EA 81 - Douglas Redrive

[asra]

Hey NickIf it aint to much trouble, send me a couple dozen of them Chook Berries. Fry up one of them things and a little Hog butt with some ground Hominy, a man could have a right smart breaky.[^] Humm I bet them HOT wings is MONSTER good.[8D][]Cheers MateSonnyIntelligence is not a privilege,it is a gift and should be used for the good of mankind.

[Ken Watson]

I'll try and get some photo's of my exhausts. They both work well and are very quiet. I tried out straight stubs and they were noisy and hp was way down. You need a bit of back pressure.Tapered headers have good hp advantage and can be blown with a porta power pump although the initial cuts in flat plate are tricky to get the right bends. I leave this stuff to a mate who blows expansion chambers. My exhaust stubs meet in a muffler in the centre under the sump. The muffler is hard up against the sump. Ken

[NiqueNaque]

Ken,Thanks, I think I know what you mean, you flat sheet template an exhaust system and roll it into pipes including the bends. Not easy, requires lots of thought and plenty of stuff ups on the way there. Chris Locker described the systems you are speaking on and said they were found most usually on motorbikes. He said they don't increase the power much but they make the trottle response really crisp.As to back pressure I can achieve that by welding some washers into the ends of the pipe to do so. rough I know but effective.The muffler under the sump is difficult with mine due to the radiator configuration, at idle the heat off the muffler may be an issue as well as on climb out.I appreciate your input Ken and look forward to seeing some pics of your machine.Bob.K; Sonny,Sorry, no bum nuts for you, quarantine forbids, and you might try to breed some Jurrasic chooks .. you know how deep the water is in Texas..Cheers,Nick.

[asra]

Nick, we picked up 300 engine rpm on one ea-81 due to the builder welding incorrect size washers in the exhaust. That was a cheap gain in hp!!!!The washers probably have to be specialy made with a larger than normal hole size for the washer diameter.Aussie Paul.[]www.firebirdgyros.com

[asra]

its a contrivisial subject,,, some people say back pressure is needed some say do away with all the back pressue you can .... due to a fella in Northern Florida I now too march to the diferent drummer .... get all the back pressure ya can off the cylinders/valves and alow it to BREATHE you'll increase your fuel efficiency and horsepower at the same time bigger exhost pipes than that are stock is the key to doing this Yessir ,,Old Walt knew what he was Preachin about .... he routinely got 35mpg out of his 400c.i.d Chevy .... he had in a TOW TRUCK !!!! i figure listening to the feller was a good idea ! ....and it works fer me too !take it er leave it no biggie .............Oh RATS ! Quarantine on the Chooks ????? I was thinkin of training one to be a watch dog..... bet those skunks would stay out'a my yard then !!!!hehhe C ya !Bob...... " Momm'a alwayse told me , Son the impossable is only a little bit Harder... and ya know I do believe She was RIGHT ! "

[NiqueNaque]

Paul,Thanks for the info, I will try to do some work to plot the static thrust when I get the opportunity to test it, I might even try putting a couple of gate valves on the tail pipe just to see what happens and report on same.What Ken is refering to is a tapered exhaust pipe system, this was Chris Lockers suggestion as well, he explained that if the pipe is tapering to a larger size it sets up a 'relative' negative pressure where the momentum of a pulse of exhaust gas tends to drag the subsequent pulse along with it, this also may be part of Kens reference to a tuned exhaust system, where you set the length to suit the speed of the pulses and the 'harmonics' of the system assist the flow of gases through the exhaust system and thereby provide a better aerobic efficiency for the motor, it means there is less exhaust gas left in the cylinder after the engine completes its exhaust stroke, if you CC your heads you'll know this is in the vicinity of 30ml of air, if this is exhaust gas that isn't expelled then it is something that won't burn in the next combustion stroke, thereby reducing the power of the engine.So Bob.K is right about the tow truck, the bigger the pipes the better, I'll try to make my gyro a test bed for this theorem.Again, thanks for the info.Cheers,Nick.

[NiqueNaque]

Butch,When did your post sneak in.. I'd only just noticed it, how observant of me... duh.The MIG is on the fritz for sure, I've doen about 200 hrs of work using them and am pretty used to how they work, but that doesn't mean to say my welding is any good..I didn't know they used aluminium as a coating, does this mean I'll get dimentia from the welding fumes? On second thoughts, don't answer that...Cheers,Nick.

[asra]

NickCalling back onto my old rev head days, I remember speaking to many drag racing guys, and exhaust people, and the suggestions were, For low down power & torque, 6000 RPM & under run an interfearance header system, which produces some back pressure.For high end power at 7000 plus RPM use an extractor system with minimal back pressure.If you used extractors on an engine which only reved to 6000 RPM, you would'nt produce as much HP as the header option.This also applies to an engine reving at say 7000 plus RPM, and using headers on it.Hope this helps.Regards Sam.

