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DCR calculator question: intake seat timing

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Rich-L79

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Discussion starter · #1 ·
I'm trying to calculate my DCR using Pat Kelley's calculator. I'm unsure what value to enter for "ADV Intake Closing Angle". My cam card reads:

Seat timing, int 34 BTDC 62 ABDC 104

Does that mean I should use the value "34"? If so, my DCR is way too high and I'll have a hard time getting it much lower (bummer!).
 
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#2 ·
It is the 62 degrees after bottom dead center number that you should be using. Think of the combustion cycle. The piston drops in the bore during the intake stroke, pressure drops and fresh fuel and air fill the void, entering through the intake valve. At high engine rpm's the fuel/air charge can hold so much velocity that the air can continue to enter the cylinder even after the piston changes direction and begins to rise, so the intake valve stays open to allow for the ram effect to occur at high rpm. In your case, the valve stays open for 62 degrees after the piston reaches bottom dead center, hench the 62 ABDC. Hopefully that shows better numbers for you.
 
Discussion starter · #3 ·
Whew! That shows a whole lot better number. Now the DCR is on the low side at 7.91 but I could bump that up by using thinner head gaskets or milling the heads a touch to get the combustion chamber cc's down slightly. I'd rather go the head gasket route.

For the moment I'm assuming I have the pistons 0.020 down the hole so I need to assemble the short block and measure that for certain before I chose my head gaskets.

*EDIT* Wait a sec, the way the (handwritten) cam card is written it reads:

seat timing, int: 34
BTDC: 62
ABDC: 104

But it can't be 104 ABDC.

I think I need to make sure my cam numbers are accurate.
 
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#4 ·
Rich - intake opens at 34 BTDC, intake closes at 62 ABDC, total intake duration is 34 + 180 (from TDC to BDC) + 62 = 276

Intake centerline = 276 / 2 = 138 - 34 (BTDC) = 104 ATDC
 
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Discussion starter · #5 ·
Okay then, my second round of calculations were correct. That nets a DCR of 7.91. I'd like to get that a little higher, or am I just asking for trouble if I shoot for 8.25 or so?
 
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#6 ·
Rich, you may be shooting a little low on the DCR. With only 8.25 and without any power adder the motor will most likely seem a little sluggish for the cam you have choosen. Running about 9.5 should get you where you need to be and still handle pump gas just fine. The dist curve is critical however,and you will probably have to play with it for awhile. Also the better cooling system you have along with a good waterless coolant to eliminate flash around the exhaust valve area will help eliminate any detonation especially in your iron heads.
Mark
 
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Discussion starter · #7 ·
Okay, now I'm confused again. Pat's DCR calculator page said to not go above 8.25-8.50 if you want a streetable combo that can run on pump gas. He went on to say that 8.8-9.1 was in the realm of race engines which would need racing fuel.

I need to figure this out now so I can get the heads milled now if I need to. As things stand now I'm looking at a static compression of 10:1. The cam is a Ultradyne (now Lunati) 276/284 solid flat tappet. A thin head gasket (.018) would get me in the 8.2 range, to go higher than that I'd have to mill the heads.
 
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#10 ·
This is for the race car? I'd say shoot for all of 8.2 then. You certainly can make more power as you get higher, but gas becomes an issue, as it has with me at 8.6 and old alum heads. 8.2-8.3 seems like 93 territory. I was shooting for 8.3 and overshot some. Might dial the cam back or run a lower temp. Either will hurt power.
 
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Discussion starter · #11 ·
No race car, just the wagon project. I want it to be streetable, able to cruise distances where only pump gas is availalbe.
 
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Discussion starter · #12 ·
Oh, and Harold suggested I install the cam 4-6 degrees advanced. If I want to calculate for 6 advanced, do I add 6 to my intake seat timing figure of 62 and instead enter 68? Do I also have to change any other figures to calculate with the cam advanced 6?
 
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#13 ·
Ok then, IMHO, stay where you are to avoid pinging on those few bad batches ( i.e. stale) gas. It might , likely it will, allow you to run 91 as well. Hey, who knows if 93 will be available in the future?
 
