Dear Mr. Know It All:

The author of this article would appear to agree with me that valve overlap is in fact a consideration in what static CR can be sustained without detonation. While the intake closing point appears to be more important, stating that overlap is "wrong" is not supported by at lesat one publication. There are plenty more publications that state essentially the same things. What published authority are you relying on, Mr. Know It All?

http://www.kitcarmag.com/techarticle...ion/index.html

So get your hand off the horn button.

 

Camshaft timing also plays a huge role in dynamic cylinder pressure, especially with street-driven performance engines. As you increase intake duration, this means the intake valve now closes later than it does with a shorter-duration cam. This later-closing intake valve bleeds some cylinder pressure back into the intake manifold at lower engine speeds. The longer the duration of the cam, the later the intake closes. This reduces cylinder pressure at lower engine speeds, which reduces the tendency for the engine to detonate.


You can also experiment with camshaft overlap. Unfortunately, this requires a new camshaft. Tightening the lobe separation angle, from 114 degrees to 110 degrees, for example, increases the amount of overlap since the exhaust valve closes slightly later and the intake valve opens a little sooner. This tends to bleed off cylinder pressure at lower engine speeds, which could be beneficial since this is a little like built-in exhaust gas recirculation (EGR) in the intake manifold.

Isn't this fun stuff? Jeff Smith hit a lot of key points in that article and I can't really disagree with them all. But they take some reading to think it through. The EGR comment will bleed off some pressure to a degree because it will cool the cylinders....but that's a heck of a way to do it. You will not want that going on at all times. It will kill power and driveability.....just like any other time you add EGR. When comparing cams you really have to look at the lobe itself as well as the I/C and LSA because there are a lot of ways to get very similar cam timing events for intake closing.

Overlap can be increased by narrowing LSA with the same lobes....but I can use different lobes and LSA and get the same overlap results. So you have to look at the whole combo of lobes and positioning to make it work out.

Generally speaking there is nothing happening as far as cylinder pressure during overlap so no change in detonation resistance. But obviously cams with different overlap figures can have different intake closing points that will have the biggest effect on pressure.

Now Jeff's discussion on inducing EGR with overlap is a little out there and generally something that most anyone would not want to do. Intentional reversion is a bad thing and controlling it to actually make that concept work is very iffy.

The big issue is that dynamic compression really does vary due to intake tract efficiency and RPM. If you have killer heads and intake...once you stuff your foot in it those cylinders are going to get real full and you will eventually hit detonation as RPM climbs if fuel is not up to it. I can drive a 13.0 motor on 93 octane easily on the street at light throttle...but once the throttle cracks WFO it's going to increase pressure enough to cause problems. Your normal compression ratio engines are the same way...it just depends on how full the cylinder can get. I know of some 9.5 motors with fast acting short duration cams that will detonate with more than 32* of timing in them because cylinder pressure gets real high real fast due to great heads.

The trick is to balance the amount of pressure you end up with for the fuel and RPM range along with all the other stuff mentioned.

Rod length can change things somewhat due to dwell time and rate of change in piston position. For example...a small bore,long stroke, short rod motor will get by with higher ratios since the piston will be moving quicker (relatively speaking) from TDC and cylinder pressure drops quicker....rather than the other combos with higher rod/stroke ratios that have the piston hanging out at higher positions slightly longer which is often designed in to allow higher pressures to push the piston. Again..there's a fine line.

Good discussion guys!

JIM

 

So low energy/stock plug gap is as good as high energy/wide plug gap?
I guess there's no need to worry about whether the plug is shrouded either with my big domes? No reason to index!
Doesn't anything that affects the exposure of the fuel/air charge to ignition have an impact?
Jim already addresed rod length.

DT

 

.......well at least I quoted an actual published article and didn't tell anyone they were "wrong" ot "fired."

Some people are just too quick on the trigger finger.

As it turns out, in the very early 70s the OEMs did exactly what was suggested in the Jeff Smith article; they took their earlier cam designs and narrowed the LSA for the purpose of lowering combustion temperatures. That was supposed to reduce NOX in the exhaust; it was before EGR valves became widely used.

 

The old saying regarding the CR is "Build it with the pistons and then take it out with the cam". It's much easier to change a cam profile than change the pistons.

Jim

 

With all due respect, counselor, I was responding to your statement in post #13, where you remarked that we had finally "gotten the concept down completely". You then go on to enumerate "what the factors are" (which implies a complete list of factors, since the word "factors" in your statement was not qualified with the words, "major", "minor" or "contributing"). In your seemingly all-inclusive post, you didn't mention the single most important factor governing peak cylinder pressure after SCR (static compression ratio): inlet valve closing event timing (in degrees ABDC).

You have not answered the previous question relative to valve overlap. Overlap has a far greater effect on drivability than it has on peak cylinder pressure. Can you name the 2 most obvious drivability characteristics affected by valve overlap?


Originally posted by AVISPA: post #13:
"It took three or four posts to get the concept down completely, but the factors in getting the maximum static CR right for any selected octane are ignition timing curve, quench volume, cylinder head material, combustion chamber configuration, operating water temperature, intake air temperature and cam overlap (duration of intake and exhaust valves being open at the same time)."

Experience is the greatest teacher.

 

Another factor in determining detonation potential is piston ring design and exactness of the cylinder bore symmetry based on honing techniques. These 2 factors when optimized, lead to better sealing, less blow-by, and consequently higher peak cylinder pressures.

There are still more factors on the table.

 

As it turns out, in the very early 70s the OEMs did exactly what was suggested in the Jeff Smith article; they took their earlier cam designs and narrowed the LSA for the purpose of lowering combustion temperatures. That was supposed to reduce NOX in the exhaust; it was before EGR valves became widely use

I gotta say I really don't remember the OEM's tightening LSA much. They DID retard the heck out of them....primarily Ford. This closed intake valves later and helped low speed detonation tendencies on the 400's and 460's. GM really wasn't using cams much different than they had for years..just kept dropping compression lower and lower. Actual measuring had some of those things well under 8.0.

Tightening the LSA would actually increase cylinder pressure by closing intake earlier (unless you change lobes)...so that would be counterproductive to anything some passive EGR might help.

I've seen magazine repeat misinformation quite often on the LSA and the effects it has on detonation and cylinder pressure. Once you think through what's actually happening in the cylinder during overlap you can see that no compression is happening at that point so it can't matter much really. Now it COULD change the way the engine breathed at some specific RPM point and how the cylinder was evacuated....but I still think that is secondary to the average street engine. Just changing exhaust/intake etc will do the same things.

JIM

 

One SBC cam, in particular, went the opposite way. The LT1 cam ("178") went from the 114 LSA used for many of the SBC designs of the sixties including the "346" ("30-30") and "151" (L79), to a 116 LSA in an effort to achieve better idle vacuum and drivability. The "929", or 300 HP cam, used a 112 LSA, as did the "097" ("Duntov").