Author Topic: CAM Timing - Advance, Retard, Early, Late, BTDC, ATDC - I'm So Confused (Read 21334 times)

RSchaefer · « **on:** June 07, 2012, 09:50:51 AM »

I'm so confused!

I have my '75F project coming along and have been, of late, focused on the engine, say the last 6-months (my hobby). I have HondaMan's book and have done most of his "cheap" engine upgrades - removed the obstructions at the inlets (semi-polish), bowled the intake under the valve, valve job, hone (stock bore), removed .010" from the head and per the book am going to "Advance" the cam timing another 5-degrees for a total of 10-degrees advance as the "F's" are stock at 5-degrees advance. Should lower the power band for more grunt from a stop, quicker around town and the porting should relieve some of the asthma as I approach the top of the power band.

So I printed off the degree-wheel from this site, already have a dial indicator from the wheel truing and went to work. My confusion is as it relates to "Advancing" the cam timing verses "Retarding?"

It seems to me that advancing the timing would be having the CAM open the intake valve BTDC (Before Top Dead Center, early) and retarding would be to open the intake ATDC (After Top Dead Center, late).

So with the variable sprocket from Dynoman ($31) I currently have the intake valve opening at 10-degrees BTDC, is that correct? Opening of the intake valve is defined as .040" measured from the top of the intake valve keeper.

RSchaefer · « **Reply #1 on:** June 07, 2012, 11:10:19 AM »

Come on, I realize its probably a stupid question but I really am trying to get a consensus!

Thanks!!

Bodi · « **Reply #2 on:** June 07, 2012, 11:26:54 AM »

You have to know how the cam was specified. The degree numbers are specified at a valve lift, not at the first movement. I don't know the exact lift that Honda cams are specified at, if you get an aftermarket cam it will be in the cam data. As I recall specifying degrees BTDC/ATDC at 0.040" valve lift is pretty common. I think the spec is with zero tappet clearance but again I'm not sure, this should be in the cam spec as well for aftermarket cams. BTW, I've found a fair bit of variation from "spec" when measuring cams with a degree wheel.
There's also a spec for lobe centre which you can measure, and then set the centre figuring the open/close points will work out. If the cam spec shows a duration of 100 degrees you can calculate where the centre between opening and closing is. Then regardless of whether you measure the same duration with your dial gauge setup, you can set the cam sprocket so the centre is at the same point - and it should then be be in spec. Advancing the lobe centre 5 degrees will advance opening and closing 5 degrees.
Advancing the cam does open the valve earlier.

RSchaefer · « **Reply #3 on:** June 07, 2012, 12:01:04 PM »

OK, so I have the stock CAM and thus the Honda spec for lift "starts" at .040" or 1.00mm, but does advancing mean that the intake valve opening starts BTDC, in my case 10-degrees BTDC? And, as I understand it, I should be monitoring the lift start to include the lash (per HondaMan's book), in my case .002-.003 intake and .003-.004 exhaust.

So is it BTDC??

MCRider · « **Reply #4 on:** June 07, 2012, 12:18:04 PM »

This stuff confuses me too, but i got thru it. Didn't read all of Bodi's post, so hope I'm not repeating. AS to:
"It seems to me that advancing the timing would be having the CAM open the intake valve BTDC (Before Top Dead Center, early) and retarding would be to open the intake ATDC (After Top Dead Center, late)."

Under no circumstance will the intakes ever open ATDC.

They will always open BTDC, just more or less so.

dave500 · « **Reply #5 on:** June 07, 2012, 12:44:55 PM »

beat me to it mcrider,advancing the cam everything opens and shuts sooner or earlier in degrees,visa versa for retarded.

