Camshaft lobe timing

[mu38&Bg#]

If the engine will never make more torque than to overcome it's own friction, it doesn't need a large carb. Glow engines do use muffler pressure in the tank, but for proper function a good venturi signal is essential. This is true of any carb throttle.

Greg

 

[Till]

Till: Also, I figured that the displacement for each cylinder was roughly 3.1 cc. Is using a carburetor designed for 2.5 - 3cc engines logical for each cylinder of my engine, or would it being a four stroke make it completely different?

Well I will start one last try to convince you of the great benefits of a single caburetor design. But to add further information, I will try answer your question first



The carb size is not linked directly to total engine or cylinder displacement. The size of the carb is determined by the amount of air-fuel-mixture needed. (Thus, four-strokes need smaller carbs than two-strokes of equivalent size and rpm).
Small carbs are good for small air requirements, but they are restrictive to higher amounts of air. With small amounts of air, a big carb will not be able to vaporize fuel into the air properly, so you can't archieve smooth running at low engine rpm, when air requirements a very small..
So a small, high-revving engine with bad idle requires a similar carb as a big, low-revving engine does for sufficient performance.


→ Long stroke engines with camshaft timings focused on lower revs with a high amount of low end torque require smaller carbs than short stroke engines with wild cams do.


I hope you now understand why you can't pick the right caburetor by simply taking one from an engine with similar displacement. You have to choose one that fits your engine's individual charateristics. Small changes on the carb setup cause huge differences.


So I strongly recommend one single carb for all eight cylinders in the beginning.
Only one carb to exchange,
only one to adjust.
And the best point: No need to syncronize.


If you still want to go for eight carbs in the beginning, a good idea is to build a singlecylinder engine with 1/8 of displacement first so you can test different carb designs and sizes on that single cylinder to archive smooth run at idle and desired maximum poweroutput (could be tested easily by using a propeller and mesuring max rpm). Then use eight of these carbs on the eight cylinder engine.


Ok, one last try: A single carb for all eight cylinders will produce a deeper and more vibrant resonance of the induction sound because of the combination of bigger induction manifold volume and longer runners.

 

[JohnnyGTR34]

Well I will start one last try to convince you of the great benefits of a single caburetor design. But to add further information, I will try answer your question first



The carb size is not linked directly to total engine or cylinder displacement. The size of the carb is determined by the amount of air-fuel-mixture needed. (Thus, four-strokes need smaller carbs than two-strokes of equivalent size and rpm).
Small carbs are good for small air requirements, but they are restrictive to higher amounts of air. With small amounts of air, a big carb will not be able to vaporize fuel into the air properly, so you can't archieve smooth running at low engine rpm, when air requirements a very small..
So a small, high-revving engine with bad idle requires a similar carb as a big, low-revving engine does for sufficient performance.


→ Long stroke engines with camshaft timings focused on lower revs with a high amount of low end torque require smaller carbs than short stroke engines with wild cams do.


I hope you now understand why you can't pick the right caburetor by simply taking one from an engine with similar displacement. You have to choose one that fits your engine's individual charateristics. Small changes on the carb setup cause huge differences.


So I strongly recommend one single carb for all eight cylinders in the beginning.
Only one carb to exchange,
only one to adjust.
And the best point: No need to syncronize.


If you still want to go for eight carbs in the beginning, a good idea is to build a singlecylinder engine with 1/8 of displacement first so you can test different carb designs and sizes on that single cylinder to archive smooth run at idle and desired maximum poweroutput (could be tested easily by using a propeller and mesuring max rpm). Then use eight of these carbs on the eight cylinder engine.


Ok, one last try: A single carb for all eight cylinders will produce a deeper and more vibrant resonance of the induction sound because of the combination of bigger induction manifold volume and longer runners.

Yeah, I see your point! I have already begun designing a single carb intake manifold for my engine that will be 3D printed with the help of a friend of mine. Should I have the inlet of the manifold be centered over the block or can it be towards the front?

 

[Till]

Suggestion for a layout:
carb-chamber-runners


Connect all intake-port runners to one central chamber, make sure all runners have the same lenght. Make sure the chamber has at least the volume of all runners combined.


This way you can assume that the partial pressure in the chamber is the same everywhere in the whole chamber, so each runner inducts the same amount of air+fuel mixture (very good!)


