I'm part-way through with a 427+0.030 build and have some concerns about DCR. I have a static CR of ~10.2 and iron heads. I am using a Howard solid flat-tappet cam which is admittedly pretty huge for this engine. The intake closing angle varies quite a bit from 0.050" to 0.006" (from 43ABDC to 81ABDC). Obviously the DCR changes a lot depending on which of the ICLs I use to calculate it--the DCR goes from too-low to too-high! I couldn't find any real consensus about which ICL number is "the one" to use. Thanks for any help.

Tony, always use the advertised duration timing events (.004 or .006) for DCR calculations. The Intake Center Line is your choice but it is best to go with the mfg's recommendations unless you have a good reason to do otherwise. Given that most install a cam between 0º advance and 6º advance, the intake closing should not vary more than 6º. More advance increases DCR, less advance decreases DCR. Forget .050" timing numbers, they are useless to both man and machine.

Hi Pat,

Thanks for the reply. I am kind of concerned about my DCR if I use the 0.006" ICA value, because I get a DCR of slightly under 7. This cam is already ground with +4 advance, and installing the timing gears dot-to-dot has actually given me an extra 4.5 degrees of advance. Not sure if the crank key was ground wrong, or what.... So I have an ICL of 101.5 with 110 LSA. I figured I will leave it since the ICL is already so late.
Actually, the 81ABDC intake closing angle that I got seemed so late to me that I checked it four times. I came within 1/2 a degree each time. I guess we'll see how it performs--I am kinda worried!

Sounds like your overcammed. You really need more compression or a smaller cam. The engine will run but you'll be leaving a lot of torque on the table.

Think of the +4º, 0º, and -4º marks on the timing set as "in addition" to any advance built into the cam. Something doesn't seem right if you're getting 101.5º ICL on a 110º LSA using the 0º advance marks.

Are you measuring at the lifter or the retainer? Check over your setup. Is the dial indicator in line with the direction of travel? Are you sure you have the correct marks on the timing set paired together? What cam are you using?

As an aside, it sounds like the cam has a lot of overlap. With low compression the exhaust sound will be rather "soft" sounding. Lots of burble but no "POP" in it. I had a SB like this once. A SB with about 8.5 CR and a 308º cam. Very rough idle but without the explosive sound you get from a high compression engine.

Tony, I have a 454 with a Crane hyd where the intake closes at 79 deg ABDC. With a static of 9.8 my dcr is about 7:1. The car will smoke the tires at will and pulls to 6,000 rpm. If you have good heads I think you will be fine. Power takes good airflow to create, not just compression.

Hey Pat,

You know what, I think I screwed up. This is a solid cam so I don't think it makes any sense for me to assume the intake closing angle occurs at 0.006". This is a solid cam with 0.025" of recommended lash, so if I am thinking about this right, the intake valve should close at something like (0.025/1.7)+deflection error due to flex, etc of the components. BTW I am taking measurements at the end of the pushrod so it's measuring lobe lift.
I don't know why the ICL is coming up as 101.5 but all I can say is I measured it a number of times and it was consistent. The cam is supposed to be 110LSA, 106ICL. I am also getting smaller duration numbers and lift numbers than what's advertised. Don't have the cam card but I think it's 248/256 dur. at 0.050", 0.598/0.618 lift. I was seeing 240/250 dur. at 0.050" and about .570/.593 on the degree wheel. If I'm doing something wrong with the degree wheel I'd like to know! This is the first time I have used this particular degreeing kit and I'm not real happy with it.
Thanks for all the help!
Tony

You need to look at the cam lift not the valve lift and measure at the lifter (or end of the pushrod, more difficult). That takes the valve train out of the picture. The correct seat duration is measured like you said, lash/rocker ratio + .004" or .019" of tappet lift in your case. Measure the seat timings using that and see what you get. If the cam is speced at .020", pretty common, than it should be slightly larger than speced.

As for the ICL, ???, I don't know. Are you finding the piston and lobe TDC going .050 down each way and dividing for the center. There is too much dwell at TDC for both the cam lobe and piston to read it directly with any accuracy.

Wouldn't you get MORE accurate DCR calculations if you took the measurement at the valve retainer? Including as much of the valve train as possible would simulate running conditions better. Any flex that happens in real life (except harmonics that are RPM related) would be accounted for in the measurement. The question then becomes "how much valve lift is significant" for purposes of determining the beginning of the dynamic compression stroke. First guess: even with the valve open a few thousanths, you're building compression. The piston is moving fast enough to compress faster than the charge can escape. Perhaps we could use the same .006 at the valve as is built into the "advertised" duration of the cam?

I am going through this picking a little-bitty solid cam for my 283 motor!!

AND I have a real problem here that borders on anal b/c the cam makers should spec. a table of "THEORETICAL" advertised duration points at the valve tip with their recommended average lash value set AND then tell us what these numbers are when we choose to set the lash at either their recommended loose or tight max. lash numbers!!

CC publishes their Magnum solid cams advertised duration numbers at .015", (lashed I assume at the lobe?),and then say's they can be lashed from .015" to .030" with a recommended running lash of somewhere around .022" or so!!

I want to know what the actual valve opening and closing point numbers are at max. and min. lash so I can pick where I want my intake valve to close to fine tune the DCR number!!

GO FIGURE why they don't come right out and do it this way except maybe it is a "black-art" sorta deal that's left over from the old days???

pdq67