Valve overlap

[302ford]

I'd like to learn more about what happens during the valve overlap period. Like how do you determine the correct degrees of overlap. Also what things happen during the overlap period. I know David Vizard says overlap is most important event with lobe center angle being second. I know others based on there experience have different options on what events are important. So I was hoping we could have a discussion on this
Randy

[novadude]

To add to the topic, how does a restrictive exhaust (think stock manifolds) affect overlap selection and timing of IVO and EVC?

[mag2555]

Let's first discuss what your power goals are for your 302 so we can relate info about Cams / valve timing/ air flow needs better!

[RevTheory]

I believe you've got it backwards; LSA first, overlap based on performance requirements second. And all of his stuff is based on an unrestricted intake and exhaust. If either of those are restricted, it pretty-much goes out the window.

[Ron E]

Direct to your question, think in terms of pressures when both valves are open. More exhaust back-pressure will cause reverse flow to occur sooner. That is the "why". For fixing it....The cam theorists can take it from here.

[groberts101]

Very complex subject and is exactly why so many people get it completely wrong or at least partially wrong for their specific application. I'd highly suggest using google to get your head wrapped around the basic players involved here then come back for specific clarifications to the newer more refined questions you'll surely have on the matter. A local search around here will also bring up cam timing and overlap discussions.

An example of what you will learn is that IVC is responsible for the amount of dynamic compression ratio the engine develops(trapped combustion volume), but that optimum number changes with CID/valve size(port flow and its relative cross sectional efficiency) and rpm levels. Changing EVO can adjust blowdown pressures to "hit the pipe" according to exhaust spec's and pipe size/velocity profiles. Generally needs to be earlier to allow more time for poor exhausts.. or higher rpm capability(needing more time to get the larger volume mass evacuated while having less chance of intake reversion at higher rev's). Assuming an extremely efficient exhaust system, an earlier EVO can also help "pick up" too big an intake port/valve in lower lift ranges. OTOH, assuming the exhaust system is better than it actually is will lead to increased reversion on same engine when EVO is advanced too much. Obviously way more to it than that, just a quick dumbed down simplistic version of a couple of the parameters involved with cam design.

Engine size, operating range, and peak rpm matters greatly too but the cylinder heads/induction tract are THE biggest factors for determining what amount of mass and when pressure differentials actually occur. Figure those numbers out and you will be better able to pick a camshaft that controls the mass relative to the pistons movement.

[piston guy]

Which head you use on a Ford makes all the difference in the world. A Boss 302 head likes overlap , an E7TE (5.0) doesn't like it at all.

[mag2555]

The massively over size Boss heads on a 302 need the overlap to get the Intake charge moving even at 180 fps before 3500 rpm!
Talk about a head / cid Combo that can't exceed 100% VE until 7500 rpm, that's it!

[piston guy]

I know , I've dealt with it for 47 years. The funny thing is the tight LSA/ high overlap doesn't hurt the engine's ability to rev over 9,000. Increasing the cubic inches while keeping the rod short does wonders too.

[EDC]

I believe you've got it backwards; LSA first, overlap based on performance requirements second. And all of his stuff is based on an unrestricted intake and exhaust. If either of those are restricted, it pretty-much goes out the window.

Really? Lobe separation first?

It's resultant of the timing events, not a design criteria.

[Warp Speed]

I believe you've got it backwards; LSA first, overlap based on performance requirements second. And all of his stuff is based on an unrestricted intake and exhaust. If either of those are restricted, it pretty-much goes out the window.

Really? Lobe separation first?

It's resultant of the timing events, not a design criteria.

Ya, there's that! Lol

[RevTheory]

I was referring to Vizard's method.

[David Vizard]

I believe you've got it backwards; LSA first, overlap based on performance requirements second. And all of his stuff is based on an unrestricted intake and exhaust. If either of those are restricted, it pretty-much goes out the window.

Really? Lobe separation first?

It's resultant of the timing events, not a design criteria.

As things stand at present it is probably the principle criteria. I explained that in a post last year. The timing events are a result of the LCA and the duration involved. Why try to pick four points when one angle and rpm related duration can generate everything else.
Dv

[statsystems]

I believe you've got it backwards; LSA first, overlap based on performance requirements second. And all of his stuff is based on an unrestricted intake and exhaust. If either of those are restricted, it pretty-much goes out the window.

Really? Lobe separation first?

It's resultant of the timing events, not a design criteria.

As things stand at present it is probably the principle criteria. I explained that in a post last year. The timing events are a result of the LCA and the duration involved. Why try to pick four points when one angle and rpm related duration can generate everything else.
Dv

Because that would be too simple and all the gurus couldn't baffle us with bullcrap.

[RevTheory]

Because that would be too simple and all the gurus couldn't baffle us with bullcrap.

Extremely well said!