The air will oscillate in the runners, so at a small bandwith of engine rpm, this provides a dynamic supercharging effect.


For a low rpm high torque concept, choose long runners with a small cross section. This will greatly improve carburetion at idle speed, too.
For a high rpm high output power concept, choose short runners with a bigger cross section.


Hint: If the chamber is connected to the runners via flexible rubber tubing, you can figure out the perfect runner-lenght by exchanging with tubes of different length (all eight tubes must be of the same length every time, of course). Measure propeller speed at different throttle settings (1/4...½... fully open) to estimate the best solution.


With rapid prototyping, manufacturing „good“ runners is an easy task. So make sure the junctions of the chamber with the runners have smoothly rounded transition and that the runners are slightly tapered towards the inlet ports.


For a more aggressive throttle response, the chamber could be made smaller, but this makes the equal distribution of air-fuel mixture to each cylinder much more difficult to archive (so don't do this on your first modelengine).

 

[JohnnyGTR34]

Suggestion for a layout:
carb-chamber-runners


Connect all intake-port runners to one central chamber, make sure all runners have the same lenght. Make sure the chamber has at least the volume of all runners combined.


This way you can assume that the partial pressure in the chamber is the same everywhere in the whole chamber, so each runner inducts the same amount of air+fuel mixture (very good!)


The air will oscillate in the runners, so at a small bandwith of engine rpm, this provides a dynamic supercharging effect.


For a low rpm high torque concept, choose long runners with a small cross section. This will greatly improve carburetion at idle speed, too.
For a high rpm high output power concept, choose short runners with a bigger cross section.


Hint: If the chamber is connected to the runners via flexible rubber tubing, you can figure out the perfect runner-lenght by exchanging with tubes of different length (all eight tubes must be of the same length every time, of course). Measure propeller speed at different throttle settings (1/4...½... fully open) to estimate the best solution.


With rapid prototyping, manufacturing „good“ runners is an easy task. So make sure the junctions of the chamber with the runners have smoothly rounded transition and that the runners are slightly tapered towards the inlet ports.


For a more aggressive throttle response, the chamber could be made smaller, but this makes the equal distribution of air-fuel mixture to each cylinder much more difficult to archive (so don't do this on your first modelengine).

Thanks, Till! I was wondering how equal distribution of fuel would work without the proper manifold. So, if I have a 7/32" intake port diameter, is that sufficient? I would like it to have a deeper, low idle sound.

I figured out the timing for each camshaft, thanks for your help you guys!

 

[canadianhorsepower]

Hint: If the chamber is connected to the runners via flexible rubber tubing, you can figure out the perfect runner-lenght by exchanging with tubes of different length (all eight tubes must be of the same length every time, of course). Measure propeller speed at different throttle settings (1/4...½... fully open) to estimate the best solution

I think this is a truck load of work for nothing specialy on a model engine.
In theory it's sound great but in reality it's BS for all of this to be 100% effective all cylinder must be identical.:fan:
Exactly same picton,rod and crank weight( different weight=different acceleration=not same carb set up) this is also real foe valve leeking,
intake, exaust and combustion chamber scavengingthat must be identical for that to work.

In Nascar with $80,000. motor and all the equipement they have they can't do it. What they are doing after all the theory is assemble and running they
are using a special tool calculating a v belt flexing that is equal to piston acceleration and from the result they change valve lash to equal evething.

It's a known fact that nascar motor camshaft have more duration on all the four cylinder at the 4 corner then the middle one

 

[mu38&Bg#]

Tuned intake runners on model engines are fairy tales. The lengths required are not significantly different than full scale.

Luc, that's due to the types of manifolds used on Nascar engines.

 

[canadianhorsepower]

Tuned intake runners on model engines are fairy tales. The lengths required are not significantly different than full scale.

Luc, that's due to the types of manifolds used on Nascar engines.

