissimilar metals. Porsche switched to aluminum for the second generation 911 engine in order to accommodate larger displacement motors.
 
the porsche 2,7 case had the same problem the vw case had, the head studs pulled the threads out. the same case saver we use for vw works best for them also. porsches still have stud problems when there is any corrosion on them.
 
Hi Tom,
I will say: 2180cc is proven!
Take the GPAS2180 and build it.
Take the safe way, there is a big lot of work to take the plane flyable and if the plane is flying and you have the wings ;D , you can optimize the plane/engine/pilot and you have the next 5-10years enough time to work...
with best regards
Juergen
 
Von Richter said:
One really important reason not to go longer than 82mm, is the crankpin to main journal overlap starts diminishing to a dangerous point causing a weak point no matter how quality the materials and forging is. Bill

based on what data? What dimension is accepted as the dangerous point?

I built a 2332 that has close to 600 hours on it now last I heard that has not had any issues outside routine maintenance. Would build a 2387 if I was going to do it all over again.
 
It's simple engineering fact, the shorter the stroke, the greater the overlap, the stiffer and stronger the crank will be. All you're doing is compromising greater horsepower against the value of your %^% (Profanity removed by moderator) at 5,000 ft.
 
It's simple engineering fact, the shorter the stroke, the greater the overlap, the stiffer and stronger the crank will be. All you're doing is compromising greater horsepower against the value of your %^% (Profanity removed by moderator) at 5,000 ft.

Thats kinda off a false statement, since stiffness matrices or compliances matrices also depended on the modoulus of elasticity of the material, case hardening, strain hardening, surface imperfections, and stress risers in the geometry.

Also a short stroke crank would require the motor to run at a higher rpm to produce a similar torque for a short stroke crank. In airplane applications out put rpm is limited by properller tip speed, so a short storke high rpm motor would require a reduction unit for the rpm, this is just another design consideration.

A Crank shafts primary means of failure is cyclical fatigue, actually it is a combination rotating beam loading and alterneating torsional loading, so running a shorter stroke higher rpm crank actually reduces the ultimate usable strength of the crank, because it will go through more load cycles faster then a short stroke crank.


So what I am trying to get at is thats is not a simple engineering fact.

A long stroke crank can be designed to be as reliable as a short stroke crank.

I think for the VW engine the main issues will be heat, case studs, and bearing webs.
 
Von Richter said:
It's simple engineering fact, the shorter the stroke, the greater the overlap, the stiffer and stronger the crank will be. All you're doing is compromising greater horsepower against the value of your %^% (Profanity removed by moderator) at 5,000 ft.

I guess a crank with no stroke at all would be safer (turbine?) and less likely to break than a crank with 5" of stroke, but you wouldn't get very far in a reciprocating engine. I find it really hard to believe that increasing the center distance from the main journal to the rod journal by 1mm (.039") is going to make any measurable difference in the longevity of the crank or it's likelyhood to fail.

To each his own I guess, but unless you've got some kind of actual reference to an 82mm crank being the largest "safe" crank in a VW engine, I'm going to discount this as an opinion.

Bear in mind the biggest crank VW put in a 1600 was 69mm. It was not counterweighted, it was not cross drilled, it was not nitrided as the aftermarket cranks are. So with that said, why not just say the 69mm crank is the largest crank you should put in a VW engine? Any increase in displacement is going to increase power output at the sacrifice of durability. If you want to ensure you don't get hurt while flying an experimental airplane my advice is to not leave the ground in one.
 
When the stroke inceases, so does the torque, so as you are increasing power, the ability to sustain that power safely decreases, that's all. You must strike a happy medium. In a car the problem isn't as severe, but with the torsional resonance of a prop, the problem increases. Why does G.P. pretty much limit the stroke on their preffered max engines to 82MM is because they feel liability claims would increase significantly. My engines I limit to 92MMx78MM. I work in other areas to increase my power. Lighter reciprocating assy. better porting, and such.
 
