w8ye

My Feedback: (16)

Why don't you try it and give us a report with rpm and mm/hg pressure readings?

IronCross

I am thinking the "back pressure" controls the flow through the muffler... The less pressure/resistance the more flow... With back pressure in the exaust pipe you will pressurise the tank ... The back pressure wich does this limits the flow of your hot exaust gasse (and probably saves the valves) and prebents this fast gas flow we are talking about anywhere in the pipe ????.

turboapache

liquids are not compressable, gasses on the other hand are.

IronCross

Thus we get pressure

turboapache

Do we have anyone else besides me with a background in fluidics? Please feel free to jump in here. Maybe you can explain this better than I. Let me just say this, you dont have to understand a principle for it to work. As for the restrictions on the exhaust, it takes very little excess restriction to burn the exhaust valve.

Thunderchild

Not entirely true. [link=http://www.thomasregister.com/olc/63970008/liquid.htm]liquid spring[/link] but I digress.

I am guessing what you are trying to say in a constant volume/constant velocity environment which is therefore maintaining a constant pressure like the exhaust header we are discussing, is that the inserted nipple will have a syphoning or suction effect imparted to it by the gases flowing past the intrusion induced by the localised low pressure area created by the gases passing it. This would be a result mostly of turbulence set up by the intruding nipple. However, if the gases slowed, pressure would increase and not provide the intended suction, so to work perhaps the gases would either need to stay at the same velocity or even perhaps speed up as it passed the nipple?

When the same gases eventually get to the muffler section, the environmental conditions change because the volume of the chamber has increased thus allowing the gases velocity to drop and the pressure to increase. This pressure is what we tap to pump up our fuel tanks.

Edited for fuzzy brain

Thunderchild

I have been trying to think of a good practical example of this sort of thing in operation and it suddenly dawned on me.

It is a very common practice in full size aircraft to move fuel from the furthest point away from the fuel pump pickups within a tank by using what are called fuel "ejectors" or "jet pumps". This is a variation on the venturi theory we have been discussing. Fuel is tapped under pressure from the fuel pump and pushed to the tank extremities where it goes though a venturi (ejector or jet pump) and teh flow speeds up. As we know, when velocity increases, pressure decreases. This pressure drop across the venturi provides the suction and picks up the fuel from the outer tank regions. This collected fuel is then returned along with the fuel providing the motive flow to the cell containing the pump supplying the engine. From here it can be picked up by the pump and sent to the engine thus ensuring that all the tank contents are available to the engine and useable.

For the purposes of our discussion, here we could substitute the exhaust gas exiting down the header for the fuel (motive flow) sent under pressure down the line to the tank. When it reaches the jet pump (our venturi pickup being the nipple inserted into the header) the velocity increase picks up the crankcase vent contents from the nipple and draws it along with the flow return to the tank, in our case (our muffler) for expulsion overboard. Also in our case we have the benifit of a heated muffler and gas stream, so our crankcase contents would be combusted and exit as smoke.

I had never thought of doing this to get rid of crankcase venting, but like the theory and might have to drill and tap a header to try it.

It also strikes me that you would have to be careful not to have the nipple intrude too far inside the header. This could create too much turbulence and if the gas flow slowed becuase of it, the pressure rise (however small) would likely defeat what you are trying to do.

IronCross

WOW
This all sounds good... I understand what you guys are saying and I am sure it pertains to the examples given...
What I do know is if you do not have an exaust header on a 4 stroke to build up back pressure you risk damaging the exaust valve... This was dicsussed long ago in this thread... There fore with just an exaust header you develop backpressure... This is a positive pressure... You can tap it and pressurize your tank as is done with the flex header... No muffler needed as it seems you imply... This positive pressure attached to the crankcase will limit blow by and cut back on the oil in the bottom end... No degree in fluid dynamics really needed except to confuse the issue...

XJet

Actually the pressure *increases* as the area increases and (as a result) the velocity of the flow decreases.

Bernoulli's theorem proves this and this is why we have things called diffusers in jet engine -- so that the pressure at the front of the combustion chamber is *higher* than that at the outlet -- or all the hot combustion gasses would move forward and come out the intake.

The *best* place for a pressure-tap in an exhaust system is generally the position where the diameter is greatest -- ie: at the point of maximum bulge in a tuned pipe or maximum diameter in the case of a muffler.

