Lobe Seperation

with a BBC and Super charger, was Effect does increasing or decreasing lobe seperation have on power?

Say 114 to 115 or 116?

 

It will increase power but may start to cause valve control issues.

 

http://www.compcams.com/technical/FAQ/LSAproperties.asp

 

Valve Control Issues ?

Please go a little farther and explain your thoughts. May be we have been overlooking something.

Thank You Noel

 

Yes, please explain.

 

Um. I think someone hacked into my screen name on this one. That is way over my head, I have no idea on this. I did find the tables in the comp cams link interesting though.

 

As you Widen the lobe seperation, say going from the 114 to the 116 it changes your valve events and will also decrease the amount of overlap that you have. Changing the valve events is not really an issue because you can advance or retard your camshaft to put the valve events where they you think they should be with your specific combination. Bascially you can run the same valve events with more or less overlap. I have seen people run all sorts of different camshaft with all sorts of different blower combinations and basically it comes down to your specific combination and how much duration and overlap you actually need (or think you need). The one thing that is certain, as that the wider LSA idles nicer due to less overlap. Some of the turbo stuff and some of the larger NOS engines go out to 118ish. Ever notice how a nitrous pro mod idles so nice and smooth?

 

Valve Events

I agree.
The exhaust valve opening event must be carefully looked at in any application. This event determines the combustion chamber heat retention. The longer we hold the exhaust waist ,before discharge, the higher the chamber temp will be during the next cycle. Opening the valve early will result in lower chamber retained heat (Less time to distribute heat).

The exhaust duration number is less important to look at than the "opening event". Exhaust valve lift is far less important than we may think. The cylinder pressure is so high on the exhaust side that as soon as we open that valve the discharge of waste is rapid, But the ability of the exhaust port to flow air is very important in the overall picture.

"Alcohol likes pressure, but will resist heat." With that said, as the overall tuneup combination becomes more suffocated, burn rate is increased, and the time to generated heat is decreased because of the increased efficiency of the combination,the "lobe centers line" requirement will become wider. Or the events must be altered by an increase or decrease in time (duration), to accomplish the required result.

This goes way past making a camshaft selection from a catalog. You need to have a working relationship with the tech department of a camshaft manufacturing company.

And there is far more to a camshaft than just lift, duration, and lobe centers, far more.

If i can help please PM me, or call 951 674-2212

Noel Manton
www.mantonrockerarms.com

 

Nice explanation, since you are talking cylinder pressure and exhaust valve opening in a blown alky combination, is it possible if you open the exhaust valve early enough to have too much cylinder pressure that the exhaust valve will resist opening (bending, flexing, touching things, tearing up misc exhaust valvetrain parts)? I also like to open the exhaust valve on the early side, by my reasoning is more that mechanically there is not a whole lot of 'work' done on the piston past the 90* mark, but I do like your point about the time for the heat to absorb into the chamber, something that I have not previously thought about.