Yet another one, up at work and we dont have many calls coming in so I have some time to type. I saw some guy the other day post about which cams he should use on a B16A, I started thinking more about the advantages and disadvantages of excessive lift and dureation. Hoping to hear back from non-VTEC, EE_Chris, Lsos, and anyone else who cares to participate in this session.

The way I've been thinking is that you have the three varibles here, the lift, the duration, and the timing. The timing is talked about so frequently in tuning a car, since you have the ability to adjust valve overlap and tune the cars maximum cyilnder pressure closer to the time of spark, amongst other things. With the duration long, there is more overlap, but there is also more time for the piston to take in air/fuel (duh) Longer duration, as I see it, can accomplish a thing or two. One of which being clearing out burnt gasses from the combustion chamber. The other being that with overlap on a turbo motor, the excess air that does not make its way into the cylinder goes toward spooling the turbo. Although with excessive overlap on a turbo vehicle, you can begin to approach the point where there will be too much unburnt fuel finding it way into the turbo. With valve overlap at the top (end) of the exhaust stroke as the intake valves are starting to open, this increases the chance that burnt gasses will make their way into the intake tract momentarily before they are brought back into the cylinder, this is neglideable at best but can hamper performance none the less.

On the other hand you have lift, with lift comes intake velocity, or lack there of. This is another point where the faster moving piston on an all motor car can benifit the motors breathing capability, with higher piston speeds comes the increased intake "sucking" effect, allowing the cam profile to have more lift without sacrificing the velocity of the intake charge, and therefore hampering atomization. Of course, again, turbo can overcome this lack of velocity. Another disadvantage to increasing lift is the increased need for deeper valve reliefs on the piston. We all know that the smoother the surface on the piston (as well as the combustion chamber) the better the quality of the burn. A more homogenous burn equates to less unburnt fuel, the benefits of which can be tremendous. All in all, the basic thought is that the more air and fuel that can be entered into the combustion chamber, the more power that will be made. Of course there is the often unaccounted for variables though, such as atomization and quality of burn that can be comprimised.

One more thought.....about supercharged vehicles....and this is a vague thought at best. One thing that I have heard people complain about in supercharged motors with valve overlap is the amount of completely unused air/fuel that results from overlap. There is no turbo that can benefit from the unburnt air/fuel that makes its way out of the exhaust valves. The major benefit on a supercharged vehicle, is when it is able to keep as much of the air/fuel that comes from the intake tract in the cylinder, this can be accomplished through precise valve timing. Theoretically, there woud be a benefit than to have the exhaust valves completely closed from the exhaust stroke when the intake valves open. and therefore not letting any of the fresh air/fuel out of the exhaust. Accomplishing this would increase cylinder pressure and theoretically increase horsepower. I would also think that this same procedure could be accomplished by matching cam profiles properly, have a larger duration and larger lift cam on the intake side, and having a slightly shorter duration cam on the exhaust side. This way there could be the increased benefit of the intake valve staying open longer and at more lift, and not interfering with the exhaust cam's opening causing valve overlap. Although the exhaust cam would have to be large enough in duration and lift to allow the clearing of exhaust gasses from the combustion chamber on the exhaust stroke. Hmmmmm...........

Any thoughts?

 

interesting stuff riceboy...........for a riceboy you sure know your stuff.....hehehe

one think many people overlook (and so do i, mainly because alot of cam manufactures dont specfy the specs) is the lobe seperation angle.

written up my Micheal Delaney on team-integra.com. he explains just about everything on cams and specs..........very informative articals....... http://www.team-integra.net/sections...p?ArticleID=51

The shorter lobe separation angle cam produces more peak horsepower, but with a loss of low-end torque. Shorter lobe separation angle is better for a drag engine than a street machine, due to an increase in valve overlap. LSA can be changed by changing the cam gears when you dyno tune.

