1 Overlap is controled by cam event.
2 This would be true if there were a comon fuel source, a carburator. In this case there are individual injectors that are limited by size & ECU parameters.
3 The power is made by optimizing the delivery. The AF is controled by what the ECU sees, not by the enhanced flow charicteristics of the head.

 

thx but being a common collector after onlt 20 " doesn;'t some of the gas get pulled into the intake charge of the next in line to fire . when i said overlap i meant exhaust valve time open ; whether you got there by cam or stem releif or bigger valve it's still time open . so are these valves pulling in spent charge?

 

i guess it doesn't matter as the back pressure at the collector will just become part of the system and like you say the computer will just add fuel till the af's at the o2's are good . yeah?

 

but I drank too much tonight and can't post about it till tomorrow.

Matt

 

If it did you would have a fire under the hood.

Overlap is controled by the cam event not the size of the valve or the shape of the port. Cutting the back of the valve does not create a longer duration, but you would need a longer duration or a greater lift for the same result ( a larger opening or more time for the same charge ). I don't know of any cam that has an exhaust valve opened during an intake sroke. If there was such a cam it would be worthless. Reversion is from the lack of momentum at idle or low rpms, with a high lift long duration cam, not from sucking exhaust. The more radical the cam - more reversion - higher the idle - less drivability ------ more lope

 

got ya . so the only time we suck exhaust is when we slam the throttle shut ,. hence the nice gurgling sounds.

 

i read up on my cam timing and the exhaust valve is indeed open while the intake is open . the exhaust typically opens around 60 btdc but rather than pull exhaust back in ; the intake charge helps to force exhaust all the way out . overlap.

 

sorry intake opens 60 btdc. exhaust will have been open already . amazing an intake would open that soon . not fully ;just starting. cool stuff.

 

And that little bit of overlap is what is likely contributing to the exhaust burble.

 

Exactly. But not during an intake stroke otherwise you would have a fire.
The easiest way to look at this is to look at a cam & the event timing. Ramp speed is not too critical but can play a part in how quickly the intake recovers ( the exhaust must be closed ). Overlap has to do more with the lobe centers, intake & exhaust, however, the 2 valves can not share a combustion stroke which negates overlap.
The burble has more to do with the rich mixture, ign timing & short primaries. In other words unburned fuel. This is due to deceleration & lack of intake momentum. As you, in the above post, can use the overlap arguement as it is hot exhaust gas that is burning the excess fuel in the exhaust system.
You are posting some very good questions about cams & how they work.
This could be a new thread that's devoted to cams & what to look for in a profile for your car.

 

the exhaust valve is still open as the intake valve begins to open . take a look at "the black mistyc art of valve timing " part one . it's based on a ducati motorcycle but the principle's the same 4 stroke . what's interesting is that to say a valve opens doesn't mean "bingo" it's open . it's a process . as compared to the piston ; it's quite slow. so to be on time the cams need to start early. so believe it or not the intake is opening at about 60 btdc on the exhaust stroke. this not only pushes the exhaust out but as is described in the read ; it actually helps cool the exhaust valves . now get this ; it is further stated that the exhaust stays open till about 60 atdc overlap is described further on . the intake then stays open till about 80 atdc all the inbetween is overlap . the time both valves are open .

 

Yes Stevescars60,

A new thread 'devoted to cams & what to look for in a profile for your car' would be a great discussion!

My first read on Engines was David Vizard's piece: "Tuning BL's A-Series Engine!" A great read!

And yes atmoshperic air is a fluid which has inertia.........hence intake/exhaust duration overlap! Gotta keep them gases a flowing!

Jeremy

 

"the exhaust valve is still open as the intake valve begins to open . take a look at "the black mistyc art of valve timing " part one"
Read the book years ago. With the cam you are talking about, you still need to factor the engine stroke with the exhaust valve timing ( where is the piston in relationship to the exhaust valve ). This is where the "black art" is. It realy is pretty cool stuff........

jhiggs26, looking at the OE cam would be good for a start. Somewhere I have that spec, if I can find it & time permits I'll post the specs.

 

Post the stock specs.......then maybe a few vendors can chime in with some aftermarket options re: head/cam/MCS usage!

Jeremy

 

I like what you guys are talking about so please don't take this the wrong way.

Please start a new thread.

Longboard

 

LOL you shouldn't of made so much whp with a stock cam then, if you had made normal whp like everyone else they wouldn't be having the above conversation

 

Ya, I guess you are right. I never expected the numbers to be that high but boy am I happy now.

The header and exhaust will be done this week so keep an eye out for more news.

Longboard

 

This cam talk is OT & there's no way to take your post the wrong way.

Cam thread, Will do You will have a new cam soon, yes? I know 1 is being made for that head & a whole lot of thought went in to it.

 

Based on your initial numbers, anything less than 300whp would be a disappointment.

 

do you really feel the car is going to make 300whp?

 

I'll answer that one! Uhhhhh... no.

But, I don't think that means it's a disappointment at all.

 

275 or bust. With the cam & header, 380s, it will get to how well tune works. My money says close to 275 if the tune is what I think it will be.

 

i'm hearing you ; but as was explained early in the read ; as rpm's go up the rudiments of valve timing need to be adjusted or the valves would open too late . so yes as the intake is opening on a modern engine the exhaust is still open ,and is so well into the intake stroke; the reason there's no fire is the exhaust gases are still in the way,and the returning pulse from the "tuned " exhaust manifold i paid dear for , keeps the intake charge from going out the pipe .

 

if they break 260 i will truely be impressed... although... i might ask to see this car on a different dyno...

 

If this car breaks 250 whp I will be *****-canning my 218 whp wheezer and be next in line!