U20 Cam Slotted Sprocket

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Guy
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U20 Cam Slotted Sprocket

Post by Guy »

Hello All,
Had Rebello rebuild a U20 head with a C cam . Feels like a turbo kicks in at 3000 RPM! The head had been milled down ( not sure of how far) It was recommended that a slotted sprocket be install on the cam,

I just want to be sure I install correctly.... should the cam gear be advance clock wises or counter clock wise?
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Gregs672000
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Re: U20 Cam Slotted Sprocket

Post by Gregs672000 »

Glad to hear you're happy with the upgrade! If the C cam is coming on strong at 3000rpm and you're happy with the top end/higher rpm performance, I'd leave the sprocket just where it is. Since a C cam has a higher duration (300 as I recall, 280 for a B, not sure on an A) it will normally come on later in the rpm range, with less torque down low but more up high and with a higher rpm capability. This is when the cam timing is assumed to be stock, all based on a number of variables, one of which you mentioned is the thickness of the head. However, I would assume that Robello made the necessary corrections to that by shimming the cam towers. The term "cam timing" refers to its relation to the crank shafts position and thus where the piston is in its stroke. The cam opens the valves, and changing the timing of that opening makes a difference in where the engine makes power. Normally you have a cam gear that is not adjustable and goes on only one position, held in by bolts and a locating pin. The cam timing is considered "straight up" or stock, meaning neither advanced or retarded from where the factory decided it should be. A slotted cam does not have a locating pin (some allow the use of offset bushings so you can use a pin, but not all), but the bolt holes are oval, allowing the cam itself to rotate forward (advanced) or backwards (retarded), then to be securely bolted down. Advancing the cam increases low end torque and costs top end power, retarding it increases top end power but reduces torque down low. Based only on your report that it acts like a turbo at 3000 while running a higher duration C cam suggests to me that the cam timing is advanced, which is fine so long as you're happy with the top end power. Also, playing with cam timing is not something you do without thought and understanding, and even then you must be careful. Moving the cam too far one way or the other and you'll bend valves or worse. When you advance the cam for example the clearances between the intake valve and the piston decrease meaning that you can have piston to valve contact, and our engines run close tolerances, again depending on how much things were cut etc. I'd talk more to Robello, do some reading, asks questions, see what they suggest...
Greg Burrows
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Re: U20 Cam Slotted Sprocket

Post by JT68 »

"acts like a turbo at 3000 while running a higher duration C cam suggests to me that the cam timing is advanced, which is fine so long as you're happy with the top end power."-- no, I think Greg meant retarded.

The C cam is certainly intended as a "top end" cam. If you advance the cam a couple degrees, it will move the power band down a bit and may improve your idle characteristics slightly, but will still "be on the cam" between 3k and 7+. If you want more lower end, the C is not the best choice.

If you are using flat-top pistons, the likelihood of piston-valve interference is minimal by advancing 2 degrees (4 crank degrees).
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Gregs672000
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Re: U20 Cam Slotted Sprocket

Post by Gregs672000 »

Well, I guess I haven't driven a stock U20 in a while. As I recalled from talking to others a C cam 300 duration tended to come on at about 3500 or so (and why I chose a B cam's 280 duration), or that was the rpm where people felt the most change. So, I was thinking that maybe the cam was a bit advanced because of the lower 3000rpm "power on" report from the owner, as advancing the cam moves the power band lower, retarding it moves it up... obviously I haven't driven his car, and as noted was based only on his report.

Since the owner had Robello install a C cam I would assume they had discussed rpm and power output, with the owner understanding the change and desiring that. The question was specifically about installing or changing out/moving the position of the cam/gear, and given that this is not something done without some thought and caution I suggested that if the owner is happy with the power band now there's no reason to mess with it just because.
Greg Burrows
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rwmann
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Re: U20 Cam Slotted Sprocket

Post by rwmann »

Great excuse to take it to a track day and test the different cam timing setups!
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Guy
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Re: U20 Cam Slotted Sprocket

Post by Guy »

Wow, that great info. Thank you,
I'm not for sure on the C cam specks. Robello did not call it a C cam, I using the term loosely. I'm looking for the exact lift and deration now and cant find it. I know the lift was 480.
Dave did suggest the I install the slotted sprocket in improve low end torque, The way it is now the engine dose not like the hills at all, ....unless your above 3000 rpm.
Advancing the cam increases low end torque, so when installing the slotted sprocket should the cam spacer be on the right or left of the dowel pin?
Sorry probable seems like a dumb question,
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Re: U20 Cam Slotted Sprocket

Post by JT68 »

The engine rotates clockwise when viewed from the front. If the cam sprocket is in any fixed position, rotating the camshaft itself clockwise (again viewed from the front) makes all the valve events happen earlier, meaning the camshaft is advanced.

