To avoid gas fumes while designing a camshaft the overlap event must be kept short. When the engine is idling the piston speed is extremely slow, but the amount of time that the valves are open is no different than when the engine is turning 7000rpm. Because of this the engine literally experiences a vacuum leak from the valves being open for so long.

Not only that, but you will have major exhaust gas contamination of fresh intake charge. This pollutes the charge and keeps full combustion from occurring. This causes the gas fume smell you smell at idle when you install a camshaft in your vehicle other than stock. It's almost unavoidable due to the physics that occur during the overlap period at low engine speeds.

Overlap would have to be kept incredibly short(think OEM camshaft design) to keep this from happening. In the performance world, this can keep the engines full potential from being realized unfortunately.

The earlier you shut the exhaust valve the less chance you have for EGR to occur. Keep the IVO a bit earlier to boost mid-range torque, engine acceleration and to "get things started" on the intake stroke. Just shut the exhaust valve sooner and you should be able to keep the majority of EGR from occurring. This will limit how much the engine can "come into tune" from peak torque to peak horsepower though.

Hope this helsps.

 

It seems kinda slow around here so I'm digging this back up.

Two cams. One is 228/228 111+1 and the other is 228/232 112+2
Both cams have IVO of 4, IVC of 44, EVC of 2. Both have 6 overlap. Same DCR, same P to V.
228/228 holds on to the power stroke with 46 EVO but the 228/232 spills exhaust earlier at 50 EVO.
The 228/228 makes more low rpm torque and offers better low rpm drive-ability but runs out of breath around 6700 rpm because of exhaust gas contamination of the intake charge. 228/232 is a bit down on under the curve power but can carry power after the curve (5252 RPM) and run out of breath around 7200 RPM. The more restrictive the exhaust is, the bigger the 228/232 advantage is.
Does this sound right ?

 

You translated those valve events very well.

 

Literally what Martin explained on the phone to me yesterday! What are the odds haha.

 

Interesting that you quoted that SoonerBrad88. Overlap = smell , Overlap = power. Therefore, Power stinks !
Martin and Lloyd Elliott agree that opening the intake valve earlier puts that valve at a higher lift point (curtain area) when the piston starts sucking on the port (CFM demand).
Predator-Z once said that having overlap centered or slightly exhaust biased helps power carry into upper RPM. I linked to his cam recommendation for me earlier in this thread .
IVO 2 IVC 42 EVO 51 EVC 3 I adjusted for the 243 heads I have now, more compression.
IVO 6 IVC 42 EVO 51 EVC -1 This is a cam Jegs sells. It reminds me of the F13

Both have 5 degrees of overlap but go about getting it in different ways. Martin says they will sound the same. Both have the 42 IVC that JakeFusion suggested. I would expect cam #1 to have better 1500 RPM manners but after that, 2k to 7k, how would the dyno curves be different ? Which cam makes more average power ? More peak power ?

 

As I mentioned in the post Sooner quoted, closing the exhaust valve later will help the engine utilize exhaust harmonics and intake harmonics which allow it to, "come into tune" from roughly the RPM peak torque occurs to the RPM peak horsepower occurs more advantageously.

If there is a strong negative pulse (suction wave) at the exhaust port during valve overlap, there is a strong tuning aid being created which helps clear spent exhaust gas from the cylinder and bring fresh charge into the cylinder before piston motion starts on the intake stroke. Overlap does not create this effect though, it merely harnesses it and takes advantage of it.

Good exhaust tuning from proper length and diameter primaries is the strongest aid in creating this effect. Collector length, diameter and taper angle of the collector also play a part in the effects exhaust tuning can have on volumetric efficiency.

Of course the paramount to all of this is timing the exhaust harmonics with intake harmonics and finally tying those harmonics into proper cam timing. When these parameters are met, you can then go about harnessing these peaks and lows of pressure to help fill and exhaust the cylinder.

 

So with out knowing if the exhaust and cam are working as a system, a answer can not be given. If the exhaust is poorly matched, overlap bTDC is more effective. If it is known the exhaust tuning is well matched to the cam, overlap aTDC is more effective. Maybe ?
A range of 1400 rpm or more ?

edit Hmm, I think I remember. 1 3/4" headers, all primaries equal length 32" into 3" collector 18" long tunes to 5500 rpm. Shorter collector tunes higher, longer collector tunes lower. Next tuning node falls at 72" ? Bullet mufflers appear to the engine as a longer collector. The 18" collector needs to open to atmosphere or a Visard pressure wave termination box. No street car here has a exhaust like that. It would explain why some cars Hp numbers seem unbelievable while others are a dyno disappointment. I think I have my answer. Martin, you are the Einstein of the E.T.