 

one thing i'd like to add which i should have before.......

the absolute beauty of VTEC is that you can have very mild primary lobes and very agressive VTEC lobes for the crazy high RPM power eveyone loves. provided the cam manufacture can design them in a way that will not damage anything when VTEC engages by having to much lift over the non-VTEC lobes.

but not everyone has VTEC so we are stuck with a slighly mild cam that will idle and run like stock but will not justify the cost for the ammount of power your going to get. or you get the biggest lumpiest cam they make and have one hell of a time trying to tune this beast and try to get it to run right.

the whole overlap issue is not important with N/A like you stated earlier but it does "sometimes" pose a problem with forced induction.

some fellow HT.com board members running turbo find that running stock cams work best for what they need. and i recall some posts about one particular member running a high boost turbo LS using crower 62403 N/A cams and making incredible power and getting aweseome track times and having no problems at all.

on the VTEC side alot of people say use the stock GS-R cam shafts with turbo because they have less overlap and work the best. then there are those who run ITR cams and make great power with those.

the fact of the matter is that every single engine is diffrent and will respond diffrently to the type of cams used. i think it's all in the tuning..........anyone who dissagrees doesnt know what they are talking about........

i hope my rambling makes some sense........... because i think i just confused myself...?

later.......

 

Good show Rice Boy Right now I am trying to find out the stock specs on the b16a and the b18c cam timing. I am trying to tune my car without a dyno as we do not have any dynos where I live. Once I get the stock specs I will try a few recommendations using my adjustable cam gears. If you can help me get some more info on this it would be a great help .
Thanks

 

The reason that there are no lobe seperation specs on DOHC engines is because they are set by the cam installer (with adjustable cam gears) and not ground into the cam as with SOHC engines. No matter what people say about moving the cams, (both n/a and f/i), some engines just respond the opposite to theory. The only true judge is the dyno.

 

Solid stuff man.
Blow down on a turbo motor wont do anything but help keep the tops of the pistons cool. Often that is the only advantage of this in a boosted motor and will be retained just for that. Unburnt fuel and air mixture does not have enough energy to spin the turbo any faster. Blow down on a N/A motor helps breathing @ higher rpms.
I think what you say about lift seem right. I don't really know, but just thinking about it, it seems to me that the duration would be of more importance. Have the cam open the valve to a little more than stock depths and then hold it open longer would probablly make more power in a boosted aplication. Up untill the point where blow down becomes signifigant. In a N/A situation its all about velocity as you said.

 

To be honest, I'm not trying to re-invent the wheel....I really got nothing to add that hasn't been added. I'll only say this though...the Team Integra website (with link posted by non-VTEC) is one of the best tech write-up sites out there.

His articles (along with a truck load of others such as piston tech, ignition timing & flow quality/quantity) on this very same topic are pretty damn sound, along with being easy to read.

So if you wanna know my thoughts, I share Team Integra's.

But....excellent post riceboy. I always enjoy the more thought provoking type of threads, or rather, the threads that had some thought put in them.


[Modified by EE_Chris, 4:14 PM 4/18/2002]

 

How can the burnt gasses make there way back into the intake tract on a turbo motor? Don't you have positive pressure from the turbo?

LSA on DOHC cams is usually given assuming gears are at 0in/0ex. (ex. Crower 62403, LSA is 113 degrees.) Which is easily adjusted via gears. LSA is sometimes referred to as LCA(Lobe Centerline Angle), not to be confused with Lower Control Arm.

Every motor is different and will like different settings. But basics are the same, big lift is good and sucking takes longer than blowing.

 

If you are operating on the wrong end of the surge limit of a turbo (the left side of the surge line on a compressor map), you will actually have exhaust contamination into the intake charge. That can cause a melt down over a period of time.

 

What happens when the compressor surges? I've seen it on the map but don't know what that actually implies. Does the cotamination stem from the exhaust gasses not fully evacuating the cylinder, or actually entering the intake tract?

 

well ramp rate also comes into effect, piston acceleration speeds relative to cam specs, i.e. conrod ratio, bore relative to stroke and also cam centerline is important and does vary for various b-series motors.

other than that great writeup.

Greg

 

Dayum, I leave this topic for a day, and the topic has all but changed!

