Replies to This Discussion

Permalink Reply by John Austin on May 9, 2013 at 7:33am

Very nice, Ian! In developing my cooling tweaks, I considered a cowl flap, but, unlike your application, in slow/high drag airframes like the 750, I could only find a couple of instances where someone had tried it. My impression was the results were marginal at best. Unlike a faster/slicker aircraft, the 750/3300 pretty much needs max cooling all of the time, so installing an extended cowl lip works just as well without the complexity of a flap. Also, unique to the Zenith STOL designs, they're capable of such high departure angles that the cowl outlet on the bottom of the cowl can be exposed to ram air effect, actually pressurizing the lower cowl rather than providing the very necessary negative pressure/suction for cooling flow. In this situation, an extended lip can act as a baffle to the ram air, blocking it from flowing into the outlet, and help keep the lower cowl unpressurized. I added two inches to my cowl lip:

 

Another area to look at on the 750/3300 is oil cooling. I've read claims that a substantial portion of an "air-cooled" engine's heat is removed by oil cooling - maybe as high as 20+%. I had high oil temps initially and like Ed, couldn't sustain a climb for long without leveling-off and reducing rpm's. I had the 7-row cooler that apparently is adequate for the Zenith 6xx series, but most 750/3300 builders go to a 10 row. I had marginal results with the 10-row. The 3300 has a heavily finned sump, and cooling of this area seems extremely effective - Zenith makes a kit to allow ram-air to flow through a duct over the sump and this usually results in a 10-15 degree drop in oil temps. However, at least one builder reported to me that introducting additional ram-air/high pressure air into the lower cowl where the sump is aggravates the cowl pressure differential and made their CHT's worse!

 

My left cylinder bank was always coolest (? due to prop swirl ?), so I devised an oil sump cooler of my own design that "borrows" some of the excess ram-air/high pressure from the left ram-air inlet and ducts it to the oil sump. (It also works out that there is a nice flat spot beside the bump in the duct for oil filter clearance that allows mounting a duct!) I got the same immediate 15 deg drop in oil temps and my CHT's were even better!!! In fact, since I'm in a temperate climate, I had to block off some of the oil cooler for winter ops! My gut feeling is that if I had done the oil sump cooler first, I would have been fine with the original 7-row cooler. My theory is that I'm not adding any additional high-pressure air that wasn't already going in the upper cowl, anyway, so I've not upset the pressure differential balance. The oil is cooler, the engine is cooler, and the cylinders are cooler. This will give you an idea of what I did:

 

All the little cooling tweaks are additive, but I think cooling the oil gets a lot of bang for the buck. Especially since I built the oil sump cooler mostly from scrap. The other big plus is no new holes in my cowl!

 

John

N750A

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Permalink Reply by Dr. Edward L. Olds III on May 9, 2013 at 4:59pm

John,
My mother-in-law had an acute MI this morning, so I am at the hospital and probably stymied for a day or 2. I did fly for 10 minutes before her attack and added fences between the heads with aluminum tape and just generally closed up any holes I could find. #5 ran 371 cht, but everything else was within parameters. Outside air was 16 C.
The oil cooler is finally installed and I was at 196 F. I thought it smelled hot, but it seems to have worked. As always, you were a great help. Do you mind if I give you a call?
Ed

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Permalink Reply by John Austin on May 9, 2013 at 6:20pm

Hmmm ... now let's see ... your oil was at 212F and I told you to install my oil sump cooler design and that it would drop 10F minimum. Earlier you told me you did a ground run and it dropped to 201F - OK, so I missed it by one degree. THEN, I told you to fly it and it would drop 5 more degrees, you did, and it dropped 5 degrees to 196!!!! Am I good, or what? LOL!

 

I'll email you with more info.

Hope your mother-in-law recovers completely and quickly!

Regards,

John

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Permalink Reply by Dr. Edward L. Olds III on May 9, 2013 at 4:46pm

Ian,
I've opted to go with the new extension that Jabiru is using! What's money when I've come this far! I also am adding some tweaks suggested by John Austin such as "fences" between the heads. I flew this morning and only had a big issue with #5 cht temp. However, it was cool and rainy.
Ed