Of course if the outlet of the muffler/exhaust is constricted somewhat there will be a higher average internal pressure almost *everywhere* in the exhaust system -- but it'll still be highest at the bit with the largest cross-sectional area and thus the slowest gas-flow.

rambler53

My Feedback: (494)

I could use some help in locating exhaust parts for this Saito FA-90TS twin I just received. One included looked 50% pinched off from an obvious crash, and the other doesn't have anything there. I need tank pressure from both? This I assume would be the same as any FA-45 exhaust? I found some Arizona dirt packed in one of the exhaust ports, can I safely disassemble the head bolts/cylinder bolts to clean it? I don't want to just go rednecking into it like I do with two strokes and I'm a little cautious with this being a twin. Any advice, specs, and sources for parts would be greatly appreciated. I know to run a 14X6 - 13X8 prop and it should power any .60 plane fine. Not having a manual, what are the clearance adjustments supposed to be? This has a single carb, single needle valve, it's missing the ratchet clip too. Thanks in advance.

rambler53

My Feedback: (494)

Saito Muffler (Left) w/Nut:LL
$34.16



Saito Muffler (Right) Nut/Prs:LL
$36.29

http://www.southeastrc.com/Items/Std...IE090TS&tc=REP

Found something here to work with as a starting point. I thought I'd see a typical single head style fixed exhaust set up. I suppose most are working with scale looking planes using a cowl.

Hobbsy

My Feedback: (102)

Rich, I'll dig out the manual for mine, on mine the left exhaust pipe has a tank pressure fitting almost at the very end. The rocker setting would be .004. If you need one of those metal feeler guages for Saitos, I have plenty.

You can find most of what you need here http://www.horizonhobby.com/Search/D...to+90ts&CatId=

rambler53

My Feedback: (494)

Thanks. I see the parts I need. Another question, I oiled up the engine and blew the dirt out with an air compressor from the glow plug end and watched at different prop positions the oil blow out the exhaust on each side, clearing out some dirt.
I turn it over and feel one cylinder having compression now that it's oiled, but I assumed the other cylinder would give me compression 360 degrees later and it is barely felt at any point as I turn the prop two full revolutions. I assume one cylinder isn't working or the valves are stuck. I pulled the valve covers off and watched the rockers move on both ends and they are also clean. Should I just give it a whirl and fire it up or what step should I confirm first? The young man sent it to me to trade. He said it ran before he stored it and I need to get it running to be able to complete the deal as discussed, even briefly. Do the cylinders have 360 timing or close to it on the twin 90? On single cylinder 4 strokes I've commonly run into lost compression with just firing it up resolving the dry out from storage. Thanks.

rajul

My Feedback: (58)

Xjet got it right. PV/T=constant

Ken6PPC

Is the 125 Saito really much better than the 120? I'm thinking of buying a 125, but a guy I know has a 120 that he will sell. I haven't seen the 120 yet, and really have no idea how reliable the seller is.

Thanks in advance.

gjeffers

there is about 6-7 oz difference in weight, if thats not a problem then get the 120

conchohoncho

My Feedback: (120)

[>:]The 125 really rocks my Jerrys Big Boy.
#260

turboapache

I must say that it totally amazes me that I spent 34 yrs. designing and building industrial hydraulic systems, pneumatic, and lubrication systems and find out in just a few posts that I never knew anything about what I was doing. Who would have thought it. When I was taking physics and advanced physics we somehow missed the theory whereby a gas or liquid increases in pressure when passing from a small area to a larger area. We always found that the pressure dropped. I guess the professor whom I studied under (also head of the physics department) must have been missinformed. Sorry I gave someone wrong information.

tailskid

My Feedback: (34)

Turbo.....I 'think' you might have a handle on this liquid/pressure/etc. stuff....but I do have a question - can't liquids be compressed? I thought I saw a book and class in the Engineering department on that topic.

So...can it???

RVM

No, liquids cannot be compressed.

roboto65

My Feedback: (30)

I don't understand none of the pressure stuff but I do know that if you blow across a straw submerged in liquid it will draw the liquid in HMMMMMMMMM

IronCross

Interesting point... Reminds me of my carburator days [:'(]

I am sitting here looking at my .72... the ID on the exaust manifold and muffler appear to be the same or pretty close... No compresion chamber etc... The pressure fitting is in the exaust stream with the exaust going across it... Yet it does create a positive pressure on the pressure fitting and presurizes the tank... I can only deduce that the ID on the pipe/muffler is such that it offers a resistance to the exaust flow and creates a back pressure... Could be wrong... Be the second time this year already [:'(]...

imsofaman

My Feedback: (2)

Hey Saito Club! I have had a few Saitos....I went towards electric at one point and sold everything. Now....I am getting back into glow again and found a little Saito 45 on Ebay. It looks like an older style by the carb and manifold tube. So.....does anyone have any info on this? It was cheap so I jumped on it.

Dave

Jim Henley

Hi, Jim from Omaha here, have any of you located a 90 degree exhaust header or adaptor for the 150? This is my first Saito It will be mounted in a Pica WACO (someday)
Thanks
Jim

donkey doctor

Hello; imsofaman; What would you like to know? I have 4 of those 45's and think that they might be the best all time four stroke model airplane engine. You can use 5 to 30 per cent nitro, even 0% will work. I use an 11 x 7 prop, but 10 x 8 and 11 x 6 work good Too. No need to put a muffler on it either, since it's quieter then the Saito 50 that replaced it.