Camshaft lobe timing

[JohnnyGTR34]

Hello!

I am new to this forum, and I have a question. I am currently designing a SOHC 16-valve miniature V8 engine that I plan to build after I finish the CAD model. There is one major problem that I am running into, and that is how to position the camshaft lobes for proper timing. As of now the crank is cross-plane and I have no preference for direction of rotation (If there is a specific counterclockwise or clockwise direction, I'd like to know which is best) I plan on having it run on Nitro, so 8 glow plugs will be used in the favor of simplicity.

Basically what I would like to know is where can I find a chart or some reference for camshaft lobe positions? The camshafts rotate the same direction as the crank, also.

Here is a photo of the current design if anyone is interested. Thank you for having a look!

 

[stevehuckss396]

This was just covered in a thread about 3 weeks ago. I'll see if I can find it

 

[JohnnyGTR34]

Oh, was it? Thank you! I can delete this thread if necessary

 

[Dousi]

cam/valve timing

I've asked this question, this is the thread.

 

[canadianhorsepower]

try this

http://www.tildentechnologies.com/Cams/CamPerformance.html

 

[dman]

http://www.popularhotrodding.com/tech/0607phr_camshaft_basics/ this is for the specs.

as for what lobe goes where, you gotta come up with a firing order based on your crank design first. there can be 16 different working combinations for any v8. the most common is counter clockwise viewing the crank from the rear of the engine with the cylinders numbered evens on the right bank, odds on the left, again this is viewed from the back the most common order is 1-8-4-3-6-5-7-2 but 1-8-7-2-6-5-4-3 is gaining popularity on modern engines and happens to be the same as one of the two ford firing orders but ford numbers the cylinders wrong so it ends up looking different.... and the third popular firing order is 1-8-7-3-6-5-4-2

1-8-7-2-6-5-4-3 is said to have more even bearing loading and now there is some rumors that it has better thermal characteristics reducing hot spots.

1-8-7-3-6-5-4-2 is said to make the most horsepower. probably having to do with crank harmonics and intake interference. but 1-8-7-2-6-5-4-3 is also becoming a popular firing order for performance engines.

if you designed your cross plane crank the same as a conventional american v-8 one of these will work for you.

 

[Till]

in the favor of simplicity.

If you got questions on cam lobes, syncronizing eight carburetors will be a giant challenge for you. If one of your eight carburetors runs on too lean mixture, the piston will get hot, expand, seize and destroy half of your engine including the crankshaft.
In favor of simplicity, i strongly recommend to use only one single caburetor with a 8-way split manifold in the beginning. You can upgrade later. This way you can easily change different commercially available carburetors until your engine runs fine, which rules out the mayor source of problems (carburetor). Then the next step is to adapt to a two carburetor design feeeding a cylinderbank each and so on... depending on your patience... however, using an existing cam design from commercially available plans will be the easiest solution for the question in post 1.

 

[stevehuckss396]

Here it is. The numbers below are lobe centers.

Your cam would have #1 exhaust at zero degrees and intake would lag 110 degrees so it would be at 250 degrees. #8 being next would lag by 45 degrees so 315 degrees BUT the cylinder bank is 90 degrees back so the lobe for any even cylinder needs to lag by an extra 90 degrees. #8 would be Zero - 45 - 90 so it is at 225 degrees and its intake lags by 110 so 115 degrees. Draw up a degree wheel with the segments numbered clockwise and start laying it out.


Cylinder# / Intake degree / Exhaust degrees

1 / zero / 250
8 / 225 / 115
4 / 180 / 70
3 / 225 / 115
6 / 90 / 340
5 / 135/ 25
7 / 90 / 340
2 / 315 / 205

 

[JohnnyGTR34]

I agree, but I may find it more challenging with the resources available to make an effective 8-way split manifold. It's still in the development phase however, so anything can change so I may redo the intake setup.

To steve: Thanks! This should help greatly.

 

[JohnnyGTR34]

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?

 

[dman]

looking at your rendering, it appears you have direct valve actuation, no rocker arms. this may be ok but the point of rockers is to control the forces of the rotating cam trying to bend the valve stem. in full scale direct actuation works but only if the valve springs sit in a bore with a cup over them guided in the bore. the easiest way to do this is the way toyota, gm and fiat did it on certain engines which is to make the head in several pieces. a head that has the cooling passages and valves and guides, then on top of that a cam box that has a bore for each valve and a cam bore above the spring bores then either bearing caps to hold the cam in or a valve cover with the cam bore machined into it. if the cam boxes are cast aluminum and the cam is chrome plated or made from a alloy steel with good corrosion resistance and precision ground you may be able to use the aluminum as the bearing surface like most manufacturers do today.

notice that bores for the valve cups in the pic below? but you can still see under it to the springs. that's so oil doesn't get trapped in the bore and lock up the valve, it needs to drain so things move and you dont leak oil past the guides. but without casting the head it will be difficult to make this feature in one piece. which is why you want to make it in two pieces.

