Posted Thursday, July 3, 2008 @ 09:33 AM  

Elsewhere on this site, I’ve posted the comment that the upgrade from 1200cc to 1400cc is a simple matter of whipping off the cylinder heads and substituting 83mm barrels, pistons & rings for the original 77mm units – an afternoon’s work.

Well, that’s true, but if you want a safe installation, there’s lots more to consider and to do.

Increasing power by nearly twenty per cent is great. But with it comes the need to increase everything else: fuel flow, intake air flow, exhaust air flow, cooling inlet air and cooling exhaust air. You might also need to change your propeller, to prevent the engine over-revving.

Climb Speed
Climb speed will need to be increased, because Vx and Vy (best climb rate and angle speeds) depend upon engine power as well as aerodynamics, and go up with increased power. I use 60 knots or 70 mph for my 1400 Fourniers, but that’s just a guess. It also helps with cooling inside those tight cowlings.

Compression Ratio
Increasing your cylinder capacity without changing the combustion chamber volume will automatically increase the compression ratio. Depending on the number of shims under your cylinders, cooling airflow, valve and ignition timing, among other variables, this could easily take your engine into the area of overheating, detonation and pre-ignition. I don’t know. Do you?

Did you measure your combustion chamber area and add under-cylinder shims to retain your original 7 to 1 compression ratio? Do you need to change your ignition timing?

Fuel Flow
An increase in capacity demands a proportional increase in fuel flow. The only guide I have to the maximum flow needed is in the Rectimo brochure, which says ‘Consommation d’ essence maxi 13,5l’ .There is also a graph in the Rectimo manual which suggests that the fuel flow at full throttle is nearer 14 litres per hour. Dividing by 12 and multiplying by 14 suggests the 1400 motor will use at least 16.4 lph. This is at the main jet, of course, so the best way to measure it is to put just a little fuel in the tank (you want to be sure you can go around at full power after a long cross-country), whip off the air filter casting, unscrew the main jet and time (with a stopwatch or sweep second hand) the flow into a measured container.

I found that, without opening up the original 2mm needle valve jet, I only got 13 litres per hour. I drilled it out to 2.5 mm and subsequently got 74 litres per hour. Our English Fournier has a proper 2.5 mm needle valve jet, and this passes 68 litres per hour. Both are more than enough.

Depending on whether your fuel lines have ever been replaced, you might need to change those too.

Intake Air Flow
With luck, the increased suction from your bigger motor will suck more air through the carburettor’s venturi, and in turn that will suck out more fuel through the main jet. Or maybe not. Have you checked? What colour are your spark plugs after a few hours of full-throttle flying? What is your average fuel consumption? It might feel great if your 1400 motor seems very economical, but will it last very long if it is running too weak (and therefore, too hot)?

Whatever happens, you will have nearly twenty per cent more air whistling through those skinny intake pipes and nearly twenty per cent more fuel evaporating in there. This is a very good recipe for carburettor icing – especially on a warm, humid day. Remember, warm air can hold more moisture than cold air, so while carburettor icing is more common in winter, it will always be more severe in summer. And for various reasons, carb icing is more of a problem with auto fuel (mogas) than avgas.

Exhaust Air Flow
Have you opened up your exhausts by twenty per cent? I prised open the ‘fishtails’ and drilled out all the little holes to one size bigger. What have you done to improve your exhaust flow?

Now go to the next post to cover cooling problems with all this extra power and heat.

Posted Tuesday, June 7, 2011 @ 04:25 PM  

Donald

Posted Sunday, July 10, 2011 @ 12:20 PM  

Quote:

I would not turn off the engine in flight for the first 10-15 hours. It takes 20-30 mph more to air start.

Sound advice, Collin. Unless off course you plan a gliding landing (like today with XST )

Posted Monday, July 11, 2011 @ 08:08 PM  

Here is Jon Karcy's recommendation for Mira's 1400cc engine he built.

Posted Friday, July 15, 2011 @ 07:11 AM  

The two minutes full-throttle ground run is an LAA requirement, and you might find it very revealing.
My Aussie engine started to falter after 1:30 due to collapsed intake rubbers, after they had been well heated and washed internally with part-vaporised fuel.

