VictoriaVette

I have been talking to Comp Cam about my set up the told me I should advance my cam by 4 degrees. Can you all give me some pointers on doing this? They told me it would give me more bottom end and to go with 1.6 rockers to get back my top end.
Thanks

OldCorvetteKid

Its very easy,its all in way you install your crank gear on the crankshaft, the cam gear sprocket stays the same ofcourse, but you need a crank gear that has a keyway for advancing it. Some timing gear sets only have one slot in the crank gear(so then you can only install it "straight up". The simplest aftermarket ones have 3 slots, so you have a choice.(4degrees advanced, 4 degrees retarded or straight up). Comp Cams, Cloyes, Crane Cams all sell them, usually as a set with a timing chain and cam gear. Some more expensive ones have multiple keyways. Instead of sliding the crank gear on the crank in the straight up slot you install the gear at the advanced mark. There is usually instructions on which mark on the crank gear is for straight up, advanced or retarded. You will have to slightly rotate either the cam or crank when installing your timing chain to get the marks to line up. Depending on the manufacturer, for example..on a 3 way gear, there's usually a dot on the crank gear right next to the slot(for straight up) a triangle next to the 4 degree advanced slot and a square for retarding 4 degrees in relation to that slot. There are installation tools for pressing on the crank gear to your crank that should be used, "hammering on a crank gear it NOT reccomended". I'm sure there are better explanations, but I gave it my best shot. After looking at your profile, I see we have a lot of similar parts, same cam too.

LeMans Pete

Could you give an explanation on how this works, the whole advancing and retarding the cam, and how advancing the cam will help bottom end?

Imo Apita

Some will correct me if I stray too much from the truth here;
Cam specs will determine when your intake valve opens/closes and when your exhaust valves opens/closes relative to crankshaft rotation.
So when you "advance" the cam it works similar to advancing the timing.
It simply happens earlier in the crankshaft rotation.

LeMans Pete

I don't think I agree with that. If you advance the camshaft, you're affecting the flow of fuel in relation to the piston position. So instead of your intake valve opening at 29° BTDC and closing at 61° ABDC, so if we advance it we're now opening at 33° BTDC and closing at 65° ABDC? This would also mean that your exhaust valve opens later and closes later. Your timing should not be affected since thats set from your crank.

Or do I have this completely backwards, and these events should all happen sooner?

LeMans Pete

Besides, if this worked similar to advancing/retarding your timing, you would just advance or retard your timing. Much easier to change later than trying to advance/retard the cam later.

I haven't heard that timing affects your powerband, I thought that it was for getting your engine to perform at its best ability over the entire powerband. Mechanical advance for idle, vacuum advance for WOT.

But, I don't always hear everything, and thats why I'm asking.

DRIVESHAFT

Do you know what cam is in the car? Many cams are ground with several degrees (4 is common) of advance ground in,
so they are advanced even when they are installed straight up.
If you need to advance the cam, and you dont have a timing set with multiple keyways, then you can use a bushing kit like this http://store.summitracing.com/partde...0&autoview=sku to do it.
You just drill the hole for the pin in the upper timing gear oversize, and slide one of the offset bushings in.

DRIVESHAFT

Advancing the cam will result in the intake valve closing sooner.
Since the intake valve usually closes while the piston is already on the compression stroke,
closing the valve sooner results in more cylinder fill at low RPMs and makes more power.
At high RPM there is a trade off. At high RPM the cylinder is usually still filling up until the intake valve closes,
so closing it early results in less cylinder fill and less power.
It can get more complicated than that if you want to think about all of the valve timing events, but that is the basics.

VictoriaVette

My cam is a Comp Magnum 270H when I talked to Comp Tech they told me to advance the can timing by 4 degrees to give me more bottom end. He told me I would lose about 200 rpms off the top end. So instead of 5800 rpm redline it would be 5600 rpms. But it is already turning 6000 rpms and I feel no loss of power. My 77 is not performing out of the hole like it should I have a Th350 with a shift kit and running a 3.70 rear and a stock torque converter. So I'm not sure if i should change the cam timing or go with a different stall. I talked to TCI tech they told me to go with a Street Fighter Converter. Which is a 3000 to 3500 rpm stall. They said with the engine setup I have and the Vettes weigh it wold be fine. Around a 2800 to 3000 stall which would get me out of the hole and he said no problems with normal driving.

Bee Jay

Back in 1979, when my L-82 was absolutely stock, and I swore I would keep it that way, I advanced my stock L-82 cam 4 degrees to get some more low end grunt. The factory cam was retarded 4 degrees for emmisions, and advancing it put it back where it belonged. It did seem to give me more low end because I was able to break the tires loose for the first time. But I thought aftermarket cams were built not needing this. But if the cam manufacturer says to advance it, do so. It's easy enough to return to straight up, or even retard it 4 degrees.
Bee Jay

DRIVESHAFT

That cam is ground with 4 degrees advance. It should not need to be advanced another 4 degrees.
It is about the same size as the L82 cam, but with a tighter LSA, which should result in more low rpm power.
What does your ignition timing advance look like?
What rpm does your converter stall at?

VictoriaVette

I have a stock stall I was told it is 1800 rpm and my timing is set at 12 degrees and total about 36 degrees at around 3000 rpms. I did have it set at 17 degrees with no vac advance hooked up.

Imo Apita

fmp, I did'nt explain it very clearly.
It's the same concept as advancing your timing not the same effect.
Advance the timing, spark earlier.
Advance the cam, valve earlier.

DRIVESHAFT

I would think that would be plenty of ignition timing, although a bit more initial might feel a little better.
You might want to double check the converter stall. If the converter was changed, it might be tighter than what came in a Vette.
If you can get the brakes to hold while you hold the throttle down in gear, you can find out what the stall rpm really is.
Comp recommends a 2000+ rpm converter for that cam, but says that it will work with a stock converter.
If your converter stalls much below 2000 though, that is probably where you will find the most gain.

VictoriaVette

So explain to me how to check a stall converter by holding the brake? I had a 65 chevy 2 with a pretty built sbc 355 i had a TCI 3500 stall in it. When I held the brake I could feel the car start pulling as soon as I started giving it gas. But if I just nailed it from dead start it would just like a scalded dog start burning the tires and just plain gettin it. But with the stock stall and I nailed it it was like someone was holding it back compared to when I had the 3500 stall.

DRIVESHAFT

Hopefully the Vette has better rear brakes than the Chevy II did.
Just push the brake pedal as hard as you can, put it in drive, and ease into the throttle until its on the floor. Then look at the tach.
I cant say for sure that it wont break the tires loose, but if its a little weak at low rpms like your saying then the brakes will probably hold.

VictoriaVette

So if the brakes hold what ever the tach says is what the stall is??

LeMans Pete

I'll buy that. Good explanation driveshaft.

Peter

VictoriaVette

I have done a brake burn the tires break lose but when I let off the brake the tires are spinning but the engine still does not rev up very good and it seems to take a bit before the rpms get up to where the engine really pulls good. And If I just nail it with out the brake it sucks until it gets up around 3000 rpms. I will check what you said and watch the tach. I know wih my Chevy 2 I had a TCI Street Fighter 3000-3500 and i could just nail it and it would break the tires lose and get into the rpm range where the motor was kicking a$$.

DRIVESHAFT

That is the brake stall of your combo.
Your converter will probably flash a little higher, but thats not to important for us to know here.