Cam Timing
 

Hey Guys,

I don't really post often, so forgive my lack of forum etiquette.

I'm trying to design a header and exhaust system for a stock SVX motor but I need either a plot of the OEM Cam profiles, or specifically I need to know first of all the number of degrees before BDC (bottom dead center) that the exhaust valve begins to open.

Please forgive me, I'm kind of new to this, but I'm learning a lot in a very short time, but from an engineering perspective.

-Matt

good luck.

Exhaust valve opens at 55*BBDC closes 9*ATDC

Sounds like an interesting project.:)

Harvey.

What are and how do these timing's effect the design. Sorry to appear dumb but would like to understand how you arrive at a good design.
Tony

Careful Tony, the flood gates will open and the band wagon will roll again with gusto. :) Honesty the subject requires a textbook and certainly not an opinion. What is more there no single good design.

The length of time the inlet and exhaust valves are open during a combustion cycle, is a major deciding factor relative to the charge available for combustion. All related design becomes a compromise based on specific requirements and the practical criteria involved.

Yup...what he said.

You can design a 'perfect' engine for a narrow power band, but there will always be compromises elsewhere. Best to allow for a broader powerband overall, and 'design' accordingly.

Tony we have been there, done that.:)
This was your thread.;
http://www.subaru-svx.net/forum/show...hlight=exhaust

Harvey.

Tony will be exactly aware that his thread as quoted above, covered exhaust systems and not valve timing. :rolleyes:

K, well from an engineering point of view and explained simply and quickly, there are two major components of your exhaust, the exhaust flow, and the pressure "WAVE".

You want to size headers and exhaust big enough that you have as little restriction as possible, but you still want to keep a minimum exhaust flow rate.

2nd part, and what most people forget, (including performance shops here in winnipeg which I have sadly come to realise) is a pressure wave that begins right after the exhaust valve opens, when this pressure wave reaches either your collector, or the atmosphere it changes form, part of it will leave the pipe as a positive pressure, part of it will come back as a negative pressure.

Now "tuning" your exhaust properly which is impecably done on the F1 formula cars, this pressure wave will return to the exhaust valve just before it closes, sucking or "scavenging" the last little bit of exhaust out of the cylinder AND providing a negative pressure in the chamber to suck new charge into the cylinder.

Now for this test run, until I can find some way dyno testing my theories, I will be tuning to around 5000rpm, because this is approximately half way between peak torque and peak HP.

I will try and keep people updated as things progress, but I'm not much of a forum junkie, so please don't expect prompt replies or updates :cool:

SVXfreq I am interested to read your comments. Do you mean the negitive pressure is effecting the cylinder that the gas just left.
Tony

Yes, the wave travels to the back of the pipe and back again at the speed of sound, about 250ft/sec. When it first travels away from the valve it is a positive pressure wave, when it enters the collector it reverses and part of it returns as a negative wave, now IDEALLY in a perfect world you could tune a car for a certan RPM so that the wave not only returns to the valve right before it is closing, creating negative pressure outside the exhaust valve, sucking whatever exhaust out, but you could also tune it in such a way that another negative wave will return to the exhaust valve just as it is opening, creating a negative pressure outside the exhaust valve again, making it easier for the chamber to expell the gases.

All headers will do this, but for example, equal length headers will do this at some specific RPM, this RPM will be decided by the length and diameter of the primary header pipe.

The problem with this is that you can have the negative effect happen at a certain RPM, where the wave travels back and forth twice and returns to the valve as a positive pressure just before closing, this is what makes exhaust tuning extremely difficult, to tune this properly to the ideal conditions I would need an all wheel drive dyno of course, which I just do not have access to, which is why I am doing all the theoretical calculations possible to get an answer that is hopefully close enough to make some extra power out of that engine.

svxfreq I made a suggestion a whiel back to use the back pressure in the exhaust. Didn't get much feed back would be interested to get your comments.
http://www.subaru-svx.net/forum/atta...0&d=1193656116

Tony

Well I'm not quite sure what you're trying to accomplish with the primary, secondary, and vacuum pipes, but venturi's can be used in an exhaust system to accelerate the exhaust, problem with that is you REALLY start dipping into a fine tuned area of exhaust design, and if it's not used properly you can end up with nothing but another restriction in your exhaust.

If you could explain better, or find the old thread and I will read through it and try and give you my input.

I think I can bring this down to bite size for the less engineering minded among us.

When svxfreq asked for cam timing specs to build headers I knew he was on top of things. (not to say the others who build them aren't:))

By tuning the length of the pipes before they merge (primary tubes) you can control the pressure inside the tubes.
Think of the waves mentioned as ripples in a pond. When the ripples from one cylinder meet the ones from another, they cancel each other out. Time them just right, and you can use one wave to suck the exhaust from another cylinder.
Remember, an engine is an air pump, more air through, more power made.
This also means, that the collector (where the primaries merge), must be sized just right too. Get it right, and big power can be made fairly cheaply. Get it wrong, and it'll sound great, but not add much HP.
Yep, it's simplified, but accurate enough for those trying to understand.:lol:

In my experience, well tuned headers are quieter than ones that aren't. That may help get you to that point svxfreq.

Ya that's more or less what happens, I'm hoping I will be able to get a fairly good estimate from theoretical equations that I won't have to dyno tune them, and if I get good results from this, I may even consider producing more if people are interested. I'm also going to look into ceramic coating them, I just wish I could find a dyno I could use to actually see how much if any power gains there are.