Recently, I have been taken to task for saying that if you expect to get any sort of reasonably intelligent answer to engine tech questions, Speedtalk is the place. So to be fair, I am going to test my hypothesis out. This is a REAL question that I have had for sometime, and I cant reason it out for myself, so I have posted it on that "other" forum. In the interest of fair play, I am going to post the same question here, and see what kind of answers I get. My own guess is that this will rapidly disappear off the bottom of the page, but heres your chance guys. Here is the question as posted on Speedtalk. The race is on. This has been bugging me for sometime now, and I just cant wrap my head around it. Why does a port restricted application call for a narrower LSA? It seems counter-intuitive to me. Can anyone elaborate on this?
3 hours, halfway down pg 6, 59 views, zilch, nada, nothing. I'll give 'er a bump, just to make sure it gets a fair shot.
with port restriction you want to open the valves as soon as possible & close them as late as possible (in a race engine) also with restriction, maximum valve lift is a mute point after the port gets full flow - any additional lift is doing nothing & is harder on the valve train - see nascar restrictor plate engine cam specs for examples
Which is exactly why I cant get my head around this. For a given amount of cam advance, narrowing the LSA closes the intake valve earlier, not later, so I am looking alot at the overlap cycle, rather than intake closing, but I'm still not seeing it. Your second point has me thinking, you may be on to something there. Thumbs up for having the balls to step up to the plate.
Narrowing the LSA increases overlap for a given duration. The overlap cycle , when both intake and exhaust valves are open, provides a chance for the exhaust flow to help pull the next intake charge into the cylinder.. This only works on engines with well tuned headers and intake combos. That' why most stockers have a 112 or greater LSA and race engines generally are down in the 106 to 108 range. A narrow LSA on an engine with a crappy exhaust and intake just produces a poor idling, no low end torque, fussy to tune motor vs the same cam with a wide LSA
Wrong way round mate! After the exhaust valve closes on the exhaust stroke, the intake opens on the intake stroke [next cycle]. The point of overlap is at the end of one cycle and the start of the next cycle on a 4 stroke. narrow LSA is retarding the closing of the exhaust and advancing the opening of the intake [more overlap]
^^The intake valve opens before the exhaust closes, not the other way round. The cycles arent all neatly divided up. Even if we assume that your opening statement is a slip of the fingers, this answer doesnt address the question. We all grasp that narrowing the LSA retards the closng of the exhaust and opens the intake earlier, the question is why would you want to close the intake earlier in a port restricted combo? And if you open the valve earlier, as you say, its pretty clear its gotta close earlier as well, other variables remaining the same. Take a good look at a couple cam cards with similar duration and narrower LSA's.
I'm going to start my response with a disclaimer, admitting up front that I'm no expert. That being said I believe you are correct if you view the cycles of a four stroke from a static perspective. I used to. But as it was explained to me once, an engine actually draws more on the overlap then it does on the actual intake stroke. So you maybe need to think of the intake stroke actually starting at the end of the exhaust stroke. For a better understanding, go to Isky's website and read Ed's thoughts on overlap and his so called 5th cycle. Ed has forgotten more about cams than most of us will ever know. When I was younger I thought I had it all figured out on cams and then the more I studied the more I realized I didn't know. It is truly a fascinating topic. I also don't think that a person can just make blanket statements about cam designs and engine designs and have them be totally acurate. What works in one may not work in another. JMO.
In my view LSA is not the most important factor , Duration is.. You select the duration based on where you want your max rpm and torque to occur.. LSA will adjust the shape of the torque curve usually more max torque but in a narrower rpm range.. Again on a restricted port LSA will not crutch a poor flowing port unless the intake and exhaust systems are upgraded..
Yes, well, thats not the question at hand, so lets not muddy the waters. The question is, why do port restricted applications respond to a narrower LSA, not the role duration plays in cam selection, or for that matter, the role R/S ratio plays in LSA selection, or anything else. The reason I asked this question is 1) this statement appears in a number of fairly credible places, David Vizard for one, and 2) real life examples provide anecdotal evidence that supports this, for example, restrictor plate oval track motors, as David has pointed out, and port restricted NHRA legal SS motors, such as 283's, 305's and small block fords, particularly prior to the opening up of the cylinder head rules, where LSA's in the range of 98-104 were relatively commonplace. While I have pretty much accepted this at face value, it has always rubbed me wrong. I had been looking at overlap alot, but my gut reaction was that the increased overlap in and of itself, relative to say the same camshaft on a 106/108 LSA, was unlikely to be able to compensate for the earlier intake closing. Keep in mind that any way you slice it, even on a 108 LSA the cams in question pack a BUNCH of overlap. My gut reaction is that David has clicked on the key here, namely getting the valve open as far as possible as early as possible. This does tie in well with your earlier comment re: overlap helping to initiate intake flow early in the intake cycle. I think that when we put the two together, we are closing in on the "why". Absolutely agreed on your last comment. If the intake/exhaust runners are out to lunch, narrowing the LSA is going to make things worse.
