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Camshaft timing questions

Russell

Russell

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I'm working on a 355 that I'd built back in highschool, but could never quite get to run correctly. I suspected a wiped camshaft lobe (didn't run right after a new cam was installed) and pulled it all apart to inspect it. Once I checked all my valve lifts, I discovered that everything was perfectly within spec, and that all the lifters looked beautiful on their wear surfaces.

Since the next logical thing that came to mind was valve timing, I borrowed a degree wheel to check the intake centerline on the camshaft. Following the steps laid out on a hot rod engine article I found online, I discovered that my intake centerline is at 113.75 degrees, whereas the cam specs call for 106 degrees (a difference of 7.75 degrees).

I know that you can purchase timing chains with different keyways machined into them to advance or retard the cam timing by 5 degrees, but I was wondering if there was a better way to adjust it so it is more precise, or if that extra 2.75 degrees of difference would be significant to the operation of the engine.

Can a guy combine one of those multiple keyed timing chains with a camshaft gear shim? Do they make shims as small as 3 degrees?

Thanks for any help you guys can offer!

BTW engine specs for you:

350 bored .020 over with factory cc dished pistons (freshly built, less than 10 000miles)
Comp Cams Extreme energy XE262H (218 in, 224 ex @ .050, .462 in, .0469 ex valve lift, LSA 110 degrees, 1300-5600 rpm)
Vortec heads with screw in studs and aftermarket valve springs as reccomended by Comp Cams
Vortec Q-jet intake, and a Q-jet carb

Symptoms included a poorly running engine that popped through the intake and exhaust with severe idle problems, but decent higher end performance. It never made any knocking or ticking noises, though ignition timing had to be set close to 20 degrees BTDC to make the engine idle decently.

The carb needs tuning (originally from a 425 olds) and I have not messed with the timing curve any, but I knew that the problem was more than the carb, as the engine acted up exactly the same with two seperate sets of heads, two carbs & intakes, along with a TPI setup.
 
The Comp specs I have for that cam are 218/224 dur. @.050, 462/469 lift on a 110 lobe center. Says the cam makes good TOP-end power, has noticeable idle. Did you install the cam straight up, and how did you adjust the valves?
 
I don't think the lobe centerline has anything to do with the advance or retarding of the camshaft.
 
We're on the same page for the cam specs (as posted), and as Zimmer mentioned, the LSA (lobe seperation angle) doesn't have much to do with the intake center line. Bascially, my concern is how to adjust the cam so it is in sync with the crankshaft, and just how much it can be out of sync before I start to experience the problems I have.

If being nearly 3 degrees out of sync isn't a big deal, sweet, then all I need is a new timing chain that can advance / retard the timing 5 degree (dunno which way the cam needs to go). I've also seen advertised little bushings that you install on the cam gear to fine tune it's position on the cam as well (involves reaming out the bolts a bit as well), but I am not totally sure I want to mess around with trying to machine the cam gear.
 
how are you checking TDC, or your stop point of piston travel?
 
76zimmer said:
I don't think the lobe centerline has anything to do with the advance or retarding of the camshaft.
Click to expand...
Actually it does, I stand corrected....the cam you have comes 4* advance ground in to it.
Right now it sounds like your retarded by 8*. Which means your actually 4* retarded with the way the cam is ground at Comp Cams. There is a 3 slotted crank gear that can get you 4* back, by using the advance keyway.
 
The engine does not have any heads on it. I am used a dial indicator to find the exact TDC, then adjusted my degree wheel pointer to 0 degrees from there.

I did think of one factor that I hadn't considered earlier in that I hadn't accounted for any slop in the timing chain. Despite being brand new, it likely does have some amount of slop in it, and I did a fair bit of rocking the crank back and forth to get my measurements on exactly .050 on either side of the intake lifter's TDC to find the position of the crank at each point.

I took the two angles I found (150.5 degrees and 77 degrees), added them together then halved it to come up with my 113.75 degree intake centerline
 
Hmm, so if this cam does have 4 degrees of advance ground into it (where did you find that info, the spec card makes no mention of it?), where do I take that into account with my math?
 
