I have been searching around for the proper way to degree an LT1 and have been coming up with mixed reviews, so and so says "the only reason to degree the cam is to verify the grind." Well so and so says "that isn't true and it must be degreed (by degreeing I actually mean setting the cam to specs with cam spacers or offset key ways)" With this being said, so and so said "if you actually degree it, it will throw off the ignition timing and injector timing."

I was thinking (just an idea), what if you set up your engine and find top dead center, install a pointer, and a timing tape (like you can get from jegs) on the crank pully set to zero.
-Verify how much off your cam is and then use a timing light to see how much off your ignition is.
-If I am right, if the cam is zero degrees off like it should be you should read 4* BTDC because that is the way the engine is manufactured.
-Now if the cam when installed and checked and it is say 3* advanced and you put a timing light on it, according to one arguement, you should still get the 4* BTDC, which in turn verifies that degreeing the cam has no effect on the ignition timing and if you do degree it you will have to make other changes to sync everything back up.
-But if you get 7* BTDC then the arguement that stands that if you properly degree it, it will effect ignition timing and that properly degreeing the cam is the right thing to do and that degreeing the cam to manufacture specs is actually sync'ing up timing and you don't need to adjust the computer when you properly degree your cam.

- I think I got that right ?? advance = BTDC/ retard = ATDC

I am looking for input on weather this idea will hold water or not.

 

Well I don't know who told you those things, but you just need to pack your bags and hit the road, because they're not people you want to be getting advice from anymore...


First off, just for the record, it is no different degreeing the cam in an LT1 than any other motor in the world...

Secondly, cam timing, and igntion timing, are 2 completely different things, do not get them confused with eachother.

Degreeing you cam will verify's that everything was ground in place, and that you achieve the cam timing you were looking for. Remember, by just sticking a cam in "straight up", you are assuming that the person that ground the key on your crank was dead on, the person that ground the keyway in your timing chain was dead on, and the person that ground your cam was dead on. Do you really want to take those chances?

Contrary to what you have been told, degreeing your cam in will not effect ignition timing. Ignition timing is controlled by the position of the crank position sensor. The cam position sensor is for the sequential fuel injectors... and it is basically just a reference point for the ecu.

Typically on a normal sbc, you will make best power around 4 degree's advanced. You were throwing around numbers like 7, etc, and I don't have time to read into what you were trying to say... but shoot for around 4 and you should be good, and right were GM wanted it.

Also, advance and retard is figured by the relation of intake centerline vs. the lobe seperation of the cam. If you have a cam that has a 112 lsa, and you want 4 degree's advance, you will install the cam at 108 degree's intake centerline. If you have a 112 lsa cam and want to install it 4 degree's retarded, you will install the cam at 116 degree's intake centerline.

Hopefully this helps all those who have been confused about what degreeing cams is all about.

Later
Chuck

 

Degreeing the cam will change the position of the optispark, which will change the timing. You will have to compensate in the pcm program for how ever many degrees it changed. Otherwise, any cam changes need to be ground into the cam.

If you change the position of the opti on the cam, the pcm doesnt know that this change occured and will continue to fire the coil as normal, which will end up advanced/retarding your timing depending on what your cam is at

And how can the timing be controlled with the CPS when over half of the model years of LT1s dont have them? The CPS has nothing to do with timing. I believe the CPS is an obd2 thing to see misfires right?

 

Unless your having to really move the cam timing alot I would not think the ign. timing would be off enough to really make a big difference?

 

See thats the debate I am trying to solve, Chuck says that degreeing an LT1 is like any other motor and Serene says it will effect the opti and programing is required. Now giving both of you know what you are talking about someone still has to be wrong.

From what I am getting from Chuck is that follow the instructions on degreeing your cam, say you have a cam ground for a 112 centerline and when you install it and check it, it comes to 107*. By the directions of Comp Cams, you need to set it to 112* by using cam spacers or offset keyways.

Now from what I am getting from Serene is that if I do that then I will be throwing off the opti and the ignition will be off hence needing programing to fix this problem.

and now my original question is, If I was to install a degree tape and pointer to the crank pully and use a timing light wouldn't that solve these debates.

Am I being clear or am I looking at things the wrong way?? I just want to get the cam installed properly and get the most out of it,

 

*IF* Serene is right, and the LT1 ignition timing is controlled by the optispark reference signal (which I personally will not believe to be true until it is proven certain... since MOST if not all aftermarket fuel injection ecu's control timing based off the crank ps, not the cam), the effect degreeing your cam in correctly will have little to no effect at all on your timing. We're talking 1 or 2 degree's here. Something easily fixed with reprogramming which you would only ASSUME someone installing a cam is going to do anyways. So the debate of whether or not this is actually true, seems of little or no use to me for someone doing it right.

Look up GM's spec for what they want the cams in LT1's installed at from the factory. It's going to be in the 4 degree's advanced range, with maybe 1 or 2 degree's variance allowed. So why all the sudden is it bad to degree in a cam, when all you're trying to do is get it in at 4 degree's anyways? All you're doing is verifying it is in tolerance. You really think you can install a cam straight up, and it's going to be on the nose from what the cam card says is ground into it? Personally I don't believe you understand degreeing cams, if you've ever even done it, because if you really did I don't think you would be saying what you are about this... You're making it sound as if when you degree in a cam you are dramatically changing the advance or retard it was originally intended to have.... which in most normal applications like this, is not true at all. It is simply something all GOOD engine builders do just to verify their work is 100% exactly the way they want it... A simple analogy is comparing 2 ways of building engines. 1 person buys all their parts from a catolog made to fit their particular application. They have the block machined by a local machine shop and brought to within tolerance. They then bring it home and assemble the engine without a dial bore gauge, calipers, mic's, bridge, or even plastigauge anywhere in site. They are purely depending on luck that the motor will run right... Then compare that to an engine builder who fully blueprints everything. They measure every single clearance, and KNOW that everything is within their tolerance, and this motor is going to run forever and a day. So all being said, by all means, do whatever you want, I'm just giving you my opinion to ponder. I am not wrong, and I will not waver. What I've said is fact. So if you want to disagree with me fine, but I really don't care to keep debating on this issue.

