27 degree initial timing?

Hmbld · Feb 11, 2007

The patient is a 71 Z28 with original engine/exhaust but with a Lunati 401A3 solid cam. Motor rebuilt 2 years ago and continued problems with low speed performance and low vacuum. Took it to a shop some time ago and just recently checked timing. It was set at 27 degrees at idle 850 rpm, 27 deg at 1500 rpm, 39 deg. at 2000, 39 deg. at 2500 which is max timing.
Vac advance was plugged and dist. does have bushing to limit mechanical advance.

Would this timing setting affect low speed performance?

Cam specs. Int. Lft. .485, Ex. Lft. .513, Int Dur. 238, Ext. Dur. 248, LSA 112

Old motor with original Z28 cam ran great.

kustomwerker · Feb 11, 2007

with that cam, i would think vacuum is gonna be an issue...initial timing seems high, but if the car will start there, i guess why not...39 seems like alot of total though....does it ping???maybe really low comp...is the timing mark correct???did you verify this???

Carbo · Feb 11, 2007

WOW!! 27* seems like an awful lot of initial timing. Do you have problems starting the car when it's warm?

DjD · Feb 11, 2007

Sounds like the high initial and limited mechanical was an attempt to fix the poor low end performance. The timing not moving until 1500 or more is also part of the problem. The '69 DZ 302's 480 dist didn't start the curve until 1250rpm for something to compare... The typical Chevy dist starts it's curve at 900rpm...

Before messing with your dist (it does need work though) what rear gears, engine compression and if auto trans what stall? Here's what Holley says about that cam... http://www.holley.com/401A3LUN.asp

Hmbld · Feb 11, 2007

No pinging, starts fine. The shop mentioned that the cam was intalled retarded which may have been to blame for the poor performance. I have not checked this but have plans to. Car has 3.73 rear end with 4 speed and compression is about 9.3.

On the 69 Z28, I have one and its timing is, idle 17 deg at 700 rpm, 28 deg. at 1500, 31 deg. at 2000, 32 deg at 2500, and it runs great. I wish my 71 ran as good as the 69.

JimM · Feb 11, 2007

look at the cam. I think your timing settings were an attemot to get it to run better at low rpm, and that lessor settings would make it worse.

Bolt · Feb 11, 2007

Here is my 2 cents. I would start at the begining and check that the dist is installed correctly and not a tooth out . At the same time I would make sure the timing marks on the dampner and cover are accurate. Then I would set the motor up with 15 deg. initial and 36 total. mechanical adv. I would adj the springs for centrifical advance by running the car and adjusting accordingly so it will take full adv. without pinging as early as possible. Also put a vacum guage on and see how much you are pulling @ idle. If you are between 12 to 15 inches you should be fine if less than that I would check to see if the cam was degreed correctly. In a 4 speed car install the cam straight up in an auto. adv. it 2 degrees. also check that you do not have any vacum leaks (intake gasket, carb, vacum hose etc.) I have installed cams with simular specs with double hump heads and 750 DP. and they run great. Please keep a post as to how you make out. I would love to see how your problem is resolved Good luck

stevo camaro · Feb 11, 2007

JimM said:
look at the cam. I think your timing settings were an attemot to get it to run better at low rpm, and that lessor settings would make it worse.

I agree. I have the exact same issue. I beat my head against the wall for a long time, for nothing basically.
The cam I run doesn't have a radical profile per say, but cars that ran this cam in the 60's had 11:1 compression. I have 9:1. Because of the overlap & the lower compression, I have to bump my timing to 22 initial, idling at 900 RPM's with 6-7" of vac.
Yeah, low RPM driving is not great, but once she starts to wind up, look out.

I tried everything at first, manifold sucking air? No! Cam grind wrong? No!
Bad distributor? No! Went thru 3 carbs, 4 distributors & 3 intakes.

The car wants to live at 22 initial and idle at 900. Being a 4 speed car it's not a big deal with converters being an issue.

I've left her alone and flog her hard. Motor has never given me problems, even on 10 hour drives.

