PDA

View Full Version : 503 twin carb or single?



Mick_Sanderson
09-10-11, 20:10 PM
Hi guys
With all you Flash alpha 2 owners out there, I'm looking to buy my 1st aircraft soon, & really like the FA2,
The dilemma is? is the twin carb 503 better than the single carb?
what is the difference in performance etc, burn rate per hour
any help appreciated

martin sanderson
09-10-11, 20:14 PM
i had the one carb with a burn rate off about 10 letters per hour

FarmStripFlyer
09-10-11, 20:50 PM
A lot less than a word per minute then (sorry couldnt resist) .

I have a twin carb 503 and it sucks nearer 12-13 lph, (cruising at about 5200-rpm), so I guess the single carb ones are a bit more frugal

Terry Viner
09-10-11, 22:17 PM
A chap on my field has the F2a with single carb and has lot les than 10 ph much better than my 912s.

TV

marilyn
10-10-11, 10:36 AM
Educate me, guys - why single vs twin carb - what are the advantages or disadvantages?? It's all Greek to me ...

Paul Dewhurst
10-10-11, 12:08 PM
Let's more fuel and air in at full throttle, so you get more horsepower.

Can't remember exactly, but I think there is about 6 hp more from a twin carb 503 vs a single carb one.

In reply to the initial post - it depends really what you will be using the machine for - if you fly solo mainly and are not obsessed with speed the single carb one will be fine and has better mixtures control at part throttle openings so is slightly more efficient rev for rev (although not much).

If you fly dual a lot, and are regularly near max takeoff weight, you might prefer the better takeoff and climb that extra horsepower gives. The extra 6 ponies may not sound much, but as a lot of the horsepower is required for straight and level it's a high proportion of your climbing power, and will add 25% or so to your climb rate. May be especially attractive if you fly from a short field with obstructions in the climb out path.

Paul

FarmStripFlyer
10-10-11, 12:19 PM
How exactly does the single carb version have better mid-range mixture control? Power difference, dunno, but id be amazed if it was as much as 6hp

marilyn
10-10-11, 12:22 PM
I'm terribly excited - never thought a Flash II Alpha would get my little heart a'beating - but there's a bright shiny red one going to come my way - 582 engine - so I think I'll need a ballast bag, eh what? It's for my solo hours and depending if I fall in love or not is whether it stays with me beyond my first hundred hours.

Paul Dewhurst
10-10-11, 16:07 PM
46 vs 50hp according to the specs on the Skydrive site for single and dual carb 503.

As for efficiency, I think it's a case of twice the volume of air through the single carb compared to the two. More venturi effect and better low RPM atomisation? I am sure some engine techy will be able to tell us.

Paul

Paul Dewhurst
10-10-11, 16:20 PM
I'm terribly excited - never thought a Flash II Alpha would get my little heart a'beating - but there's a bright shiny red one going to come my way - 582 engine - so I think I'll need a ballast bag, eh what? It's for my solo hours and depending if I fall in love or not is whether it stays with me beyond my first hundred hours.

The 582 version is de rated in the Alpha installation with the restricted 'low noise' exhaust, so comes out similar to the 503 twin carb in the power stakes, but more relaxed sounding doing so.

Alpha not a bad ship, a bit sharper in the stall than it's contemporaries - but softer if it has the 'speed rigging' mod which moves the bar forwards and effectively limits forwards movement, giving more pull back available.

They are a bit of a handful in turbulence and for it's genre rather heavy in roll with a fair bit of adverse yaw, but if you stick to calm weather for a while it wont feel bad.

They glide well and are nice and floaty in the hold off and land nice and slowly. They can feel a bit unstable on the ground due to soft rear springs, especially if the steering block is a bit tight leading to jerky nose wheel steering inputs, but landing so slowly it isn't a problem - just avoid landing downwind:)

Twin tanks need good discipline and procedure if you are going to fly needing to change over.

good value machines just now - a lot of plane for the money.

Paul

ajojets
10-10-11, 16:30 PM
On a duel carb setup the sliders are slightly smaller oneach carb than in a single setup, therefore the mixture velocity is faster throughthe carbs due to the smaller openings giving a better fuel air mix and chargeto the combustion chamber, hence more power, also the jet sizes will be largerin a single carb unit.

Paul Dewhurst
10-10-11, 17:30 PM
Same carbs are used on the dual set up as the single, so sliders are not smaller.

only the jets are changed.

Single carb 503 does seem more fuel efficient than the twin, maybe some of that is due to using the extra horsepower, but I was told it was intrinsically better at lower power requirements - but that may not be so?

