cam timing using cranking pressure

[lorax]

Guilty!!! Last cam I put in I didn't degree in, just used the dots. Car felt sluggish for what it was, but I knew it was over cammed (cam was originally bought for a larger engine). When I replaced the cam with a custom grind, I did degree it per the company's recommendations and found that dot to dot the cam was roughly 8 degrees retarded!!! when I found this, I checked it several times, yep, way off. New cams in where it belongs and I can't wait to drive it. Makes me wonder how terribly the old cam affected the engine seeing that it was too big to start with and then retarded possibly 8 degrees!

You're making the assumption that the cause of the retarded dot to dot is caused by the crank key or the timing set. How do you know the new cam isn't just ground that far off. Its all based off that little pin's location in relationship to the lobes. For all you know, the old cam could have been anywhere.

[3window]

I'm not making any assumptions. I understand the cam could've been anywhere, I'm just stating an example of how far off a cam could be. I guess I should've put the "possibly" before "retarded." I'll be sure to proof read next time!

[CamKing]

We'd be much better off if everyone erased the word "Advanced" from their mind.
Every engine wants a given Intake Centerline( of Intake opening and closing points), and a given Exhaust Centerline(of exhaust opening and closing points).
The LSA is nothing more then these to centerlines added together, and divided by 2.
When you work with double overhead cam engines, you don't talk about LSA or advance.

Let's say an engine wanted a 102 ICL, and a 114 ECL. You would say that's a 108 LSA, advanced 6 degrees.

Now let's say, we modified the exhaust port, and exhaust lobe, and left the intake lobe alone.
The engine wants the same 102 ICL, but now wants a 110 ECL, You would say that's a 106 LSA, advanced 4 degrees.

Saying it's advanced 4 or 6 degrees doesn't tell you anything, and can be misleading.
Talking in Intake Centerlines and Exhaust centerlines, keeps everything clear.

[Dan Timberlake]

The Crane Corvair cam (with springs and beautiful golden aluminum retainers!) I bought new (from a local speed shop, not a catalog) in 1970 had a card with intake and exhaust lobe opening and closing events at some checking clearance. maybe 0.050 inch. Before the heads went on I made an extension to fit in a lifter so I could measure lobe lift directly. just like racer Brown said to do. I rigged up a blue Mr Gasket degree wheel, adjusted a pointer for true TDC, and Checked the event timing ( not lobe centerlines or anything else) and was within a degree or 2.

A buddy built a V8 Vega and bought a Speed Pro copy of some GM Hydraulic performance cam. He and I went thru the same procedure working with published opening closings at some checking clearance and felt the cam was several degrees off when installed with the dots aligned. Called the SpeedPro tech line, and they said run it. I talked him into adjusting it closer, probably with a fancier multi-key timing set.
Now I wonder if it happened to be 4 degrees advanced before we fixed it?

respectfully,

Dan T

[DaveMcLain]

Have you ever had a Chevrolet or Mopar V8 come in with the cam timed using the dot on the cam gear and the key on the crank snout? I've had 3 over the last 5 or 6 years and two of them were actually running. They sounded weird very loud and hot out of the exhaust and just sort of flat sounding. One of them I ran on the dyno but that was after the cam timing was corrected. That engine only made about 300 horsepower but it would have been interesting to have done a pull with it screwed up. I bet it wouldn't have made 200 horsepower. I didn't check it to see how much of a timing error that creates but I would say it's 30 degrees or so over advanced.

[Motor Daddy]

We'd be much better off if everyone erased the word "Advanced" from their mind.
Every engine wants a given Intake Centerline( of Intake opening and closing points), and a given Exhaust Centerline(of exhaust opening and closing points).
The LSA is nothing more then these to centerlines added together, and divided by 2.
When you work with double overhead cam engines, you don't talk about LSA or advance.

Let's say an engine wanted a 102 ICL, and a 114 ECL. You would say that's a 108 LSA, advanced 6 degrees.

Now let's say, we modified the exhaust port, and exhaust lobe, and left the intake lobe alone.
The engine wants the same 102 ICL, but now wants a 110 ECL, You would say that's a 106 LSA, advanced 4 degrees.

Saying it's advanced 4 or 6 degrees doesn't tell you anything, and can be misleading.
Talking in Intake Centerlines and Exhaust centerlines, keeps everything clear.

CamKing, why do you write, "( of Intake opening and closing points)" and "(of exhaust opening and closing points)" after the word centerline? That is contradictory, as centerline is defined as the max lift point of the lobe. Asymmetrically ground lobes will not have the max lift point centered in between the opening and closing points so your statement is contradicting itself.

[MadBill]

Can of worms time! This distinction blew up big time on an S/T thread some years back. The basic problem is in how 'centerline' is defined for an asymmetric cam. It can be argued convincingly that it is the point of highest lift and equally plausibly that it is the midpoint between some specified 0.050" or less opening and closing points.

To me the only logical answer it that for any given cam, the centerline is defined however the cam grinder says it is, period.

[David Redszus]

4.The cranking compression would increase as the cam was advanced, thus closing the valve sooner after BDC and so giving a longer compression stroke, until the effective valve closing was at BDC (i.e., 30°advanced) and the CC reached perhaps 190 psi, at which point further advance would slowly start to reduce CC. At this point, the carefully selected position of every valve event has been subverted to the single irrelevant goal of maximum cranking compression.

