engineermike

I highly doubt this, nor have I ever seen any examples proving it. In order to over-scavenge a wedge-head motor, the incoming air has to make 2 - 90 degree turns to get into the exhaust port. In a hemi engine, the air only has to make 2 - 45 deg turns to get into the exhaust port. On a hemi motor the intake port is aimed directly at the exhast valve for chrissake!

Mike

Sean Collins

Actually air makes nice u turns. here is the phenomenon Navier-Stokes and a link expliaing why air sticks to curved surfaces and continues to turn well past the radious of the turn.I again refute that wedge heads are more likely to overscavenge.

http://www.osc.edu/education/su_prog...yn/index.shtml

engineermike

Sure, that's why conventional port locations work so much better than high-port heads. . . That sharp 90 deg turn into the cylinder flows sooo well. [/sarcastic]

Sean Collins

UUUgggghhhh With alot of overlap its very easy with the piston comming up off bdc for it to push the intake charge right out of the exhuast port or vise versa. Secondly the proximity of the valve to each other is what makes it so easy. At low lift the air come out of the valve sidways which is why the chamber shrouding is so critical.the point were overlap occurs the most is at low valve lift. I don't think valve location much matters in regard to overscavenging but id be willing to bet its far worse on a wedge then anyone imagned. Splayed valve heads due have breathing benefits but it has to do with the chamber wall shrouding.

GIGAPUNK

iTrader: (2)

Still hoping to garner something usefull out of all this.

please please...

So prostock engines have huge duration and consequently monster overlap/LSA...
Good to know.

And stock LS1s with shitty log manifolds have no overlap and wide LSA because of emissions and the log manifolds are usually higher pressure than intake track at low/early valve lift.

Now for the usefull part...

On a well built LSx engine how do we determine where [between those two extremes] a particular combo is going to make the best power over a defined rpm range with regards to overlap.

I found this somewhere online:

"If you don’t change the exhaust system, you only pick-up 2 hp with the ASA cam, but if you drop the back-pressure to something near zero (ie: a racing exhaust), then it’s more like 20hp. So, that’s the back pressure effect I keep talking about.

What about that ASA cam? It’s got only .525-in. lift but, at .050-in., it’s got 226° intake duration and 5.5° overlap versus the LS6’s 204° and no overlap. Clearly, it’s designed for higher rpm and more power but it needs a very low restriction or open exhaust. "

So does anyone have an opinion on the statement that seems to be made above that 5.5 degrees of overlap on an LS1 is only/most usefull with a wide open race exhaust. And that a more street oriented (mufflers and cats) exhaust system dictates a smaller overlap?

Sean Collins

The ASA cam might be onto something. the LS6 intake and consequently the headers good ones that work with it might be worth looking into. the 5.5 overlap might be ok but might not depending on duration. Sometimes more is less and less is more. Dpends on alot of factors. I am working n a useful set of equations but its going to take at least a week or two until i can even begin to validate the model and make the Math work. Its not the port in the head thats the issues is doing all the Math for the damn manifold thats killing me. Up till now we have had a dyno mule car to test stuff out with and its a real pita to tear a car up and down.

Adrenaline_Z

Sorry, I didn't see this earlier. The lobe on the right is far more aggressive
and has a bunch more area under the curve. That's the sort of testing that
would help realize the effects of minor changes, as opposed to throwing in
a totally different cam with all valve events changed.

Big-DEN

Adreneline,

One of the cam mfg's could show this. Same combo, and the testing done as I said.

Us normal folks would wonder how the entire tq and power curves are affected - even to low rpm. Vacuum, response.

engineermike

Am I the only one that see's something very wrong with this statement?

Adrenaline_Z

Well, I'll be totally honest with you. I think this thread started to take a
turn at about post #22. I don't know if you were being sarcastic with the
comment , , then followed up with: I don't know if you missed the first page, or didn't understand the conversation,
but it was brought up. I politely answered your hypothetical/rhetorical question about the possibilities of efficienct cylinder filling without overlap.