Paul

ajojets
10-10-11, 18:16 PM
That maybe so on the rotax engine but in most cases the venturesize is smaller on a TC engine, still the fuel mixture velocity will be faster,the other thing to consider is the timingof the engine induction, (in most cases ) the porting is changed to suit, is there a differencein the two engines in that respect.

Mick_Sanderson
10-10-11, 18:17 PM
Glad I asked the question now, It opened up a whole can of worms, & Gives me FUEL (sorry couldn't resist) for thought.
Thanx for the input, I've seen a few advertised on AFORs sadly the twin carb has been sold, but I'm in no hurry as the weather is pretty miserable at the mo, but hopefuly I'll have my own by December.

AndyJ
10-10-11, 23:31 PM
Not sure of all my facts here, but I beleive the twin carbs all come with twin coils, so there'xs a good safety point here, dcdi, if one coil goes down you are still firing on the other, (both fire at top and bottom of piston travel).

Paul Dewhurst
11-10-11, 00:15 AM
No difference in porting - barrels are the same and it's just an intake manifold change to mount the second carb.

No difference in ignition systems. They used to all have points ignition, and then they all changed to CDI.

You can convert a single carb to dual and vice versa. Just buy the second carb, and manifolds and re jet the first carb.

Paul

Mick_Sanderson
11-10-11, 09:44 AM
Not sure of all my facts here, but I beleive the twin carbs all come with twin coils, so there'xs a good safety point here, dcdi, if one coil goes down you are still firing on the other, (both fire at top and bottom of piston travel).

Surely can't be right why would the Ignition be at Bottom Dead Centre as well as T,D,C?, the new mixture is entering the combustion chamber through the transfer port ready to be compressed on the next stroke of the crank/piston thats why is called a 2 stroke,
I think your confused with the 2 mags it has = 2 coils = 2 plugs per cylinder.
A spark at the bottom of the piston stroke would cause pre ignition & backfire or blow a crank seal,
Have a look in the cosgrove manual it explains how a 2 stroke works.

AndyJ
11-10-11, 18:42 PM
Have a look in the cosgrove manual it explains how a 2 stroke works.

I prefer to read the rotax manual when its specific to a rotax engine.

ajojets
11-10-11, 19:16 PM
The system is called a wasted spark ignition, the spark at the bottom dose absolutely nothing it just simplifies the ignition circuit, if it was an ignition system with points you would require two sets of points.

Mick_Sanderson
11-10-11, 20:19 PM
I don't mean to P**s on your chips here but I cant see how a wasted spark would work on a 2 stroke at BDC, as the piston is forced down the cylinder on the power stroke the cylinder is already filling up with the fuel / air mix, pushing out the Exhaust gas (Scavenging), if there was as SPARK at this point it would pre ignite the partial fuel mix already filling the chamber, thats why it's called 2 stroke (everything happens faster than 4 stroke), I can't find where it says this in the Rotax book, Yes you need 2 CDIs or points 1 for each cyl, the wasted spark works on 4 stroke. Like I said risk igniting the fuel in the crankcase with the potential of blowing crank seals.

Rick Moss
11-10-11, 21:11 PM
Mick, you might want to look into who you're correcting before you correct them!

They are both correct in that there is a spark at the downstroke; the point in the cycle just after combustion has occured and the cylinder is full of burnt gases....

Mick_Sanderson
11-10-11, 21:28 PM
Mick, you might want to look into who you're correcting before you correct them!

They are both correct in that there is a spark at the downstroke; the point in the cycle just after combustion has occured and the cylinder is full of burnt gases....

Like I said I don't want to p**s on anyones chips........... But if that is the case in these 2 strokes, can someone point me in the right direction as I cant see for myself in the Rotax Manual. on the single Mag or dual mag engine. and you moved the goalposts a little as now saying the spark is on the power stroke not at BDC as I originally corrected. I'm not trying to be clever or pretentious, I want to know how the 2nd spark doesn't cause pre ignition and not damage the engine?

Rick Moss
11-10-11, 21:33 PM
I think everyone's chips are still dry.

The sparks occur at each 180° of revolution; the spark on the upstroke occurs before TDC, ergo the spark on the downstroke does the same, presumably (and this is my assumption) before the mixture in the cylinder has reached it's volatility limits.

FarmStripFlyer
11-10-11, 21:34 PM
I'm no expert, But like Mike, I just dont see how a wasted spark could work in a piston ported 2-stroke

Mick_Sanderson
11-10-11, 21:52 PM
I think everyone's chips are still dry.

