383 still pinging

[Heavy Z]

Hello everyone, I'm hoping to draw on some of your knowledge as I've got an ignition problem that has yet to be solved. My 383(10.5:1 cr) was having problems with the old points-type distributor so I upgraded with a rebuilt HEI w/ vacuum adjuster and despite this and a lot of playing with different springs this thing still does the same thing my old distributor did. It wants to add all sorts of advance(44 degrees or so) at fairly low rpms and the result is that on the road it'll ping in 3rd gear of a turbo 350 at 40 mph under a slight load. Not good at all. The cam is a comp XE268H just installed recently, could cam timing do this? Any ideas welcome at this point, thanks. :|

[Zoldman]

OK, here goes:

 

I have a '79 Vette that I stroked to 383; 9.6/1 compression; cam similar to yours; Edelbrock 790CFM Quadrajet carb rejetted properly. Heads are AFR 180cc street heads with 2.02/1.60 valves and rollers.

 

10.5 is a bunch of compression for street gas, even supreme. You can get away with it if you have alumuminum heads, but will probably continue to have problems if you've got cast iron heads. This is not something you can get around.

 

Here are a few things I did:

 

1) Set mechanical timing to 36 degrees advance at 2500 to 2800 rpms. Most HEI's will give you max mech advance at around 2500, maybe a little more. Your looking for your total mechanical advance to be as close to 36 as you can get without pinging. Plug off vac advance at the source and go drive the hell out of the car at all, punching to throttle from various rpms and in all gears to make sure it doesn't ping anytime you really jump on it. If it does, back off to 34 and try again. You will find the max mechanical advance your car will accept this way. This sets your wide open throttle timing.

 

2) Next, you need to make sure that your vac advance is hooked up to the right place. You want a vac source that is high at idle and drops off as the throttle is opened. In other words, when you punch it, the vac advance drops out, leaving the timing to your mechanical advacne springs. Usually, there is a vac port on the carb body underneath the primary butterflies; or you can use one off the manifold. It depends on the carb and how much vac you're producing as to where you hook up.

 

3) Get an adjustable vac can. Crane makes on that you adjust with an allen wrench. This way you can tune you vac to give you only the advance at part throttle that your motor can handle.

 

4) Now, with vac plugged at source, get a reading of your timing at idle; it will be only the mechanical at this point. Remember to do this will car fully warmed up and at the idle speed you want. Now hook up vac unit and check timing at idle again to see how much the vac unit added to your mech timing (this will be your total timing at idle). OH, don't forget to readjust your idle speed after hooking up vac unit; it will jump higher and will need to be lowered to the same as before. Do this before taking the second timing reading.

 

5) Whatever the difference is is what the vac is adding to your timing at idle and part throttle conditions. You now drive the car and see if it pings on you at part throttle when under load like you decribed. If it does, adjust the vac can to give less advance and test again, and again, and again until you get it right. When you're happy, record the total timing at idle for reference and enjoy.

 

There are other things about vac cans that can be experimented with. Some are not as sensitive as others, meaning that they don't open as easily under a given level of vacuum. I have never found one of these that worked as well as an adjustable one, but some people hunt them down from junk yards and swear by them.

 

You may also find that with 10.5/1 compression, you have to leave the vac advance out of the system and run on only mechanical. Lots of people do this and it doesn't hurt wide open throttle performance at all. It does lessen part throttle response, which can make a car feel more sluggish when cruising, and it will hurt your gas mileage some.

 

Good luck; hope I helped a little.

 

Bill

[Guest Nic-Rebel450CA]

10.5 is a bunch of compression for street gas, even supreme.

 

 

Maybe if you live on top of Mt Everest where there is almost no oxygen in the air.

(You shouldnt be pinging)

 

Do you have any trouble with pinging if you disconnect the vacuum advance line? Have you checked to make sure you are picking up vacuum from the right location? (Should be somewhere below the base of the carb, not at or above the base).

[z ya]

Heavy, Your problem is the cam. With 10.5 to 1 comp ratio and the small 268 cam you have way to much cylinder pressure. Too use that kind of compression on pump gas you will need to run at least the 274 extreme or the 280 or 286 magnum cam. Alot of guys are running into this problem. Give comp cams a call and I am sure they will tell you the same thing. That cam is a great cam but it only likes maybe 9.5 to 9.75 comp on pump gas with aluminum heads. With steel heads up to 9.5 MAX!

[Guest Nic-Rebel450CA]

Could someone explain how the cam could have any affect on whether the engine would ping or not? Pinging comes from gas igniting due to pressure and temperature. The cam does not change the compression ratio or the temperature, so I dont follow on how it could make the engine ping. The cam would have to allow the engine to take in enough air to change the compression ratio in a way such as forced induction systems. (Basically making the ratio at BDC greater than 1:1). Am I missing something?

