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!).

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.

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.

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

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?

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

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.

Where can one find Pat Kelley's calculator?

http://members.uia.net/pkelley2/DynamicCR.html

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.

No race car, just the wagon project. I want it to be streetable, able to cruise distances where only pump gas is availalbe.

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?

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?

You're already 4 deg advanced, 6 deg advanced is 2 more than what you have now. So the number you use is 62 - 2 = 60.

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?

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? 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 ;)

Mike to the rescue graemlins/waving.gif , 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.

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

Originally posted by Pat Kelley:
Mike to the rescue graemlins/waving.gif , 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. 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!

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.

Well, the block was decked a few thousanths to clean up the surface a bit. I think he said he only took 2-3 thousanths off.

Even if he took it from .025" (the nominal clearence) to .020, the .018" will be fine. As long as the quench is between .035 and .045" you'll be fine. If it turns out the deck is a bit too short, Victor makes a .020" gasket. After that, AFAIK, .026" is the next thickness, Victor again, GM has a .028". You shouldn't have any problems finding a suitable thickness.

There isn't a law that I know of that says you can't paint the aluminum radiator black. I am using an Optma battery which only comes in several colors depending on the amperage rating.So, I painted it black. You really have to look hard to realize it's not a stock battery. Being a completely aluminum radiator including the welds I don't know of any reason you couldn't powder coat it a satin black. Hmmmm!

I wouldn't powder coat it but spray painted black is fine.

It's a Science/Physic's class thing, the black paint and radiators to up their emmisivity or however you spell that word?? Radiation emitting ability to help rid them of heat..

pdq67

I'm sure it could be painted, but it would seem a shame to paint a a nice shiny radiator! I have a long time to think it over though given the speed at which this project is progressing.

Think over using the mid 80s Blazer rad, or whatever "look-alike" the racers are using that's alum with two plastic tanks.

Pat helped me with DCR and my car ended up at 8.19. I have out of the box Canfield heads, I run 33 deg total timing and 91 octane on my 355. Quench ended up at .043. My car cam is 218-230 @ .050 113 LSA I have so much bottom end ( even with the 3.07 gears) i'm going to try a vic JR next summer.

Mike

7.9 DCR is actally pretty darn good for an iron headed combo. In some instances, 87 octane can be run with numbers like that, but in your case I would say 89 to be safe. I am assuming we are talking about a fairly heavy wagon, and the weight will add to engine load as opposed to a lighter car, and if you have milder gears(3.23 or something) I would not raise the DCR any cause you might put yourself in a position where you need 91-93 octane to be safe in all driving situations. If it were an aluminum head application, Id say shoot for 8.2:1 max to be totally safe. And once again, depending on the grand scheme of things 8.5:1 and aluminum heads with premium fuel has been done(probably even more), but it might not be perfect in all situations. This is why most people reccomend 8:1 dynamic compression ratio with an iron head, and about 8.2:1 with aluminum to be safe cause its plenty enough to make power, yet its still low enough where you can be a little liberal with the fuel choice. DCR isnt the only determinig factor either as to fuel choice. You can have two motors with identical DCR, yet one runs fine on 87 and the other wont run on anything less than premium. Sounds strange, but its true and the efficiency of the top end itself will also be a determining factor.

Pretent you have two small block chevies, both identical DCR, but one is a hydraulic cammed motor with 283 power pack heads(small port, small valves, not a lot of flow), and the other has some trick aftermarket iron heads that flow great and fast, and a properly matched roller cam. As you probably know, the reason that the intake valve stays open even after bottom dead center is because the fuel air charge builds up a good amount of velocity at a higher rpm and can actually force itself into the cylinder even after it changes direction and starts to rise(you could call it ram tuning, inertial superchargine....Ive heard many refer to this phenomenon as many different things). Anyways, when you apply this to the two examples I gave, which motor would you think can better take advantage of the longer intake valve opening? The roller cam deal, cause at high rpms the head/cam combo is probably moving some serious air and will allow lots more to keep pushing itslef in cause its moving so much, but the crappy iron heads and hydro cam probably are acting as more of a restriction, and although there will still be some of this ramming effect, its not going to be as high as what is happening in the much better top end motor. In turn, even though they both have the same DCR and are compressing the mixture for the same number of degrees, the mixture in the better motor will be packed tighter before it starts to be compressed. So at high rpm where this effect really starts kicking, the motor with the real good top end would actually build more overall cylinder pressures so it needs higher octane. If that doesnt make enough sense, pretend both motors have a seemingly lowly 7:1 DCR. By that number youd figure it would run on lower than 87 octane. But here is the monkey wrench. Although DCR(number of degrees its actually being compressed based on IVCA) is identical, one of the two motors is running a turbo at 20lbs of boost. All the sudden, just like in my first example, that DCR number doesnt fit into the standard mold and the forced induction motor will see much higher pressure even though its only compressing for the same amount of time. All this goes back to the fact that your DCR is very good for a heavy driver, and I would not push it much more cause there isnt a positive guarantee that you will get away with a lot more just cause somebody else did(who might not have as efficient of a top end).

one last thing just for a further example. If you look at Mike1985 post right above mine, you can get an idea of what could happen if you boosted your DCR. He has an aluminum headed motor(good heads I might add, and aluminum is less sensitive to detonation) and just about 8.2:1 dcr and it runs on 91 fuel. If Mike swapped to an iron head that flows identically to his canfields but with no other changes, he would probably have to either retard ignition timing or step up the grade of fuel to make it work like it does now. Not guaranteed, but its more than likely. Being that your wagon is probably pretty heavy, if you go with an iron head and similar DCR numbers to mike, its safe to say that you are putting yourself in a position where it would probably need premium fuel all the time with optimum timing advance. You could richen up the mixture and retard timing and get away with less fuel, but that wouldnt provide as much power either. Better safe than sorry so get everything in check before any changes.

Good synopsis Greg! Welcome aboard and thnx!

Well said, and the reason why I'm more than a little anxious about my lil 8.6 dcr 427 when its working best at 6000-7000 rpms. ANd anything I do, retard cam, jet up, or reduce timign, will clearly affect the rip and it likes it hot, lean, with moderate timing (36). I'm running around at 32 now, just to be safe, and the motor is almost a stone, like it lost most of the two outboard carbs. :(

Anyone have a set of GM 074s with 115cc chambers, swap my 112.5's with ya!? smile.gif

Sorry to tread Rich, but a bit conservative is better 7.9 OK, 8.2 optimal on premium given gears, etc, 8.6 too high for 93 even with "porous" old GM alum heads.

And BTW, I use the cheap GM 6768 gasket ( L88-L89), which requires retorquing, yet, after sneaking up on final at 5 lbs increments, not one bolt moved during retorque. So, I can confirm that the longer incremental procedure seems to work well.

Greg makes some good points. One of the keys to avoiding detonation is the quench distance. Keep it tight (.035-.045) and you'll have less problems.

With a .018 gasket I get a DCR of 8.3 and a quench of .038. A .025 gasket would net an 8.19 and .045 respectively. I need to confirm my piston to deck clearance and then I'll be able to make an informed decision.

Thanks for the great calculator Pat!