Why does a tighter lsa need more compression?

[digger]

it should be effective static compression ratio. ESCR instead of SCR

[randy331]

This thread should have been titled "LSA myths" .

Randy

[GARY C]

Maybe the term just needs to be changed from DCR to IVCCR.

Yes, the name DCR is unfortunate as it is misleading. I was thinking CACR, Cam Adjusted Compression Ratio.

It works.

Yes, it is just a phrase used to define a subject.

[GARY C]

This thread should have been titled "LSA myths" .

Randy

That will only happen when you quit confirming DV's LSA theory with your testing.

[MadBill]

it should be effective static compression ratio. ESCR instead of SCR

A two stroke engine's CR is usually quoted as calculated from the piston position when the exhaust port is fully blocked, analogous to IVC in a four stroke. <6:1 per the former might equate to 15:1 or more per the latter..

[mt-engines]

Why. Is this thread still going.. And why do people care what your engine cranks for compression? Im not building air compressors..

[Geoff2]

Agree, DCR is misleading. It will change as more/less air is inducted into the cylinder at different rpms.

[GARY C]

Why. Is this thread still going.. And why do people care what your engine cranks for compression? Im not building air compressors..

Not sure but if any of the Credited Super Stock engine builders matter, they only focus on cylinder pressure and could care a less about compression ratio. The same could be said for boosted and nitrous applications.

[plovett]

Agree, DCR is misleading. It will change as more/less air is inducted into the cylinder at different rpms.

DCR doesn't change with rpm. DCR is just a calculated number like static compression ratio. Actual cylinder pressure does change with rpm. Are we discussing semantics? Absolutely, but it is important to clearly indicate exactly what we are talking about. Many people incorrectly use the term DCR to indicate actual cylinder pressure in a running engine, yet the two number are very different and only indirectly related.

As to the original question, I agree that all else equal a tighter LSA tends to want less static compression ratio, not more. A cam with a tighter LSA tends to have an earlier intake center line and an earlier IVC. That drives toward less static compression ratio. Also, a tighter LSA can make more mid range cylinder pressure and so wants less static compression ratio.

paulie

[swampbuggy]

Great topic, and after reading it all it causes me to think about the bigger picture. The main objective in building horsepower is when the intake valve closes have the highest possible cylinder pressure ABOVE the appx. 14.7 atmospheric pressure outside the engine. The better the cylinder heads and intake manifold, the more air that will be trapped in the cylinder at Intake Valve Closing. Combine the correct amount of fuel and we have a more powerful power stroke. This is why as induction systems (intakes and cylinder heads) have gotten better and better camshafts don't need as much duration. Camshaft selection is just about always a BIG question on exactly what to choose. CGT for example, he has his engine built, now he has tried a second camshaft in search of a little bit more power. Very impressive by the way CGT. One thing is for sure camshafts are application specific. My 2 cents Mark H.

[My427stang]

I hate to keep this going too much longer because DCR and LSA discussions usually end up in wrenches being thrown LOL

I never like to discuss LSA itself, except for when I place an order for a custom cam. To me, it's a manufacturing number to position lobes. However, my experience if we were to discuss LSA as a reference, if combined with the same intake centerline and lobe pair, a tighter LSA may like a little more compression to deal with the diluted chamber at off-idle speeds, and mechanically would fill the cylinder a little more, maybe much more, so in theory, at higher RPM and loads, would need slightly less static compression if the exhaust is working well. In those cases though, I think we are talking the margins, unless you are right on the edge

The discussion becomes misleading though, because the average guy pairs LSA as the determiner of an off the shelf cam's ICL, usually 4 degrees advanced from the LSA value. Crank an intake lobe 4-6 degrees forward, regardless of LSA, and assuming the same lobe pair, it does change things noticeably, again not "lose your birthday" kind of changes, but it will change and on a street car likely more noticeable good or bad.

That is not to say an LSA change with the same lobe pair will not be noticeable, it is VERY noticeable, especially in off idle torque and vacuum as overlap decreases and mid range if overlap increases, but in terms of compression and detonation, I believe intake centerline has more effect.

Start combining those changes, LSA and ICL, that's when you get into magazine cam testing and false results IMHO

I will say, I am a DCR guy, but I consider it like a check figure in a long algebraic equation, so call me "DCR-light". I understand it's limitations (especially it's mis-naming), and I actually consider it no more beneficial, or less beneficial, than SCR, because SCR never "happens" in a running engine either, but because it's named static and has been around forever....people don't mind SCR as much. You can't really use one or the other alone, and if you do use SCR and avoid DCR, you are likely adjusting for the cam under another means, or from experience, which is just fine too.

I will say though, DCR only works as a single input of many, when you have experience with a single calculator over many engines with similar design, and your inputs were actually measured. A tight chamber wedge will not be the same as a big open chamber, and all the other rules of engine building still apply for quench, etc. For some reason, people think if you like DCR as a check figure, you forget all the other stuff and just bolt engines together and kill compression with the cam. Not sure why, because I see MORE guys who want to build a 10:1 SCR engine with a 268 degree cam for their truck then I do guys who try to kill SCR with a big cam, maybe not the case a few years ago, but certainly a trend I see now.

[CamKing]

There's nothing Dynamic about DCR, and it has no mathematical significance to what's going on in a running engine.
DCR is based on the misconception that you need the intake valve to be closed, to start compressing the mass in the cylinder, and that's far from the truth. As long as the pressure above the intake valve is higher then the pressure below the intake valve, the open valve will not bleed off any compression.

[Stan Weiss]

Two engines

one 9.0:1 CR

one 11.0:1 CR

same 290 Duration lobe

What would the LSA be and where (RPM) would each make peak HP?

Stan

[CamKing]

Two engines

one 9.0:1 CR

one 11.0:1 CR

same 290 Duration lobe

What would the LSA be and where (RPM) would each make peak HP?

Stan

Need more info
Bore:
Stroke:
Rod Length:
Valve size:
Port Flow @ .400", .500", .600"
What's the valve lift for this cam?
Is 290 measured at .050"?, if so, what's the seat duration ?
What size carb?
What intake manifold.
Is the exhaust restrictive ?

[Stan Weiss]

Mike,
Yes you would need some more information. My point being the original question is flawed. In that I would not just change the CR and keep the duration the same (or some other parts) and just adjust the LSA.

Stan