In my mind, the biggest cons are the amount of clearancing of the case the larger cranks require, and the inability to USE the extra power for very long due to CHT issues.

My engine (which hasn't run yet...) is a 2180, 92mm X 82mm. I might be tempted next time to use a smaller stroke (less clearancing, say, 78mm) and BIGGER (94mm) barrels turned down at the base to avoid having to gouge out the case all the way to the case savers - which is another limit on how big you can go with a Type 1/2/3 engine.

Chad has built and run them bigger though, and I have a lot of respect for his work, considering I've never laid eyes on it personally ;)
 
Karmarepair said:
In my mind, the biggest cons are the amount of clearancing of the case the larger cranks require, and the inability to USE the extra power for very long due to CHT issues.

My engine (which hasn't run yet...) is a 2180, 92mm X 82mm. I might be tempted next time to use a smaller stroke (less clearancing, say, 78mm) and BIGGER (94mm) barrels turned down at the base to avoid having to gouge out the case all the way to the case savers - which is another limit on how big you can go with a Type 1/2/3 engine.

Chad has built and run them bigger though, and I have a lot of respect for his work, considering I've never laid eyes on it personally ;)

My 2332 required LESS clearancing than a conventional 2180. Connecting rod selection is key. My cam required zero clearancing at all and the case had more than enough clearance for the crank with standard 2180 work done. I bought a CB supercase clearanced for stroke and machined for 94mm pistions and didn't touch a thing other than drilling and tapping for full flow oil.

Cooling in a sonerai was a non issue. I ran a ton more timing and more compression than "conventional wisdom" experts and my hottest CHT on an 85 degree day in climb was 305 (thermocouple under head nut). Cruise temps were usually in the 270 range.

Like I said earlier, to each his own I guess. Unfortunately the internet is full of experts and people that have never actually done anything that offer advice freely based on what they've heard or read elsewhere. Facts get quickly diluted into a false truth.

To the original poster, I'd recommend building the biggest displacement engine you are comfortable with and focus on airframe mods for increased performance.
 
I appreciate all the input here. It has provided many views from different perspectives, and that's what I was after.

Since I already have an engine with 94 mm pistons, I will follow Chad's advice and build the largest engine I am comfortable building, a 2276. I have a great deal of respect for Chad's experience, knowledge, and his demonstration of what really works. I would need to understand much more (and have more experience) to build any larger, so it will be a 2276. I'm not building a Formula racer, I just want to have a little better performance on high density altitude days, and an engine I can count on. I will say if I could afford to have Chad build it for me, I'd be very tempted to take it to the max.

Thanks for the spirited thread guys.

Tom
 
thanks Juergen and Chad. i used porsche rods in my last 2180, but they are getting pricey now. the chevy size journals will get the longer stroke i want past the cam.
 
Doing a little research, came upon this in one of TheSamba forums:

"With the H beam rod, the rods from the same manufacturer are the same size outside. The chevy rod just has a smaller hole on the big end than the VW journal. The clearencing would be the same. The difference in the 5/16 and 3/8 bolt in clearencing in the spigot hole is negligible. For the street with a 78 I would stick with the VW journal. Just my 2 cents."

So, just because a rod has a Chevy Journal, does not necessarily mean it will be any easier on the clearance...

Chad, where did you buy the rods you used?
 
Karmarepair said:
Doing a little research, came upon this in one of TheSamba forums:

"With the H beam rod, the rods from the same manufacturer are the same size outside. The chevy rod just has a smaller hole on the big end than the VW journal. The clearencing would be the same. The difference in the 5/16 and 3/8 bolt in clearencing in the spigot hole is negligible. For the street with a 78 I would stick with the VW journal. Just my 2 cents."

So, just because a rod has a Chevy Journal, does not necessarily mean it will be any easier on the clearance...

Chad, where did you buy the rods you used?

I used Eagle rods. Don't remember, but I think I sourced them from a seller on the Samba. I also have a set of Scat rods that required no additional case or cam clearancing so I tend to believe that the above statement is too broad to consider to be a completely true statement.

Chad
 
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