Have you ever spec'd a Comp RPR lobe for exhaust ?

 

Where the majority of the overlap event occurs, or where it's bias is based towards is all a result of how the engine or cam designer wants the power curve to be shaped. Where the valve timing occurs in relation to intake and exhaust harmonics will determine where the engine receives a "torque boost" from added volumetric efficiency. Enhancing the scavenging effect from a well tuned exhaust system will result in increased removal of spent exhaust gas, and increased cylinder filling. Thus an increase in torque output and volumetric efficiency.

Even if we have a great scavenging exhaust system that is very well tuned to the engine's parameters and operating range, the cam timing is still based on the end goal. So if the customer wants a power band from 2500-6500, I wouldn't go about timing the valve events to exceed this operating range. Say his exhaust system is really well matched to keep pulling to 7000rpm, but he doesn't want to turn 7000rpm. I would not arrange the timing events to continue pulling to 7k, I'd stop at 6500 as the customer wished.

Of course the best course of action in that case is to change the exhaust system to best suit a 2500-6500rpm operating range, but that is never as easy as it sounds.

From peak torque to peak horsepower is where the engine comes into tune the strongest. I've provided some diagrams in the thread I made that is titled, "Why LSA doesn't matter" that better show this phenomena. That said, there are spots in the power curve below this range where for brief periods of the time the engine also "tunes in", it just doesn't occur for as long.

You've got the general gist of the exhaust tuning deal. There are formulas that will tell you where this "torque boost" of added volumetric efficiency can and will occur. These formulas are based on primary length and primary diameter among other variables. Collector length, taper angle and diameter also plays a part in where the engine comes into tune.

I have never used the RPR lobe. I don't like slow opening fast closing lobes, it just seems backwards to me.

 

Maybe on the intake it could be decent if the exhaust events are planned around it?

A slow opening intake, I think, would be ideally opened earlier than usual. If the exhaust also opens earlier, this would cause a depression in the cylinder sooner. Then you could hold the intake valve open longer, and then shut it faster, thus ending up at the same IVC event, therefore allowing more time for the piston to act on the intake port on the downward stroke instead of holding the valve open for longer on the upward motion of the piston. Now, I know you have pointed out that there is a delay between the actions the piston takes on the intake port, so how much earlier you open the intake valve would depend on this delay. But, This would seem to give an earlier start on cylinder filling if the EVO is also earlier as well. With a standard exhaust lobe, you could still close the exhaust at the same point, therefore holding cylinder pressure for the same about of time after EVC. Seems like a no compromise approach to intake valve timing. You get an earlier start on cylinder filling, and you close the valve at the same point.

 

My reasons were not so scientific. 225 LSL/227 RPR 111+2 EVO would be @ 46.5, not early enough to clear the chamber at higher rpm. The slow opening would be like using a larger duration lobe yet the XE-R close would hold down overlap.

Quote from Vizard :
The exhaust system on a well-tuned race engine can exert a partial vacuum as high as 6-7 psi at the exhaust valve at and around TDC. Because this occurs during the overlap period, as much as 4-5 psi of this partial vacuum is communicated via the open intake valve to the intake port. Given these numbers you can see the exhaust system draws on the intake port as much as 500 percent harder than the piston going down the bore.

I have LS6 intake so IVC around 42* may work well. Seems like my next step is to find out what my exhaust tunes to.

 

I would offer to you that choosing a lobe is all about getting the proper acceleration rates for your valve train set up. Choosing valve events is where you try to create the desired power band.

For 99% of people, there is no reason to mix the two up. In general, I don't think that those changes in lobe design will produce any measurable benefit in performance, but they could produce problems in longevity or valve train noise.

 

You mean the RPR ? I worry more about the LSL .612 lift, stock rockers and my valve tips, yet this is the same lobe and intake event as the Street Heat 1 v2 for LS1. The 227 RPR is very close to the 228 XE that the Sh1 uses, it just has a XE-R close. Compared to the XE-R it eliminates the 49 jerk intensity that stresses the lifter plunger and spring, the bending of the pushrod and rapid acceleration of the valve spring. All that's left is the valve bounce from closing too fast. The EVO should limit power to 6600 so I will set the limiter to 6800. I already have LS6 hollow stem valves, PSI ls1511ml springs, stock retainers & locks and a Avon Progear pg4210 timing set. I plan to clearance for Trend 3/8" .080 pushrods and am considering Morel 5290 lifters. Light and stiff.
Where do you think the weak link will be ? I do need longevity more than power.

 

I don't use any of those comp lobes anymore. I have come to believe that the most all of their popular lobes are not as smooth as they should be for 99% of most common LS applications.