Let's throw a wrench into it from a off topic point of view,sort of.. I build old Triumph 650 cc engines. These 40 cubic inch twin cylinder pushrod engines have a classic 90 degree 2 valve Hemi head,1.66 intake and 1.5 exhaust valves,reasonably straight ports that at a glance look quite large.Compared to a typical OHV car head these flow quite well. Triumph performance cams have a LSA's of 102-104.Typical cams specs are 250-270 degrees duration at .050 lift and .400-.450 total lift.A hot street engine peaks at around 7200 rpm.A stock head engine can make 1.5 HP per cubic inch and be somewhat streetable. Not much by modern Crotch Rocket standards but similar to 450 HP 302 Chevy,quite hot.Yes,it's easier to make power with asmaller engine. So what does the narrow LSA on the bike engine have to do with narrow LSA port restricted car engines used in drag racing? Something common for making power obviously...
You have to think about it differently here. A four stoke cycle consists of [ in order ] 1: Intake stroke 2: Compression stroke 3: Power stroke 4: Exhaust stoke Then.......................................... 1: Intake stroke [ this is where the intake follows the previous exhaust stroke ] Valve overlap and LSA are at the point where 1 cycle ends and another cycle starts [ not during the middle of a cycle ]
Heard from Mike Jones on Speedtalk. His position is that I am overthinking it, the restricted port will cause power to drop off very quickly after it peaks anyway, so there is little to be gained by closing the intake valve later, So it makes sense to narrow the LSA as much as possible to capitalize on the resulting mid-range torque gain. He also said that if you want the power to hang on, widening the LSA is the way to go, so it looks like the "rule" is not as solid as it first appears...So my initial dubiousness on this "rule" was justified.
I've been told alot of things over the years, and I never accept them at face value, hence this thread. Sit down and really think about this statement. Map out where the piston is through overlap, and how quickly its accelerating. Then think about where it is, and how quickly it is accelerating as the exhaust valve closes, and the piston moves down the bore.
something else to consider is that fuel/air is constantly trying to separate & fuel will always follow the long side of the port while air will always follow the short side of the port
In your NASCAR resticted engine example, I can see why a narrow LSA works.. It makes a higher torque peak over a very narrow rpm. Restricted motors only make power in about a 1000 rpm range. Everything in the intake through the exhaust is optimized for that rpm.. NASCAR cams also have extremely fast opening ramps esp the restricted motors... I think the pull of the over lap in filling cylinders is more important than later intake closing. After reading your other post, Mike Jones captured my thought much more succinctly
Hey falcongeorge you are not overthinking it. It is just difficult to explain in writing. They issue is that the breathing is reciprocating and the timing is rotational. In an idealistic situation the valve would instantly open to max lift at TDC and close instantly at BDC with only 180 duration. Now lets look at the scenario. At low RPMs the engine breathes in pulses [ or gulps ] but as the engine revs go up the airflow smooth's out in volume [ but there will be pulses of pressure spikes in the manifold as the valve closes ] With a poorer flowing head design, the valve needs more duration at maximum lift. so the ramping stage of intake valve lift is usually during the overlap period Which is why the intake is advanced so the intake stroke has more timing at maximum lift. At idle with low intake pressures,the exhaust gases will flow out the intake [and get gulped back in ] this is the cause of rough idle that we all love. As the engine revs higher port velocities increase to the point where intake pressures reach a point of equilibrium with the escaping exhaust gas pressures. and the engine smooth's out Advancing the cam can be a useful tool for tuning. But the problem is that it also advances the exhaust, causing the exhaust valve to open too early on the combustion cycle of the engine. Closing up the LSA is a way to advance the intake and retard the exhaust at the same time [ the trade off is idle quality ] Tight LSA’s are very common on oval track racing where the engine needs to pull off the corners at lower or mid range RPM’s . [ “pull like a schoolboy” is the Kiwi saying ] Wide LSA’s are better for High port velocities [ high rpm ] , this is because these velocities create a “Ram” effect so less cylinder pressures are lost due to overlap. but too wide LSA can also lose too much cylinder pressures at BDC on the compression stroke similar to retarding the camshaft In both scenarios engines with lots of duration usually need to have higher static compression to compensate for a bit of lost cylinder pressure.
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