Anyways, I spent some time looking over the team-integra post, looked good. But the more I looked at it the more I noticed that the higher lift and duration cams (eg. Toda C, Skunk III, and Jun 3's, and the second level cams as well from those manufacturers) The exhaust cam duration on those cams is close to, and sometimes the same as the Intake cam duration, which would obviously increase the ease of overlap while operating in the VTEC lobes. But, when I look at the specs for the Honda manufactured cams, be it the GS-R or the Type-R cams, I notice that the exhaust duration is much shorter than that of the intake cam. This I think could be the reason that supercharger manufacturers reccommend the useage of stock profile cams with their systems. To increase the ability to keep the fresh intake charge in the cylinder through lack of overlap.

And as for the dyno charts posted on team-integra, it seems obvious that an increase in the duration and lift provides large horsepower increases at the top of the RPM band. Although on the dyno graphs presented there was no low RPM loss of power due to increased duration. Granted, the torque peak was moved higher in the rev band than with the cam with less duration, but looking at the charts, you will notice that at the point of torque peak on the cam with less duration, the cam with more duration is producing the same amount of torque and still climbing for another 1000 RPM until it's peak at 4500 RPM. Yes, indeed, some of the lower RPM (below 3000 RPM) torque is lost, but how can this matter when the offset is a more than substantial increase in top end power? On that same dyno as well, horsepower numbers are identical until the higher RPMs are reached, then there is the showing of increased horsepower. Keep in mind that the cam gears can be used to change the lobe seperation angle here, moving the torque curve and horsepower peak while substantially reeping the benefits of increased flow.

And, on to the dyno on lift, I think that this one is blatently obvious to anyone that looked at it. When LSA is identical between the two cams, torque in lower RPMs is not effected by the increased lift. (At least not in these dynos, I would be interested to see the flow bench numbers for the particular engine that the test was performed on, as well as the actual cam specs) The lift of these cams could not have been "enormus" or there would have been s drop in low rev power due to velocity, that is just simple science. But for the most part, this is an accurate report over lift, lift results in an increase in horsepower production in the higher half of the rev band. Thus the reasoning behind VTEC.

On to the last dyno graph, the more interesting one to look at. The LSA dyno. In this dyno you will see, like stated on the post, that the cam with shroter LSA makes more peak horsepower, while loosing some in the low rev torque area. Yes, this is true, but look at the dyno closer and you will notice that the shorter LSA cam (with dots) has the benefit of horepower taking over when torque begins to fall off. With the cam with the longer LSA, torque AND horsepower both fall off considerable at in the higher end of the rev band. Although, these dynos were performed on a normally aspirated application, there is alot that can be learned from them. Though, looking at the dyno graph for LSA, the usage of a turbo for the effect of forced induction could benefit here on the cam with the longer LSA (no dots) For when the torque peak to horsepower transition I mentioned before takes place and both then begin to "fall off" this is at the time that the turbo would be at full boost, thus horsepower would take over due to the fact that the engine is not scavenging for air but being provided with it.

And one thing about VTEC, has anyone else ever noticed the horsepower and torque drop when VTEC engages? This (as I am sure this is no secretto many of you) is from the fuel "dump" so to speak that the ECU gives at the engagement point to overcome any differences in velocity that may occur. Thinking about this for a moment, if you were to run larger non-vtec lobes, then there would be less of a transitiion factor to the VTEC cam lobe, and less need for fuel dump. The only issue here is getting this low RPM lobe to idle correctly, pass emissions (if you care about that), and and not create a lack of velocity/swirl on the lower half of the rev band, this obviously would defeat part of the theory of VTEC as to producing good swirl and good emissions at low RPM.

Still thinking on the topic of flank angle.............

 

One thing that I noticed is the idea that overlap causes fresh air to make its way into the turbo....

I've heard from sources that this is the opposite of what happens, as overlap actually causes reversion of the exhaust gases back into the combustion chamber. Sure the air in the combustion chamber is pressurized, but very rarely does that pressure exceed the exhaust pressure (only on hardcore racing engines). This is why you need a large exhaust A/R on VTEC motors, to minimize the exhaust backpressure. Either way, the tendency on VTEC motors is for the higher pressure exhaust to make its way back into the combustion chamber, not the other way around.

