DannyBlake10

hey guys
been wondering, do your cam specs have any effect on the compression you can run assuming all parts are the same?

I can mostly see duration possibly having something to do with it and if so, would it be longer duration enabling you to run a higher or lower compression??

I personally think longer duration would result in a lower possible dynamic compression being that it lets more air and fuel into the cylinder which would increase the compression at tdc

garygnu

you are correct .you also have to take inconsideration the LSA ,and cam advance.

bluedawg

Overlap allows you to run higher compression, as well as lsa and advance. Other factors such as aluminum heads, air fuel ratio and octane play part as well. Their are dynamic compression ratio calculators all over the internet, pat kelly being one as well as wallaceracing.com im some what new to the concept of of d.c.r., but if i recall you want a dcr of 7.5 for cast iron heads and 8.3 for aluminum heads.

DRIVESHAFT

As a cam's duration gets longer, the engine's dynamic compression ratio goes down due to the typically later intake valve closing point.
In order to not have the engine get lazy from the lower cylinder pressure, you need to increase the static compression ratio.
That's a bit oversimplified, but its a good basic description.

pauldana

So, help me understand also.....you can run higher static compression with a longer duration and not get ping, but a lower static compression ration is needed for cams with a shorter duration? correct?

i.e., for no ping (numbers just for understanding) lets say 10.5:1 static compression won't ping with a 240 duration, but all thing being the same it would ping with a 205 duration? Is this correct?

REELAV8R

First of all kind of funny that I should be giving any explanation on a cam since I was in a fog about this only 10-12 months ago. However I think I have a pretty good handle on it now and will make an attempt here which if I get it wrong I'm sure will be corrected by someone with greater understanding than mine.

Essentially this is correct.
Static compression is needed to reach a certain cylinder pressure that will produce the desired torque level of an engine. Torque is developed through cylinder pressure. Horse power is developed through a combination of torque and RPM.
High torque requires high compression given all else remains the same, gearing, displacement etc. The limit to compression for a gasoline engine is cylinder pressure and octane of the fuel. Go too high and the gas explodes and does not burn. This explosion is known as detonation or ping.
so a longer duration cam, one that keeps the intake valve open longer, lowers the cylinder pressure at lower rpms. This is where detonation is most prone to occur due to the slow speed of the events in the combustion process. As rpms increase detonation becomes less of a problem because now the fuel does not have enough time to explode instead of burn...basically.
The longer duration cam lowers the cylinder pressure at lower rpms by allowing some of the intake charge to escape out the exhaust during the period in which the intake and exhaust valves are both open at the same time, ie "overlap".
The shorter duration cam does not allow as much of the intake charge to escape since this overlap period is of a shorter duration. This allows more of the intake charge to be compressed and thus raises the cylinder pressure. If the duration is too short and the static compression too high then the cylinder pressures get too high for the octane of the fuel and you get detonation.
On a longer duration cam it is desirable to have higher static compression so that more of the intake charge can be captured during the overlap period. Higher compression = more charge captured. Lower compression = less charge captured. For the same cam.
DCR (Dynamic Compression Ratio) is an estimation of the compression ratio at low rpms due to the loss or lack thereof of the intake charge during overlap.
You'll notice on dyno charts that the longer duration cams make their max torque numbers at higher rpms. This is due to what is know as "capture speed". This is the rpm of the engine at which bleed off due to overlap ceases to be a factor. In other words at this rpm and higher little or no cylinder pressure is lost due to overlap. So you could say that at that rpm and higher, static and dynamic compression are roughly equal.
The tighter the lobe seperation angle (LSA) for a given cam the more overlap your going to have and thus a lower DCR. More duration with a given LSA will also lower DCR.
The opposite would also be true.
A 280 duration cam on a 114 LSA may have the same overlap as a 270 duration cam on a 108 LSA and give the same amount of overlap and therefor the same number of degrees of overlap giving you similar cylinder pressures. (I didn't do the math just an example).

DRIVESHAFT

Overlap actually has nothing to do with DCR.
Since overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke, there is no compression going on at that point to bleed off.
The only valve event that affects DCR is the intake valve closing point, which is quite a ways away from overlap.

Since the intake valve typically closes when the piston has already begun to rise on the compression stroke, some of the cylinder contents will get pushed right back out past the intake valve at low speeds.
That's the compression "bleed off" that you hear about with big cams.
At higher engine speeds there is enough inertia in the intake port to continue to fill the cylinder even when the piston is rising.
In a perfect world you would close the intake valve right before intake flow stalls and begins to flow back out.

bluedawg

That makes sense, but my question is as the exhaust gas rushes down the primary tube and creates suction would that pull intake gasses past the exhaust valve? isn't the purpose of overlap to create velocity of the intake charge? If not what's it for? The intake closing point makes sense, and would lead me to ask if the out flow of the intake gasses past the intake would be related to reversion?

DRIVESHAFT

You have it figured pretty well. Overlap is used to assist in pulling the intake charge into the cylinder.
It will start to pull on the intake port even before the piston moving downward has significant affect.

DRIVESHAFT

Per Isky, thats exactly what causes reversion.
Here is a tech report they published on that. http://www.iskycams.com/techtips.php#2002

I tend to believe though that reversion is also happening during overlap, at least at low engine speeds.

bluedawg

Do you think that any intake charge is lost during over lap, if so would that affect dcr, maybe if it does its negligible.. Does decreasing overlap raise low end torque? Reversion, I seem to remember some thing about step headers fighting reversion, I don't know how true it is.

cv67

Valve opening and closing events I imagine would have more of an effect on torque when/where it happens. Actually combination of both meainig timing events and overlap.
It can work for or against you depending on your needs from you motor. Im not the one to explain the science of it though lol.
Some real good tech stuff over on speedtalk about this enough to thoroughly confuse any gearhead

Belgian1979vette

Don't forget VE of the motor. Even though you use a long duration intake, you can still get spark knock (detonation) if you run into the 100% VE range.

REELAV8R

one of these days I'll get that dcr vs ve percentage thing right in my head. I always equate the longer duration cams which have later intake valve closings with greater overlap. Hence my error, sorry bout that. Although the end result may be less cylinder pressure, driveshaft is right overlap and dcr are not directly related since it occurs during a time of no compression.

Belgian1979vette

If alu heads, limit SCR to 10,5/1 and with steel heads 9,5/1. Approximate DCR is in between 8 and 8,5 optimally.

Cam is way more than just the numbers of duration (either adv or .05). the profile can pick up what you've lost on duration because of CR limits. If you want to go to the edge consult a competent cam manufacturer that grinds custom cams and do not rely on customer service. Otherwise stick with what works for a given combo.

DannyBlake10

ok sorry but whats ve?

and were getting 10.5 with steel heads no problem. i mean it takes like a 110 degree day and bogging the engine to see ping and even still its hardly happening

ok so overall, longer duration or overlap makes for a higher cr?

Belgian1979vette

Volumetric efficiency.

Longer duration reduces actual compression, therefor you need more in order not to loose too much low end torque.

It's possible not to run into detonation with steel heads and 10.5 depending on the condition of the engine, the intake (efficiëncy) and the cooling capacity of the system. If the engine is in new condition with good ring/valve seal and the intake allows for a 100 % filling in a certain rpm range, you will run a big chance getting into detonation with the current fuel and steel heads.

On the street with inconsistent fuel quality you normally stay away from the limits.

DannyBlake10

ok cool thanks guys