How does cam timing affect DCR?

blykins · #1 (**permalink**) 06-08-2006, 04:18 PM

Just wondering...I'm doing some figuring on what my SCR and DCR will be and I'd like to know what affects cam timing would have on that. If I retard the timing by 4 degrees to raise the peak rpm a little.....what would that do to the relationship between the piston on the compression stroke and when the intake valve closes?

Carnut427 · #2 (**permalink**) 06-08-2006, 11:01 PM

If the intake valve closes later, your DCR will be lower, as the “static” compression will be lower. If the intake closes earlier, DCR will be higher. You’ll have to figure your DCR for each position the intake closes.
Dan

Carnut427 · #3 (**permalink**) 06-08-2006, 11:08 PM

Here's a link to a "Brand X" site that has a good discussion on DCR.

Excaliber · #4 (**permalink**) 06-08-2006, 11:54 PM

...same link as above, but this one actually works...

http://www.chevytalk.org/threads/sho...28/an/0/page/0

Carnut427 · #5 (**permalink**) 06-09-2006, 12:10 AM

Ooops

...thanks Ernie.

blykins · #6 (**permalink**) 06-09-2006, 08:35 AM

Guys...stupid question....but I'm trying to think this out in my head....what I need to do is make a drawing...BUT....

Picture the engine with the piston half way up on the compression stroke.

If I retard the cam timing 4 degrees (by keeping the cam in the current spot and rotating the crank CCW to accomplish it), then the piston would move lower in the cylinder. Correct?

If that's true....then essentially the intake valve on is going to be open for a shorter period of time relative to the piston than with the cam timed straight up.

If I'm still thinking on the right track.....

With the cam timing straight up, the intake valve closed at 50 deg (@ .050") ABDC.

Now with the cam timing retarded 4 degrees, then that number should decrease.....correct? It should be 46 deg @ .050 ABDC.

Does all this sound right?

Some of you thinkers chime in on this....Ernie, you should be good at picturing this stuff in your head...

So theoretically, if I choose to retard the cam timing, then my DCR should go up....and I'd have to choose a thicker head gasket to compensate.

Right?

Carnut427 · #7 (**permalink**) 06-09-2006, 12:11 PM

Quote:

Originally Posted by blykins

If I retard the cam timing 4 degrees (by keeping the cam in the current spot and rotating the crank CCW to accomplish it), then the piston would move lower in the cylinder. Correct?

If that's true....then essentially the intake valve on is going to be open for a shorter period of time relative to the piston than with the cam timed straight up.

If I'm still thinking on the right track.....

With the cam timing straight up, the intake valve closed at 50 deg (@ .050") ABDC.

Now with the cam timing retarded 4 degrees, then that number should decrease.....correct? It should be 46 deg @ .050 ABDC.

So theoretically, if I choose to retard the cam timing, then my DCR should go up....and I'd have to choose a thicker head gasket to compensate.

Right?

Sorry to disappoint you, but I'm not Ernie.

There's no such thing as a stupid question, especially on this subject, as it's confusing...Everything you've said is correct except what you're describing is advancing the cam, because you're making the valves open earlier. Really easy to get confused here. This would increase low end and decrease top end power. It would also have a greater tendency for the intake valve to hit the piston, as the intake is opening faster than the piston is moving down. Always check piston to valve clearance when moving a cam, just as you would when changing cams.
Moving the cam as you explain (advancing it) would indeed increase the DCR, as the piston has more time to build compression.
A cam closing @ 50* ABDC is fairly mild...what's your compression ratio and resulting DCR with the cam straight up?
Dan

blykins · #8 (**permalink**) 06-09-2006, 12:42 PM

Hey, anyone that can use their brain will suffice. Unfortunately, mine's not working right now....because I used the .050" durations when I was supposed to have been using advertised durations on the DCR calculator.

One question first though. When I look at a timing chain sprocket, the "retard" numbers are towards the clockwise side of the sprocket. Say 0 is at 12 o'clock.....4 degrees retarded is at like 2 o'clock or 3 o'clock. This is where I'm really confused.

By looking at the timing gear this way, and not moving the cam shaft at all, you would have to move the crankshaft CCW 4 degrees....right? Or am I totally looking at this the wrong way? And if that happens, then yes I can see where it would look like advancement of the cam instead of retardation.

I must be looking at something totally wrong here.....or I have too much time on my hands to think theory.

Anyway:

The cam IVC is at 50 degrees ABDC at .050". Seat to seat, it's 69 degrees ABDC.

My SCR is right at 10.95 I think. DCR with the cam straight up is 8.45.

With the cam 4 degrees retarded, DCR is 8.16.

I do think I want to run it 4 degrees retarded...just to help out a little on piston/valve clearance and top end power.....but now I'm thining that the combustion pressures may not support a DCR of 8.16 and a SCR of 10.95. So I may have to get a little thinner head gasket...

I do plan to take my time and check piston-valve clearances, etc. However I just wanted to know where I stand before I start.

blykins · #9 (**permalink**) 06-09-2006, 01:29 PM

I think I just figured out my question about the timing gears....when you time an engine, you keep the dots pointing to each other...if you want to retard the cam timing, you would then turn the crank so that the key would line up with the keyway on the 4 deg retard mark. So in essence, instead of rotating the crank CCW to retard it, you're moving it CW.

Tada.

Carnut427 · #10 (**permalink**) 06-09-2006, 01:39 PM

Steel or aluminum heads? If you're running steel heads, you've got too much DCR either way...no more than 8:1 is recommended, and a bit lower if possible. Aluminum heads can support another .5 compression. Installing a thinner head gasket will increase compression, I believe that was a typo and you're thinking of using a thicker gasket to lower your compression, and thereby your DCR. The trouble here is your quench is also an important part of the equation. Proper quench (around .040 give or take .005) will cool hotspots in the combustion chamber, allowing the compression ratios mentioned, as well as keep the fuel/air in better suspension.
Glad you were able to figure the dots on the crank gear out...my brain is getting frazzled.

Dan

blykins · #11 (**permalink**) 06-09-2006, 02:18 PM

Yeah, mine is too.

The heads are aluminum. That's the reason I mentioned putting thinner gaskets in....I was going to try to bump the DCR up a little bit to help combustion pressures.

But some of the other guys on the FE forum were running 8.01 DCR on some pretty healthy engines....so I may save myself some trouble and money and just run the gaskets I have.

If everything measures out and matches my numbers that I've been fooling around with.... 8.16 DCR should be fine.

Carnut427 · #12 (**permalink**) 06-09-2006, 04:48 PM

Makes good sense to me. The closer you get to the maximum recommended compression, the more you have to adjust the timing etc. to get the motor to run on pump gas. With a lower DCR you may not have to run premium.
Sounds like you're set.
Dan