[NiqueNaque]

Sam, and Ken..The question at hand is how much back pressure?The subi on my machine will probably do no better than 4800 RPM... so by your advice it falls into the 'needs back pressure' design. Any ideas?Cheers,Nick.

[asra]

FWIW we built a few quick 4-stroke exhausts for historic racers - had requirements like "must not rev on beyond 7000" and "lots of welly at 4000".The results have been good so far, but it has been almost as much art as science - everything else has to be right too. We have seen BMEP over 200psi on old, modified pushrod OHV road car engines used in historic kit-cars.We applied experience from (thousands of) 2-stroke expansion chambers to these 4-strokes, and it took some nerve to build the first one, since the lengths came out way bigger than 'all the boys' used for racing. The technique works on the principle that you want a negative pressure pulse (eg suction) to arrive at the exhaust valve at around TDC on overlap. This both scavenges the cylinder and initiates the inlet flow, since the inlet valve will be starting to open before TDC.A one-pipe-per-cylinder arrangement works by timing the pulse speed down the pipe, and arranging the pipe length so that the positive pulse from the opening of the exhaust valve has time to travel all the way to the open end, where it is reflected as a negative pulse back up the pipe. Yes, really. Nature is wonderful.IF the length is right for the RPM, this negative pulse arrives back at the exhaust valve as the inlet valve is opening, and Robert's your relative.Now this works best at one value of RPM, or thereabouts, so it can be peaky. Fortunately, you can make a step in the pipe by adding another (short) length of larger diameter to the open end. This reduces the first pulse in size, a bit, but provides a second one when the original pulse reaches the new open end, and starts back again to the exhaust valve.If you are following so far, it's a small conceptual leap to a pipe that can grow in a large number of small steps, to spread the returning pulse in time - a tapered expanding pape or megaphone.Image Insert: 3.66

[Adrian S]

Ben,The most popular engine used in Gyro's here in OZ is the Subi EA81 with most guys looking for peak power at 4600rpm or 4800rpm depending what prop and redrive they use. I would be keen if you could explain the formula and recommend a design we could experiment with.Regards,Adrian

[Ken Watson]

It's all suck it and see. I raced the fastest Laverda sfc in the world and believe it or not the exhaust was a 2 into 1 that finished just under the crankcase', in all about 2'long. This was found to be the best in dyno testing. Max rpm was only about 7500. There is no real formula that works it is more trial and error.The fella who built this bike also use to make bits for historics, G50 Manx, etc. On Barry Sheenes he put the tapered headers with great success. Exhausts are weird things and a lot of the figures manufacturers give are optimistic. We had just as much success if not more with the intake side using a quality filter, K & N, and different stack lengths.We also found that a 6mm bolt through the end of the silencer helped us get through noise meters. My 2 carb ea81 made it's best power at 5200rpm. Ken

[asra]

Indeed ! Don't forget the other end of the breathing idea to an engine ! the carberator and intake manifold are extreamily important ! Back in my 2 stroke Motorcycle days of porting and polishing I was alwayse amazed at the amount of metal that could be removed on a DT1-250cc YamahaPiston port 2 stroke both intake and exhost... I alwayse tried to increase the intake as much as I did the exhost side and alwayse had a great deal of performance increase.... even if the old stock exhost pipe was still used ! For A quick increase of power in an engine I alwayse looked to change the carberator to a bigger size ... however.... this doesn't alwayse work so good ! realibitysuffers... and getting the jetting spot on is sometimes next to impossable .. Instead of changeing the carberator I began to taper the intake manifold takeing 1/4" off the inside near the cylinder ( opening the cylinder to match the intake of corse) and tapering it smothely out to the carberator attachment point.... Jetting was alwayse needed to get the engine to run right again... but the increase was almost equil to putting on a bigger carberator in the first place , so I discovered that changeing the carberator may not be needed at all ! .... the important thing is to realise that an engine is a Hudge AIR-PUMP ...and anything you can do to increase the flow of air into and out of will increase the engines preformance If it were my EA-81 I think I'ed have a muffler shop make me up #2 3" exhost pipes with all the bends needed to just bolt on to the frame one piece thingsand weld them to the headder flange myself perhaps a small glass pack Quite-er -downer on the end and run with that ...I doubt I'ed mess with the carberator or intake manifolds as its more truble than what its worth if its running good and clean now ...and I'ed still have the realibilityinherrant in the good little EA-81.I have an EA-81 in a 4x4 sadan in the front yard... it is a very well machined piece of equipment ...unlike the VW engine ( I also have) both engines are fuel injected which i detest to an extreamily High degree... I can't modify the computer controlers at all here. ( no eprom burner or program ) but no doubt there are Many aireas of which one can imporve the effeciency of a given engine if your carefull.... Ignition is another place that can be a nice bolt on improvement ...in preformance .I have experimented at length with a timeing light and a vacume guage and found I could time an engine with a vacume guage and get better gasmilage ! in the car... it was about 10 degrees more advanced than the specs called for but it worked and worked good .... that car is a toyota tercell with 350k miles on it before i parked it in general , I have found that engine specs are only CLOSE for what that pecitular engine needs... timeing is crucial if your engine is going to last , as well as its breathing ability .... the easier it is for it to breath the less work it has to do ... Also timeing and carberator adjustment is needed for the FUEL you burn ... if you know what your doing and adjust your engine not to the specs given by the manifactures but adjust to what the engine is doing ... I feel your alot better off ! Just my 2 coppers fer what its worth !C yaBob....." Momm'a alwayse told me , Son the impossable is only a little bit Harder... and ya know I do believe She was RIGHT ! "

