|09-08-2010 03:08 PM|
|09-08-2010 02:11 PM|
|09-08-2010 02:08 PM|
All the theoretical discussion going on here is fun, but I think this gets to the meat of the original question:
The general consensus among folks who seem to know is that installing the cam 4 degrees advanced from straight up is usually optimal for street applications, and - as mentioned - most street performance cams are ground this way. Most. And most timing sets are made so that installing dot-to-dot doesn't give any additional advance/retard. Most.
But then, the cam grind could be off a bit, or the timing set could have an advance or retard built in that you're unaware of. Or you could have an adjustable timing set and not be sure how to set it. This is why we "degree" cams when installing them. To make sure they're installed in the intended phase relative to the crankshaft.
FWIW - I've always heard that some of the blame for low-compression emissions engines being dogs in the 70's and 80's was because of retarded (at least relative to what we consider normal, i.e. 4 degrees advanced) cam timing.
In general - if all other aspects of the engine are supportive of the change - retarding the cam pushes the power curve farther up the rpm range (moving valuable power out of the lower rpms), while advancing the cam slides the power closer to the bottom end (due to closing the intake valve sooner, letting the engine build more cylinder pressure, i.e. torque).
|09-08-2010 01:43 PM|
Well we were talking about putting the reeds in the head next to each intake port on the head I think, thats the only way it would work I reckon. As far as them being an obstruction in the air flow, (compared to no reeds at all) that may be true, but at higher rpms when they are no longer needed, they would simply be open all the time so it would be a matter of having a big enough reed cage to flow the cfms needed for the hp desired. My 38 mm carb on my husky for example could theoretically flow a whole bunch, not sure exactly sure how much and at what vacuum but if each cylinder had a reed cage like a rm250 suzuki that could make 57 horsepower, that would be about 456 horses right? To flow more you could have a reed cage with 6 reeds in it per cylinder (my husky reed cage only has four reeds) and get more flow that way. There may be bigger reeds that would be better I dunno. Its all just kinda a new idea I guess! Anyways for sure at least it would let you run that long duration cam without the reversion at low rpms and still have the high rpm magic once the intake runner inertia kicked in at higher rpm. Kinda the best of both worlds I guess.
As far as handling a four inch piston, I am pretty sure the positive(reverse) intake pressure on a bigger piston is not that much more than the two stroke motor with no camshaft at all. If it is any credit to this, my husky motor has nearly a 3.5 inch piston. The reed valves they make for these motors are made out of some crazy amazing stuff, they can go for lots and lots of miles "flapping in the breeze" or whatever you wanna call it. The ones I took out of it when I upgraded to boysen dual stage reeds looked like the original ones from 1981 according to Andy at the husky shop I go to.
Between this reed stuff and the spanny chambers we are talking one crazy motor here... Crazy!!!!!
|09-08-2010 01:10 PM|
Flow rate of reed valves,----
I like the reed valve idea, but feel that locating them between manifold&head port would be more effectual, however, I do not know of any " reeds" that could control up to a 4in pistons airflow or pressure "needs"
|09-08-2010 12:33 PM|
|Adam65||Straight up timing is defined as when the lobe separation angle and the intake centerline angle are the same. Most racing cams are ground with no advance in them, so if they are installed "dot to dot", that is straight up: a 110 lobe separation cam will be installed at 110 intake centerline. Most aftermarket street cams are ground with 4 degrees advance. If you have a aftermarket street cam with 110 degrees lobe separation angle and put it in dot to dot, your intake centerline will be 106 degrees. You would have to put in a 4 degree bushing and retard it back to 110 degrees intake centerline to be straight up, even though dot to dot is what everyone thinks is automatically straight up timing. It is dependant on how the cam is ground.|
|09-08-2010 11:54 AM|
I really think cam of the future is no cam
Either solenoid actuated or pnematic ,a La formula 1.
I believe we will all see that n also very high comp ratios just as soon as they figure out how to alloy ceramics into metals,50:1 on gas engines probably.
|09-07-2010 08:57 PM|
|LATECH||Look at many of the new cars. A lot of them are using variable cam timing They can vary the timing for different operational conditions. A lot of it is to help make horsepower as well as help with keeping down emmisions.Older cars like my wifes camry had a centrifical advance on the cam drive pulley.You can feel it pulling at about 3500 rpm and up. (passing gear LOL)|
|09-07-2010 12:43 PM|
When an engine is running there is chaos inside the intake due to.
Intake dilution from exhaust,intake draw through to xhaust,flow reversal in intake,dry manifold as driver lets off throttle to go into corners,wet flow in manifold(small rivers of fuel).
I know how both cranckase charged and rootes scavanged 2 stroke work.
I just dont know how you would hold reeds inside a manifold so that they wouldnt create all kinds of other problems like interfering with the flow etc.
I think just about the only way to get rid of some of the chaos is to have forced induction and then has to be more than 10 psi or so.
Just got back home today.
|09-04-2010 04:40 PM|
Now that is what I am talking about!!! Yes that would be easy to do!! The reed valve cages I work with on the motocross bikes flow alot, and by alot, I mean the carb on my husky 430 is 38mm. It is actually small, the carb on a modern ktm 250 is about the same size but my bike is older and is designed for more tourque at a lower rpm. I still pray before riding it, and yes sir my dog still rides in the back. That equals 1.5 inches for us americans. We could put one on each intake port and run as much intake duration as you need for the power you are after yet still have good low end tourque, thats the whole point on the two strokers and it works just fine! Tech Inspector, you just made my day.
|09-04-2010 04:24 PM|
So to make the cam straight up then, like if I want to put a stock factory 262 cam in my 283 motor (which is what I think I am going to do becasuse it gives more power than the stock 283 cam on almost every range with stock two barrel manifold, the single hump heads, and 1.5 inch i.d. 32" length headers which is what I have to work with here) I want to degree the cam in so that the overlap of the intake and exhaust valves, that they are open the same amount I mean, at tdc? If so that is good to know. If not then I misunderstand.
