Posted: 8/24/2010 23:02:18
Modified: 8/25/2010 09:44:36
For years the only way to be able to swing a nice size prop on a VW ( 72" or so) was to use a redrive. The first one I liked was on the PL-4 and now Valley has a nice one. There is also the Maximizer redrive that Stewart designed years ago and is a very nice one too. With a redrive you can go with a stock stroke--69mm and build a nice dependable 1775 or 1835 and still swing a 72"
For years I have thought about--What if you could build a Big stroke crank VW for lots of torque to turn a big prop. The late Gene Berg use to build street engines with 86-88 and even 90mm stokes and short (5.4") rods that would last and be dependable.The expert told me it couldn't work becouse of all the mods to the case. Well now the VW aftermarket has the new Bubble Top case that can take 86-to-90mm stroke cranks. So now I can build a 88mm Stroke crank with short stroker(5.4" with chev.2" jernals) rods and 90.5Barrels and have a TORQUE ENGINE! Will it be able with a 88mm arm to swing a 72" prop? A 84mm crank can swing a 64"x20 is there enough extra torque with a 88 stroke to swing the 72"? If not would it handel a 68" prop? With this combo I can keep the rpm down to 2800rpm tops and cruise at around 2500rpm.
I ask becouse I like building engines and feal that no one has built this size engine before--but it should work great with the right cam. I'm looking for feedback pro and con. But tell me why it will or will not work! I had an expert (still selling VW engines and the same one that told me I could do the stroker engine) tell me 16 years ago that I couldn't put a VW engine with the PL-4 redrive on a Fly Baby as VW's were not ment to have a redrive and the PL-4 redrive was not a good one. He also went on to state he would never build a VW for use with a redrive so he wouldn't sell me any parts or help me with my project! He now sells(has for a numer of years) the Valley redrive--an pushes it as the way to go --for planes like the Fly baby!!!
So I know what the so called expert says. Now I want to hear from real EAAers who have built VW engines with and without redrives. How big did you stroke it? How big did you punch it out 90.5-92-94?? What kind of prop hub setup did you run. How big a prop did you run direct drive. If you did a redrive which one? What ratio did you use and what size prop are you swinging.
Thanks for any and all feedback
Posted: 8/25/2010 09:30:05
The issue is not the torque or horsepower, it's the tip speed of the propeller. VW engines generally run in the 3200 to 3500 rpm range in cruise. That would put the tips of a 72 inch prop at around 750 mph! That speed is too close to mach one and will severely degrade propeller performance. (It also might set up undesirable vibration characteristics.) You want the tip speed to be around .9 mach max for maximum propeller efficiency.
Propeller tip speed is the main limiting factor for propeller length on a direct-drive VW engine.
Lasciate ogne speranza, voi ch'intrate
Posted: 8/25/2010 21:49:39
Why use a 72" prop if the engine swings it too fast?
I would guess a 58" or 60" prop turning at 3500 RPM could stay within the recommended tip speed. That would save the weight of the redrive on the front of the engine.
Posted: 8/26/2010 00:38:47
I am a big guy (6'2") and while on a diet I still weigh more than my goal weight of 200-200lbs. I feal that a large prop for the Replica I may build is what is needed. I can swing a 72"--78" prop with a redrive. I feal the 88 or even 90mm stroke with a low RPM torque cam should be able to swing a 68"-72" prop at 2500rpm at take off and cruise of 2100 to 2300rpm. Looking for VW engine builders who have built Redrive engines or Big stroke engines to get there feed back.
Posted: 8/26/2010 19:28:33
Hi John, I hope you are talking prop rpm above. To get decent hp out of a vw you need to spin the engine 4000rpm or so. When trying to get "big" hp from a vw cooling becomes critical. You did not state how much hp you need. What type of replica or how fast you want to go. You can swing a big prop with a low hp engine with a low pitch but it will not fly an airplane.
Posted: 8/26/2010 19:40:14
Hey John, You are definatley on the right page. I build Harley stroker motors on a regular basis. It's going to be the cam that determines the RPM range. It's gonna have to have enough duration to fill those bigger cylinders with a pretty late exhaust valve opening to run at those lower revs. The critical area in those engines are the shims used to set up end play on the crank. I haven't done it yet but I feel that you could machine the flywheel and use a ball type thrust bearing(I wouldn't feel safe with roller bearings) that could handle the thrust loads imposed in a direct configuration. I guess bottom line is build it, hang it on a test stand and flog the living (well,you know what) out of it. Isn't that what experimental means?
Wish you luck, J.
Posted: 8/26/2010 22:31:46
I have been flying a VW powered Kolb MKIII for 12 years. The first engine was a direct drive 2180cc. It had just enough thrust to get me off the ground solo. I switched to a Valley redrive VW I fly faster, climb faster, burn less fuel, run lower engine temps, and can carry a passenger. The engine is very inexpensive, reliable and very smooth. Bushmaster is making a long stroke VW for Just Aircraft with the Valley redrive that they are rating it at 114 HP turning 4400 RPM and cruising at 3800 RPM. I think they may be asking too much of the VW engine but It does show what the VW is capable of. My VW performs right along side a 80 Rotax so I figure it has 80 HP and I cruise at 3200 engine RPM turning my 72 inch prop 2000 RPM.
Posted: 8/26/2010 23:18:54
thanks Rick and Jay,
That's the kind of info I'm looking for. Rick you didn't say what size engine you have the redrive on. Can you give me ther CC's 1775 or 1830? 2180???
Posted: 8/26/2010 23:22:11
I am also using a VW with a Valley redrive on my Kolb Mark 111 Xtra. After talking with Rick Neilsen I decided it was the way to go and I am very satisfied.
My engine is 2276 cc and I swing a 70 inch 3 blade Warp Drive prop with a 1.6 to 1 ratio. The engine turns 4250 on take off and about 3400 cruise. I am still fine tuning the prop so this will probably change. Great Plains rates this motor at 105 HP. I don’t know if this is accurate or not but it sure feels like it has enough power.
Posted: 8/27/2010 10:38:26
My redrive VW is a 2180cc engine. It is a 82mm stroke 92bore. The new 94mm Nikasil cylinders like Craig Spoke has on his engine is the thing to have on a VW. They are 10Lbs. lighter, transfer 4 times the heat and give more power. They are a bit more expensive butwort it.
Posted: 11/27/2010 07:08:14
For you guys with the Valley Engineering/GP redrive a couple questions
How is your belt life?
What weight flywheel are you using?
How much does the thing really weigh?
Also for better VW cooling Search on Fat Fin mod on the following sites, it really does work.
Posted: 3/14/2011 12:05:04
I have a 2276 VW with a 1.8:1 valley redrive driving a 80 X47 Culver prop that was matched to my engine. The engine seems to handle it ok produces about 360 lbs thrust. on uneven grass. I haven't flown it as I abanded the project, airframe issues. I plan to now put the engine on a Volmer VJ-22 much more rugged airframe. The complete engine redrive unit weighs 185 lbs. The flywheel is whatever Great plains sent me with the engine kit. Roble
Posted: 3/18/2011 22:27:46
Yes it's flying well on the Highlander and is made by www.bushkingperformance.com
The Warp Drive Prop is still a issue it seems with a bad vibration, may switch to Power fin or wood prop soon.