squalor

iTrader: (3)

Like the F13 (230/232 112+4) with it's IVO of 7 and EVC of 0.
So reversion is a high RPM problem ? The F13 would hit a reversion wall at a certain rpm and a tuned collector length (backwave) at that point could help it gain a few hundred more RPM ?

Another question has been bugging me Martin. The Tick Street Heat 1 V2 for LS2; why such a low lift on the exhaust valve ? It's events are similar to F13 and the page says "Basic Operating RPM Range: 1500-6400" . Is this because of reversion ?

Would leaving the swirl ramp in ported heads help with reversion problems ?

Sales@Tick

iTrader: (3)

Geoff is a smart man! Bret and I do not choose our camshafts in the same manner, but he and I both have a mutual respect for one another and how we go about choosing our cam profiles.

Reversion is a problem wherever the engine is running "out of tune". That usually is at low engine speeds and at engine speeds above the RPM where an engine makes peak power.

If you take a look at the in cylinder pressure graphs I posted on page 3 you'll notice this.

Look at the graph I posted first that shows the engine running "in tune". This is the "sweet spot" and normally occurs between peak torque and peak HP as this is where volumetric efficiency is highest due to the engine being "in tune".

Look at the exhaust pressure, look at the cylinder pressure and the look at the intake pressure during overlap. This would be in between the letters IVO and EVC.

Exhaust pressure at this RPM is lower than cylinder pressure and cylinder pressure is lower than intake pressure. Notice how exhaust pressure stays lower than cylinder pressure and intake pressure almost all the way up until the exhaust valve closes. Because of this, cylinder pressure is able to stay below intake pressure during overlap. As a result intake charge velocity and intake flow into the cylinder is jump started before the piston even starts pulling on the head when it starts the intake stroke.

This is the true benefit of utilizing valve overlap in an internal combustion engine. As you can see, it's a huge benefit to increased cylinder filling in a N/A engine that has to work against atmospheric pressure to fill the cylinder.

Flow cannot exist without a pressure differential present. It works the same forwards as it does in reverse. If there is lower pressure in the exhaust and in the cylinder than what is in the intake, flow from the induction system will naturally migrate to those areas of lowest pressure. The opposite is also true and if cylinder and exhaust pressure is higher than what is in the intake during overlap, reversion can occur. This is noticed most in turbocharged engines as they can easily experience exhaust pressure that is 2 times higher than intake pressure. Overlap must be greatly reduced in those engines to allow cylinder filling to achieve the highest levels of VE possible.

Now look at the second graph I posted which shows the engine running at an engine speed above peak power where it is "out of tune".

Notice how much higher the exhaust pressure is after BDC on the graph and notice how it does not drop nearly as sharply as it did in the previous graph after BDC. Now notice how high the cylinder pressure is as a result when the intake valve opens. This is causing reversion and port stall. All the added benefits of valve overlap are negated.

To really put this into perspective and show just how much cylinder filling is lost when this occurs, look at the differences in cylinder pressure at the IVC event in both graphs. It's huge! That is power output being lost right there in front of your eyes.

If you've ever seen a "dip" in the torque curve on a dyno graph when a larger cam is being used, this is what causes that dip.

Steve, I also want you to look at how much lower the cylinder pressure is in the second graph where peak piston speed would occur. Notice how much more of a depression is created? This is also a good correlation to show you guys that the faster the piston speed is(higher rpm and/or longer stroke) the higher the depression in the cylinder will be. The piston will want more CFM and it will pull harder on the induction system exposing any potential shortcomings of the intake port shape/design.

The exhaust port cares much more about when you open the valve and where you close it. Since the exhaust port operates under such an extreme pressure differential versus what is at the end of the pipe(atmospheric pressure) it doesn't care nearly as much about area(lift) as the intake port does. Only when the exhaust valve is purposefully shrunk to fit a larger intake valve does getting enough curtain area at the exhaust valve become of utmost importance.