Bodi · « **Reply #6 on:** June 07, 2012, 01:53:15 PM »

Well, I don't have the book.
The lash is tough to include because the rocker is not symmetrical - the valve end isn't the same length as the cam end. That's why I thought you measured with zero tappet clearance.
Also I don't remember if you measure lift at the rocker or the valve - on cars one usually measures the rocker lift not at the valve. That's why I like the lobe centre setting, it eliminates all those variables.
The intake valve opens before TDC, while the exhaust valve is still open. Advancing the cam does this earlier, so the intake valve is fully open sooner and more air-fuel mixture gets sucked in for more power. This stops working at higher RPM when cylinder filling is imore nfluenced by pressure pulses rather than pure suction: the pressure pulse is delayed and you want the valve open later to let it push mixture into the cylinder. So advancing the cam generally makes more midrange power and reduces top end power. Aftermarket cams change the lift, open/colse ramp angles, and overlap to trade things around in a similar way. There's no free lunch, and a cam made just for maximum top end power will suck at low and midrange power. A torquey cam for midrange punch will lose top end power. The stock cam is a compromise, and Hondaman has noted several changes over the model years - these change the power band to discourage riding habits that would cause engine damage and warranty claims.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #7 on:** June 07, 2012, 02:26:45 PM »

I think what Rich is trying to grasp is strickly the numbers here first before trying to grasp the full context. Correct me if I'm wrong Rich.

ie If he sets his timing at, say, 5^o but wants to advance it 5 more degrees to a total of 10 degrees would he set it at 5^o PLUS 5^o to 10^o as it appears he has done or 5^o MINUS 5^o to 0^o on his wheel?

RSchaefer · « **Reply #8 on:** June 07, 2012, 03:04:35 PM »

Yes your right Jerry!

This all started because HondaMan suggested the 5-degree ADVANCE based on the suggested improvements he mentions in his book. In the book he suggest rat-tailing out the CAM sprocket bolt holes (notching in an arc) as a way to advance the sprocket on the CAM. Problem with my situation is the "F" sprocket has various holes in the sprocket to reduce the overall weight of the sprocket (quicker throttle response) and your not able to arc the hole far enough to pickup the 5-degrees, you run into the next hole.

So I purchased the Dynoman adjustable sprocket, which also has the holes to reduce weight, but a nice arc hole from which to secure the sprocket to the CAM with the two bolts (use Blue Locktite). So I never went back and re-mounted my old sprocket and check the stock advance. That would be #1-4 at TDC, CAM line horizontal, notch up, rotate to intake lift at .040" - what does the degree wheel say?

So I guess I am screwing up here as I made a rash assumption that 0-degrees was the stock opening for a "K" and 5-degrees BTDC was stock "F." And that retarding was ATDC. THIS IS ALL WRONG!!

The degree wheel I printed off this site says it is for a Honda CB750 SOHC, Web-Cam 63A Grind. In the notes on the wheel it says "Valve Timing is Checked with Zero Valve Lash @ .050" and it indicates the Intake opens at 27 degrees BTDC.

So apparently a stock "F" cam would open at 32 degrees BTDC and I should be setting mine for 37 degrees BTDC, does that sound right?

Bailgang · « **Reply #9 on:** June 07, 2012, 03:06:53 PM »

Quote from: RSchaefer on June 07, 2012, 09:50:51 AM

It seems to me that advancing the timing would be having the CAM open the intake valve BTDC (Before Top Dead Center, early) and retarding would be to open the intake ATDC (After Top Dead Center, late).

So with the variable sprocket from Dynoman ($31) I currently have the intake valve opening at 10-degrees BTDC, is that correct? Opening of the intake valve is defined as .040" measured from the top of the intake valve keeper.

ATDC and BTDC can be confusing but you've got the right idea.

Bailgang · « **Reply #10 on:** June 07, 2012, 03:12:14 PM »

I better be more specific so I don't confuse you more. What I meant on my last post was that you had the right idea in regards to what direction to go in regards to BTDC and advancing the cam timing but I can't verify what the exact cam timing figure would be.

RSchaefer · « **Reply #11 on:** June 07, 2012, 03:23:59 PM »

Really confusing!!!!!!

Here is the quote from HondaMan's article on the forum "More on Cam Timing"

"Stock timing (we’ll use ony intake numbers for simplicity) is: valve opens (.1mm lift) at 5 degress BTDC, closes at 35 degrees ATDC. This is the “sweet spot” for the system.