[Guest Phil1934]

Here's a few tips to figure what you've got.

Centerplate=======Weights=======Dist.Degrees

368=================139=============10======

406=================139=============12======

349=================139=============15+=====

371=================139==============9======

489=================139==============14=====

375=================139==============13+====

364=================139==============12=====

397=================139==============12=====

370=================139==============12=====

346=================139==============11=====

396=================139==============11.5===

398=================139==============10.25==

383=================139==============12=====

 

If you are experiencing part throttle pinging with the HEI, you probably have a 20 degree vacuum advance can. Most HEIs have a ridiculous amount of advance here, which is a detriment when you start experimenting for optimum timing. Pull the distributor cap and you will find a number stamped on the top of the support bracket near the can. This is the cans advance, it can be anything from 8 to 24 degrees. A 10 degree vacuum can is ideal. It is available from AC Delco Parts, or your dealership. Canadian AC Delco part number: D1370A

 

If you take your example with the 371 center plate and 139 weights we see 9 dist degrees. Double that to get crank degrees and we get 18. Say you want 34 total. Then 34-18=16. Set the initial at 16 and you will get a total of 34 at what ever rpm you have the curve brought in.

 

The use of 045 or 053 weights will knock exactly 1 dist degree from each of those numbers.

 

Finally you could get an MSD 8682 retard box, which will allow you to pull up to 15* out of the timing with the twist of a knob. This, coupled with a digi set timer, will hopefully be my secret weapon at the next drags to allow a good launch without excessive tire spin.

 

Lastly, as to the question of how a cam affects compression, a later intake closing allows compression to be bled back into the intake, so the dynamic comp. ratio is less. There are formulae on the net to allow you to calculate this.

[Guest zfan]

Figure out your dcr compression. I am running a 11.2 to 1 motor on pump gas, 93 octane. I am on the borderline myself and that is with a Comp cams Hydraulic roller with 236/242 duration and 550/540 lift at 110 lobe seperation. I degreed the cam in 4 degrees retarded to get it straight up.

 

My dcr is 8.25 and that is pushing it. You on the other hand are running a relitively small cam that bleeds little compression off. I think you should step up your cam. Mine is parts number 12-433-8. You could use the cam that Corzette is running I believe it is 230/236-510/520 and retard it 4 degrees and be ok.

 

Of course this is just my opinion/idea. Hope others will chime in.

 

 

Mike

[Guest Phil1934]

His cam closes at 60* ABDC so DCR is 8.7 As opposed to the cochise calculator everyone uses I like this one http://www.rbracing-rsr.com/comprAdvHD.htm as it has altitude, too. Doesn't help here with Sacramento at 42', but you see a lot of advice from someone in CO telling someone at sea level that combo works for him. And this one allows boost which should be fun for the turbo'ed Datsun 6 crowd.

[Heavy Z]

Thanks everyone for the advice! The efficiency of those who frequent this site never ceases to amaze me. I do remember about a year ago when the old dizzy was working it had an even smaller cam(comp 268 - .218 duration at .50/.454 max lift/110 degree ls) and had less trouble pinging. I thought the new cam might help to bleed off a little more comression than the old one, maybe going a step bigger would've helped more. I'll try your tuning recommendations first and see if I can't get this problem solved, once again thank you all!

[Guest Nic-Rebel450CA]

What would be the point in getting a cam that would let the intake charge back out to lower the CR? Is that any better than just having an engine with a low CR to start with?

[Guest Phil1934]

Going at it that way it does seem like a band aid fix. Crower sold a cam with a deliberately late IVC for people buying the high compression LS7 engine in the early '70's, but it was not as good as swapping pistons. Look at it the other way. Big cams require higher compression or they will be extremely lazy. This is because a longer duration will have an earlier IVO and later IVC. It's a case of matching the combo. And while you can make small changes in your favor like retarding the cam 4*, you can't solve it all this way. Ford retarded the 460 cam 8* for emissions about the same time. Swapping an early timing chain set would transform a truck that could not turn a tire into one that would spin as long as you wanted. However if you have a combo that pings, you can put in a colder thermostat, retard the cam, back off the timing a little and get by. There's also water injection, which solved it for me in the early '70's, but again at a power loss. I tried octane booster, toluene specifically, but it's a pain to add every time to a daily driver. A few years ago I had one with a mild case and carried a distributor wrench with me and backed off the advance on hot days. Here's where the MSD box would be easier.