Think Pro-stock, Comp Eliminator or any other very high RPM N/A engines where they are trying to get everything they can out of the engine while working with atmospheric pressure. Getting a big intake valve centered in the combustion chamber is paramount and moving the exhaust valve out of the way and making it smaller accomplishes this. It also tends to shove the exhaust valve up against the cylinder wall, but again due to pressure the exhaust valve doesn't care as much about this as the intake valve does. Just give it a bit of added lift in this situation for good measure and you're good to go. That still doesn't mean exhaust lift should be as high as intake valve lift or higher, it just means it needs more than it normally would.

Darth_V8r

iTrader: (4)

I have read this whole thread several times, and my head is still spinning, but I think I'm starting to understand. So, I'm asking for input on a thought experiment looking at very similarly spec'ed cams to understand why one would be better suited than another in a typical 346 CI LS1:

---Cam------specs at .050-----IVC---EVO--Overlap
1. Custom - 227/234 114+2--45.5---53--- 2.5
2. SNS S2 - 227/235 110+3--40.5---50.5--11
3. VRX4 --- 228/230 112+4--42-----51---- 5
4. Titan 4 - 227/232 113+4--42.5---53----3.5

At quick glance, it looks like the SNS2 would have best torque (earliest IVC and latest EVO) and carry power past peak very well (highest overlap)? VRX4 and Titan 4 should perform quite similarly to each other, but maybe not pull quite as hard as SNS at the low end or carry as well above peak?. The custom cam (currently in my car) would be the easiest to drive, but likely anemic on the lower end due to later IVC AND earlier EVO?

Which would have the smoothest torque curve, and which would hit a certain RPM and suddenly take off like a shot of nitrous? And most importantly, why?

Sales@Tick

iTrader: (3)

I take it that everything would be the same in this test aside from the camshaft themselves?

Darth_V8r

iTrader: (4)

Correct. I'm assuming stock displacement and a set of good flowing heads. Maybe we figure it's ported 243's with 64cc combustion chamber, 1-7/8" long tubes, and whatever intake is appropriate. Full bolt-ons. But our theoretical engine would only receive a cam change between imaginary dyno's.

Or maybe another way to look at it, what changes would need to be made to accommodate the changes in valve events - but sticking with stock displacement.

Sales@Tick

iTrader: (3)

If the VRX4 didn't have XER lobes, IMO it would offer the best power past peak HP due to it's 5 degrees of overlap(it still matters where the IO and EC event occurs and not just "total" overlap), later IVC and earlier EVO.

The Titan will probably have the smoothest lobe profiles out of the group, and even though it only has 3.5 degrees of overlap its 42.5 IVC and 53 EVO will carry power very well past peak.

squalor

iTrader: (3)

Each additional degree of positive overlap is worth 2 additional HP at peak

How true is this statement ?

Darth_V8r

iTrader: (4)

So then, what drives the decisions to design the SNS2 with so much overlap? What is it about those valve events that make you as a cam designer say, "This is a great idea" - to be fair, I have yet to see someone post they don't like that cam, so the results are obviously there and speak for themselves.

I imagine with the early IVC and late EVO, that thing packs some torque. Did you decide to hold those events for torque, and overlap was a result of duration? Was it to help with scavenging to maximize a power peak? Would increased compression help "absorb" some of the overlap, or would it cause detonation due to the early IVC?

Sales@Tick

iTrader: (3)

I would never make a generalized statement like that.

If that statement had been precluded by something along the lines of, "In this combination", or "When using this cylinder head", or "When using this style collector", I'd agree with it.

Making a generalized statement like that will get you and anyone who listens to it in trouble.

Overlap wasn't the only driving force in designing the SNS2. All of the valve events played a part.

I wanted an earlier IO event to get the lift@TDC I wanted, I wanted an earlier IO event to help get intake velocity going earlier in the power band. I wanted an earlier IC event to build cylinder pressure at lower engine speeds and I wanted an earlier IC to help keep peak RPM down.

I wanted a later EO event to utilize the added cylinder pressure from the earlier IC event by keeping the power stroke long. I wanted the EC event I did so that the overlap triangle it created allowed the engine to "tune in" in the RPM band I felt would correspond with the other valve events I chose the best.

