Yep, I got drug into the DCR game and forgot the basics.

 

Sport Side when I decide that I can afford a cam you are the guy I'm gonna hunt down for advice. You seem to really know your

 

he has way too many hours into researching cams

 

Can you explain this? I feel like I am on the verge of grasping new knowledge, almost.

 

He means that you can move the intake and exhaust lobes around in relation to each other any direction you choose, but if the centerline of the intake lobe stays the same in relation to the crank then the DCR will be the same.

 

I was thinking that when you move the ECL closer to the ICL you would loose DC. I can't seem to think through what is happening. I'll do a little research online and see if I can find some pictures to post up.

I have a lot more reading to do, I don't want to mess up the thread with the newb questions. I'll come back more educated .

 

Nope. Go to a real expert. I can just yap about a few things.

 

I've always read between 8-8.5 is where you want to be with the DCR. Anything over 8.5, detonation is more common with pump gas.

 

so what about boosted apps?

 

BOTH LSA and ICL are important because LSA can effect the ICL (and ussually does unless you spec the lobes yourself, or tell your grinder you want this ICL and that LSA -- and the trade off you make there is by shifting ALL exhaust events, which in some cases won't be possible). Having said that, and as I understand DCR, the ICL is what actually can change *without replacing the cam shaft* the DCR. Retarding the ICL decreases DCR (delays IVC) and advancing the ICL increases ICL (accelerates IVC).

The IVC is what we should really be talking about, if we're getting down to brass tacks. IVC is determined by intake duration, ICL and LSA. ICL is virtually ignore when people talk about spec'ing a cam because it can be adjusted for (and people are freaked out about learning about cams for some reason).

Look at it like this:

After the power stroke, the exhaust valve pops open and the piston pushes the exhausted cylinder charge out as it moves towards TDC. At some point as the piston moves back towards BDC (depending on the cam geometry) the intake valve opens and the exhaust valve closes (these may occur IVO .. time .. EVC and this is the 'overlap' we hear so much about). When the piston reaches BDC and starts moving back up (at the start of the compression stroke) the intake valve is still open and still sucking in charge. At IVC, some portion of the cylinder volume is left (remember there is NO compression yet as the intake valve just closed) and that volume is what we need to calculate the DCR (as opposed to the total cylinder volume used to calculate the SCR).

Another way to say this is that the SCR uses the full stroke of the crank, and the DCR uses only the portion of the full stroke that is actually used to compress the cylinder charge.

I've always heard that street engines (e.g. non-race gas) are best with 7.5 - 8.5:1 DCR and that anything over 8.25:1 requires exacting detail in the build and piston design (quench needs to be at the lower limit when running high DCRs) and must be run cooler than lower DCRs (or lowered timing). I've also heard that race engines use DCRs of 8.8 - 9:1.

As you can guess from the numbers I've heard and what Mark has heard, there are no real hard and fast rules for DCR.

Here is a handy chart posted by Zick in another thread:



I've read that DCR doesn't care about boost, so you have to adjust your DCR to accommodate the extra pressure. To do that you multiply your DCR by your pressure ratio (14.7 + boost / 14.7). You still want to be in that 7.5 - 8.5 range after the multiplication. But that means running 18psi is 12.xx DCR for me (8.65:1) and I don't believe that number. I can't seem to find an authoritative answer on boost DCR -- but as Mark suggested way back when