firestorm

In a 305 with headers and automatic, which would yeild a better idle, the Compcams XE262 with 110 LSA or the XE262 with 114 LSA? This is with a computer controlled Qjet and all the smog stuff-have to pass visual AND sniffer They are listed both at hydrolic flat tappets.
Thankx

Stekman

The 114 will have the better idle quality due to the decrease in valve overlap.

The 262 is not too terribly large, the 110 may perhaps be able to pass with a proper tune. I am not sure, though.

scottland

if you want, it is only a couple dollars more, have Comp grind the XE262 profile with a 112 or 114 LSA for you. Just give them a call.

Stekman

They do offer the EFI cams with those larger LSA's. I believe the EFI version for the 262 has a 114 LSA.

Zerstörer

Will the powerband change on you going from a lobe seperation of 110 to 114 dgrees??

firestorm

Yeah, compcams offeres the XE262 in both 110 and 114 LSAs. The 114 is $20 or so more for some reason. Since both are offered, I figured I might as well try to get as smooth an idle as I can for the same power.

87Formula4bbl

What exactly is the purpose of the closer angle (110) as opposed to a wider angled (114) lsa? If they make the same power, or do they? My cam is ground on a 110 lsa and it seems to have trouble idling right, rpms jump around from time to time and just seems to idle just a little rough, overall. I thought my carb was way out of tune but I'm not so sure anymore...it's within operating specs, the computer should be controlling everything fine, but maybe the 110 lsa is a little extreme, I dont know. Oh well, next cam I get will be ground with a wider angle, assuming I still have the computer stuff.

BTW, Firestorm, I have the cc Qjet (LG4) setup. I'd go with the 114 or close to it if I were you.

IROCaholic

The tighter lobe seperation makes for more torque. Also I think that going from 110 lsa to 114 will change the power band but I am not sure of that. I know you will defenetly loose a little off idle torque I would go with a comprimise and go 112 if you can get a custom grind. Good Luck.

RB83L69

The wider lobe separation lowers the peak torque and the lower RPM torque (below the peak), and flattens the curve out toward higher RPMs.

A properly designed race car or other high-performance setup will keep the engine RPM within a relatively narrow band. The tighter lobe separation (later exhaust valve opening) narrows and raises the peak torque band, and the goal is to match the cam to the required engine RPM range. 106° is pretty common in racing cams. 110° was common for a long time on the. Computer controlled engines need all the help they can get at idle, and so 114° is common for replacement cams for those. Some factory cams have the LS as high as 117°. But in those the high lobe sep merely improves idle quality at the expense of torque, because their intake lobe is so small, that there's no higher RPM torque to flatten out in the first place. The engine is already useless by the time it reaches the RPM at which the LS would improve the curve.

Depending on your gears and converter, the car might be considerably faster with the XE256 instead. If you still have the stock 3.08s and the 1400 RPM converter, stick with the lesser cam. Too much cam in that low-compression motor will make the car a dog off the line.

Stekman

Expanding on what RB said, the higher the LSA, the sooner the exhaust sequence starts. This bleeds cylinder pressure at the very end of the down stroke of the powerstroke. This is what reduces maximum torque. Beginning the exhaust sequence sooner starts to bleed the pressure sooner, in effect, taking away the last little bit of 'umph' that can be drawn from that particular combustion cycle. This means the higher the LSA, the less cylinder pressure you have.

Like i said above, the idle quality will be better with the larger LSA as well. This is also related to valve overlap. The longer the 2 valves are open, the more of a chance the exhaust gas has a chance to get back into the intake. Computer controls don't like this too much. This is also caused, in part, by cylinder pressure. With the reduction in cylinder pressure due to the exhaust valve opening sooner, in conjunction with a smaller valve overlap period, there is less of a chance for exhaust gas to get forced back into the intake. Computer controlls do not like it when this happens. Which is why the computer controlled cams have the larger LSA and don't go to any extremes when it comes to duration.

You also said you have emissions. The wider LSA helps to reduce emissions, most particularly, hydrocarbons, wihch is essenstially fuel not getting burnt. The wider the LSA, you effectivly reduce valve overlap. When both valves are open, the air/fuel mixture has a chance to go right into the cylinder and out without combusting. The smaller the valve overlap, the lesser the chance for the mixture to do this. I'm not sure how much of a role that will play when looking at a cam with the duration of the 262, I know it can be a factor when looking at the larger durations.

