On intake vs exhaust duration spread with camshafts.

Not looking for exact answers since many motors/exhausts are different from each other, but looking for some discussion with thoughts / real test data / etc,etc.

Wet exhaust - manifolds/marine type 'headers' and even mufflers, not dry full length headers that are open.

I would love to hear results from someone / some people a real life wet exhausted motor with playing around with intake/exhaust spread. Say, if you added more exhaust duration and it picked up power, did you then add more intake duration and keep the exhaust the same ? Did you just keep adding exhaust duration until you found more or less power ?

Would love to hear thoughts/theories/etc,etc also. This subject is not talked about much and I think more 'assumptions, regergetations (sp?)' are presented as fact, than really is fact.

So, no exact engine build to give you as example - just looking for trends and new ways of thinking.

SB,
I look at the following factors when camming an engine:

Cylinder head I/E ratio at given lift points.
Why? This ratio will give you an idea of what you have to work with coming in the engine and going out. Since the lobe creates a valve path as to which air and fuel enter and by product exits the I/E ratio gives you the duration at which the valve must stay open to accomplish this.

Intake runner size in comparison to the size CID.
Why? We can over head or under head an engine. Based on which scenerio you have will play apart in determining the duration split. If engine CID is under headed but the desired max rpm is not going to "pull" on the head to hard because the cross sectional area can fill the CID then the split on duration is based more on the I/E ratio. Now same scenerio but the rpm range will stress the cross sectional area of the intake runner then we would have to compensate with more intake duration to help "crutch" the intake that is working over.

Just a couple of things I look at.

What about wet marine exhausts ?

My thoughts say these should be treated different than full dry with full length headers that have no mufflers.

We know with car engines, exhaust manifolds vs headers and different exhausts thrown into the mix, valve events should be changed to maximize things.

Dynoing with dry headers doesn't tell us much about the valve events when wet exhaust manifolds/headers/ typical style wet mufflers are thrown in.

An engine is from the air cleaner/flame arrestor all the way thru the exhaust tips into the atmosphere. It's all connected, therefore all of it is a part of the same system.

So, let's discuss what's going on in these and what possible things can happen when lengthening/shortening spread between valve events.

Again, I realize every motor is different, but just looking for some more thinking / results / facts / theories / etc,etc on this subject.

Okay, it's monday, I'm sure atleast 50% of people are back to work. LOL.

Who wants to jump in ?

I find your question very interesting. My guess is no one has ever really done the type of testing you're asking about. Reason being- most development work is done for the high end poker run and race engines, where headers and dry exhaust is the norm. Nobody wants to spend the money and time for R&D in the pleasure boat world. A few HP one way or the other doesn't make much difference. Taking this a step further, I recently acquired a quality prop shaft dyno. After doing some testing I"m beginning to believe there is a lot more HP lost once installed in the boat than most folks would believe. Similar to what chassis dynos are demonstrating in the automotive world. I hope to do some more comprehensive testing this winter on a couple of engine builds I'm doing.

Bobl - thanks for your interest. I'd like as many people as possible to jump in this round table sort of deal.

I feel that it may be more than a few hp in many applications.

As we add exhaust duration, we of course open exhaust valve earlier, but we are also closing it later. Exhaust valves close after TDC when intake valve is already been opening for a little while.

Let's take 6 degrees off our typical 10 degree split. Typically 6 degrees means 3 degrees off the exhaust closing. So, 3 degrees removed from ATDC. Note: Keeping LSA/centerlines the same.

Okay, so this is one aspect of a handful we must think about.

I'll bring up other aspects + more thoughts in a while.

BTW: If people haven't noticed by my poor typing, what I'm concentrating on here is EVC . I think this is crucial with our wet exhausts.

Bob, would you happen to know where any other propshaft dyno's may be that are for sale?

I've seen a few of the hydraulic units but all they really do is load the engine. I bought mine new with good software, 02 sensor, fuel flow, weather station, etc. so I can do comprehensive prop shaft testing. I had a customer bring in a boat Saturday. We spent 4 hrs. and picked up 50 HP by tuning. Probably the cheapest HP he'll ever get. Of course it was tuned pretty badly to start with.

Bob

Ok, Monday work rush is gone.

Anybody been atleast thinking about this ? Share your thoughts/questions - no matter how good/bad you think they are.

There is no 1 answer that will apply to every motor/combo. But, there may be trends we must / should consider.

Am I the only 'dipschit' that thinks about thesse things ? LOL. Just playin' myself.

A big % maybe...yes.

But, there are a lot of people out there that will spend the $$$. And I'm just talking about a few hundred $$ if that.

You know the one's - many are your type of customer.

Oh well, I guess I'll let this thread die and dig deeper into it myself.