But she has bent 1 set of axles, bent 1 driveline & tweaked a leafspring. LOL

kustomwerker · Feb 11, 2007

you cant install a chevy distributor a tooth out.....as long as the rotor is pointed at the #1 wire on the cap you can install the distributor wherever you want...i usually drop the dist. in at the 3 oclock position with the #1 wire to the right of the coil connections...(on hei)that way, theres plenty of room for timing...

JimM · Feb 11, 2007

kustomwerker said:
you cant install a chevy distributor a tooth out.....as long as the rotor is pointed at the #1 wire on the cap you can install the distributor wherever you want...i usually drop the dist. in at the 3 oclock position with the #1 wire to the right of the coil connections...(on hei)that way, theres plenty of room for timing...

Exactly. There is no such thing as a tooth out. You could give the distributor a wild spin and drop it in, and I could make it run. #1 plug wire might be in an unusual place, but it would run. If it runs, it's right.

kustomwerker · Feb 12, 2007

lmmfao...

Lost in the 60's · Feb 12, 2007

Yup, quite common to have 21 + initial with a cam profile like that. If the cam is installed retarded, it could account for the additional advance to 27 and be the cause of poor low end performance. Keep in mind that the cam you have isn't going to produce much power below 2500 rpm.....just too much profile for that. If the power loss you are referring to is below that figure....it's normal..

I had a cam very similar to yours in a LS6 years ago and it needed 21 degrees initial with a 1200 idle.. I had 36 total...all in at 2,800, which is where the cam started to come alive. After that, when the tach hit 3000 you better be ready to steer, cause the rear axle wasn't happy following anymore..

DjD · Feb 12, 2007

Hmbld said:
No pinging, starts fine. The shop mentioned that the cam was intalled retarded which may have been to blame for the poor performance. I have not checked this but have plans to. Car has 3.73 rear end with 4 speed and compression is about 9.3.

On the 69 Z28, I have one and its timing is, idle 17 deg at 700 rpm, 28 deg. at 1500, 31 deg. at 2000, 32 deg at 2500, and it runs great. I wish my 71 ran as good as the 69.

Well you have the gears and a 4speed working in your favor but you could use a bit more compression, I think the recomendation was 10:1 min... Work with what you have and get your ignition dialed in. Start with getting the timing curve to start moving around 1000 rpm. Then you want about 20 degs mechanical, right now you have about 12 degs.

With 20 in the dist you could set the total between 34 - 38 and you would have 14 - 18 initial. With the curve starting at 1000 rpm and being all in by 2800 - 3000 your ignition will be there and will be very tunable... The next thing I would do is look at your carb. A smaller carb will usually give better throttle response and that may help your low end some.

I think the real answer though since you believe the cam to be installed retarded is to re-install it using a degree wheel to get it right. Retarded cam timing may increase your high rpm power but it will kill your lowend. I'd run the cam straight up or possibly 2-4 degs advanced to make up for your low compression.

dubs68camaro · Feb 12, 2007

The other thing we haven't concidered is that you may have too many Z/28's!!! I wish to trade you 2 goats, a rabbit and this really neat rock I found down by the river for the '69.

This is all really good info. I have the same, yet opposite problem. Tons of low end but not the top end I should have for my setup. I think mine is an issue of not enough total and not at the right time. Hope these guys help you get off the line quicker!!!! They are all super helpful.

Hmbld · Feb 13, 2007

Thanks for the input. I am going to check the following:
How cam was installed - It is my understanding from Lunati that if the cam is installed at 0 advance, then TDC #1 the intake should have 20 -30 thousands more lift than the exhaust.

Recurve distributor.
Check valve lash.

The big issue with the car is I have to really rev it and slip the clutch just to get it going.

27 degree initial timing?

Hmbld

kustomwerker

Carbo

DjD

Hmbld

JimM

Bolt

stevo camaro

kustomwerker

JimM

kustomwerker

Lost in the 60's

DjD

dubs68camaro

Hmbld

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