I see threads all the time where guys are saying they want to use this killer lobe or that killer lobe because they want to make big power, but they don't even talk about the lobes relation to valve train weight, potential deflection or valve train stability at a given engine RPM and power range. I believe that is just wrong thinking.

I say choose the lobe to suit the valve train design and application, then pick the valve events for your desired power curve.

 

No doubt the RPR is a strange lobe and the 225 LSL/227 RPR 111+2 is just way to phrase a question DREAMZ28 and myself have; how would it be used.
Luv em or hate em, Comp does have a wide variety of lobes and they don't keep secrets about how they measure.
A .006 to .050 difference of 55 is not for everybody. Kip knows this and now a buyer can also call their .006 durations (but still not the .200). Soon Comp will learn that people will pay more for a 8620 core and cam doctor report and offer it for a upcharge just as Kip now offers a 1050 core for $50.00 off the 8620 price.
I thought choosing valve events at .006 and at .050 would dictate which lobe to use; is this where I'm making a mistake ?

 

In my opinion they are two completely different considerations.

You likely don't have the data you need to choose the correct lobe for your engine. To really choose the correct lobe, you would need to know the acceleration rates, velocity for each lobe (which pretty much nobody gives you), the mass of all of your components, and you would need to be able to mathematically quantify what this would mean in the real world in regards to component load and deflection as well as the resulting loft and seat bounce. Hobby hot rodders not only do not have this data, 99% of them have never seen it or have any frame of reference to what is optimal.

There are tons of monkey-see, monkey-do trends in hot rodding. From cam selection, to tuning and everything else. In the past we have done things wrong for decades before finally realizing we had it wrong the whole time. I think technology and the internet have shortened this cycle, but it still goes on plenty.

Specifically, I think there is a lot of monkey-see, monkey-do that goes on with lobe selection. For a while the buzz was that a rodder had to have the most aggressive lobes that a vendor would sell them. Many LS guys had the idea that the LSK was the baddest lobe they could get therefore it would make the most power. It did not matter if they were breaking stuff, they just thought that was the price of being a baddass. Most of these guys had no idea they were actually making less power because their particular valvetrain was out of control with loft and bounce.

While this may be a new conversation for some in the LS world, this has been going on for decades. Back in the 80s when I raced Buick engines, the class racers got hot on one company's "cheater" lobes. These lobes had much more aggressive acceleration rates and .200 up durations for the given seat timing and lift. The lobes damn near looked square. I can't tell you how many guys had these things in there engine. Then somebody actually did some definitive dyno testing and found out these "square" lobes were losing power like crazy at high RPM. Oops.

This is why I choose a lobe that has optimal stability for the valve train, then choose the valve events for the desired power curve characteristics.

 

Would you agree that the events at .006 lift tell us more about what's going on than the events at .050 ?

I found a formula relating header primary to EVO

P = [(850 x ED) / RPM] - 3 Where:
RPM is the engine speed to which the exhaust is being tuned.
ED = 180* plus the number of degrees the exhaust valve opens before BDC.
P = Primary pipe length

It seems over simplified by not taking pipe diameter into account. Still, I have questions. Is the 850 chosen to indicate the speed of sound inside the hot pipe ? Is the -3 a guess of how long the exhaust port is ? What RPM should be used ? Is ED the EVO at .006 or .050 ? The cylinder pressure is released and the sound wave launched as soon as the exhaust valve cracks open. I want to turn this formula around ; known primary length to find ED.
I remember collector length as 24" tunes the wave to 5000 rpm and each inch shorter raises rpm 250 . If the collector is going into a X or Y, the pulse is much weaker than if the collector is going into atmosphere.

My next question is probably simple but I don't know the answer. How much negative overlap would a mild cam (224/228) need to be able to tune out all lope ?

It really is. Understanding the why is much better than finding a band wagon of monkeys to climb aboard. And remember Martin, Steve; your not just helping me. 10 years from now, searches will still bring views to this thread.

 

probably a stupid question, but can the fumes in the exhaust be countered by telling the injectors to fire later?

 

I have always been of the thought that primary diameter tunes the point where peak torque occurs and length shapes the curve before and after peak. If I could have a perfect header for a street car, it would be smaller in diameter such as a 1x3/4" stepped to 1x7/8" and it would be 25-27" in length.

The smaller primary keeps the torque peak at a lower RPM(4600-4800rpm) and the step helps to broaden that peak torque "area" and give it a flatter appearance. The shorter primary then allows power to hang on upstairs with good valve timing.

I would assume that the formula is based on seated duration figures and not @.050" or higher lobe lift figures.
Yes, you can fire the injector later once the exhaust valve has closed which helps with fumes. This also helps with IPW and IDC as you're not using as much fuel to achieve your desired AFR.