Ah, very good read!

I see that their instructions for degreeing the cam take into account the slop of the timing chain (all measurements taken by turning the engine clockwise, while I rocked it back and forth till I got directly onto .050"). It also seems that the 4 degrees of advance need not be factored in, and just to set the cam to the 106 degree intake centerline spec called for...

I'll re-do my measurements tomorrow, and if it is still way out to lunch, I'll go pick up a timing chain kit
 
are you stetting TDC taking into account piston dwell?


TDC is not when the piston stops traveling upward. its 1/2 way between stop going up and start going down. the crank travels quite a few degrees in that time. so its important to get the correct starting point.
 
do you have the correct length pushrods? If they are wrong that could throw off your valve timing some and cause some issues
 
I actually think its better to take it 1/2 to 1" down in the cylinder, while the piston is still in motion.
 
DEMON44 said:
are you stetting TDC taking into account piston dwell?


TDC is not when the piston stops traveling upward. its 1/2 way between stop going up and start going down. the crank travels quite a few degrees in that time. so its important to get the correct starting point.
Click to expand...

Ah, another source of error! What I'll do next time is pick a certain position on my dail indicator and record the angle of the crank on either side of the piston's travel, then use the midpoint as TDC
 
to find true tdc

rotate till top dead. note degree on wheel.

rotate backwards till top dead center. note degree on wheel.

take 2 and divide and you get your true #

search last weeks hp tv i think or 2 guys garage. thay had a 2-3 min thing on how to do it.
 
thats exactly it,


measure piston rise till stop travel. make note. back crank up till piston come up the other side and stop travel. halfway between the 2. thats actual TDC.


when I set the injection timing on a b cummins thats what I do too. except theres no degree wheels. just the dial indicator and makeing marks on the damper.
 
Don't waste your time trying to mess around with measuring TDC with an indicator, at the top or almost, it doens't matter, the piston can wobble in the bore to much to reliably use a dial indicator unless you measure at 4 different points on the top of the piston for each measurement, not worth it. You need to use a solid piston stop. If you don't have one you can use two short bolts in the head bolt threads and a flat piece of steel or aluminum, about 1/4" thick x 1" wide x 6 - 10" long. Just stagger the plate beteen two short bolts with washers so that it is pinched on opposite sides of the plate and the bore. If you have flat top pistons then take some sort of a solid shim anywhere from a 1/4" to 1/2" thick and place it under the plate you have bolted down (with dome pistons you wouldn't need this). Now, with the degree wheel on, rotate the crank one way until it comes to a solid stop, and note the reading. Then rotate the crank the other way until it stops and note the reading. Then remove the piston stop and rotate the crank exactly halfway (within half degree) in between there and that is true TDC. You can then bend you little wire indicator to read 0 degrees on the wheel. That is the most accurate way to reliably find TDC. And you will get the same thing every time.
 
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On another note, after I degree the cam and get the timing cover (with tab) and balancer on, I usually do the same thing with the balancer. Using the piston stop rotate it both ways and mark the balancer at zero according to the timing tab. Then make a mark directly in the center of those two marks. If that mark doesn't match the TDC mark on the balancer, then your ignition timing won't be exactly what you think it is. If it doesn't match, I move the crank to TRUE TDC, and then either buy an adjustable timing tab and point it at the factory TDC mark, or just make a new mark or cut a deeper notch in the pointer you have. Many times the factory tab can be a couple degrees off.

If the head is already on you can do these things with a piston stop tool you screw in the head, I usually take the rocker arms off that cylinder though so I know the valves won't hit the stop tool.
 
I should clarify. I use a dropped valve as a stop with the head on. dial indicator is on the plunger in the injection pump.
 
Well, after taking all the above suggestions into practice, I came up with a number of 106.5 degrees intake centerline. Close enough for me, but also ruins the only thing that I could think of that'd describe the kind of crappy performance I'd gotten in the past.

Oh well, I'll have the Vortecs ready to pop on within a month or so (gonna be a very slow build), then I'll see if I can get it up and running with a known good Edelbrock performer carb and a brand new Mallory distributor.
 
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