I feel that knowing what I know now, I'd of closed the intake valve one to two degrees later and kept the EO event where it is along with IO and EC. Instead of using the piston to create more cylinder pressure by compressing the charge longer, I'd let the slightly later IC event create that cylinder pressure for me by filling the cylinder with more mass. Thus decreasing pumping losses from the earlier IC event and still making the same squeeze and increasing peak HP by a few numbers.

Who knows, maybe you might see a V.2 SNS Stage 2 one day soon.

redtan

iTrader: (5)

Speaking of the "pros/cons" with every set of valve events being basically a fine balance between what you want and what else is happening.

I have a question about balancing IVO and EVO with the advance/retarding of the ICL.

For example let's say a cam is ground on 114 LSA with +1 advance (so 113 ICL). At 0.050 lift the IVO happens at 0* BTDC and the EVO at 50* BBDC.

If you re-grind that cam with a +4 advance (so 110 ICL) you gain some torque from compression (due to an earlier IVC) and you also have the IVO happen 3* earlier BTDC (which someone mentioned is a good thing as it opens the valve earlier and thus is at more lift when the piston starts going down the intake stroke). However this also advances the EVO by 3* earlier to 53* BBDC. From reading the forums, the earlier you open the exhaust valve the less low end torque you have because you basically "let out" the potential on the power stroke.

So how does advancing the cam in the example above do in terms of power and where it's delivered. Does a certain setup care more about an earlier set of valve events vs. later?

Sales@Tick

iTrader: (3)

That is why choosing performance based on a given amount of advance, LSA and duration leads you astray.

If you want the events where you mentioned, just grind them to occur that way.

I know you asked a different question, and wanted a different answer, but if you want a certain set of events you don't advance or retard the cam to get one or two of those events and sacrifice the others, you grind the cam to have the events you want.

If you want a 3* BTDC IO, and a 50* BBDC EO along with whatever IVC and EVC you wanted, that's what you grind.

Darth_V8r

iTrader: (4)

If that happens and you need a guinea pig, I'm perfectly willing

speedtigger

iTrader: (16)

From a pragmatic standpoint, the IVC will have a greater impact than the other valve events in the scenario you mentioned. When advancing or retarding a cam, just be careful of the IVO and EVC as they happen at TDC which could literally have an "impact"...... with the piston. LOL

redtan

iTrader: (5)

Well it's not that I want those exactly, I was wondering what the pros or cons were of advancing the cam those 3* and changing the valve events. Or better yet, what type of setup would like more advance and an earlier set of events vs. later?

squalor

iTrader: (3)

I liked what you said earlier in this thread, "Understanding that the mass of air and fuel rushing in to filling the cylinder is like compressing a spring. We want to close the intake valve at close to the exact moment the spring is most compressed (most air and fuel in the cylinder) before it springs back and pushes the air and fuel back out of the cylinder. I think that visualization really works."
The visualization works when one realizes that the spring is weak at low RPM and gets stronger as the engine approaches torque peak.

Martin, you introduced a term in this thread I have not heard before, the "overlap triangle" ? You said, I wanted the EC event I did so that the overlap triangle it created allowed the engine to "tune in" in the RPM band. Could you tell us more about this ?

Darth_V8r

iTrader: (4)

I was sort of playing around with a valve events calculator following this post, and found this...

I took the SNS stage 2 torquemax specs at 227/235 - 110+3 and calculated the valve events

IVO-6.5
IVC-40.5
EVO-50.5
EVC4.5

Then, I kept everything constant, except the IVC, which I moved to 44.5 to make the grind numbers come out in integers, and I got the following:

231/235 - 111+2. (look familiar, Martin? )

So, yesterday I would have looked at these as completely different cams, and today I realize the only thing different between them is the IVC event. Crazy, huh?

squalor

iTrader: (3)