5. For best dynamic cylinder filling at high RPM, a radical race engine might have the valve closing at 100° or more after BDC, so that even with 16:1 CR, the compression stroke is so short that CC is still only ~ 170 psi. Ignoring piston to valve interference issues, if you started to advance this cam, cranking compression would rise to probably 450 psi or more as the cam was retarded 100° What are the odds the engine could even be cranked, much less perform better?

Building upon what MadBill said above, and along the lines of Stans graph, here are some representative calcs.

SCR = 14.0:1

IVC
BTDC. TCR.... psia
94.... 8.8.... 266
96.... 9.1.... 276
98.... 9.3.... 284
100.... 9.5.... 293
102.... 9.7.... 301
104... 9.9.... 310

180.... 14.... 489

I have not included the compression tempertures at each IVC position.

For a single camshaft, changing the IVC will also have an effect on all other valve events such as overlap, exhaust blowdown, etc.

[CamKing]

CamKing, why do you write, "( of Intake opening and closing points)" and "(of exhaust opening and closing points)" after the word centerline?

I write that for people like you that are still using the 1960's definition of "Centerline".
I write it so people understand that I'm talking about the centerline between the opening and closing point of the valve lift curve.
This is how I define "Centerline" with all my cams.
Opening and closing points are way more important then where max lift happens, and are the one common way to compare symetrical and asymmetrical cams.

[lorax]

CamKing, why do you write, "( of Intake opening and closing points)" and "(of exhaust opening and closing points)" after the word centerline? That is contradictory, as centerline is defined as the max lift point of the lobe. Asymmetrically ground lobes will not have the max lift point centered in between the opening and closing points so your statement is contradicting itself.

How did you determine the definition "centerline" to mean peak lift?
The terms "centerline" and "peak lift" are self explanatory and are just what they say.

[Stan Weiss]

I guess I am still back in the store age. While theses to lobes have just about identical area and the same opening and closing points I would not say they have the same ICL.

Stan

ab-cam-sym-asym.gif

[CamKing]

I guess I am still back in the store age. While theses to lobes have just about identical area and the same opening and closing points I would not say they have the same ICL.

Stan

They have the same centerline, and will peak at the same RPM. If you move the asymmetrical so they had max lift at the same point, but the opening and closing points won't be the same, they would not peak at the same RPM.

That's why we don't use max lift.
If you use max lift as centerline, the asymmetry of the lift curve would change the optimum centerline forthe application.
Using the centerpoint between the opening and closing points allows you to calculate the correct centerline for symetrical or asymmetrical profiles.

[Motor Daddy]

This is how I define "Centerline" with all my cams.

So every cam manufacturer has a right to make their own standards on how to measure cam specs, and screw the system that's already in place? Isn't that confusing the heck out of people and just plain wrong? Why not just use the industry standard that is already in place so you are on the same sheet of music as everyone else?

[Motor Daddy]

CamKing, why do you write, "( of Intake opening and closing points)" and "(of exhaust opening and closing points)" after the word centerline? That is contradictory, as centerline is defined as the max lift point of the lobe. Asymmetrically ground lobes will not have the max lift point centered in between the opening and closing points so your statement is contradicting itself.

How did you determine the definition "centerline" to mean peak lift?
The terms "centerline" and "peak lift" are self explanatory and are just what they say.

Let me know if you need more links that say the same thing, there are plenty of them.

http://www.holley.com/data/TechService/ ... shafts.pdf

CENTERLINES
The intake centerline is the point of highest lift on the intake lobe. It is expressed in crankshaft degrees
after top dead center (ATDC). Likewise the exhaust centerline is the point of highest lift on the exhaust
lobe. . It is expressed in crankshaft degrees before top dead center (BTDC). The cam centerline is the point
halfway between the intake and exhaust centerlines

http://www.cranecams.com/faqview.php?s_id=7

The centerline of either the intake or exhaust lobe is the theoretical maximum lift point of the lobe in relationship to Top Dead Center in degrees of crankshaft rotation. (They are shown at the bottom of the camshaft specification card as "MAX LIFT.") The centerline of the cam can be moved by installing the camshaft in the engine to an advanced or a retarded position.

http://www.iskycams.com/degreeing.php
Thus far, we have seen how it is possible to use the cam timing diagram to determine the camshaft position in the engine with symmetrical cams only. But what of camshafts that are asymmetrical in shape? (The opening and closing sides of the lobes differ in profile). For the answer let us consult Figure 22 where we see that even though the tappets are equally open at T.D.C., indicating a split overlap, the timing diagram indicates the camshaft is five crank degrees retarded because of the extended closing ramp of the asymmetrical cam lobe. Therefore, it is evident that with an asymmetrical cam, the timing diagram cannot always be used to check for split overlap. A more accurate method then would be to check the theoretical "Centerline" (point of maximum lift) position of the intake and exhaust lobes. In Figure 22 notice that regardless of how the cam timing changes with the addition of higher closing ramps, the centerline of the intake lobe remains 110 crank degrees after T.D.C. and the centerline of the exhaust lobe remains 110 crank degrees before T.D.C. Therefore, we find that splitting the difference in crank degrees between the centerline of the intake and the exhaust lobes either side of T.D.C. is a more reliable means of phasing an asymmetrical cam.

[CamKing]

3 arc cams were the industry standard. You should go buy one.
There is no industry standard for today's cutting edge, asymmetrical cam designs.

If you want to run your Duntov 30-30, you can degree it any way you like.