The other way to look at it (still viewed from the front), is if the cam is left stationary, you would position the offset dowel to rotate the sprocket counter clockwise making the sprocket position "later" relative to the the cam.

Be aware that this will most likely improve things, but only to a point. If the cam is a very long duration grind, other valve timing events begin to work against you when the cam is advanced too much and both power and torque will fall off. In other words, every cam has an "ideal compromise" position- you can find it empirically as rwmann suggests, or time the cam on an engine stand using a degree wheel.
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Gregs672000
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Re: U20 Cam Slotted Sprocket

Post by Gregs672000 »

The only dumb questions are the ones not asked! Yes, what JT said. The C cam is a good design for those who want more top end power but it is always a trade off (the advent of variable cam timing and the ability to switch over to different lobes in modern engines was a dream come true). The cam is really the brain of the engine, determining where the engine makes power in the rpm band and where the "sweet spot" is, and that's mostly determined by duration.

What adjustable sprocket do you have? Some have just the oval holes and no locating pin where you measure (or carefully guess... be careful!) how much to rotate the cam and lock it down with the bolts. Others use a bushing kit for the locating pin that are designed to rotate the cam a certain number of degrees by offsetting the pin location, advanced or retarded. Sounds like you have the bushing kit one. That's probably safer and you can try a couple different degrees if you want. It takes VERY little movement to make a difference, but like JT said it won't be night and day although it will help.

I'll walk you through it. Please be careful and if needed ask questions. I dont know if you're installing a new adjustable gear or if you're just adjusting an installed one, but the main issue is to not drop the chain down the motor or mess up the links etc.

Put the engine at TDC. The cam pin should be at the top of the gear. Wipe the oil off the chain a bit and use a permanent marker and mark the link on the chain that lines up with the pin just in case location gets lost. Carefully remove the tensioner (I stuff a rag down the front of the motor just need case something drops in) and if it's not already shimmed now would be the time. Be careful of the tensioner gasket as you can likely reuse it if intact, if not one can be made out of thin gasket material but it must be the same, same holes etc (not hard). Unbolt the cam gear and bolt it to the chain guide using the center bolt hole in the cam gear, or if the "evil L" is already cut off (like mine) FIRST shape a thin piece of wood (a paint stir stick works well, about 3 inches long, narrowed slightly at the end) and slide it firmly between the chain to pin the chain against the guide just below the gear. Then unbolt the cam gear and gently lay the chain down out of the way (the firmly placed wood will hold it). If the gear is held up by the guide as originally designed with the Evil L, you don't have to worry about the chain falling in. However, if the gear IS the adjustable one, the you're going on need to remove it for prep and secure the chain to the L with wire so it does not fall in, and if something does happen your chain-to-gear mark will save you). You need to prep the adjustable gear for install and you don't want to be messing with little bushings that can drop down the front of the motor.

You want to advance the cam in your case, so as you face the engine from the front that means the cam needs of move slightly to the right. That means the pin in the gear needs of move to the right as well, so which ever bushing you use should be thicker on the left. With the bushing set correctly and the pin thus moved, install/reinstall the gear on the chain, lining up your mark on the chain with the pin on the gear (be careful to not drop the chain while you're messing with this). Grab ahold of the cam (careful use of vice grips or similar on the shaft to more easily rotate the cam) and rotate it slightly to the right until it sets onto the pin (won't be a lot of change, much less than a link) then use some locktite on the cam bolts, replace the tensioner etc and you should be good. It's always wise to hand turn the motor with the plugs out to check for interference but you should be fine. Go for a drive and see how it does. JT or Robello should be able to suggest the maximum degrees of advancement you can go before problems occur.

Hope this helps. I had good results playing with cam timing before.
Greg Burrows
'67 2000 #588
Tacoma, WA
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Guy
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Re: U20 Cam Slotted Sprocket

Post by Guy »

Parts should be here for a weekend project.

Thank you Greg and LT.

Your explanations and instructions are very very very! much appreciated!

Thank you for taking time out of your schedules to help a fellow Roadster Guy.
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spyder
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Re: U20 Cam Slotted Sprocket

Post by spyder »

If the head has been shaved and no cam tower shims were installed the cam is most likely retarded. In my case with my last engine, I advanced the cam and it improved the bottom end quite a bit. With my current engine which has a different head and pop up pistons the advanced cam caused knocking so I put it back to no advance.
If you are going to run a big lumpy cam like I do, an Isky Z-196, it works better with a lower rear end gear ratio so the cam gets out of reversion faster.

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