I'd like to take a stab at a answer. Maybe if I'm totally off base, others can learn from my mistake.
If you had a 346 with a LS6 intake, 241 heads unmilled, 1 3/4" headers into cats and a stock Y, stock-ish muffler and you found a deal on a 224/228 112+1 cam but you wanted a smooth daily driver you could advance 2 degrees. If later, you wanted a street/strip car and you got a Vic Jr and a free flowing true dual exhaust you could retard back to +1. When you advance, not only do you close intake sooner and open exhaust earlier but you move the center of overlap closer to before TDC.
(maybe) there are two ways to get low rpm torque. Earlier intake close or less overlap. There are two ways to get hp to carry past peak, earlier EVO or more overlap after TDC. Since this 346 starts with restrictive exhaust, there is less chance of resonance tuning so your better off giving the exhaust a head start. 3 41 49 -1 After the true duals you go for the resonance tuning with equa-distant overlap. 1 43 47 1 After the Vic Jr. you trade lifting the intake early in order to take advantage of the raised peak that the Vic offers.

Darth_V8r

iTrader: (4)

In re-re-reading this, I picked up on your discussion of overlap and turbocharged engines and the higher exhaust pressures working against earlier IVO, and that makes sense.

On websites I tend to see FI cams lumped together.

A supercharger would have higher intake manifold pressure, but also -assuming long tubes- a free flowing low pressure exhaust like any NA engine.

Given the different natures of the exhaust, does this change the way you specify an FI cam, or do the higher intake pressures change something else that make the turbo and supercharged engines end up running similar cams?

06blackGTO

iTrader: (20)

Hey Martin. I know you say lsa doesn't matter. But my cam is a 219/232. .625 .599. 109+0. Lsa. The car has been a bear to tune in the lower rpm ranges. And the hp peaked at 5600 on the dyno. And I noticed bank 1 runs just a hair leaner like +2 on the stft's. Would the same cam but on a 112lsa have totally diff. Characteristics then this lsa? And would it still have the midrange hp but possibly peak later?
And I guess the drivability would be alittle bit better? Thanks.

Sales@Tick

iTrader: (3)

The overlap triangle is created by curtain area during overlap.

The earlier you open the intake valve, the more curtain area it will obtain during the overlap event. Thus the effects of overlap becoming more pronounced, for better or worse. I.E. reversion, creating intake port velocity before the intake stroke, filling the cylinder before the intake stroke etc.

Same goes for the exhaust, the later you close it the more curtain area it will have during the overlap event. I.E. more short circuiting(blowing charge out the exhaust), allowing the suction on the exhaust port to pull harder on the intake port and cylinder during the RPM's where resonance tuning occurs etc.

With a given valve diameter and valve lift (the two main variables in determining curtain area) during overlap you're literally creating a triangular shape resulting from each valve's lift curve, which in this instance forms the overlap event. This is what allows the resonance tuning I have laid out in this thread to occur. Either at greater effect, or to a lesser effect. Of course the other factors I mentioned play into creating these greater and lesser effects as well.

Good investigative work there Darth!

Absolutely!

You first have to group turbo engines into two categories.

The first category would be street engines using smaller more responsive turbochargers that are more oriented towards quick spool times, faster transient response and are not geared towards high RPM operation.

In this category it is not uncommon to see exhaust pressure upwards of 2x higher than intake manifold pressure. Remember as I've said many times in this thread and elsewhere, flow works just as well in reverse as it does forwards. If you have 2x higher exhaust pressure than intake pressure, and you increase the overlap event's duration, you're increasing the amount of time that the engine spends in reversion. This kills power output from heating up the intake charge from the hot exhaust gasses. Not only does it make the intake charge less dense from heat, it also can cause ignition timing issues and cause the engine to become knock sensitive. This will only be fixed by reducing ignition timing which also hurts output.

The other negative to the engine running in a reversion state, is that no cylinder filling is occurring while reversion is present. These hot exhaust gasses literally block the intake port from flowing air/fuel into the cylinder. Thus the longer your overlap event, the longer the reversion period and the more degrees of crank angle you're wasting to fill the cylinder. The more air and fuel we fit in the cylinder the more power can be made.