Yes I know but I guess "faity tailes" is a better wording for model engine

 

[JohnnyGTR34]

Hey guys! It's been a long time. Thanks for your support, I figured everything out! Right now it's a twin turbo SOHC mini V8, the ball bearing turbos from turbokeychains.com. (I know it won't create any meaningful boost, but it's just so cool)

 

[gbritnell]

Johnny,
You have been given some useful information from all the respondents. I have been building miniature engines for a long time and still don't have all the answers. Each engine has it's own characteristics and therefore it's own fuel/air needs. Your last posting about putting toy turbos on and engine is upsetting to me because the people on this board were trying honestly to help you with a problem and you more or less throw all that aside. All I can tell you is when you get your engine finished to the point of putting some type of fuel system on it you think that trying to adapt and tune a carb to it is tough then adding supercharging of any type will be a nightmare.
gbritnell

 

[JohnnyGTR34]

Well this thread is about camshaft lobe timing and I can say that with the help of the kind individuals on here, I figured it out. So I'm not throwing it completely aside, but I have taken into consideration all of the carburetor and air intake suggestions that everyone has provided and modified the engine as such. So I am very sorry you are disappointed but I am extremely greatful that I was able to receive help from you folks. As for the turbos, They are up in the air at the moment. The turbos are cast and include a twin shield chromium steel ball bearing for the turbine shaft, and a "race" model is available for custom order (about 75$) that includes multiple strengthening and performance enhancements (like a stronger compressor blade), so I wouldn't exactly call these toys, but I have the plans modeled up so that I can remove them at any time.

Anyways, thanks for taking your time to comment on this thread and voice your opinion

 

[John Rus]

Doesn't a turbo charger or super charger need a lower compresion ratio to run properly? Other wise you might get into of worms!

John.

 

[Septic]

I hope that you wont take this comment as any sort of personal attack, or insult, as that is certainly not my intention and I can understand to an extent what you're trying to do because I too have been that way inclined (even if it was 30 years ago)

It's important to push boundaries and explore limits (especially our own) but its just as important to mount our engines securely before firing them up next to the wife's china cabinet.....

I am slightly concerned that, as with a huge number of enthusiastic, talented and intelligent people, you may have been sucked into devoting your efforts and energies toward an over ambitious project that will leave you disappointed and disheartened... Especially where the project itself will consume a huge amount of time and effort on your part.

Your design focus, from what I can gather seems to be centred almost exclusively on techniques used primarily in the full sized world to increase performance e.g. V-8, multi-carb, turbo, etc ideas which, as I'm sure many people will agree, don't necessarily react as you might expect to being scaled down to the sizes we're talking about without using exotic materials, special fuels, incredible levels of machining accuracy and certainly not novelty turbochargers that would most likely stop the engine working efficiently enough to do anything but build up enormous levels of heat, destroying themselves and possibly damaging other components...

If my assessment is wrong, or if I've misread/understood your thread you have my sincere apologies, but otherwise, I'd encourage you to either concentrate on a far simpler V-8 if (it must be a V-8) or better still, focus your obvious enthusiasm for performance on creating a highly tuned single, or twin which would offer you a far greater sense of accomplishment and leave you with a much greater understanding of how induction, valve timing and fuelling work, so you can then go on to design powerful, multi cylinder power plants successfully.

 

[stevehuckss396]

Doesn't a turbo charger or super charger need a lower compresion ratio to run properly? Other wise you might get into of worms!

John.

Not really. The lower the compression ratio is the more boost you can have without excessive cylinder pressure. That's the short version but there are advantages to low compression and high boost.

 

[John Rus]

Whoops! I meant " a can of worms!".

Thanks, now that you mention it I do recall somthing about still being able to use higher compresion ratios. Thanks for the clarafication.

John.

 

[JohnnyGTR34]

Thank you septic. I do agree with what you have advised me to do. I haven't started building it, it's still in the design phase and I do agree that a single cylinder would be a great deal easier. This project unfortunately has been put to the side though because I am working on my car. I hope to resume it soon!

 

[JohnnyGTR34]

Hello all, after a long break from this forum I am briefly returning. Unfortunately, I don't think this engine will ever make it to a physical model. Ive been extremely busy with my car, robotics, and have development. If I ever find time in the future, I'll pick up this project again but in the mean time I am too busy. School is keeping me very busy as well. I would like to thank all of you for your experienced advice and good intentions, and I would also like to apologize for immaturity on my part with sticking on fake turbos to my cadd model, which I quickly realized would most likely be a disaster. Anyway, thanks a lot to everyone.