The following reply about having valve sizes on par with bore area and large
ports would do nothing for efficient cylinder filling. It would actually destroy
velocity and lower VE. According to Bernoulli, the massive port volume doesn't
work...certainly not on a street engine with under 500 cubes and 7000 RPM.

Post #36 carried no value in my opinion. You even talked about LSA on Prostock
motor as if LSA determined the overlap? At least that's how I read that line.

It wasn't until post #41 and #43 that I agreed with a majority of your statements
and the discussion seemed to get back on topic.

My Post #54 was not to dispute your claims, it was to support and give solid
info from a very good source. You seemed to get defensive...not sure why?

Your post #62 goes against all fluid dynamics theory and dyno/track testing
of cylinder head ports and chambers. R&D is working harder to design ports
that are more line of sight, changing valve angles and working with taller ports
to get a more efficient radius to reduce turbulence. I have yet to see a
head, or Intake/Exhaust port that flows well with 90 degree bends.

Post #64 That's a very generic statement, and is not necessarily true if the overlap is
tuned for a specific RPM. Prostock is getting over 100% VE with 64 degrees
of overlap @ 0.050" and 9000+ RPM!

I just don't see much credible info in your posts having scientific texts and
professional engien tuners who have proven race data to show otherwise.

Sorry for the incredibly long post, and no flame intended.

engineermike

Let me repeat myself one more time. . . a minimum of 20 degrees of overlap on an LS1 is most useful with a wide open race exhaust. I say "minimum" because any more overlap and P/V clearance becomes a problem. Increasing overlap to this point only gained power and increasing overlap beyond this point will continue the trend.

And, yes, restrictive exhaust will dictate less overlap. How much depends on how much backpressure. Backpressure in psi at the collector can be anywhere from 0 (open headers), to 5 psi (good mufflers), to 10 psi (Flowmaster), to 25 psi (pellet cat and stock muffler). I can tell you for sure that 14 deg of overlap was too much for 9 psi backpressure in one of my old combos. So, just how much backpressure do you have?

Mike

Sean Collins

reflected wave from the face of the piston.I forgot to mention on the wider LSA cams we have using that we hold the IVO constant and push back the EVO point. Seems to pickup alot of power this way and tons of TQ. Would suggest overscavenging and the AF charts and BSFC have improved as well.

Adrenaline_Z

It's pretty incredible when you think about it. Having that cam in a regular
street engine of small displacement probably wouldn't even idle under 3000 RPM.

I guess when exhaust waste is getting thrown off the piston at sonic speeds,
you can take advantage of the extreme overlap and not worry about sucking
exhaust back into the chamber at 15-20 ATDC!

Sean Collins

I take the no flame intended just fine. Understand something here this is comming from over a yr of cam testing on the ls1 engine family. We have tried about 40 cams with the same intake exhuast etc and have come to the following conclusion.

Wider LSA's help to a point but and the big but is that A i am not going to give away a yrs worth of work and B that i will say its definately on the longer duration lobes. C would be that we do our testing with full exhuast vehicles becuase thats how they really are in the real world.

Its hard to swallow becuase it flys in the face of everything everyone touts but the widers LSA have vastly improved BSFC which to me is the hall mark of engine efficiency and the TQ output is substantially higher as well. Yes we dropped a few HP but the trade in avg Tq was well worth it. If you are making alot of power per lb of fuel then something is going your way. I made the prostock cam reference to make a point that focusing on a 112 or a 119 or a 108 LSA is a backwards approach to cam science and If Dave Vizard would just shut up on the matter alot of people would have better running engines that produce more power per cube.

It is very easy to get cuaght up in the LSA battle but really the LSA will always be a moving target. The truth is that one particular lobe head manifold combo will need a different LSa then a different lobe with the same duration but different ramp profiles. Yes there might be a best mean avg but agian this si going to depend on sevral factor including exhuast.