In a boosted engine you do not have to rely on piston motion to start cylinder filling. Since there is already higher intake port pressure than cylinder pressure due to compressing the intake charge, cylinder fill will instantly occur when the intake valve opens. Granted exhaust gas isn't blocking its path!!! In this category we have to design the cam to close the exhaust valve early so that we can trap those higher pressure spent exhaust gasses where they belong, in the exhaust port and exhaust manifold. Since we do not have to rely on piston motion for cylinder filling as I mentioned above, we want to open the intake valve as early as we can without incurring reversion. This is so we have the maximum amount of crank angle possible for cylinder filling to occur. I've already mentioned several times how an earlier opening intake valve event allows for more intake valve lift(curtain area) by the time the piston reaches TDC and then begins its intake stroke. More lift means more airflow available from the induction system. More airflow available from the induction system means the airflow demand created by the piston on the intake stroke can be better met and more power is produced as a result.

So generally speaking in this category you will see camshafts with much shorter overlap events, even having no overlap event @.050" lobe lift and higher. Very little overlap seat to seat. In some instances the exhaust valve may close so early along with the intake valve opening so early that a reverse split duration camshaft is created. Meaning intake duration is larger than exhaust duration. A lot of times you'll see single pattern camshafts used in turbo applications as well.

With today's turbochargers getting better and better engineered, and back pressure dropping lower and lower as a result, you'll see camshafts start to trend larger and larger in duration and overlap. Even in street applications. Overlap in a turbo engine is directly related to backpressure vs. boost pressure.

The other category would be race engines using larger single and twin turbochargers. These turbochargers tend to have much larger turbine wheels, exhaust housings and compressor wheels. As a result, the back pressure created by these engines in this category is much lower than the back pressure created by the engines in the street category. It's not out of the ordinary for a 400"+ small block engine with twin 88mm turbochargers to have 1:1 boost to back pressure ratios, even at higher RPM's. Obviously at low engine speeds, back pressure is much lower due to mass flow through the engine being much lower. As mass flow and piston speed(demand) increases, so does back pressure.

In this category of larger turbochargers and race engines, the cams will more closely resemble the camshaft you'd utilize if the engine were N/A. Just with slightly less duration and slightly wider centers. It is not uncommon for very high RPM, large displacement turbo engines with 1:1 or similar back pressure to boost ratios to have upwards of 10-15 degrees of intake to exhaust duration split. In the traditional sense that is, and not in reverse. I have a customer with a 434" LS engine that utilizes twin GTX5588 Garrett turbochargers. His cam has 45 degrees of overlap @.050" lobe lift and has 16 degrees duration of intake to exhaust split @.050". This combo has been 4.40@173mph weighing 3200lbs with a lot left in the tank.

Since back pressure is much closer to boost pressure in this category, you do not have to be as worried with reversion and having the intake charge going the wrong way upon the IVO event. That said, you face a different problem altogether which you touched on when you mentioned blower motors and overlap. Now that boost pressure is equal with exhaust pressure during overlap, the concern becomes preserving as much intake charge as possible, and not short circuiting the charge out the exhaust valve. Anytime you increase overlap, you're increasing the amount of airflow through the engine. That doesn't mean you're actually trapping that airflow in the cylinder though. The more we trap in the cylinder the more power we make(I know I sound like a broken record).

As for blower motors, they have to contend with much the same as a race turbocharged engine does. Blower engines always have higher intake port pressure than exhaust port pressure during overlap. These engines do a very good job of purging the last bit of spent exhaust gas from the cylinder by utilizing the higher pressure intake charge to cleanse the combustion chamber of spent exhaust gas during the overlap event. Promoting good gas exchange of spent exhaust gas with fresh intake charge are just one of the many goals I strive to achieve during the overlap event.

These engines also do a very good job if you're not careful of blowing precious air/fuel out the exhaust without trapping it in the cylinder where we need it the most. When running smaller blowers that do not have a large amount of airflow capacity, this is a huge negative. The smaller the blower and the smaller its airflow capacity, the less overlap you want to run. This is so that the maximum amount of air/fuel is trapped in the cylinder making max power output.

On the other hand you have a Top Fuel funny car engine with an absolutely enormous blower and airflow capacity. These engines have such a long overlap event and so much positive manifold pressure that they literally pump raw fuel out the exhaust ports straight into the air while STILL producing 8,000 HP+!!! Quite the visual if you have ever seen one run before.