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Permalink Reply by adrian dines on May 14, 2013 at 8:05pm

did you know , the piston's in these engines are installed incorrectly , arrow to prop, they should be installed arrow to flywheel. the piston pin has a .040"offset   , this offset should be toward's crank rotation , as it is in all auto engines . this provides suitable angularity of the con rod to crank, provides more dwell as piston reaches tdc and after tdc ,providing much smoother transition of power to the crank, peak cylinder pressure should be 15-17crank deg after tdc.. With pistons installed as they are from factory , i have demonsrated with model's that peak cylinder pressure is occuring befor tdc - tdc , same effect as advanced timing , also same affect as early valve timing eg higher compression , earlier ex valve opening[ egt],large moments of deceleration btdc   very high rates of excelleration atdc this is causing most of the engines problem's , flywheel 's and prop hubs departing crankshaft , i have every reason to beleive the thrue bolt problems are caused by this also .It is also not recomended to run ground adjustable prop on these engines either . because of high moments of decel. and excel. We have changed piston's on a 2200 with amazing results,much lower cht/ egt oil temp's and thats flying in amb temps of 42 deg. c  The manufacturer dosn't recomend any flying if temps go above 34deg.c. we have now done 200 hrs since piston change , with no ill effect's, infact blowby is minimal , catch can has not been drained in this time, and only contains approx 1/2"of oil in it at the moment. Ihave also found other proplems with these engines which i 'm working on at the moment but will not have any proof result's befor end of yr.

I can honestly say this is a worth while modification , you will notice the difference the moment you start it . other benifits of note , 50 rpm increase static very little viabration,less noise, cooler engine all round,small increase in power at top end less fuel burn.

I have been engine rebuilder [reconditioner] for more than 35 yrs. and hold 22 repairer's licenses relating to the motor industry [auto], i have researched more than a 1000 engines and i have not found one with reversed piston's. Ihave seen pistons that have been fitted reversed by d.i.y builder's which have all presented with problems sim. to the above engine.

ANY WAY have athink about it ,consult a eng rebuilder [auto]  see what they say . We are changing some more engines in the next 3 weeks, PLEASE NOTE!  engine should also be in good condition when performing thi modification.

KEEP THE SKID MARKS  of your plane seats while flying behind this engine!

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Permalink Reply by Ian McClelland on May 14, 2013 at 11:17pm

That's very interesting information Adrian. Have you had any communication with Don Richter of Jabiru Australia regarding this issue? He should answer an email to info@jabiru.net.au

When you say they are offset do you mean the piston is offset 0.040" relative to the connecting rod center?

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Permalink Reply by adrian dines on May 15, 2013 at 1:23am

Hi   Ian, I have contacted rod stiff  at jab but he wasn't interested at all.  piston pin offset is .040"or piston is still on center line of crank but pin is offset , should be into direction of rotation of crank , in jab case pin offset is with rotation of crank, opposite the normal engineering convention, and is causing all of the above problems.

it is mainly thought that the offset in the proper direction is to avoid piston slap, this it does , but is only secondarybenifit . the main for offset is to provide smooth transision of power to crank .some engines employ centraly located pins , but generaly have different desighne pistons and heavier flywheel  [ subaru ea 81 example]   . somepeople have fitted forged pistons big in nz  forged pistons only come with centraly located pin [ unless specificaly orded.  ] this also makes a big change for the better in the engine. Jab motors have avery small conrod/ratio ,the shortest i'v seen, which makes offset very important for this engine, if it is in the right direction . to be effective the offset with this c.r ratio should be in the order of .125"

however this would cause other problems with piston load imbalance. some manufacturers use as much as 14mm .560"offset by offsetting the crank to center line of cylinder  [ honda being one example] this makes the engine very smooth the use of lighter construction to out put more power and greater thermal efficiency.

the jab engine has offset opposite to all other engines i know,which causes very unfaverable moments in the crank , which the prop and flywheel have to try and absorb via their connecting fastener's, also moments of avoidable load in cylinde liner's/thrue bolts ,cyl head bolts , power canot be efficiently transfered to the crank ,that power is used in the form of heat which the cooling sysem has to deal with.

This is aquote from a tech bulletin that i have: there is a mechanical advantagefrom piston offset. Since the piston dwellslonger near top of stroke, the crankshaft armswings over further befor the combustion cavity begins to open. This allows the pressure of combustion to be more effectively transmitted tothe crankshaft arm during the period when the pressure is the highest. This increased leverage exist's throughout the power phase and the result is a smoother engine tha produces more effective work during the power stroke.     there is more to this as well  .  

THIS IS PART OF THE SMOKING GUN TO JAB ENG. PROBLEMS. 

ADRIAN DINES    ,   DINES MECHANICAL SERVICES, MOORA    WEST. AUST

0437511002              dmech@westnet.com.au

 

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Permalink Reply by Ian McClelland on May 15, 2013 at 2:10am

How do we get Jabiru to comment?

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Permalink Reply by John Austin on May 15, 2013 at 6:42am

There has been a good discussion of this on the Yahoo jabiruengines group forum. Pete from Jabiru USA did comment on that discussion:

 

... It's in the overhaul manual JEM-001-3.
Put the pistons in the way the manual tells you to.

