First couple of questions are basic..

1. Does anyone know the intake runner length of the LS6 manifold?
2. Same for the LSX manifold.

3. What runner lengths would you guys like to see for Forced induction apps?
4. Same for N/A or N20 apps??

This is for a market survey for a new intake manifold setup I'm thinking about making...Just want to get some info...

Chris

 

I don't think it would matter much with forced induction, since the boost will take the place of any ram charging effect. You're going to turn up the boost until just before detonation anyway.

More important, particularly on high boost apps, is elimination of any overlap on the cam.

 

it depends. on a street car you probobly wont care. but on a racecar- boosted or not you want a tuned- welll everything.

length is dependant upon powerband location

 

good point...The only problem with that whole thing is there is never *one* sweet spot with every motor...URGHH..I hate doing one offs every time...

Chris

 

its what you are shooting for, is it not

 

"More important, particularly on high boost apps, is elimination of any overlap on the cam."

I have to question this quote.

Understanding that the manifold will always have a higher pressure than the
exhaust in a boost application, do we really want to eliminate ALL overlap?

Wouldn't a slight amount of overlap help clear the cylinder of waste charge
and even help the VE% of the next intake stroke?

I'm going to take a wild stab and state that some overlap is needed to help
scavenge the cylinder. The exahaust stroke pressure alone with a closed intake valve would probably lower the outgoing exhaust gas velocity.

Analogous to plugging the end end of a tube as fluid is flowing through it.

 

I always thought supercharged cars liked overlap, where the turbo cars hated it.

 

I would think opposite due to the fact that S/C are belt driven and do not
rely heavily on exhaust flow to make boost.

 

no, overlap is not required in supercharged cars.

the exhuast is under so much pressure it doesn't need much pesuading from the intake charge

 

On a NA application the overlap is helpfull at mid to high RPM, below that it can accually hurt TQ production. So much for the lower LSA improves low end TQ theory huh.....

In a blown setup it helps because even when there is overlap some of the intake charge is blown thru to the exhaust which is not a problem. What you are getting though it more time (duration) to feed the motor. That's much more important than the small amount that escapes thru the exhaust.

Your never going to get any short circuting (or exhaust in the intake tract) with a blower motor so it's actually not a bad thing to have some overlap on them. They can have overlap, but you don't want a huge amount.

Bret

 

I disagree.

Well first let me clarify. When we are saying "more/ less" overlap, I am assuming we are changing the IVO/IVC but keeping durration constant. Otherwise its not really a fair copmarison.

second, overlap on a boosted application is rarely useful.

in race it lowers your BSFC because part of your fuel is dumped into the intake tract. In street cars its more lope. In OEM its more emissions to worry about with little benifit.

I fail to see the point of using overlap just because its not very harmful. Maybe I am missing something

 

I wont speak for Bret, but I'll clarify my thoughts:

I'm thinking IVO and IVC points are changing, but there is still a certain
amount of overlap.

My example: Put a straw into a glass of water. Put your finger over the
open end. Pull the straw out of the glass with the end plugged. There
is water trapped in the straw.

I believe the amount of overlap would be a minimum to allow the pressure
pulse to reach the collector so that the next exhausting cylinder pulse
will be able to "push" the remaning charge down the collector/exhaust system.

Having an intake valve close too early while the pressure pulse resides in
the primary runner will create a drop in velocity. This will build pressure in
the primary hindering the next pulse from exiting the chamber.

Can anyone find a supercharger specific cam from Comp, Holley,Crane, etc. that does
not have any overlap?

 

Just for comparison sake...And n/a cam *typically* has about 250+ duration on the intake and exhaust on a 112 or less LSA with a centerline of 110 or less... Now as an example look at the turbo cam I'll be getting from cammotion....250 + duration on both the intake and exhaust and on a 114 LSA with a centerline of 108...!!! There's a lot more lobe seperation for the forced induction but it's not totally gone. If you totally eliminate the overlap you totally lose the cylinder clearing scavaging that the over lap provides...Yeah you lose some fuel, but the motor is still just as efficient since it burns most everything in the cylinder, you just blow some through to make sure it's "fresh"...

That and Overlap provides some of the cool lope that everyone loves so much...Yes it's more duration, but they both play a big part into just making the car sound cool....Just thought I'd add that...

 

the amount of overlap you want is VERY minimal. JUST enough to send out those residual exhuast gasses at the end of the exhuast stroke. VERY little. and that is mainly to get the heat out.

just my 2.

 

In the turbo cars especially, the exhaust pressure is gong to be greater than the intake pressure, sometimes by a factor of 2:1. Remember, you're running the turbo via the exhaust Ten pounds of boost, in other words, results in twenty pounds of exhaust pressure. So overlap, even a little, forces hot gasses BACKWARDS into the intake path, increasing sensitivity to detonation. I don't have Vizard's cam amd valvetrain book with me here at work, but that's where I got the above viewpoint. He points out that the faster your car comes on boost, the higher the exhaust pressure is going to be at high rpm, because the A/R is smaller.

He gives some interesting specs for a turbo cam that he ran in a 1000 hp, 1100 tq motor:

200/235 duration, 126 LSA, installed 9 degrees retarded. This closes the exhaust right at TDC and doesn't open the intake until 35 deg after TDC, because up until then the pressure in the cylinder is greater than the amount of boost in the intake.