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Discussion starter · #15 ·
4 degrees of advance is built into the grind of the cam as you know. He recommended installing it 6 degrees advanced. I now wonder if he meant 6 beyond the 4 it already has or 2 more than it already has?
 
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#16 ·
Originally posted by Rich-L79:
4 degrees of advance is built into the grind of the cam as you know. He recommended installing it 6 degrees advanced. I now wonder if he meant 6 beyond the 4 it already has or 2 more than it already has?
Click to expand...
Rich,
No, just add 2 degrees & then check to see where you actually are.
You don't normally want to go more than 6 degrees total with almost any cam.
If you found it ran better moved any further than that it means the cam is wrong for the combo ;)

Mark,
I think you are confusing static compression which is the total volume in the cylinder & head vs the volume in just the head & effective compression which is the volume from when the intake valve closes ;)
 
#17 ·
Mike to the rescue
Image
, thanks. Yes, 62º is the number to use (or 60 with 6º advance). I agree with Mike that 6º is about the max. If it needs more use another cam.

Rich, 7.91 DCR isn't bad at all. 8.2-8.3 will add more bottom end. Measure everthing up and see what you have. If you can get to 8.2 without too much effort, do it. If you need to re-piston or something like that, well, I wouldn't. If this is a race car, it might be worth the extra effort to up the DCR.

Harold's cams like 6º advance. Has to do with the un-symmetrical shape of the lobes. What is the LSA of the cam. The difference between the LSA and the ICL is the advance. Example, 108 LSA installed on a 104 ICL is 4 degrees advanced, installed at 102 ICL is 6º advanced. Don't confuse these numbers with the numbers on the timing set. The +4 (or maybe 2 for cam degrees), 0, and -4 (or 2) marks are in addition to any advance built into the cam.
 
#18 ·
Rich, Sorry to confuse you. I re-read your post and I got it wrong. Mike is correct for DCR not static. Go with the 8.2 and you should be ok. Timming curve and coolant statement still stands since your static compression will be above 9:1
Mark
 
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Discussion starter · #19 ·
Originally posted by Pat Kelley:
Mike to the rescue
Image
, thanks. Yes, 62º is the number to use (or 60 with 6º advance). I agree with Mike that 6º is about the max. If it needs more use another cam.

Rich, 7.91 DCR isn't bad at all. 8.2-8.3 will add more bottom end. Measure everthing up and see what you have. If you can get to 8.2 without too much effort, do it. If you need to re-piston or something like that, well, I wouldn't. If this is a race car, it might be worth the extra effort to up the DCR.

Harold's cams like 6º advance. Has to do with the un-symmetrical shape of the lobes. What is the LSA of the cam. The difference between the LSA and the ICL is the advance. Example, 108 LSA installed on a 104 ICL is 4 degrees advanced, installed at 102 ICL is 6º advanced. Don't confuse these numbers with the numbers on the timing set. The +4 (or maybe 2 for cam degrees), 0, and -4 (or 2) marks are in addition to any advance built into the cam.
Click to expand...
LSA on the cam is 110.

Mark, thanks for the follow up, I was really confused there for a second. I plan to run a large aluminum radiator (which will ruin the stock look under the hood I'm shooting for but sometimes you gotta do what you gotta do). I haven't decided on an ignition system yet but I do expect to have to spend some time dialing it in. I am definitely going electronic but probably not HEI. A small body HEI by Dave Ray may be used or I might just install Pertronix and their MSD-like box with a built-in adjustable rev limiter.

All in all after I get the exact down-the-hole measurement once I get the short block together I'll know what gasket I'll have to use to get me in the 8.1-8.2 range. If my current guess now on that is correct (0.020) a .045 head gasket will net about a 7.91 DCR and a .018 head gasket will net about an 8.25 DCR. Knowing all this I don't plan to mill the heads.

Man this thing is gonna be fun!
 
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#20 ·
I like that .018" Fel-Pro head gasket. Cheap and it works great. I highly recommend it. Plus, it gives the correct quench on an untouched block. I have them on my driver. I installed them in 1998 and have never even re-torqued them. The secret is to sneak up on the final torque. I initially torque all to about 30 lbs, then go up in 5 lbs increments. I go around the pattern twice for the final pass, just to check them. Takes a while but a lot less time than re-torquing.
 
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Rich-L79
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