Forgive me for my last post, but I am very confident that I could make expansion chambers that would work with a four stroke engine with the right camhaft timing. I realize that you may be unfamiliar with two smokers, but I am very sure that you can grasp the concept of the expansion chamber. The point is increasing the volumetric efficiency of a naturally aspirated engine far far above the swept volume during a narrow rpm range and with the least amount of moving parts possible. Just like forced induction, but with no parasitic drag or bearings to replace or lag etc. Again I appologize and realize this may be the wrong thread, or even wrong forum, to propose such ideas. I believe it would work, tho you would maybe need a cvt transmission of some kind to get the power to the ground and keep the engine at the right rpm.
FWIW, Although two stroke engines have no camshaft, they still have intake and exhaust events just like a four stroke, the overlap where the intake and exhaust are both open (and connected to each other via the combustion chamber) is used with the expansion chamber to create more power. During the opening of the exhaust port, the pulse of hot expanding gasses travelling into the megaphone exhaust chamber creates a vacuum which (scavenges?) draws out exhaust gasses but also fresh fuel air mix because of that overlap. Then the intake event ends and the intake port closes. The exhaust event still continues tho. There is a sharper angled reverse megaphone where the end of the megaphone would be that reflects that energy back into the engine, forcing that fuel air mix that it drew out previously back into the exhaust port before it closes. When it works, it works really really well. Scares you half to death going from 22 horse power at say 2500 rpm on a 250 cc bike motocross bike to like 57 horse power in the blink of an eye at 3000 rpm, only 500 rpms later, when the set of events lines up perfect. That is the best I can explain it on short notice. Plenty of good reading on it tho out there, the expansion chamber was invented around 1961 by a german dirty rotten stinking biker dude.
Expansion chambers were first designed by East German Walter KaadenWalter during the cold war. They first appeared in the west on Japanese motorcycles after an East German motorcycle race in the 1961 Swedish Grand Prix. He hid the blueprints under his racing leathers and defected during the race by riding off the track and claiming asylum. He did not finish the race. He later provided the blueprints to Japan's Suzuki....
|09-04-2010 04:01 PM|
im with u .....cell just gave up the ghost talkin to phone co,,
u brg up gud point n i wanna answr but tlkn with fone co.
|09-04-2010 03:35 PM|
I'll have to think about this some before I give a response. In the meantime, I'll give you something to think about that I have considered doing but never got around to.
One of the problems with a 4-cycle motor is the balancing act between closing the intake valve either too early or too late to take max advantage of the incoming column of air/fuel mixture. Close it too early and you lose power from a reduced charge entering the cylinder. Close it too late and you lose power because the ascending piston pushes some of it back up the intake tract and out the carb throat.
What if a guy cobbled up an intake manifold that used 2-cycle reed valves at the head ports? I mean, leave everything as is except just add the reed valves between the carb and head port. Seems to me that this would optimize the whole mess. What do you guys think?
Thinking about it further, you would want some pretty agressive intake timing on the cam so as not to limit the effectiveness of the reeds.
|09-04-2010 03:10 PM|
I knew somebody like Tek insp. would know it.
Phone got answered!
I dont know what you mean Drty B.??? whats the point?
@ tech inspector
I start my cam timing process at t.d.c. xhaust stroke and go from there
I think the most important event is the point at which"crossover" occurs,this is the point at which both valves are open the same amount,from the factory I find most cams when installed at factory specs this point occurs from 5 deg atdc to 5 deg btdc.
Please remember Im not drag racer,just round tracks.Well...... oval anyway!
|09-04-2010 02:49 PM|
You guys are very smart, and I would like an opinion on a crazy cam idea I have. If it is too crazy then I understand. ready? here goes: What if a cam is made that duplicates the intake and exhaust duration of a known good two stroke engine, like a cr500 honda motocross bike, only for a sbc, then 8 suitably sized expansion chambers are fitted to the engine.
Would these spanny chambers not have a similiar effect as on the two stroke engine?
That is to say, drawing the fuel/air mixture out of the cylinder while both valves are open then with the timed pulse of the expansion chamber pushing it back in at the last second before the exhaust port is closed, and because the intake valve closes earlier than the exhaust valve it could thereby increases the amount of fuel air mix in the cylinder than the piston actually could draw in.
Organic turbo boost.
I love two cycle motors sooo much because of this. The power comes on all at once at a certain rpm largely based on header pipe length but who cares.. It gets your adrenaline pumping and puts grey hairs in your beard and thats ok with me. Castor oil keeps you regular too.
I have been wanting to ask this question to somebody for about 15 years. I don't know any body who could even understand what I am talking about on this thing except maybe you guys.
A good expansion chamber design provides the equivelent of about 7 psi boost they say, it could be like a turbo with no moving parts if it worked... I realize two stroke engines are not the specialty of this forum but I wouldn't be surprised if you guys had already thought this very same thing before.
Im out there I know, sorry.
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