I tried a search, but practically every engine thread comes up when you punch this title into the search.

I'm trying to learn more about the relationship between these two things. I get the basic gist, more duration, more compression. can someone go over why that is. feel free (please) to get as technical as you want.

Thanks in advance for any input

 

Do a search on dynamic compression ratio and that will explain it best.

eg http://en.wikipedia.org/wiki/Compression_ratio

in simple terms, the valves are not both sealed at the start of a compression stroke in a larger duration cam.... so some of the static compression is bled off, necessitating a higher static compression to compensate for torque loss at lower rpm.

look at the timing details of these 2 cams... both Edelbrock.

#2207
Duration at 0.006" Lift: Intake: 305° Exhaust: 314°
Duration at 0.050" Lift: Intake: 242° Exhaust: 240°
Lift at Cam: Intake: 0.396" Exhaust: 0.396"
Lift at Valve: Intake: 0.594" Exhaust: 0.594"
Timing at 0.050" lift: Open Close
Intake: 14° BTDC 48° ABDC
Exhaust: 57° BBDC 3° ATDC
Centerlines: Lobe Separation - 112° Intake Centerline - 107

compared to

#2102
Duration at 0.006" Lift: Intake: 278° Exhaust: 288°
Duration at 0.050" Lift: Intake: 204° Exhaust: 214°
Lift at Cam: Intake: 0.280" Exhaust: 0.295"
Lift at Valve: Intake: 0.420" Exhaust: 0.442"
Timing at 0.050" lift: Open Close
Intake: 5° ATDC 29° ABDC
Exhaust: 44° BBDC 10° BTDC
Centerlines: Lobe Separation - 112° Intake Centerline - 107°

In particular, look at when the intake valve closes to allow compression of the mixture.
#2207 is at 48* after bottom dead centre...
#2102 is at 29* after bottom dead centre.

#2207 cam requires more static compression than the #2102 to restore a good dynamic compression ratio!

Also, if too high a static compression is used with a stock type cam, then the elevated pressures within the cylinder can cause pre-ignition of the mixture, especially with std ULP fuel. In that situation, there is a need for higher octane fuel.

The high compression muscle cars of the 60's (eg 12.5:1) with BIG solid cams had access to high octane leaded fuel. For those engines today, you would need a quality racing fuel / av gas.

As with everything about good performing engine designs....
all the components need to match.....

 

Good explanation. For our Vettes we are mostly looking to run on pump gas. This means a DCR lower that 8.5. 7.5 to 8.5 will work. The lower end will be a little lazier.

To achieve your goals everything must be measured and measured correctly. Nothing is assumed. I run 91 octane in 600+HP small block because the motor was planned down to the last bolt and nut

 

I'll try a little bit.

An engine operates on airflow and cylinder pressure. To get more airflow into the motor with a given set of heads etc, you can increase duration, increase lift..or both. OEM's usually went with relatively mild lift and just made longer duration cams because it's easier on parts. Less spring pressure etc.

The increased duration on the intake lobe opens the valve earlier and closes it later. Opening it earlier before TDC as the piston is rising and the exhaust stroke is ending....allows the valve to get a *head start*. The exhaust is still flowing out the ex port and if timed right the exiting exhaust flow can help the intake charge begin to enter the cylinder by creating a lower pressure area that can help *pull* in the fresh air/fuel charge. This is *overlap*.

As the piston passes TDC and begins downward, the intake valve is already moving quickly (because you started it earlier). The quicker you can get the valve open a large amount while the piston is moving its fastest on the descent, the harder it will *pull* on intake charge and get the cylinder more full of fuel. So this is where you hear about *fast or quick* opening lobes. They get the valve open more and earlier in the intake stroke. A bigger window allows more fuel in.

The fuel continues to rush in as the piston goes to the bottom of its stroke. After it hits the bottom and begins the move back upwards on the compression stroke...the intake valve is closing. You have the piston rising while the intake valve is still open. This works because you have the airflow column moving very fast into the cylinder and you get a *ram effect* that allows the mixture to still enter as the piston rises seemingly *pushing* the mixture back into the intake port. But that doesn't happen as long as the cam isn't too big.

This is THE critical event. The timing of the intake valve closing. For all real purposes, no compression is actually occuring as long as the intake valve is open. If you close the valve earlier, you trap more of the intake charge in the cylinder. Whatever gets trapped in there will be compressed as the piston rises before the spark plug fires. If you look at the degrees listed on a cam card, you will see the number for the intake closing is listed as degrees * After Bottom Dead Center*. The longer duration cam will have a number that is later than the shorter duration one. This gives you an idea of how much higher the piston will be in the cylinder bore as the valve closes as compared to the milder cam. Closing it later means there is going to be less mixture to compress.

Now that sounds counterproductive doesn't it? You really want to trap all you can in there right? Yep....but there are limits. If I have 11.0 pistons in it, and use a cam that closes the valve early, I will end up with cylinder pressure that is too high and I will get detonation. This is why you get detonation and have to use higher octane fuel or move to race gas. The higher octane fuel burns slower and won't burn so fast that it is trying to push the piston backward as it's rising. Now I can still run it by retarding spark timing and light the mixture later...but there goes the power.

Now, let's use the same 11.0 pistons, but use a cam with a longer duration that closes intake valve later. The good part is the longer duration gets more mixture in the cylinder, yet the later closing combined with the high compression allows the mixture that actually gets trapped in the cylinder to be within the workable range and operate on available fuel. Whatever gets trapped in the cylinder will be squeezed to an 11.0 ratio and create a better fuel burn to create higher pressure as the piston descends on the power stroke.

So as you increase duration, you will reduce cylinder pressure. That's why *big* cams are dogs if stuck in stock low compression motors. They will run OK after you finally get some RPM and airflow really starts rocking, but at lower RPM, there isn't really good *ram effect* happening AND you have lower cylinder pressure. Just not a good combo.

Now if I use the same cam, but increase compression some...I get the best of best worlds. I get the increased high RPM power from the longer duration cam that gets all of that extra fuel in, AND I get the the good/better low speed TQ because the net effect is that I end up with at least as much cylinder pressure as the low compression motor with the small cam.

Man...all of that and we just lightly talked about one lobe and one part of the 4 stroke cycle! We could go on all night!

Hope it helps!

JIM

 

Lots of info in this article too..... it is referenced in the wiki-compression ratio link above.

http://victorylibrary.com/mopar/cam-tech-c.htm

It also deals with rod length influence too. Interesting tech info.

 

Looks like everyone else already has this one covered.

 

Cool, thanks for all the info guys!

so its basically about maintaining proper cylinder pressures. So, in a smaller cam, lower static compression engine would the cylinder pressures be similar to those in a large cam, higher static compression engine?

 

Well said.

Planning for the intended application produces the desired effect. Bravo.

Thanks, Jim. I appreciate the time you took to break that much of the process into a readable chunk. Clears up some ambiguity I still had.

Thanks OzzyTom; also appreciated.

Thanks for a great question, dp9. I'll lurk to see who else offers information.

I feel so much more confident in the pieces I selected for my project will produce expected results in my performance window.