So, there's been a rash of 305 build up posts lately(maybe because no one searches)...
but I got to thinking...
if the 1.94 intake valves on 305 are too big and causes valve overlap, how does a longer duration cam come into play to lessen the effects of overlap?

i've tried reading some stuff, but i can't reach an understanding of how/why this works...

thanks.

 

It's not overlap, but shrouding. This is an ongoing debate regarding putting 1.94 valves on a 305. The problem is in the 305 heads, the edge of a 1.94 valve is so close to the wall of the chamber, that it will effect flow coming out of the valve. Some say that it is such a problem that putting a 1.94 valve on a 305 is a waste of time and money, while others say that regardless, the 1.94 will still flow better than the stock valve.

Overlap is a cam profile parameter, it is the amount of time (in crankshaft rotation degrees) that both the exhaust and intake valves are open. It happens at the end of the exhaust stroke and beginning of the intake stroke. Overlap is a good thing! The amount of overlap you want depends a lot on your intake and exhaust systems, how much power you want to make and at what RPM. Long overlap is for race engines, low overlap is for mom and pop daily drivers. Long overlap is usually associated with cams that provide high lift and long duration, for max power at high RPMs. Overlap can also be adjusted by different cam LSA angles.

Regarding the 305 heads, I wonder is anyone has tried to put in a 1.6 exhaust valve, keeping the stock intake valve? Would that be shrouded as well? Anyone know??

 

Overlap has nothing to do with valve size. It's the time when both valves are open.

As the exhaust valve is closing, the intake valve is opening.
See the area in the middle where the two lines intersect? That little "triangle" under that point is the overlap.



Increasing duration (the 'amount of time' each valve is open) while leaving the center of the lobes in the same spot causes the overlap to become larger (makes the triangle bigger). The only way to fix this is to spread the centers of the lobes (the two hills in the picture) farther apart, therefore decreasing the overlap.

For example... A cam with 250* duration and 110* Lobe Seperation Angle (AKA "LSA") will have 30* degrees of overlap.

Increasing the duration to 280* and keeping the LSA at 110* will result in 60* overlap.

The only way to keep the overlap to 30* is to spread the LSA to 125*. That's A LOT!!! And that defeats the purpose of having a cam with a lot of duration anyway, so it isn't normally done (or not that I've ever seen).

That's what overlap and duration are.

If you really want to investigate what it all means go buy a book by David Vizard about Valvetrains. You'll learn more than you ever thought you'd want to know.

 

my bad, wrong syntax...

so....

we want more 'duration' if we have 1.94 intake valves to be able to stuff more air in, during the time that it should be getting shrouded....right o.

 

To sum it up in one word. No.

Shrouding is interference. Because the valve is so close to the cylinder wall, there isn't a lot of room for the air/fuel mixture to get by. SOOO... you're actually flowing LESS air than a 1.84" valve would. (Please don't turn this into a "I put 1.94" valves in my 305 and I got more HP". This is physics, not engineering)

A bigger valve is not always good. You have to do other things when using a bigger valve to make up for the additional shrouding. If done right, then 'yes', there will be an increase in flow.

Here's a link to something I think you'll find useful about cams, overlap, etc. It's pretty involved, but if you read it a few times you should grasp it.

http://www.iskycams.com/degreeing.html

 

Many assume that a 305 head cannot be improved with a larger 1.94" intake valve. Cause someone else said so.

The flow bench and dyno say different.

The so called shrouded area is a relitively low flow area . Most of the flow is concentrated
towards the plug as it enters the chamber. Lots of room there.

Yes, a head (any head) will flow a little less on a 305"s small bore than it will on a larger 350 sized bore.
But the modified 305 head (with larger valve) will still flow more than a stock 305 head on a 305.
Don't believe every thing ya read....

The yellow arrow shows the high flow path .
The green area is the spot that needs deshrouding.
No-where near the bore wall. Minor grinding at the purple arrow helps too.

 

ditto that

 

Damn.

What part of this...
didn't you understand?

That wasn't my point. I just used common numbers of valve sizes so the guy would understand that shrouding and overlap are not the same!

You can't explain something in Greek to someone that doesn't speak it.

*in announcer's voice* Let the pissing contest begin!!! *end announcer's voice*