Now, let’s move the whole cam forward 5 degrees Is this advancing? to open at 10 degrees BTDC, close at 30 BTDC This Must be a Typo, must mean ATDC? . The early torque now comes to a peak at about 25% of redline, and there is more torque than before. But, the HP peak is also moved earlier, to about 75% of redline, but there is LESS HP than before. The engine is a little harder to start, idles a little less steadily. In the CB750K2, this drops about 3 HP at 6800 RPM as compared to the previous peak at about 7600 RPM.

Now, let’s move the cam back 5 degrees Is this retarding? to open at 0 degrees TDC and close at 40 degrees BTDC Is that a typo again, should be ATDC? (like the CB750 “F” and K7-K8 camsI thought the "F" was advanced 5-degrees from stock , like 10-degrees BTDC? ). The engine idles better, and the torque peak comes on at about 40% of redline. The HP peak comes on at about 85% of redline, and there is MORE HP than before, by a little over 4 HP. The overall “feeling” is of increased power, because the power peaks are closer together with less “droop” in between the peaks.

Bailgang · « **Reply #12 on:** June 07, 2012, 03:35:00 PM »

Quote from: RSchaefer on June 07, 2012, 03:04:35 PM

So apparently a stock "F" cam would open at 32 degrees BTDC and I should be setting mine for 37 degrees BTDC, does that sound right?

I apologize for the multiple posts but here's an idea to consider that will help eliminate the guessing as to what the timing for the "F" cam should be from the factory. Simply reinstall your factory cam gear and time everything the way you would if you were setting it up stock THEN see what your degree wheel says and use that figure as your benchmark.

Now I'm picking these numbers out the air just to demonstrate but say for instance after you've reinstalled your factory cam gear and the degree wheel says it's opening at 32 deg BTDC at .040" and you wanted to advance it 5 deg then once you put the adjustable cam gear back in your target would be 37 deg BTDC at .040". Make sense?

Bailgang · « **Reply #13 on:** June 07, 2012, 03:47:04 PM »

Quote from: RSchaefer on June 07, 2012, 03:23:59 PM

Really confusing!!!!!!

Now, let’s move the cam back 5 degrees Is this retarding? to open at 0 degrees TDC and close at 40 degrees BTDC Is that a typo again, should be ATDC?

0 deg is TDC plain and simple. You don't start getting into BTDC or ATDC until you go 1 way or the other

RSchaefer · « **Reply #14 on:** June 07, 2012, 04:42:37 PM »

I appreciate what your saying about going back to my original sprocket but I kinda hate to take everything back apart. Torqued and oiled cam towers, set valve lash throughout, etc. I could, I've done worst and will if necessary. Should be able to get the Honda "book" on what the stock "F" cam intake degree start is from somewhere?

Bailgang · « **Reply #15 on:** June 07, 2012, 05:33:40 PM »

Quote from: RSchaefer on June 07, 2012, 04:42:37 PM

I appreciate what your saying about going back to my original sprocket but I kinda hate to take everything back apart. Torqued and oiled cam towers, set valve lash throughout, etc.

Yeah I forgot about that detail. I advanced the cam timing last weekend on my 550 that has a 650 cam. However I had done the cam swap earlier this year so the engine had already been up a running beforehand and I used the degree wheel to tell me where the cam was at with the stock cam gear before I installed the Dynoman cam gear.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #16 on:** June 07, 2012, 06:20:54 PM »

Rich,

What do you mean?? "Should be able to get the Honda "book" on what the stock "F" cam intake degree start is from somewhere?"

RSchaefer · « **Reply #17 on:** June 07, 2012, 06:52:45 PM »

Well I should! You must have it, can't be a mystery? In other words at what degree does the #1 intake open - 10-degrees BTDC or 37=degrees BTDC?

scondon · « **Reply #18 on:** June 07, 2012, 07:08:20 PM »

I've been looking around to see if I can scare up the timing specs for a stock F cam grind. Haven't been able to find one yet. You will need to know these numbers BEFORE you even start trying to advance or retard your cam.

