Best way to extend the power - lift or duration?

[andyf]

Assuming one has a reasonably well sorted out bracket type engine and they want to move the peak hp point up a little more. Mathematically it is possible to come up with more area under the curve by either making the valve lift curve wider (more duration) or making the curve higher (more lobe lift or more rocker arm ratio). I've used both tricks on a number of engines but I've never done a good A-B-A comparison. I know there are rules of thumb on how many degrees of additional duration will raise the hp peak rpm by but I've never heard a rule of thumb for additional lift. Does extra lift move the peak power point up in RPM or does it just add more power at the same RPM?

[ptuomov]

I'd be interested in hearing people's thoughts on this one, too.

I would naively just think that if you keep the duration constant and increase the lift, the one number that would be a good summary statistic about the increased breathing ability of the engine would be the head flow (CFM@28") at the lift at the maximum piston speed, usually in the neighborhood of 75 crank degrees ATDC. The peak lift probably doesn't matter itself that much, but if you're keeping the duration the same and have a higher peak lift, you'll be lifting the valve on average faster. That means that the valve will be more open at the peak piston speed point. Whether that's going to help a little or help a lot would seem to me be reasonably well captured by how much the head flow capacity goes up at the maximum piston speed point in the cycle. But others know more and better.

[groberts101]

lift can obviously make the power range fatter.. but it can also move it up slightly as well. Just not to the same extent as the additional duration can. IOW, they're both rpm/time related, but the added duration is whats needed moreso at higher engine speeds than just the additional airflow potential gained from the higher lift which MAY allow the motor to peak higher than it did with the lower lift setup.

Myself?.. I always prefer to add rocker ratio whenever possible because it typically helps everywhere in the rev range(makes the lobes more aggressive) and still keeps the power from nosing over a bit better up top as well. Adding too much additional duration to an already larg'ish profile almost always comes at the added expense of at least slight loss in the lower ranges. Just depends on what you're willing to trade to get that higher peak, is all.

[ptuomov]

Plus once consideration is that often the valvetrain is at some sort of acceleration and/or velocity related reliability limit, and the only way to get a little bit more lift reliably is to add a little bit of duration.

[bigjoe1]

My experience is that the duration will move the peak speed, but lift does not move it at all. This seems to be true in mils as well as serious race engines.


JOE SHERMAN RACING

[modok]

Another trick question?
you'll get the best results fixing what needs fixed, go after the weak link in the chain.
If there isn't one, then you'll want to increase everything equally.

[mag2555]

Duration will move the power band far more far more than lift!

If your heads are not flow saturated at the lift you are running and you add more lift you will end up lowering the rpm of peak torque and fattening the current rpm power band you have .

[hoffman900]

Duration will move the peak up, lift can help anywhere.. BUT

Adding lift might require adding duration due to the lobe shape/intensity/dynamics.

[Stan Weiss]

Telling about duration (time) - lift (area)

http://www.yellowbullet.com/forum/showt ... 130&page=3

Stan

[pdq67]

Might go back in time and read the write-up on the "great big" old car "Heavy Metal" that onna the mags, (CC or HR?), built that shortly got stolen and only a front finder was found from it.

It was about a 496 turned 498 after changing stock rect. heads to Merlin Ovals and EFI as well as a bump up to some "MORE" compression and finally a roller cam that was almost identical to the one they started with except for LIFT!!

It is a very informative write-up.

pdq67

[groberts101]

Assuming one has a reasonably well sorted out bracket type engine and they want to move the peak hp point up a little more. Mathematically it is possible to come up with more area under the curve by either making the valve lift curve wider (more duration) or making the curve higher (more lobe lift or more rocker arm ratio). I've used both tricks on a number of engines but I've never done a good A-B-A comparison. I know there are rules of thumb on how many degrees of additional duration will raise the hp peak rpm by but I've never heard a rule of thumb for additional lift. Does extra lift move the peak power point up in RPM or does it just add more power at the same RPM?

guys.. maybe reread the intro posts wording in more detail and don't carry it too far beyond the original questions and their phrasing.

I certainly don't claim to have the experience that some others posting here have but I will say that there has NEVER been one single occasion where I have stuffed a larger ratio rocker on ANY engine and not seen the peak power rise another 100 or 300 rpm higher up the rev range. Go from a 1.5 ratio arm to a 1.8 with a stiffer spring and I assure you that it's easily towards the higher end of the range. Works especially well with stock/rv style cams if you get enough spring in it to keep things under control. It is commonly the case to add a higher ratio rocker to a stock cam along with a couple degrees of retard and see an easy 500 rpm peak power rise. I've done it dozens of times and it works well when you don't want to change cams out.

Sure, we need more duration(time) to raise peak power rpm significantly(though I didn't see that word used above).. BUT.. gaining additional airflow through higher lift ranges will also give the engine the ability to make more power through increased breathing potential higher up the ladder too. And as mentioned, also allows the motor to carry power just a tad longer like increased exhaust duration would also allow. Add the two together in smaller amounts and the rev range can be extended without the larger tradeoffs that leveraging only one or the other would typically incur.

[Walter R. Malik]

I remember putting a 1.8/1 rocker on a big block Ford engine where a 1.73/1 ratio used to be and lost power everywhere.
That 288/302@.050" camshaft just didn't want more lift than the .883" it already had.

Ended up that it needed a shorter runner intake manifold to move the power higher in the RPM range.

[bigjoe1]

I have seen that same thing soo many times myself--- Theory's are all well and good, but the results do not always agree with them.


JOE SHERMAN RACING

[turbobaldur]

Any port is going to have some point where increasing the lift doesn't increase the flow, let's call that minimum lift for full flow. However as people mentioned above, in order to limit valve acceleration it's going to be beneficial to run more lift than necessary for maximum flow, in order to maximise the time spent at or above that lift. The greater the acceleration can be allowed, the greater the surface area under the lift curve for a given duration and lift.
That being said, increasing the peak lift without altering the acceleration profile or time spent above this minimum lift is not going to have much effect.
If you increase the rocker ratio for example, you are increasing the duration spent at useful amounts of lift even though the seat to seat duration remains unchanged.

[andyf]

The engine I'm working with at the moment is making very good power but I haven't been successful in moving the peak RPM up much. I've dyno tested 3 cams and 4 intake manifolds and a bunch of different carbs. I keep finding more and more power at the same RPM peak so that is going good. When I try a longer duration cam I lose power down low but don't pick up more RPM so I might be head limited. PipeMax says I need more lift, Controlled Induction says I need more duration, etc.

I'm going to try adding more rocker arm ratio since that seems logical to try. Worse case it probably just adds more power at the existing peak point which is something that is fine with me. I just installed a fully ported intake manifold and picked up 30 hp at the peak point, but the peak RPM didn't move up any. I was thinking that the intake was the choke point and I guess it was to some extent, but it didn't allow the peak power point to move up any further. Easy answer is that the heads are choking the flow but I've talked with other engine builders who have pushed similar combos to higher peak RPM points so that answer is iffy.

Anyway, thanks for the replies, gives me some additional thoughts to chew on.