Dialing in camshaft timing-or "degreeing"

What are the effects and results-
good-bad?

http://www.explorerforum.com/forums/showthread.php?p=1859704&posted=1#post1859704
^^^^^^^^^info here^^^^^^^^^

lets keep the comments in this thread please-

Josh,

for my reply im going to assume you aren't aware of the ins-and-outs of camshaft timing. say, the average joe with a family who loves his explorer and just wants a bit more grunt out of it. a guy with better things to do then sit at the computer into the wee hours of the morning reading about cams. please understand that by no means am i trying to call you stupid. yes, cam timing changes many variables (and i suppose it can change DYNAMIC compression), but i wouldnt say it's your simplest power adder. im going to try to make this as simple as i can, but it's still huge. if you want Comp Cams' opinion, or dont understand mine, go here.

just so you know, my cam knowledge comes from magazines, Desktop Dyno, a passion for a thump to my '69 chevy's 350, and a shop teacher who has built motors for 30 years. (im a high school student, big deal.)

cam timing crash course (from a guy who learns about cams into the wee hours of the morning):

im going to assume that we know how a basic OHV 4-stroke engine works. from now on, im also going to use the terms TDC, BDC, IVO, IVC, EVO, and EVC - any cam junkie is familiar with these as they make or break a cam. the first two are "top dead center", and "bottom dead center", which refer to where the piston is in it's stroke.

the next 4 stand for (and represent the actions of) Intake Valve Opening, Intake Valve Closing, Exhaust Valve Opening, and Exhaust Valve Closing. these figures are always represented in degrees of either [before or after] TDC/BDC. this might sound like mumbo-jumbo, but bear with me.

okay, so big deal, the motor turns and we get a sequence:
combustion, EVO, IVO, EVC, IVC, next combustion.

"wait, what? are you saying that the intake opens before the exhaust closes?" you might ask. yup! look at this comp cam graph:

those curves look a lot like cam lobes, dont they? coincidence? hell no.
the valve may only be cracked open 0.050" at TDC, but the valve is still open and air is still flowing. why? because on a naturally aspirated engine, air&fuel is introduced through vaccum, right? the open exhaust port creates that continuing suction through the chamber as the piston rises. when both valves are open, this is called VALVE OVERLAP.

your cam is designed to cause the valves to behave in a specific manner, corresponding to the different times of the crankshaft's rotation. makes sense, right? im basically saying the valves move in harmony with the crank.

that said, by changing the cam timing, you're moving the whole cam graph by whatever amount of degrees. for arguement purposes, and to make it easier on me, let's say you're moving by 10 degrees ADVANCED.

so, at the TDC in the middle of the cam profile at officially 0 degrees, instead of only a little bit of each valve being open, your new TDC behaves as it would at the old -10 degrees. why?
because by changing your timing, you're telling everything to happen 10 degrees before it was designed to. an appropriate analogy is a carefully designed Olympic runner who starts precisely early at the 100m sprint.

still with me? good! now that we have our newly timed valves (or olympic runners, if it helps) starting early, let's look at what this changes.

look at 0 degrees, TDC. our intake valve is closed, but the exhaust valve is open a bit more. this means that we're kicking more used gasses out, but injecting less fuel into the combustion chamber at the moment. thats fine, but how does that affect flow?

unfourtunately for you i cant tell you what formula tells you because i have no idea. just take my word for it that flow will change. and when flow changes, what else does? horsepower, and compression!

that depends on what compression you're looking for.

if you're looking for a ratio like 9.0:1, or STATIC COMPRESSION, your answer is no. but if you're looking for a PSI value at the spark plug, or DYNAMIC COMPRESSION, the answer is yes. why?

look at the cam profile. when the valves are overlapped, there is no compression occuring. why? because any air that tries to compress gets kicked out of the exhaust valve and stays at the same pressure.

what happens if we re-time the cam to the point where when the piston moves up the stroke towards TDC, no valves are open and the air MUST compress? well then pressure would increase because more air molecules are being forced together and none have anywhere else to go.

"so my answer is yes, right? alright, cool! ill go whip out the tools!" is what i hope you arent saying. there's a few more things to consider.

chances are very high that by blindly advancing a cam you can gain some dynamic compression and your engine will pickup a few ponies. this is a pretty good roll of the dice because to be fair, the factory designs a cam with other things besides power in mind. idle quality is a big factor, only a select few like their SUVs lumpy at a stoplight. soccer moms would rather have their trucks purr at a stoplight because it vibrates less and makes less "weird noises", yes? so your cam is designed to purr at idle like a happy kitty.

but that's besides the point. whole new set of conditions: assume your cam has been re-timed and the exhaust valve is fully open when the piston is at TDC. is your OHV motor what's called an "INTERFERENCE" engine? -insert blank confused stare- that means that if your timing belt/chain were to snap and come out of harmonius timing, your valves can physically collide with the piston. yeah. so that means if you dick with the timing to the point where the valve can touch the piston, and at 250rpm when you hit the key, there's a lot of force involved. unfourtunately, valves bend before pistons stop moving, so then you have a bent valve which can lead to a whole multitude of problems i hope i dont have to explain.

i could go into the process of degreeing the cam, in an OHV engine, it involves tearing the intake, valve covers, pushrods, lifters, and rockers out. in short? it pretty much is a total engine overhaul!

finally, lets assume that the compression increases and all goes well.
how much does it increase by? can you still run pump gas? will it detonate?
what happens if it detonates and you fry a piston? uh-oh... total engine overhaul...

so, to conclude this amass of jargon that i've spelled out?