I said i will work on the formulas to validate the results and post up the equations to make it work. their are definately tighter LSA cams that rock out the power i can think of a 218 109 lsa cam with xer lobes that made surprisingly good power. but the overlap is different from a 255 cam with a 109 lsa. As for LS1 engines liking 20 degrees of overlap wow unless your running a single plane intake i just don't see it.

As for information content I have been starkly honest. Don't think in one direction. that has been my theme on every post.

Sean Collins

Yes they run crazy cams. the RPM is crazy everything is pretty much to the extreme for sure. Outside of F1 and prostock i can't think of many engines that run at these efficiencys aside from the 4v ohc honda guys.

GIGAPUNK

iTrader: (2)

Mike,

Thanks for the real world numbers. I'm heavily leaning towards dynomax bullets to side dumps. Hoping to get under 5 psi of back pressure. Do you think 14 degrees would still be too much overlap on such a set up?

engineermike

With LT headers, bullets, and dumps, I think 14 deg overlap would be fine, but it could probably use more.

Mike

J-Rod

You can go back to my posts in the Camshaft discussion thread II and see where a lot of this has been touched on before.

Its interesting to say the least. I really see that there are a couple of camps on this topic.

There are the folks like my buddies in Australia who belive in big duration and tons of overlap (read that as narrow LSA). Engineermike's post on the shop that keep adding overlap and kept making power was interesting.

As an example of this line of thinking, my buddies in Australia recently made 1,000hp + @ only 6,600rpm with just 12lb’s of boost and a mere 18deg of total ignition in a 408 with a left over cam that had come out of a 422. The cam was a Crow 258/262 .644/.661 107.5 LSA. Most folks wouldn't call that a supercharger cam. But, with the narrower LSA it makes the same power at 12psi that a wider LSA cam would make, but need several more PSI of boost to do so.


On the other end of the spectrum you have guys like Steve Lowe. He is a wide LSA / High Spring Pressure kind of guy. CollinsAutomotive in this thread seems to like wide LSA. Obviously when you get into high RPM wave tuned motors with 127% VE in a narrow window, all sorts of wizardry takes place. I love listening to guys like them because there is so much you can learn.


I'll be the first to say that the more I learn, the more I learn that I don't know. As long as you are willing to approach the system as a complete system and learn from it, the better off you'll be. Too often we are stuck using old assumptions rahter than questioning of why X always works, and why it is best.


I think however on most LS1's you can make certain assumptions about some of the valve events based on the engine modifications being similar to one another across the vast spectrum of the engines.

I think one of the things that is helping us tune motors is the avaliability of better and better data acquisition gear.Things like Wideband o2s, EGT probes, pressure transducers, etc.. all make for being able to make max power on an engine combo.

The discussion earlier was on overscavenging. If you go back to the 60's and look at the Ramchargers. They did overscavenge a 354" Hemi with 392 heads. The car had 8 independant megaphone exhausts. The car had a huge power "hole" down low. The car would overscavenge the intake charge, and power suffered. Here is a discussion on speed talk which goes over that part of the discussion.

http://speedtalk.com/forum/viewtopic.php?t=1163

On of the persons (Billy Shope) in that discussion was one of the Chrysler engineers who worked on the team that developed the formula for determining intake tuning.

I think optimally testing and verification are what is going to find best power. Here is some food for thought on that subject.

http://www.tfxengine.com/software7.html

Valve Overlap Graph

The conditions that exist in the intake and exhaust ports during valve overlap on a racing engine have a tremendous influence on engine power. Any exhaust gas left in the combustion chamber or pumped back into the intake port, not only reduces HP by reducing the amount of air/fuel mixture that can enter the cylinder, but the exhaust gas also slows down the combustion process. The result is a two fold gain or loss in HP. Exhaust gases also tend to heat up the air/fuel mixture resulting in detonation.




Intake/Exhaust Port Pressures at Valve Overlap

The red curve shows the pressure (left side scale) in the exhaust port and the white curve the pressure (left side scale) in the intake port relative to crank angle in degrees (bottom scale). Intake valve opening is indicated at IVO and exhaust valve closing at EVC. TDC is TDC during valve overlap.

