Lobe seperation question

[Metrobilly]

Panic, I like your term "chamber flush", a very descriptive phrase that I hadn't head before.

Here is a contradiction that I cannot figure out. I have read here that often engines with relatively poor (stock) cylinder head flow need tighter lobe centers to make best power.

However, in the case of a head that doesn't flush well we may open the exhaust valve earlier for more "blow down". We also want to minimize reversion so may open the intake valve later giving the chamber more time to flush. However these changes would widen the LSA and are at odds with narrow LSA for stock heads that I read about.

I suspect there are other factors that I'm not considering.

[CARRITAS RACING ENGINES]

Chevy-power another instance you can be more aggressive with a flat tappet over a hyd roller would be in a (track rule restricted low lift) engine.

[kirkwoodken]

"Chamber flush" is one of the things that happens when the engine "comes on the pipe". Residual exhaust gas is a real killer of power. When all the exhaust gasses are being sucked out, good things start happening. It is also the reason a slight leak at the intake valve kills power so easily. Just blowing a small amount of spent gasses into the intake kills power.

[Metrobilly]

It seems "chamber flush" at high rpm is something my particular race engine doesn't do too well. There is a restrictive elbow in the drain pipe just above the exhaust valve. So I'm trying to figure out how to fudge the valve events to minimize the impact of this. I think there is BIG MOJO happening around TDC and the overlap period, but I just don't understand it all. Wasn't it UDHarrold that said the most important thing is the degree before IVO or something like that.

I like that "Comes on the pipe" phrase too.

[panic]

Re: "chamber flush"

I tried "purge", but that suggested that the exhaust was removed, but not that the agent doing so remained.
Generally, the overlap triangle window in a motor with poor breathing will be larger in proportion to the nominal events. If the port doesn't work, give it more time.
All cam/port/rocker stuff looks remarkably similar to the (then revolutionary) Naitoh-Nomura SAE paper using time in milliseconds + event duration based on rod angle + port window area to analyze, predict, and compare 2-stroke port function over 30 years ago.

[RCJ]

On my seat of the pants dyno a 106lc 224 @.050 is rough idling and choppy to about 2000rpm.Then it comes on instantly and pulles good to 5500rpm.A 112 LC idles smoother is easier to drive below 2000rpm, but doesn't come on as hard.Will keep pulling better after peak.As with all things your combo has to match to get the most out of either.

[SStrokerAce]

Is this why gm has gone to wide lsa's for their ls engines? Or is it to perserve the idle quality.

It IS ONLY FOR IDLE QUALITY, you would get better fuel milage with more overlap than the OEM cams have. The guys in GM Powertrain know this but the platform guys want smooth idles.

Bret

[Joe Mendelis]

Lobe separation angle effects (from chevy hi performance mag)

NARROW LSA
Increased overlap
Increased low-rpm torque
Narrower powerband
Reduced idle quality
Increased cranking compression
Decreased piston-to-valve clearance

[/b]WIDE LSA
Reduced overlap
Improved top-end power
Wider powerband
Improved idle quality
Reduced cranking compression
Increased piston-to-valve clearance

My questions are, if running a narrow lsa, the increased overlap should help top-end horsepower and decrease low-rpm torque/cranking compression because it would bleed off cylinder pressure at lower rpms right?
If running a wider lsa, you would have less overlap and therefore more low-rpm torque. But they say a wider lsa produces more top end power?

I'm sure thats the way it is but I'm just confused on how it works.

Any comments are greatly appreciated.

What a load of crap. This is why I don't buy those magazines.

[Procision-Auto]

Does that article mention anything about installed centerline, or duration
while making those generalizations?

[SStrokerAce]

With those generalizations how could it?

[Stan Weiss]

Does that article mention anything about installed centerline, or duration while making those generalizations?

Just a wild guess. Duration stayed the same and Intake centerline was the same as LSA.

[Procision-Auto]

My thoughts exactly (Bret).

Even further, are they comparing or mentioning the lobe and lifter type?

Some poor kid comparing a flat tappet hydraulic against a solid roller
with similar durations wont realize the overlap particulars on the same LCA (among other points listed).

Having said that, I wont be too quick to criticize as I have not read the
entire article.

Edit:

Would this be the article that is being discussed?

http://www.chevyhiperformance.com/techa ... index.html

[chevy_power]

No this in the article: http://www.chevyhiperformance.com/tech/ ... with that?

[randy331]

Is this why gm has gone to wide lsa's for their ls engines? Or is it to perserve the idle quality.

It IS ONLY FOR IDLE QUALITY, you would get better fuel milage with more overlap than the OEM cams have. The guys in GM Powertrain know this but the platform guys want smooth idles.

Bret

Why is that? I would have thought, with full ex, reducing overlap would help mileage. Too early of EVO?

Randy

[randy331]

What a load of crap. This is why I don't buy those magazines.

Come on Joe don't hold back, tell us what you really think.

I buy the magazine, I just don't always buy the info.

Hey, did you see the new artical where low lift flow is the most important, because the engine sees it twice, and high lift once?

Randy