If you just set your cam to open at 10 deg BTDC at 0.040" lift then who knows where you are. Hopefully the bike will run and the valves won't kiss the pistons at full rev $:-\$

I went through all this for the first time some years back and wrote a post about it. Several entries down on the post I have written the timing specs for the cam I used(not stock). This is the same type of info you'll need to properly time your cam.

http://forums.sohc4.net/index.php?topic=3148.msg22871#msg22871

Can you give us the open/close numbers you set your cam at?. Need measurements from both intake and exhaust lobes.

scondon · « **Reply #19 on:** June 07, 2012, 07:16:55 PM »

Found it! It's just as you posted, so never mind

Intake:
Open- 5degBTDC
close- 35 deg ABDC

Exhaust:
open- 35 deg BBDC
close- 5 deg ATDC

EDIT: After reading, re-reading your posts it looks like you have it figured correctly. Would just like to confirm how you took your measurements i.e. at zero lash, or with valves gapped to spec. Also, did you use a piston stop to determine "True"TDC

Jerry Rxman Griffin aka MuthaF'er · « **Reply #20 on:** June 07, 2012, 07:24:52 PM »

Sean to the rescue! Yep, that's it!

So from this discussion as I am NOT a cam expert if you want to advance the cam 5^o you would set the cam at 10^o BTDC intake opening. Unless I'm wrong

This would advance the intake opening by 5^o because it would open in advance 5^o sooner. And since it's a single stick you don't have to do anything to the exhaust as it will follow suite.

Anyone say different? Please, it's open for discussion. I am not yet comfortable with my cam dynamics.

scondon · « **Reply #21 on:** June 07, 2012, 07:40:59 PM »

Quote from: Jerry Rxman Griffin on June 07, 2012, 07:24:52 PM

Sean to the rescue! Yep, that's it!

So from this discussion as I am NOT a cam expert if you want to advance the cam 5^o you would set the cam at 10^o BTDC intake opening. Unless I'm wrong This would advance the intake opening by 5^o because it would open in advance 5^o sooner. And since it's a single stick you don't have to do anything to the exhaust as it will follow suite.

Anyone say different? Please, it's open for discussion. I am not yet comfortable with my cam dynamics.

Jerry! All the numbers and dynamics really help confuse my thought process when it comes to "advance" and "retard." It's actually a lot easier to wrap your head around if you just simplify and throw out all the extra info.

Advance the cam timing = move the cam in direction of travel while the engine remains stationary. Valves begin opening sooner. The BeforeTDC number increase.

Retard the cam timing= Move the cam opposite direction of travel while the engine remains stationary. Valves begin opening later. The BeforeTDC decreases

Same thing with ignition timing. Advancing ignition timing makes the spark happen sooner(in relation to the rising piston)

KJ790 · « **Reply #22 on:** June 07, 2012, 08:01:22 PM »

I'm not sure if it's been mentioned, but keep in mind that when you are talking degrees with a cam you are really talking crank degrees. Advancing a cam by 5 degrees would really only be rotating the sprocket 2.5 degrees on the cam, which would equate to 5 degrees of crank rotation.

As a rule of thumb, advancing the cam timing generally shifts the power curve towards lower RPMs and retarding the cam timing generally shifts the power towards higher RPMs. Just be careful with how much you advance the timing, as this will decrease the piston-valve clearance on the intake valve. Too much cam advance and your piston will contact the intake valve.

Hinomaru · « **Reply #23 on:** June 07, 2012, 08:19:52 PM »

This chart helps illustrate the cam dynamics of advance / retard into a visual perspective:

Jerry Rxman Griffin aka MuthaF'er · « **Reply #24 on:** June 07, 2012, 09:01:18 PM »

Sean brings up something interesting that may be confusing. We have to seperate cam timing from ignition timing!! 2 different animals.

If you advance it you do it sooner/advanced. If you retard it you Jerry it

errr you do it later.

News:

Author Topic: CAM Timing - Advance, Retard, Early, Late, BTDC, ATDC - I'm So Confused (Read 21334 times)

Hinomaru