In this example the pressure in the exhaust port is greater than the pressure in the intake port during valve overlap, indicating that the combustion chamber is left full of exhaust gases on this cycle. An easy gain in HP of several % can be made by changing the exhaust header dimensions to provide a vacuum in the exhaust port during valve overlap, over a wide range of engine speeds. Pressure analyzer data has shown time and again that at high rpm, exhaust header design based on selecting certain primary tube lengths and diameters, has no bearing on reality. This often leads to a reduction in peak power and a reduction in the rpm at which peak power is attained.

The pressures during valve overlap are often overlooked on turbocharged engines. Just as with a naturally aspirated engine a good flow of intake mixture into the cylinder and out the exhaust valve is need to flush the exhaust out of the cylinder and cool the exhaust valve. Conventional pressure sensors that are often used on the intake and exhaust systems of turbocharged engines are not fast enough to measure dynamic port pressures. The pressures in the ports during valve overlap can easily be the reverse of what the conventional pressure sensors read, due to the dynamics in the engine. These types of sensors read essentially an average pressure. The sensors used by TFX Engine Technology Inc. to measure intake/exhaust port pressures are made by the same companies as the cylinder pressure sensors and are designed specifically for very high speed response.


Exhaust Pressure Wave Graph

The conditions that exist in the exhaust port during the exhaust stroke and when the intake valve opens can also have a significant effect on engine power. The goal is to minimize the pressure in the port during the exhaust stroke so that less power is spent pushing the exhaust out of the engine and to ensure that the pressure in the cylinder (as dictated by the exhaust port pressure) is low when the intake valve opens.



Exhaust Port Pressure

The blue curve shows shows the pressure in the exhaust port (left side scale) relative to crank position in degrees (bottom scale). Of particular importance is what happens from the time the exhaust valve opens at EVO to the time the intake valve opens at IVO and finally when the piston reaches TDC at the end of the exhaust stroke.

In this example an unconventional exhaust header is used to generate a tremendous vacuum after the exhaust valve opens (EVO) and part way through the exhaust stroke (right quarter of graph). A conventional header would generate a high pressure in this region blocking the flow of exhaust gasess. This vacuum helps the exhaust gases escape and is maintained until a little before the intake valve opens. Unfortunately a high pressure is generated in the exhaust port leading up to when the intake valve opens. It is obvious that by further modifying the exhaust to extend the exhaust port vaccuum to include not only the exhaust stroke but the valve overlap period will result in significant power gains.

The theory of how an exhaust header works can be complex and the actual results are almost always different from the theory. There is no way to accurately predict how an exhaust header will affect the port pressures. The pressures must be measured and with a little experimentation, based on fact not theory, significant power gains can be realized.

Sean Collins

nice presentation. Thanx for the comments i appreciate that. As for that tight LSA supercharger cam which way did they move the intake centerline and how was the exhuast system ? just curious ?It funny that since a supercharged engine offers up so much in intake pressure that sometime it can be used to clear the chamber with overlap. I would geuss that engine had to small of a header primary and collector ?

Adrenaline_Z

I wouldn't compare N/A motors to boosted motors when it comes to overlap.
Not sure if you're making that parallel, or taking the discussion toward another
induction type?

I'd also suggest to stay away from the term LSA when comparing overlap. There is not enough information contained in LSA values alone to determine overlap, unless we're talking about a specfic set of lobes.

Hence a lower overlap is required.

With a naturally aspirated street motor, you might see up to 5 PSI at the
intake port during the overlap period on a well tuned engine (at a specific
range of RPM).

With boost, the port pressures are higher and do not rely as much exhuast
pressure to initiate the flow into the chamber.

The higher the differential between the exhaust port and intake port at TDC,
the better chance of filling the cylinder during the overlap period. Intake
pressure being higher of course, and the flow is from high to low pressure.