Cam timing

[trbig]

I remember a big discussion on this, but couldn't remember the verdict. To advance the intake sprocket one tooth, or just one link in the cam chain, is this too far in advance (As in over 5%). I remeber there being some question as to if the 5% counts towrds one rotation of the cam sprocket, or two. And just to make certain... you advance towards the rear of the engine.. correct? I'm just about to re-time things, so as prompt a reply as possible would help.


Tod

[mro]

Thought you were going to slot the sprockets.
Slotted ones I have allow adjustment of less than one link.
On a hot engine would think you'd want the intake to open sooner and the exhaust later.


mro
not sure changing sprocket posistion on chain is good idea
bent valve?!

[TopCatGr58]

Hey MRO,

I just did a search and found several threads where Dan Hodges posted info about the difference in cams and such, and using slotted cams, advancing the timing 5 degrees of the intake, not 5%, which would aide in the bottom/midrange torque levels, BUT didn't find any info about how many degrees 1 cam sprocket tooth is worth? I don't have a cam sprocket handy to look or count, but I bet TOD does! Take the number of teeth, and divide it INTO 360 and you should find out how many degrees 1 tooth is worth. PS: 5 degrees is only 1.38% advanced!

Also, hitting the valves with the pistons was more of a concern for the exhaust cam since the piston is racing up as the valve is open and closing, and if the Ex timing is retarded too much, then the piston can catch the valve!

But with the Intake, the piston is already on it's way down when the intake valve is starting to open, and so advancing the cam timing to let it open a little earlier isn't as much of a concern....
according to what I read! YMMV!
T.C.

[trbig]

Cam sprocket is 32 teeth, crank is 16. So if one rotation of the cam is considered 360 degrees... not the crank spinning 360 one time, it's 11.25% for one tooth, but only 5.6% if it goes by the crank. So which one do they go by when they say they advanced the timing by 5%?

Tod

[TopCatGr58]

Hey Tod,

You're using the %(percent) symbol when we need to be talking about degrees! I just know that they slot the CAM sprockets for doing the degreing, but it's been a while since I last read the HOW TO degree the cams, so can't remember if they have the cam sprocket marked in degrees, or the crank, or both!?

But, since the crank has to turn twice as many turns/teeth as the cams, then it requires 2 teeth's worth of crank sprocket to move the cam sprocket 1 tooth's distance, and since it has 1/2 as many teeth, the degrees of crank rotation will be twice that of the cam.

So...if the cam sprocket were moved 1 tooth=11.25 degrees, that would be 22.5 degrees of crank rotation. But Dan and others only mentioned about 5 degrees of advancement of the cam timing, and I would think it was the cam sprocket that was moved via the slots that amount of degrees, so it looks like 1 tooth is too much anyways!
T.C.

[TopCatGr58]

Followup!

Hey Tod,

Here's a thread you posted a while back on this subject, and TURBOPETE has the answer as the last reply on the last page...above link takes you to the last page!

The degrees are measured at the CRANK, as in BTDC, and 1 tooth is over 20 degrees of crank timing....not good!! So...you'll either need to find a machinist to actually slot your cam sprockets, and get a degree wheel that mounts on the crank, OR...just be content with the 78 cams with your XJ heads....if that's what you have right now, can't remember?
T.C.

[mro]

Check out this link

http://www.101sportbikeprojects.com/Resources/101SportbikeProjects_89.pdf

mro

[TADracer]

Like TC said, cam timing is measured at the crank with a degree wheel. A 2 or 3 degree change in the CAM itself can be a big adjustment that would equate to 4 or 6 degrees at the crank. Degreeing your cams should be done with a bit of study on where you want your performance to stand because with the twin cams you can also adjust your overlap and total durtation a bit.

Slotted sprokets are the way to go and you will need a degree wheel mounted to the crank with TDC positioned accurately using a piston stop before you begin any cam adjustments. I definately would not try skipping a tooth on the sproket. That is the equivelent of 22.5 degrees at the crank. And a word of caution, if you RETARD your exhaust cam at all, better check your piston/valve clearance because the piston chases the exhaust valve closed and that is the main place a collision will occur. Should be about .050" as a rule of thumb.

BTW, MotionPro makes a nice degree wheel for about $20.00. If you are fooling with cam timing it will be worth its price.

[trbig]

Nope... not gonna fool with it. I was just wondering about the one tooth thing. That reminds me...(the one tooth thing..) we will be riding through Arkansas a bit in a couple weeks??


Tod

[MAXIMAN]

Tod

The only "sane" way to dial cams is with a slotted sprocket and a degree wheel.

RULE OF THUMB:

1 advancing intake and retarding exhaust builds pumping compression and thus low-mid torque.

2 retarding intake and advancing exhaust increase flow and top end horsepower.

And yes...since the Eleven spins backwards advancing cam rotation would be rearward.

When you say 5% this is meaningless. 5% of what?

5 or 6 degrees of intake advance is about all you'd want to go. I have Zilla's cams dialed in with the intake advanced 6 degrees and the exhaust retarded 3 degrees.

It is likely I will take Sansei's advice and install the '79 exhaust cam dialed in straight up. Since I have grunt to burn on that bike I am going to shoot for more top in hopes of breaking the magic triple digit HP number. After the addition of the '79 exhaust cam and the oval K&Ns I hope I'll see about 105 to 107 on the dyno.

But only time and money will tell for sure. If it works you'll be the first to know because I'll be right back here posting the results.

[trbig]

I still have the 78 cams in mine. It seems to run very well with them. I just put them back on time. When I said 5%, I meant 5 degrees, Cody. Get off my back! Listen to what I mean... not what I type!

Just a run-down... I put a spare motor together because mine was knocking, and did the tranny fix on it while I was in there. I couldn't get the carbs dialed in at all and rode it to Texas to John and Wildkat's (You know... the gay guy) and it got even worse there and had to ride home @ 200 miles basically on 2 cylinders. Compression showed to be only in the 120's. The knocking motor was in the 150's.

So.. I took the engine head and cylinders back off and put back on the '78 cylinders that I had been running in the 1st motor, and also swapped the rings on the pistons so that the same rings were running in the same cylinders that they had been. I didn't want to just swap pistons because I figured that knock was somewhere high like a wrist pin or piston skirt slapping.

I slapped everything back together tonight and threw the carbs on and it busted right off when I bumped the starter. I haven't done anything to the carbs, but they are pretty close. It is running SOOO much better. I threw the seat on got to go for a nice little evening ride. So now I am back to the 82 motor, pistons, and head and 78 cylinders and cams.

As the old song goes... "I'm back in the saddle again... "


Tod