[ianmac]

I have a question regarding the potential to mix up the timing of a cam by using a cam meant for the exhaust side on the intake, or vice versa, and using the original timing marks to set the timing. I have a '77 Spider which I have rebuilt after the PO blew the headgasket. After cleaning up the head etc the cams were put back in the original places, exhaust where the exhaust cam was originally and the intake where the intake was. The car has never run well, in spite of several attmpts to tune the SPICA, new distributor and charging system. I was reading Pat Braden's book and he described a syndrome, where the car is gutless below ~3000 rpm but pulls fine above 3000, that sounded similar to mine. He suggested that this syndrome could be resolved by retarding the intake cam by a few or more degrees. I know my cams are timed according to the marks on the cam, but I also know that the cams don't exactly 'match'. One is an ALFA cam, the other unmarked. I have described the ALFA cam, and its characteristics in the diagram below. It is a 105020320001. The other is presumably an aftermarket match to this one. My question is in the next post, since I need to post another picture to address it properly. (this diagram is incorrect ...read the rest of the thread - IM)

 

[ianmac]

Cam Question Part 2

My intake cam has markings on both sides, presumably 180 degrees apart, but one is a heavy factory marking and one is simply scratched in, further evidence that the cam is not an original match for the exhaust cam. My question is this, I have heard that one can use paired exhaust cams and intake cams interchangeably, but this doesn't make sense to me. The timing marks would result in the cam being out of time, correct? For example, the picture below, an exhast cam used in the intake position would result in the cam being out of time by 24 degrees, correct? (this diagram is incorrect ...read the rest of the thread - IM)

 

[ianmac]

Cam timing 3...

And similarly, if one were to use the intake cam on the exhaust side, you would have a cam 24 degrees out. Am I correct in these assumptions?

I believe I could run the cam that far out since it only has 9.6 mm of lift, and I have 10.4 compression pistons with valve cutouts. Does this seem possible?

Also, have I drawn my diagrams correctly?

(this diagram is incorrect ...read the rest of the thread - IM)

 

[Gordon Raymond]

Send your querry to Alfar7 and he will know, and possibly correct your situation.
Gordon Raymond

 

[papajam]

Nice drawings, Ian!
Unfortunately though, this is not what happens. Lobe center refers to the angle between camshaft lobes on a cam and is fixed once the cam is ground. Lobe centerline, which is very often shortened to lobe center, refers to the number of degrees of crankshaft rotation from Top Dead Center (TDC) when the valve is at maximum lift. Lobe centerline is the cam or valve timing. For example, if the cam spec says to time the intake cam to 102 degrees, this means that the crankshaft will be 102 degrees After Top Dead Center (ATDC) when the valve is full open. If the exhaust cam is timed to 100 degrees, the valve will be full open when the crank is 100 degrees Before Top Dead Center (BTDC).

 

[ianmac]

How about this?

So if I understand correctly the correct layout for the cams would be represented like this (cams drawn as if you are sitting in the drivers seat looking at the engine):

(again the bottom half of this diagram is incorrect ...read the rest of the thread - IM)

 

[ianmac]

Both open at 102 degrees, one BTDC, one ATDC

And when the valves are fully open the cams would look something like this (I have drawn the tappets at an arbitrary 45 degree angle):

 

[ianmac]

If swapped or wrong cam is used?

And according to this, the exhaust and intake cams are not interchangeable (correct?):

(incorrect ...read the rest of the thread - IM)

 

[papajam]

This whole thing can be difficult to visualize so please refer to the below drawings. This is the intake cam as viewed from the rear of the engine. The first drawing shows the 102 degree cam timing marks with the engine on TDC of the power stroke. The nose of the cam will be 96* away from the timing mark on the cam journal. As the engine goes through the power and exhaust strokes, the crank has turned 360*. But the cam will have turned only 180* and will be in the position shown in drawing two, 84* past the timing mark. Note that the cam has contacted the follower (the beginning of the intake stroke) and has slightly opened the valve (arrow). The exhaust valve at this same time will be open as well. The time that both intake and exhaust valves are open is called valve overlap and is measured in degrees of crankshaft rotation.
Since the follower is at a 45* angle and the cam is 84* past the timing mark (piston at TDC), the cam needs to turn an additional 51* to fully open the valve (drawing three). Since the cam turns one half crankshaft speed, this means that the piston will be at 102* ATDC when the valve is fully open.
All original Alfa camshafts that I know of have two timing marks 180* apart. This means that the cams will work on either the intake or exhaust.

 

[ianmac]

You spin me round....

Ahhh ... foiled by the 720 vs 360.... thanks Papajam!

I am getting this, slowly. I just pulled the cam cover and checked the cam timing again, the intake is advanced perhaps a mm or more from the line. I'll check the marks on the cam caps tomorrow PM using centerlines templates to see if they are at 102 as the cams are supposed to require. Then I may try to retard the intake cam a touch.

 

[Alfar7]

Cam timing marks

ianmac:
The cam marks are at 90 degrees from the lobe center. each direction so they can be used on either side. But depending on grind of "non-stock" it might be best in other position.
The cam # you have listed is for a late 1600/early 1750. 9,5 lift.

I would seriously doubt that the other cam is a "copy" as it is a fairly low performance cam, but better than the US 2L cams. But most non factory cams are different from stock.

What is lift on other cam. Can you take a Pic of both sides & post?
The INTAKE 10502 cam should be set exactly on the 102 mark, or about advanced 1/2 of the cam notch inside the cam mark on the Cap. (100 degrees) (My choice for overall best performance start at 102)

The exhaust can be set at 102 but I would suggest retarding it to 100 degrees which would be again 1/2 the timing mark to the inside of the cap mark. (both cams moved torward the center of the motor) (I would suggest the 100 degree setting for a start.)
Since you have a template you can set very close. But the cams will not get close to piston tops unless seriously off from intended settings.
Cap marks are actually 102 BTDC INTAKE, and 102 ATDC EXHAUST

All these drawings!!!!! Don`t tell me you are engineers! This will never be resolved!

 

[TS_turbo]

...The exhaust can be set at 102 but I would suggest retarding it to 100 degrees ..

Richard why retarding exhaust cam ? is this for max power , low torque ot best overal ? my expirience with 10548's 2L nord 10.5:1 CR for good mid range and overal performance is advancing exhaust cam(decrease overlap and early exhaust blowing actualy)