It's pretty clear that conflicting info for each offset method has been
presented in this forum. There are advantages and disadvantages to either
method. You want a clear statement from Jabiru as to the way they want the
engines assembled - READ THE MANUAL - often times referred to as RTM ...

 

John

N750A

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Permalink Reply by adrian dines on May 15, 2013 at 9:37pm

Hi John,

           I'V read the manual many times.  Who designed the engine, who built the engine, who market's the engine, who wrote the manual,   The same people!!. the designe is wrong with regards to the pistons and c/rods and so is the manual.

Please , someone ,tell me what the thrust of the prop. has to do with piston pin offset.

 a 1000 people will tell you the engine has serious problems very few will say they don't.

We have reversed pistons in  two engines and were extremely happy with the results, one having done 200 hrs so far. i also know of other's that have been done as well.   I hav'nt heard of any being swapped back as yet.

Interestingly, jabiru in s affrica are turning pistons in new engines before delivery to customer's , have been for nearly ten yrs. they say they are consistingly acheiving tbo 's  of 2000 hrs. you could contact them and find out , talk to the dealer principal.

I'v been involved with engines all my life , my father befor that, so i have plenty of back ground. i know what piston pin offset is designed for and i know these pistons are in the wrong way, i have also done alot of research on this matter and have a lot of engineering data and models and working models to  demonstrate this.

These pistons come from a buick engine , the same as used in vn holden comodore.

in the holden engine the ref. mark's on piston go to the front of engine, when veiwing this engine from front it turns clockwise.     I n jab engine the same pistons, marked exactly the same are also fitted with ref. mark's to front/prop.when viewing engine from the front it turns counter clockwise[opersite direction] . in my automotive engineering schooling day's we were told   offset always goes toward rotation of crankshaft.which it has to, to do what it was designed for! My engine  is stripped down because of a circlip coming out of piston[ another common jab problem ] when this  engine is rebuilt it will have the pistons fitted,  marks to flywheel, and longer con rod's.

,

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Permalink Reply by Don Burkholder on May 17, 2013 at 12:02pm

Hi Adrian
Are you saying that with the notch of the piston facing forward, the pin is closer to the low thrust side of the cylinder? As seen from the pilot's seat, the pins on the left side of the engine are high in the cylinder bore and lower than centerline on the right side cylinders? All ifo I have seen indicates the pin is offset to be closer to the major thrust side as this angles the rod more at peak MEP. How much room is there to improve the rod to stroke ratio without increasing the compression ratio?
I also thank you for the information given.....DonB

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Permalink Reply by adrian dines on May 17, 2013 at 9:05pm

Hi Don

 you seem to be one of a very few that seems as though they have a handle on the rod angle bit at peak cylinder pressure , most think pin offset has only to do with controling piston slap/noise.

looking into the prop end of motor pistons are fitted ex factory and as stated by their engine manual, with notch to front [all pistons]  this puts offset toward's minor thrust side, don't forget this engine turns anti clockwise , not clockwise as in the car engine.    in reality the pistons should be fitted notch to rear, or flywheel , this would put the offset to the major thrust side and make the angle of c/rod correct , this provides a dwell period as the piston approaches tdc -- after tdc  alowing time for combustion to get organised and application of full pressure on piston at 15-17 deg of crank arm travel AFTER tdc, thus providing a smooth transition of power from reciprocal to rotory motion. with piston fitted notch to front this is all reversed causing very destructive forces into the crank then prop and flywheel as they then try to absorb these moments of decel and excelleration of the crank .

Also high frequency pulsing [ more cyls the higher frequency] I feel this leeding to other structural problems  as well, thrue bolts being one!

my working model shows that pistons w/offset fitted wrong way round will cause peak pressure at arround o deg of crank arm , rod on opposite angle trys to cause a moment of reversal to crank arm, then rapid forward motion when crank passes 0 deg, also has same effect of increased ign. timing, ealier valve timing , ealier inlet opening  means higher compression pressures at lower speeds , earlier ex valve opening causing hotter gas escape to exhaust system instead of being used in providing power in th remaining power stroke.

CONRODS

increasing c/rod length won't increase compressionn ratio. to accomodate longer rod's , new cylinders have to be made, or spaced out from c/case , not a good way to do it! it's quite common to see it done  on m/bikes . I'm  making new c/rods and cylinder's in my own machine shop , Longer cyls also mean longer push rod tubes and longer push rod's . As you can see quiet a big job. However , changing pistons arround about 2hrs  and just doing that will make  a very big differnce to everything .

deffinately all positive. I'm  half way on the rods , but don'expect to have any results befor the end of the yr.

Regard's

  Adrian

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