The sparks occur at each 180° of revolution; the spark on the upstroke occurs before TDC, ergo the spark on the downstroke does the same, presumably (and this is my assumption) before the mixture in the cylinder has reached it's volatility limits.

There is no spark on the down stroke, the spark occurs approx 16 degrees before TDC and thats the only spark for each cylinder, No twin spark 1 spark per cycle.

www.800-airwolf.com/pdffiles/ARTICLES/part22.pdf

Mick_Sanderson
11-10-11, 22:12 PM
I'm no expert, But like Mike, I just dont see how a wasted spark could work in a piston ported 2-stroke

I can't i even started to doubt myself... weather its dual or single mag points or CDI one spark and one spark only at approx 16 deg Before TDC for each Cyl
I might be a student pilot learning to fly but I do have a lot of knowledge on how engines work.

AndyJ
11-10-11, 22:19 PM
Mick, just accept some additional knowledge from people who do know, These mixtures with no compression don't fire, its a fact so listen, you need to make sure of your facts before speaking out strongly.

ajojets
11-10-11, 22:37 PM
I can't i even started to doubt myself... weather its dual or single mag points or CDI one spark and one spark only at approx 16 deg Before TDC for each Cyl
I might be a student pilot learning to fly but I do have a lot of knowledge on how engines work.

Obviously

Mick_Sanderson
11-10-11, 22:47 PM
Mick, just accept some additional knowledge from people who do know, These mixtures with no compression don't fire, its a fact so listen, you need to make sure of your facts before speaking out strongly.

Its ok taking hearsay Andy, show me where I can read the info on a second spark after TDC? I added the info for all to read, on timing & it clearly states 1 spark per cycle. its no good saying take it as read, show me in black and white, or where I can read the twin spark theory & I'll thank you for the help.

if the mixtures don't ignite with no compression explain how you get a Backfire if you get the timing out by 180 deg. ie firing at BDC

AndyJ
11-10-11, 23:09 PM
Its ok taking hearsay Andy, show me where I can read the info on a second spark after TDC? I added the info for all to read, on timing & it clearly states 1 spark per cycle. its no good saying take it as read, show me in black and white, or where I can read the twin spark theory & I'll thank you for the help.

if the mixtures don't ignite with no compression explain how you get a Backfire if you get the timing out by 180 deg. ie firing at BDC

I'm getting a little tired of your "expert opinion", therefore here is a brief summary:

When one cylinder fires in compression the other fires on the exhaust stroke. As both spark plugs are fired at the same time from opposing ends of the same coil they have opposite polarity; the spark “flows” from the center to the outside of the compression plug and from the outside to the center of the exhaust plug. The exhaust sparks have lower energy as there is no compression. One advantage is there is no need for a distributor so no moving parts, so they are ligght, simple and reliable. As for your backfire, you get pre-ignition when you have unburnt fuel, compression and hot embers to ignite it, but I suppose you know that too. So if most of your content is burnt and you don't have compression you don't get it, get it!
I don't have time to look for more info. find it yourself.

FarmStripFlyer
12-10-11, 08:20 AM
I'm getting a little tired of your "expert opinion", therefore here is a brief summary:

When one cylinder fires in compression the other fires on the exhaust stroke.

Hold On a second Andy. If this wasted spark is at 180 degrees to the firing spark, then its not going to be on the exhaust stroke of a 2-stroke, it will be on the induction stroke. Sounds like your info was lifted from an explanation referring to wasted sparks in 4-strokes. P.S., Please dont take my post as confontational, I'm trying to learn, not gain internet points

ajojets
12-10-11, 09:07 AM
For f----s sake, On the cyilinder with the wasted spark at the firing point the piston is covering the inlet port from the carb, the transfer port to the bottom of the crank to the cylinder is open and the exhaust port is open,to get any combustion to take place the conditions have to be just right (asyou know when trying to start your engine. LOL) stoichiometric value has to bebetween 14.6 to 14.7 for combustion to take place this value will never beachieved with the two ports open, also at this point the exhaust scavenging has not completed its cycle to charge the cylinder with fresh fuel. as well as using displacement to fill the cylinder the pressure waves setup in the exhaustare used to replenish even as the piston is travelling in the upward direction.

Mr No Limits
12-10-11, 10:00 AM
Hi Guys,
Maybe I can help here.

The Bosh points ignition 503 engine (one spark plug per cylinder) has a single spark per cylinder per revolution at 19° before Top Dead Center. Ie each cylinder sparks every 360°. Each cylinder has its own set of points, condenser & coil. The timing is controlled by a cam on the crankshaft opening the points at the preset point.