 

>I've heard from sources that this is the opposite of what happens, as overlap actually causes reversion of the exhaust gases back into the combustion chamber. Sure the air in the combustion chamber is pressurized, but very rarely does that pressure exceed the exhaust pressure (only on hardcore racing engines). This is why you need a large exhaust A/R on VTEC motors, to minimize the exhaust

IME you are correct sir. On my car the exh backpressure has been in the range of 45+ psi with say 25 psi in the intake. The exh pressure will win in this case yes it does have reversion into the intake. Though maybe not an ideal situation, no it does not make the car slow. Part of life with a turbo.

But on a street/strip car IMO and IME it's far better to trade a smaller a/r exh housing with better response than go with an oversized a/r to try and minimize backpressure. The response loss will be huge, while the backpressure gain will be minimal. Not worth it IMO for a street/strip car, especially with an automatic. Even with like 1.5 x pressure ratio from exh to intake the turbo car can still make tons of power

turbotr

 

I have never thought about the point that was made earlier about the cooling effect that the air moving over the piston at the point of overlap has. That is interesting but true. And as far as backpressure against a turbo goes, that is just up to the discretion of the user, cause you can't have your cake and eat it too Since a larger A/R reduces backpressure, this can have its benefits, there is less wear on the motor and the turbo, your parts will last longer. But also you will have a cleaner burn due to the reduction of exhaust gasses in the combustion chamber, but at the sacrifice of spool time. The pistons upward force on the exhaust stroke can always inflict reversion, but ports and combustion chambers can be designed to reconcile its effect on performance. Of course, then again backpressure does not become one of the more major factors in horsepower production until you reach the twenty psi and upwards range of boost pressure. Personally I'm a fan of a higher quality burn and parts that have less stress induced on them by the motor, but then again sacrificing power can always make me re-think this.

Thinking about flank angle for a moment, here you have the oppurtunity (through extensive tuning, and a budget that probably none of us have) To theoretically control the ammount that the valve is open whenoverlap occurs. Generally thinking, overlap is look upon as the time that both the intake and exhaust valves are open, but how much are they open? Are they at full lift? The opening flank and the closing flank can be different angles, so theoretically, the valve does not neccacarly have to be completely open at the point of overlap. (This I have known, but have never thought about how to possibly take advantage of until now.) The more I think about this, the more that it makes sense to take advantage of these angles, but theory is one thing, utilizing it is another, for flank angle to be utilized, and a specific amout of valve lift at point of overlap to be constantly held, a new cam profile would have to be ground for all timing changes. Throw the timing off one degree on either cam, and now the valve lift occurs sooner/later in the overlap cycle. Hmmmm, need more time to think about this one.

 

Thinking about the flank angle more, and how it can be used to optimize overlap. Lets say that you have an exhaust cam with a thirty degree flank angle on the closing side of the cam lobe, and an intake cam with a fifty degree opening flank angle, for the sake of argument, lets say that they are both midway (or there about) down (or up, whichever you would like to refer to it as) the flank at the point of maxium overlap. The intake valve will at this point be open more than the exhaust valve while the motor is completing its exhaust strok and beginning its intake stroke. The benefit of this is that the more of the intake charge will stay in the cylinder, due to intake valve being plenty enough open for the intake charge to flow into the chamber well, while leaving enough of an opening in the exhaust valve to clear the last bit of the exhaust gasses. Now there is one readily appearent downside to this that I can see immediately. At the combustion point, it is better to have the combustion directed toward the exhaust side of the motor, now, with the above stated cam specs, on a normally aspirated application with high compression domed pistons, there will be the need for larger valve reliefs on the intake side of the dome, this will immediately increase the volume that is on the intake side of the combustion chamber, and the combustion will naturally find its way here and be more concentrated in this area. This of course is not good for many reasons. Although, on a forced induction motor, this dosent matter since the pistons woouldnt have domes. Hmmmm......still thinking.......anyone got any opinions on this?