Chuck

 

Are you sure you're not confusing lobe seperation with intake centerline? What you just described is the cam has 5 degree's more advance ground into it than the cam card states (which would be a prime example of why cams should be degree'd to verify everything is correct, and not way out of wack like this)...


Chuck

 

I have done alot of searches on this sublect and have heard of cases where the cam was off as much as 9* so thats talking about more than 1* - 2* off, I am not trying to waiver you, I am doing what I think every GOOD engine builder does before building, "research"! If i go ahead and put my cam in and I am faced with this problem of being off more than 1* or 2* then i would like to know what to do. After all I thought that is what this board was intended to do, help get a job done right.

Since I don't understand degreeing cams, from my research advance vs. retard effects top end vs. bottom end and to get the most out of a cam would involve being dead on (yes I know technically). I am not the type of person that would disreguard what an expert would say and go based on what someone else might say.

So with what has been said, do you according to Chuck, degree the cam and program the computer for the changes or do you program the computer for how far off the cam is.

 

I'm not confusing the two, I threw 112 out there because it was in my head. Looking at my card, it says to install the cam at 108 intake centerline, now on sbc's, not LT1's, normally you would set the intake centerline to 108 using spacers or offset keyway and set the timing using the rotation of the distributer. With LT1's you can't do that.

So whats the proper way of setting up a cam, find out how far off of centerline you are and make up for it through programing, degree your cam to centerline and program the computer so it knows what you have done, or degree your cam to centerline and the ignition should follow.

another question, is the cam says to install the cam at 108 centerline, giving that GM says 4* advanced, do you install it 4* advanced from 108 centerline.

 

Hmm, I've never seen Warren Johnson doing research on the internet? Good engine builders know this stuff, but beginners definitely need to do research, which I am happy to see you doing. Too many people assume they know it all, and get themselves into trouble. So doing research is highly comendable.

My 1 or 2 off numbers were not set in stone. Obviuosly there are cases, like yours here that are exceptions... Here's something to think about regarding this example you gave... How do we know for sure it's 9 degree's off? What probably happened in this case was user related. The cam this person used had 4 degree's advance built into the cam. Then when they installed the timing chain crank sprocket, they used the advanced keyway, which advanced it another 4 degree's... That would be 8 degree's advanced simply by the person not knowing what they were doing... the other 1 degree was probably from the parts used not being exactly perfect... Not that this has anything to do with your question, but if this was a car I was working on, that would be the first thing I'd check...

Well, in basic terms yes... but most adjust their timing according to what makes the best power. Typically for a sbc spinning less than 7500 rpm, around 4 degree's advance is where the best curve is. You haven't givin any details about what size motor this is, how much the heads flow, and how big the cam is, so I can't tell you if this will work for your combination. But for most, that is what you'd want to shoot for.

You do both. And my question to you is, why can't you do both. What is your reason for only being able to do one or the other? If you can't do it right, I don't recomend doing it at all. And I really feel this whole discussion is highly overrated. But it's cool that you are asking these questions, because most on here probably wouldn't.

If the lobe seperation of this particular cam is 112 degree's, and you degree it in to 108 degree's intake centerline, the cam is in 4 degree's advance. Loctite the bolts, torque them down, and go get some dyno tuning done.

Chuck

 

No no no, I did not see this the first time I replied... Being dead on would not be good for whatever you are trying to do, unless you are building a very high end 8k+ rpm racing engine.

Chuck

 

when I degreed my cam it was 110 centerline but I needed 108

I just advanced the crank gear 2 degrees. So I should change timing -2 degrees ? Thats what I did.

 

Here are a few "facts" that I have personally experienced.

If you alter the phasing of an LT1 cam with offset keys or cam bushings, you will definitely alter the injector & ignition timing. This may not hurt anything, but in some cases where the cam was moved 4 or more degrees, it can have an effect on part throttle drivability (hesitation/knock). This is due to the injector timing change. Changing the ignition timing (with programming) did not cure the problem. I have seen this problem 1 time & also discussed it with Mike from (Modern Musclecar / Ramchargers) who also acknowledged seeing the problem with altered cam timing. Now to prevent self abuse, I will not try this anymore.

To correct, you can also elongate the optispark bolt holes & rotate the unit. If you are really getting picky I suggest you check this as well.

The typical cam will be ground close to perfect so this may be nothing to worry about. Personally, if it is off by 4 or more, I would question the quality of the cam/timing set anyway. Also a stretched timing chain will retard the cam phasing slightly as well, so it will automatically retard itself as the chain streches. Keep this in mind.

Hope this helps.

 

Just to clarify.... In the LT1, spark and injector timing is based on the high and low res signals produced from the cam-driven optical sensor in the OptiSpark distributor. The 92-95 LT1's so not even have a crank position sensor, so they are in effect, relying on a "pseudo" crank postion derived from cam position sensor.

The 96/97 LT1's added a crank position sensor, but it is used only for misfire detection. The PCM still takes info from the OptiSpark sensor for ignition and injector timing.

I run a MoTeC M48Pro ECU, and it too uses the high and low res pulses from the Opti to set injector and ignition timing. It does not rely on a crank position sensor, since I do not have one.

 

Alright, thanks for the clarification Lonnie and Fred...


Chuck