[asra]

Hi Adrian, glad to oblige. Ken and Bob - excellent points - it all needs to be validated by testing, and everything else needs to be in harmony for the engine to work well. That's for sure.No sense in porting a head, grinding a cam, and piping the beast if the original carby is still bolted on!The exhaust theory is pretty simple, but getting the right numbers to plug in is a lot harder. If we take the EA81 as a basis, I'll have to guess the valve timing (found no info on the net so far), and make a guesstimate for the speed(s) of sound in the pipe.Lets's give it a 25-60 cam with symmetrical valve timing around TDC, and optimise for 4700 rpm. The exhaust therefore 'opens' 60 deg before BDC, but not that fast on a 4-stroke, and there will probably be a quietening ramp that gives slower initial opening - see diagram (NOT TO SCALE!)Image Insert: 7.7 KBSo we can assume that the main body of the positive pulse starts some time after the valve is cracked open - say 20 degrees after? This is where testing is important - a time-pressure history taken at the exhaust port is really useful to see what is happening. Problem is that electronic pressure sensors tend to be very sensitive to temperature....! [xx(]Next we need to estimate the speed of sound in the exhaust gas. This varies with temperature and gas composition. Typical values for a front pipe start at over 2000 ft/sec and can sag to 1400 ft/sec as the gas cools further down the pipe. I suggest that with the 20 degree allowance for valve opening we use a block value of 1800 ft/sec, which corresponds to air at 475 Centigrade.OK, let's use the assumed figures to estimate how long a pipe must be to deliver the negative pulse at TDC at 4700 RPMDelay needed in degrees is: 60 + 180 - 20 = 220 degrees Pipe length 'L' in inches = 1800 x 220 / 4700 = 84.2"Call it 84" and that'll be close enough given the estimations used.Now we need to go 2:1 and provide some mid range bmep. The front pipes are assumed to be the same internal area as the exhaust port, or a fraction larger if that is easier to make.The inside diameter of the collector pipe needs to be at least 2x the ID of the front pipes - this provides 4x the pipe area. We are providing a step up in area, as in my last post with the single cylinder.How long is this bigger pipe? The simple way is to run the numbers again for the engine, but this time (say) at 4000 rpm. L = 1800 x 220 / 4000 = 99"This is the overall length, so we subtract 84, and get 15" of collector pipe. The tricky bit is adding exhaust silencing. Much easier to silence 2-strokes in my experience - less weight is required to build the smaller boxes usually needed to absorb the higher frequencies of higher RPM and twice the power pulses.I do have a solution for silencing the exhaust - the road-legal one, but it is the sort of weight that is (just) acceptable on a roadable sports/racing car, but would probably overload a gyro. Needs more thought.The observant will spot that the collector pipe really needs to be longer than calculated beacause of the declining gas temperature.REMEMBER....everything else needs to be in harmony for the engine to work well. So check the compression and borrow an exhaust gas analyser to check your mixture - it should be richer than stoichiometric for full power - about 12:1 is good for most engines, and be sure to play with ignition timing to see what variations you get. We have seen some engines need a lot of advance under load.None of this will work worth a doodly squit if the carby has a tiny bore or the throttle is not opening fully. (Done that!)All the best, BenEdited to add these EA81 performance links: http://www.subarubrat.com/engine.htmhttp://www.subarubrat.com/airscooby.htmhttp://www.subarubrat.com/Retrofitti...0DGAV%2032.htmhttp://www.subarubrat.com/manifold.htmhttp://www.ausubaru.com/ausubaru/wik.../ENGINE:Hitachhttp://www.ausubaru.com/ausubaru/wik...p/ENGINE:WEBERLooks like the first port of call is that carburettor...... Ben

[asra]

WOW NEETO !!!! thank ya Ben ! moochas gracious muchacho ! and all that stuff ! fun numbers to play with indeed ! Thanks ! Bob...." Momm'a alwayse told me , Son the impossable is only a little bit Harder... and ya know I do believe She was RIGHT ! "