Just food for thought on the "why?" aspects.

ditchbangr

iTrader: (1)

I don't agree with the above post. The wider 114 will produce more cylinder pressure because there is less overlap. Idle quality given between the 110 and 114 will be much better with the 114. It will be more responsive and more torque in the lower rpm also, not as much choppiness as a 110. The wider lobe seperations will have a wider powerband but make less peak horsepower than the 110 or narrower cams. Another reason why engines with alot of duration and narrow lobe seperation require more compression and gear, they bleed it off and need the gear to get into a higher rpm. Then let the scavenging affect of having the exhaust valve open to pull the intake charge into the engine with a large cam.

Not an expert, just built a few engines and talked to alot of people.

RB83L69

Sorry to hear that. But it doesn't really matter, because it's wrong.

Cam timing is all measured off of the intake lobe centerline. That means that as you increase the lobe separation, what's actually happening, is that the exhaust valve opens sooner; thereby allowing the combusted gases to begin escaping sooner. That's why it lowers torque.

On the other hand, that's also why increasing the cam's LSA improves idle quality and lowers emissions. Opening the exhaust valve early allows the exhaust gases to escape out of the cyl through the exhaust valve for just that much longer, which produces lower cyl pressure when the intake valve opens, which in turn produces less poisoning of the incoming intake charge. Exhaust gas contaminating the intake charge creates random misfiring, and that's what causes the "lope" we all enjoy so much in a motor with a big cam. Misfiring also increases emissions by dumping raw fuel or incompletely burned fuel out the exhaust, which is why a wide lobe separation helps lower emissions levels when using a cam that opens the intake valve early (i.e. has a large intake lobe, i.e. is set up for high RPM operation / performance).

What happens at higher RPMs as far as scavenging and all that is a whole different matter. The speed of sound becomes an issue; that is, how long it takes for the wave of increasing pressure from the piston beginning to move up in the bore, to reach the top of the cylinder. If the material right behind the valve doesn't "know" that the piston is coming back up, it won't begin to move in accordance with the piston motion. And even before the RPMs reach a point at which that is the predominant effect, the inertia of the exhaust gas rushing out of the valve helps scavnege the cyl of exhaust gas, which is more effecitve if it happens for longer; and if it happens that the intake valve starts to open while that's still going on, then it may help fill the cyl with fresh mixture.... if all the exhasut manages to get out first. The mixture always flows in the direction of higher pressure to lower pressure, so it won't go into the cyl while there's still pressure in it.

"Overlap" has nothing to do with it, although it is possible, depending on the cam design, that the issues in question may occur during that time. But it's not a matter of cause and effect, only a matter of coincidence. Valve event timing in relation to crankshaft (piston) motion and location, is the key to how cam specs produce the results they do.

RednGold86Z

iTrader: (1)

At idle, I'm thinking it IS the exhaust closing point determines a lot about idle (but also intake closing point to trap as much charge as possible). It's not only the amount of exhuast gas that's left in the cylinder that dilutes the intake, it's the pressure of it. During "overlap" the higher pressure exhuast can push up the intake port (or sucked up, however you want to look at it - above ambient exhuast pressure versus partial vacuum intake). This dillutes it really bad, and lowers intake vacuum simultaneously.

The 114 will be much easier to pass emissions with, but with the conservative size, the 110 may pass if everything else is tip-top.

Good luck.

RB83L69

One more time, let's take a hard look at the myths...

"Overlap" is the period of time that the exhaust and intake valves are open at the same time. OK...

Assuming a decent exhaust system on the car, i.e. lower pressure in the exhaust than in the cyl, "overlap" CAN NOT cause reversion into the intake. The fact that the exhaust valve happens to be open at the same time as the intake has NOTHING TO DO WITH exhaust gas going backwards into the intake manifold. That is, having the exhaust and intake valves open at the same time, doesn't create higher pressure in the cyl at that moment.

An early opening point of the intake, i.e. long duration, DOES however allow for the possibility of the spent gases in the cyl travelling back into the intake manifold. However having the exhausat valve open at that time DOES NOT cause reversion.