https://encrypted-tbn0.gstatic.com/...Ge4sLTJZaD5gNbFIZQms7988yIZhYxM42W9Iu9T3zuZWS

if you dont want to use cups and shims to set the lash you want to add a rocker and adjuster. you can move your cam bore over a little and add a pivot. the ford mod motor design would be easiest to adapt you blueprints for. you could add adjusters to either the pivot or the valve end.

http://image.hotrod.com/f/techartic...6_02_o+valve_arrangement+on_ford_pl_heads.jpg

 

[JohnnyGTR34]

Another question, What would you recommend for lift distance? I'm not sure what is sufficient and what is too much. I'm trying to design the lobe but I don't want to design it wrong so that it may break the valve stems. Thanks for the reply dman! I'll have to look into this more.

 

[dman]

Another question, What would you recommend for lift distance? I'm not sure what is sufficient and what is too much. I'm trying to design the lobe but I don't want to design it wrong so that it may break the valve stems. Thanks for the reply dman! I'll have to look into this more.

the flow area equals the valve area at just under 25% the valve diameter but you can go a bit more for better CD (coefficient of discharge) and more area under the curve. so 25%-30% the intake valve diameter should be good and you can make the exhaust as big as the intake even though the valve is smaller, infact the exhaust lobe is often a bit bigger than the intake for more duration and area under the curve since the exhaust timing is less critical to velocity and pressure that helps make horsepower. under 25% valve diameter and you are under using the valves, 35% intake valve diameter is about as much as you will ever see.
the exhaust valve diameter is usually 75-80% the intake valve diameter.

 

[JohnnyGTR34]

Great, sounds good. Compared to the .625 bore, how large should the valves be? As of now I have them set at .28 (about the diameter of a #5 countersunk flathead machine screw, what I will be using for the valve stem with a few modifications), is this sufficient?

 

[dman]

Great, sounds good. Compared to the .625 bore, how large should the valves be? As of now I have them set at .28 (about the diameter of a #5 countersunk flathead machine screw, what I will be using for the valve stem with a few modifications), is this sufficient?

sounds about right. maybe you could make the intake a tad bigger but not much. basically if the valve flow charicteristics were perfect you would need an area around the valve that was 41% larger than the valve (less actually because the seat is smaller than the valve). but that's just theoretical based on cross section and many other things get in the way but it's a place to start. once the valves are large enough that you have less than that much extra room around them the flow is less efficient but the actual flow still increases with increased valve diameter to a point. just not at a rate proportional to the increase in valve area. the number you have puts you pretty close to the ideal size. and it's just a model. you're not engineering a race engine. it's certainly going to run with a valve that size....

 

[mu38&Bg#]

Have you designed the glow plug hole yet? Might be better to make room for it before working on valves.

The 2.5CC carb will be far too large for a 3.1CC four stroke cylinder. I ran a 2.5CC Rossi slide carb on a 15CC single cylinder four stroke (OS FS-91S) and it made full power.If the engine will be run without load, even smaller will be better. Carbs are really sized for the amount of power the engine makes(airflow). Putting a carb from a 0.5-1.5HP engine (most 2.5CC glow two stroke car engines) onto a cylinder that will make maybe 0.1-0.2HP, will result trouble in tuning and fuel draw.

Greg

 

[JohnnyGTR34]

Hmm, I guess I never really thought of that. Will it run off a single 2.5cc carb if the engine is 25 cc? (roughly).

dman: I agree. Thanks for your input guys! it really helps.

 

[JohnnyGTR34]

Have you designed the glow plug hole yet? Might be better to make room for it before working on valves.

And to answer your question, yes I have! It will accept a standard 1/4-32 glow plug. Thanks for the support you guys! I'm learning quite a lot that I did not know about IC engines ;D

 

[mu38&Bg#]

Hmm, I guess I never really thought of that. Will it run off a single 2.5cc carb if the engine is 25 cc? (roughly).

I would say a single carb of that size is enough for display running.

Will you be using bucket cam followers?

Greg

 

[dman]

Hmm, I guess I never really thought of that. Will it run off a single 2.5cc carb if the engine is 25 cc? (roughly).

dman: I agree. Thanks for your input guys! it really helps.

if you have a common plenum the air demands are much smaller, individual cabs need to be big to supply the peak air demand but the air is only flowing about 180deg out of 720deg rotation. with a common plenum and 4 or more cylinders the air demand is more constant so a 2.5cc 4 stroke carb could in theory power a 10cc 4cyl engine to similar speeds. also the rpm range may be different and the effective displacement of a 4-stroke is half that of a 2-stroke since it take 2 revolutions. that and glow engines have pressurized fuel tanks so they dont need velocity in the carb as much... it all adds up.