High compression, small cam

[cudaman]

What would a high CR with a relatively short duration camshaft do power wise in a spark engine? Very high cylinder pressures, I guess, but does this equate to power? I figure that the high compression is useless if the valves weren't open a sufficient period off time to allow enough cylinder filling. I suppose that this is what superchargers do; effectively create high dynamic compression without a lot of duration, at least compared to a NA engine. Will the cylinders not fill enough to actually generate high cylinder pressure/DCR, thereby wasting the high SCR?

I know the engine would need to be pretty stout, etc., and one may think,"Why would you want to do this to begin with," but I'm just curious.



Let's say for argument sake a 13:1 SCR with a 272 advertised duration cam.

[cudaman]

Maybe this should be in engine section. I thought this would be more appropriate, though.

[trackrat79]

I could be wrong since the hole engine dynamic of this hobby is still a bit of a mystery to me but from my limited understanding. Yes there would be some power gain from a high compression engine and a fairly stock cam, a lot of it would be in the increase in efficiency of the burn. Like u already stated that with a mild cam you will not be pulling in as much air/fuel mixture as possible and not taking full advantage of the increase in combustion. And probably unless running electronic fuel injection you'll have problems with detonation, and possible over heating at low rpm, Increased burn efficiency means increased heat. The fuel air mixture is just so much more critical as the CR goes up that a carborated setup just wont cut it.

I mean if you look at modern production engines they are slowly going that way. Take some of the import 4 and 6 cylinder engines coming from the factory at 12/1 , 13/1 compression. Even some of Chevy's new LS series of engines are coming with upwards of 12/1 compression with fairly mild cams. Its all in an effort to increase efficiency and with that comes some power improvement.

[cudaman]

I was thinking the biggest effect would be an increase in efficiency, which is sort of why I started thinking about it in the first place. I think direct injection engines are in this arena (I know diesels are). I don't know that much about DI technology, however. Just that the rail pressures are incredible.

Let's assume alcohol or some other fuel that can tolerate lots of compression.

Why would EFI be necessary? Is the A/F ratio that much more critical at higher cylinder pressures? If this is the case, then how do high compression engines with carburetors run all the time with no problems? Are they relying on bleed-off from the cam? If so, what's the point of building a bunch of pressure, only to bleed it off? Is that the only way to ensure the cylinder is completely filled?

Apologies for the stream-of-consciousness questioning. Must be the James Joyce I had to read in high school.

[Nessumsar]

"They" aren't bleeding off compression for the sake of it, those engines usualy turn high rpm, which means lots of duration for proper cylinder filling at those rpm's. They raise the SCR to combat this.

If you took an advertised 13:1 SCR piston set, and matched it with a fairly short duration cam, the DCR would be much higher than 13:1; from my understanding.

You could easily do it with a carb., it would just need the right octane fuel, and proper tuning. How about E85?

[Twentyover]

There was a pile of discussion on something like this about 30 years ago. The premise was use high mechanical compression ratio, like 15:1 or so, and then close the intake valve late, to reduce dynamic compression ratio. The plan was ton use the 15:1 expansion to extract more work from the fuel used. One problem they had was at low speeds they were pumping alot of mixture back into the intake manifold, and it was tough to get jetting right transitioning from light load (but low vacuum) to high load low vacuum.

FI could be programed to mitigate the problems of the mid 70's

[MonzaRacer]

no no no ,had a pontiac with 256 duration and 10.5 to 1 heads, rattled its head off above 3500.
sucks watch cylinder pressures above 200psi cranking, unless you love 8.00 a gallon race fuel.
basicly best trade off is around 190psi cranking.

[cudaman]

[QUOTE]If you took an advertised 13:1 SCR piston set, and matched it with a fairly short duration cam, the DCR would be much higher than 13:1; from my understanding.

So what would be the result?

[cudaman]

So if the long duration is necessary on a high compression NA engine in order to properly fill the cylinders, then is a supercharger the best way to increase efficiency with a short duration cam? Or does it matter because some unburned mixture will be pushed out of the exhaust side at the same rate as it would on a high duration NA engine.

BTW, sorry about screwing up the quote in the previous post. I guess I don't post on the internet enough to know what I'm doing.

[cudaman]

Again, for the purpose of the discussion, assume 100 octane fuel or higher - race gas, alcohol- it doesn't matter. I'm not actually paying for it.

[MonzaRacer]

NO NO NO without specific overlap of larger duration cam your cylinder peak pressure will be stratopheric, think of this, even with high octane and water injection, I had an engine thsat was making 255 psi cranking pressure, and it would not even run above 3500 rpm without rattling its self to death.
OK tip one, ever think of WHY they say thast increased compression is required for larger cams, because they do bleed off cylinder pressure.
don t try crutching a bad idea, it wont work, well atleast not propperly.

With 13 to 1 static you will need to run a lot of overlap, call Comp Cams and ask same question.

Like I posted, try to keep your cranking cylinder pressure around 190psi for best torque.

[cudaman]

Monzaracer, I read you the first time. I know that lots of overlap reduces dynamic compression through pressure bleed off, etc., and I believe that this is done in order to adequately fill the cylinder. I got it. And if diesels and direct injection are bad ideas, then OK.

I'm asking theoretical questions regarding efficiency gained from high cylinder pressures. I know you're only trying to help, but please stop posting as though you're correcting children.

I'm also trying to verify my thinking that lots of overlap is just the means to the end of cylinder filling and that high SCR is necessary to compensate for the pressure loss created by the high overlap; as opposed to the CR being of primary importance. This makes sense instead of the converse where CR is most important and then the valves are opened for a long time to release pressure that you just spent time and energy building up fro no good reason.

I'm sure this seems like elementary combustion theory to some. I thought I understood it correctly, and I think I do, but I just wanted a refresher on the CR-valve overlap thing, because I started thinking about it from another perspective and it began to not make sense. Sort of like staring at something for a long time and suddenly you can't determine how far away it is.

[Pro Stock]

Cudaman;
Some of the replies to your post are close to being correct just as mine will be close, your assumptions on efficiency are correct, choose a cam that you want based on what your goal is (driveability, performance, ect.) and then go with at least 10.5:1 with the smaller cams and increase compression as you go up on duration. The important things to look at are timing, timing curve, carburetor calibration and hot spots (sharp edges) in the chamber, I've probably made over 300 dyno pulls with 92 octane gas in the last six months and have had no issues with detonation or pre-ignition on engines up to 11.5:1 compression, I also have a 68Z that has a 355 small block with 10.5:1 that I drive regularly in 100 degree heat.

Dale

[cudaman]

In a roundabout way, my original question was: how would one make high cylinder pressures work to increase efficiency? I'm guessing direct injection, or carburetion/FI with super-high octane fuel. Or is it only practical with direct injection?

[TnBlkC230WZ]

To answer the question, why are they doing to day? High compression will increase power and efficiency. It also increases heat. From an emmissions perspective it decreases carbon dioxide, but increases oxides of nitrogen NOx. It is the violation of NOx standards that keep high compression gas and desiels for passing emissions standards.

PFI, especially SPFI, allowed the compression rations to creep up a little. Direct fuel injection allowes extremely precise control of the combustion process which allows the auto companies to raise compression without raising NOx levels. The added benefit is the CO2 levels are lower.

All of the emissions legal passenger car desiels sold today are direct injected.

[cudaman]

Thank you for that simple answer. That was the info I was looking for.