There is also futher info on airborne VWs on the LAA web site, and more on running them in in the PFA Handbook.

You have informed the LAA or BGA and received their advice, haven't you?

Yours, Bob

Posted Friday, July 22, 2011 @ 04:02 PM  

In any case do not baby your engine or you will glaze the cylinder walls.

Posted Friday, July 22, 2011 @ 06:21 PM  

I seem to remember Jamie Oliver complaining that it was impossible to try to be an international chef cooking dishes in Italy since it meant competing with the cooking of everyones Mother- and there was just no way of winning!

I've heard about the risk of glazing cylinder walls, is that just during breaking in of the engine or could it happen if you "Baby" your engine later? (he asked worriedly after flying on 2200 rpm for the last 20 hours or so...)

May the 4's be with you/ Jörgen

Posted Saturday, July 23, 2011 @ 10:41 AM  

May the 4's be with you/ Jörgen

Posted Monday, July 25, 2011 @ 06:02 PM  

However, while we're on the subject of replacing pistons and barrels I'll share a few things I've learned, concluded along the way.

The piston crowns are marked with an arrow which, on assembly, must point towards the flywheel end of the crankshaft. Odd, the pistons look symmetrical but it turns out the gudgeon pins (wrist pins) are not located through the piston centre but are offset. OK, I read that some pistons are like that but I wondered why. My guess is that it's so that the geometry of the opposing conn rods and pistons, or big ends and small ends, can never line up with the axes of the cylinders to 'straight-arm' the engine at TDC. If the pistons are at TDC the wrist pins, being offset, put the cranks slightly out of line with the cylinder bores so the articulated geometry can be more readily collapsed and folded into the next cycle. I imagine this could only ever be an issue at start up but it's a feature so it's there for a reason.

When assembling the pistons and barrels the first thing is to fit the rings and I'm not going to describe that. Follow the instructions, use proper tools. But the trick is to then fit the pistons into the barrels on the bench. Don't try to fit the piston to the crank on the engine then slide the barrel over. Do it on the bench where you have control, where you can easily verify the ring gap spacings and piston alignment, where you can use a piston ring compressor, where you can see what you're doing. When it comes time to fit the assembly to the engine, back the piston out the bottom of the barrel just far enough to slide the wrist pin in but not so far as to pop the oil control ring out the barrel. When it comes time to fit the wrist pin circlip, you've already fitted the one that won't be accessible, right, stuff the crankcase opening with clean rags, paper towels, whatever so that when you ping a circlip it doesn't vanish inside.

Enough. I only hope I've learned enough not to make a pigs ear of it.

Posted Tuesday, July 26, 2011 @ 04:08 AM  

You can get away with the piston-first-barrel-second method with a broken in engine but I very much doubt it would be successful with all new parts and, as you observe, a ring compressor cannot be removed after use without modification. Doing it on the bench there is no need to modify anything. No need to whack anything. It's just a push fit. Altogether a more benign method.

The one caution I would add is that the ring fitting instructions tell specifically to align the oil control ring gap with one end of the wrist pin and the other two rings 120° either side of that. Since the rings spin easily in the piston grooves but with difficulty inside the bore, to maintain that alignment relative to the piston the piston must be inserted into the barrel aligned as exactly as possible to it's final rotation. If you were to rely on spinning the piston into alignment with the wrist pin the rings would not follow and the oil control ring gap would not be in the right place.

Posted Wednesday, July 27, 2011 @ 03:42 PM  

I would probably have put the oil control ring gap at the top like you except Mahle's instructions are quite specific about where the gap should go, so I'll stick with that even if the rings decide later to migrate the gap elsewhere. Your tip about installing rear cylinders first for ease of access to the circlips is noted and is perfectly obvious when stated. I might even have figured it out myself when I change the cylinders, but maybe I might not, so thank you for that one. Taking it a little further therefore on all the pistons the rear circlip can be fitted before offering up the piston and cylinder to the engine so that when working from the rear to the front the final circlips are always inserted on the forward, open side. That makes sense and should prevent a few cuss words.