-cam timing can increase dynamic compression.
-cam timing is a pain in the ass to do.
-cam timing depends on many factors, including cam spec, head volume, and whether your engine is an interference-type or a non-interference-type.
-cam timing is a pain in the ass to do.

to be honest josh, i would look at other things to do before re-degreeing an OHV motor. do you have a cold air intake on your truck? hows the exhaust? how's the plugs? what is your rear axle ratio? changed the computer yet?

i've probably skipped something elementary, but hopefully this somehow helps.
ian

 

[rookieshooter]

I used to use offset bushings in my cam sprocket so I could advance or retard my cams. Now this was on chevy engines but same principle. It was just a way of rotating the cam in the sprocket while the sprocket stayed in the same geometric plane as the crank sprocket. In other words, lets say that your engine specs call for both timing marks on the cam sprocket and crank sprocket to be exactly facing each other. Cam mark on bottom of sprocket and crank mark on top of sprocket. Now with this offset bushing installed in the cam sprocket, it lets me rotate the cam a few degrees. Either advance or retard, depends on which way I install the offset bushing.

So now I can have more low end power or top end power, depending whether the cam is advance or retarded. A cam with a lot of split overlap is more designed for higher RPMs. This is where you will get more beneficial air flow as the momentum of the column of outgoing air is pulling (by vacuum) fresh air thru the intake, even before the piston starts it's downward travel. So your now getting some free air as to speak. But now you will have that real lopy sound at idle and may not have as much air in your air booster for brakes. Your vacuum may suffer at low RPMs.

 

[malohnes]

Old thread, I know, but I'm surprised at the lack of responses from either this thread or the referenced thread. 'Im reviving it in hopes of more activity form those who have experience in cam swapping and tuning their 4.0L OHV engines.

In my research and understanding a general rule of thumb is: retarding a cam a few degrees will reduce HP and increase torque while advancing cam with do the opposite, that is, boost hp and reduce torque, all things being equal. This isn't gonna necessarily add actual hp or torque so much that it changes the natural hp or torque curves so that they build sooner or later in the rpm's. When you buy an aftermarket cam this effect is all ready calculated in to the profile fo the cam itself, so it can be very difficult to compare the effects of a stock cam advanced or retarded to a aftermarket cam. to further complicate things, our OHV engines dont have adjsutable cams nor aftermarket support for adjustable performance cams. This changes dramatically when Forced Induction comes into play as boost complicates the airflow characteristics and therefore the fuel requirements for proper combustion.

I personally think that you can change the alignment of the chain and sprocket to achieve this effect, by literally skipping a tooth on the cam sprocket plus or minus so the alignment dots on the crank and cam gears don't line up exactly. A machine shop could custom make an adjustable cam sprocket but I would expect it to be a very expensive mod for few or any gains gains. However, it would be a very educational project to see exactly what happens when one advances or retards our stock cams.
But, just because you can do a thing doesn't mean you should. I'd rather throw a 410 or 422 cam in and mess with 1.6:1+ rocker arms to gain some actually airflow and ponies.

 

[massacre]

Are the 4.0 OHV engines "interference engines"?

 

[CDW6212R]

I never put any thought into the idea of an interference engine. I'm not an engine expert, at all, but I know enough and I can't imagine an engine that if the timing chain broke, the valves wouldn't hit the pistons. Every engine I know of the valves will hit the pistons if the timing chain breaks etc.

Anyway, advancing or retarding the cam is all about synchronizing the valve timing events to what the intended ideal was, by the cam designer. There are manufacturing variances with any OEM cam, and the best high end billet cams can be a hair off. Race engines will have the closest to perfect cams(least error in manufacturing). Typical custom cams are all billet, and checked more carefully than regular cheaper cams, and rejected if they aren't very close to designed cam specs. That what you get for spending an extra $100(that's for cheap *******s who'll spend $2k+ on an engine but mix and match cams they get a good deal on).

Advancing or retarding a cam basically will rock the power curve, around the rpm just below the torque peak. So thus you get a little more bottom end, and less top end, by advancing a cam. But it's still a guess, an assumption that the cam isn't ideal for the engine at hand.

I don't know what is available for the 4.0 V6's, but I'd hunt very hard for a custom cam, before ever considering any off the shelf cam, or the stock parts. A custom cam made by a pro cam designer can gain you power you can feel, at low rpm along with mid range and top end. An OTS cam will never succeed that well.

 

[fastpakr]

I never put any thought into the idea of an interference engine. I'm not an engine expert, at all, but I know enough and I can't imagine an engine that if the timing chain broke, the valves wouldn't hit the pistons. Every engine I know of the valves will hit the pistons if the timing chain breaks etc.

Lots of engine designs won't make contact in the event of a timing chain/belt/gear breakage. The Lima 2.3/2.5, Vulcan 3.0, Cologne 2.8/2.9/4.0 (OHV) and 5.0 all come to mind. The path of travel of the piston never goes above the lowest point of the valve travel, regardless of time. Within the Ranger family, I'm only sure of the SOHC 4.0 being an interference design. Not sure on the 2.3 Duratec.