This whole "overlap" myth needs to go away. It's right on up there with "gotta have backpressure".

daverr

iTrader: (1)

i agree with ditchbangr
for a given cam duration the wide lsa has the good idle.the narrow lsa will produce more power(narrow peak) at the expense of idle.here are some examples of real world lsa differences. my honda cbr900rr has a very peaky motor,it idles at 1500 rpm and is a dog below 5000rpm. my truck idles at 500 and has real broad powerband from 1000 to 4500rpms .

ditchbangr

iTrader: (1)

For a street car you will want a wide LSA, Z06 is 117 I believe and I think they produce pretty stout off idle torque and a top end charge. Hence a wide powerband.

Ever drive a car with a 106 or tighter LSA and alot of duration. Sometimes they are a dog just to get moving. But watch out from 5K to 8K, their intended poweband.

firestorm

Wow, alot of info here. Thanx guys!!!! What would you guys recommend for this combo?

Here's what I'm going to run:
305 LG4 with computer qjet and dist
Performer EGR manifold
1 5/8" headers
3" cat and catback
corvette stall convertor
3.42 gears
700r4

Here's some of my choices, all avail from summit:
XE262 110LSA
XE262 114LSA
XE256 110LSA
XE256 112LSA

I figured the bigger cam with the wider LSA. But would I be better off with the XE256 like RB mentioned? I dont mind loosing a small amount of power to get a smoother idle. Plus lower emissions would be good.

RB83L69

I'd recommend the XE256 with the 110° lobe separation.

AM Racer

THis may sound really off the wall, but how about calling Comp directly? Tell them your setup and what you want it to do ( if within reason). I've spent a bunch of time with them on the phone in the past without disappointment in their recommendations.
Since it's their name on the cam, they're not going to steer you wrong since they're the first to be bashed if it doesn't work.

daverr

iTrader: (1)

i disagree.the more overlap u have the more reversion(rougher idle).my car idle at about ten inches of vacuum.I also run open headers.



so with that being said, i say the exhaust will go towards 10 inches of vacuum (intake manifold) during overlap .

scottland

the vacuum is drawing INTO the CYLINDER not up into the intake manifold.

exhaust gases aren't going to travel against the flow of air back up into the intake. They will take the path of least resistance, out the exhaust valve.

RB83L69

You can sure tell who was paying attention in high school physics, and who was sleeping.

chev496

ill tell ya right now that they all pretty much, 106, 108, 110, 112, 114 all have about the same power. A magazine, called i think best of tech just had an article about overlap. Search the web, its probly on here. I have a 107 cam in my 454, dont listen to the dawg comment. my car is a bat outta hell and it screams all the way to 5k. just ask 87formula.. hes seen it.... The idle is not all that unsteady, but then again it is a pretty small cam. I.E. 230 @.050 292 ad. .540 lift.

daverr

iTrader: (1)

ok take my cam for example the intake opens at 41 degrees btdc(begining of overlap).The crankshaft rotates 41 degrees to top dead center while the intake is open.The piston doesnt really move that much up.the remaining exhaust will go thru the intake valve at idle due to insufficient exhaust velocity.At hight rpm the exhaust velocity start to scaveng the intake charge at overlap.
Some of todays cars have variable cam timing in place of the egr valve.they change cam timingso they can dilute the intake charge with exhaust(overlap).

ditchbangr

iTrader: (1)

Your also talking 500 cubes vs. 305. And it "screams" all the way to 5K, impressive. Warren Johnson and Greg Anderson better watch out for you.

firestorm

I already know that Comp does not recommend the XE262 for a 305 because they claim the vacuum will be too low or something, or at least may people posted thats what Comp said. But many on this board are running the 262, so that why I was asking.

firestorm

You caught me. Twas I that was asleep. But it wasnt just limited to physics. math, english, history. Fact the only thing I was awake for was autos and shop classes.

chev496

look here ditchbanger.. dont matter the size of the engine, all camshafts work the same, so dont critsize me. Im just pointing out the facts, if you fail to listen to me, maybe ill find my magazine. There was about 5" of vacumn difference between the cams, and about 10 hp/tq in the power band. It did look better to go with a average 112. Anything lower than a 110 would be ungreatful on the street.

scottland

once again, path of least resistance, out the exhaust valve.

AM Racer

On a non computer controlled engine, the requirements are more relaxed as to what it can tolerate. Computer controlled engines aren't as friendly.
Why doubt the people that designed it?
They could have saved a few bucks in r/d if 2 different models weren't called for.