The Ducati ignition 503 engine (two spark plugs per cylinder) has two sparks per cylinder per revolution at 16° before Top Dead Center and 16° before Bottom Dead Center. There are two coils both providing a spark for both cylinders. The ignition timing for each coil is controlled by a trigger external to the flywheel which detects two bars on the flywheel spaced 180 degrees apart. There are two triggers & two coils both providing a spark at 16° before TDC & BDC on both cylinders.
You will have to take it from me that it’s a wasted spark as I cannot find a description of the firing detail in the engine documentation anywhere. Knowing the engine ignition systems well it cannot be anything else.

The wasted spark at BDC occurs during the induction/exhaust cycle which in a two stroke engine is the same thing. Just to expand slightly.
After the engine fires the piston travels away from the cylinder head. As the piston travels away from the head it opens the exhaust port allowing burnt gasses to pass into the exhaust system, as the piston travels further away from the head the transfer ports open and allow a fresh charge of fuel/air to be transferred from the crankcase into the combustion chamber, hence the exhaust cycle & induction cycle occur during the same stroke.

On the engines with a wasted spark - At DBC there will be with fresh charge in the cylinder as a spark occurs, but due to very low compression levels the charge does not ignite. A two stroke with low compression (badly worn rings/pistons/barrels) can be very difficult to start as the fuel/air charge doesn’t get compressed sufficiently & just doesn’t want to ignite.

Conrad
ConAir Sports

FarmStripFlyer
12-10-11, 10:26 AM
Calm down ajo, I'm not an idiot, in fact I know enough to understand that the "stoichiometric value has to be between 14.6 to 14.7" that you refer to is the mixture strength and has been achieved allready by the time the mixture is in the cylinder and has nothing to to do with whether the mixture is compressed yet as you imply It seems to me you are trying to say that the mixture wont fire till compressed, I can understand that, no need to go using them big words in the hope of sounding all superior mate

Mick_Sanderson
12-10-11, 10:56 AM
Well guys I do apologise!!!!! see it's not hard to do, if one of you had pointed me in the right direction it would have made it so much clearer, But after speaking personally to Conrad Beale, he confirmed on the Ducati there is a spark to (wasted) at aprox 16 deg BBDC, If one of you guys who might I say should be helping instead of dictating, the matter would have been resolved days ago.
Telling someone just to take it as read, is like telling somone for the 1st time to Pull the Bar IN to recover from a spiral dive, if they dont know better they will do it,
In the end I helped my self by phoning Conrad.
Time to put the matter to bed.

As for the expert opinion, I never claimed to be an expert, If you look closely at the post It's in "Frequently asked Questions" aimed at newbies.

One of you could have PM me

Mr No Limits
12-10-11, 11:21 AM
I thought I put a posting on replying to this thread (before Mick rang me) explaining the ignition systems on the 503, but it doesnt seem to have appeared yet (maybe something to do with my log in status)


Conrad.
ConAir Sports.

Sam
12-10-11, 12:02 PM
Top entertainment in an uncomfortable fashion !!

But just so you know, I am far bigger than all of you put together and have been known to knock slighter woman unconscious if I wave it about to much !!!

:jumpeveryone:

ajojets
12-10-11, 12:23 PM
Yes but i have a bigger one than you.

Sam
12-10-11, 12:41 PM
I had heard that ajo but you dont count because the stenchometric value of yours is tooo high ! :guitarred:

marilyn
12-10-11, 13:36 PM
It is precisely this sort of thread that threatens this forum with extinction. The other thread about the election which has been running simultaneously is also a front runner for useless drivel. Please fellows, act your age for once. And stop comparing willy sizes. I suspect they are all sub-standard.

FarmStripFlyer
12-10-11, 13:45 PM
Gulp, Guys I think we've been told :imsorry:

Seriously, Cheers Marilyn.

marilyn
12-10-11, 13:55 PM
I'm smiling. There's something about this weather which makes everyone grouchy. I'm off to warmer, sunnier climes in more benign wind conditions - check out www.xcweather.co.uk and the weather at Montmorillon and eat your hearts out.

Bye for now
Marilyn

Sam
12-10-11, 16:06 PM
check out www.xcweather.co.uk and the weather at Montmorillon and eat your hearts out.

Looks beauuuuuuutiful, apart from tue /wed next week. When you going ? Flying ?


I suspect they are all sub-standard.

Looks like you went to the same school as PJK, just looking to stir up trouble :poke: