Spitfire Spitfire 1500 Cam timing

[Morris]

Okay I lied. It's really a Midget 1500.

I have a stock cam, and I just want to make sure it is timed correctly.

What I have done so far:

I have aligned the marks on the cam and crank and installed the chain. I have used a dial indicator to find TDC. My dial indicator is now following the intake lifter for #1 cylinder. I have a degree wheel on my crank. What I am having a hard time understanding is how to read my degree wheel to determine if my cam is aligned properly.

Here are the numbers I have for my stock 1500 cam (from Paul Tegler's site).


Lobe centers 112°
Intake timeing 18°-58°
Seat duration 247°
.05" duration 205°


How do I translate my degree wheel readings so that they relate to these numbers.

Lemme throw this out: Starting from TDC before the intake stroke, I take a degree wheel reading when my tappet reaches .05" lift; I take another reading when it returns to .05" lift. I add these two readings then divede by 2, and if everything is aligned correctly, the sum should be 205°. Is that right.

If so, what numbers would I look for if I wanted to advance my cam timing by 2°.

Thanks.

 

[RomanH]

Sorry can't help with specifics since I just followed the cam grinders recommended settings.
I did find an article that might help you.
Camshaft Basics

Good Luck!

 

[Dave Russell]

Here are the numbers I have for my stock 1500 cam (from Paul Tegler's site).
Lobe centers 112°
Intake timeing 18°-58°
Seat duration 247°
.05" duration 205°
How do I translate my degree wheel readings so that they relate to these numbers.

Lemme throw this out: Starting from TDC before the intake stroke, I take a degree wheel reading when my tappet reaches .05" lift; I take another reading when it returns to .05" lift. I add these two readings then divede by 2, and if everything is aligned correctly, the sum should be 205°. Is that right.

If so, what numbers would I look for if I wanted to advance my cam timing by 2°.

Thanks.

Not quite, expressed in crankshaft degrees,
The intake timing given above means that the intake valve opens 18 degrees BEFORE tdc near the end of the exhaust stroke. The valve remains open until the piston has reached bottom & stays open as the piston is rising on the compression stroke until the piston has reached 58 degrees AFTER bdc. Thus the intake duration is 18+180+58 degrees, a total of 256 degrees open duration. This is NOT the 256 degree seat duration & is not the 205 degrees duration at .050 lift.Quite possibly it is the duration at something like 0.015 running valve lash.

The statement "112 degree lobe centers" (plural) is the angle between the peak of the intake lobe & the peak of the exhaust lobe expressed in CAM degrees. Generally a wider angle such as 112 degrees gives better low rpm torque while a lobe center angle (LCA) of around 106 degrees gives better high rpm torque.

Do not confuse this number with the sometimes given lobe center (singular) angle which is the point where the intake lobe is at maximum lift. Expressed in crankshaft degrees. Although many folks time a cam by this latter "intake lobe center angle" I find it hard to precisely determine the exact lobe center due to the very small valve movement at this point.

With the numbers given, advancing the cam itself two degrees would make the valve events occur four degrees earlier in the crankshaft rotation. the cam advanced numbers would now be - Intake opens at 22 degrees before TDC & closes at 54 degrees after BDC.

Since the point of intake valve closing has the most effect on engine characteristics, it controls the actual effective compression ratio, since no actual compresion can begin until the intake valve is closed, advancing the cam to give earlier intake closing will give more low rpm torque. (higher effective compression ratio) At higher rpm, the late intake valve closing is offset by the fuel column inertia still cramming fuel into the cylinder even though the piston has reversed direction & has started back up on the compression stroke.

A nominal 10 to 1 compression ratio engine may have an actual effective compression ratio of 7 to 1 at low rpm due to late intake valve closing. Part of the mixture is pushed back out through the still open intake valve. This, in large part explains the lumpy idle with more radical cams. That & the fact that the exhaust valve is still open as the intake stroke begins. The compression ratio will approach the 10 to 1 ratio as intake ram effect overcomes the the reversed (piston rising) at higher rpm.

The whole thing is confused a bit by having cam degrees & crankshaft degrees both in the discussion, & the fact that many degree wheels only show 360 degrees while the specs call for numbers before & after TDC & numbers before & after BDC. You have to keep careful notes as you go & translate the degree wheel numbers into actual valve event numbers. Drawing a couple of pictures helps.

The exhaust valve events have similar effects on engine operation such as the exhaust valve still being open after TDC while the intake valve has already started to open before TDC on the exhaust stroke. This is called overlap. The realier mentioned angle between lobe centers directly affects intake to exhaust valve overlap. The wider the angle, the less overlap.
D