Hello, I have just finished building a 383 stoker on my 79 vette. I am using a comp cam CS270H, headman elite headers, Edelbrock RPM performer aluminum heads-60899 62cc, Edelbrock performer air gap manifold, with an 600cfm Edelbrock carb. We are using speedpro pistons and the compression is ~9.7 to 1. The problem is that I am getting engine ping under load. I am using 93 octane with an octane booster. Driving normal there is no ping. We have played with the timing, pulled the vacuum off the distributor and still ping under load. My mechanic has suggested that we replace the stock distributor with an MSD to help elimate the ping.

The question is which MSD ignition products to purchase?

 

Your compression is a little high for a pump gas engine. Ideally it should be around 9:1 for an iron head engine and around 9.5:1 for an aluminum head engine. In some cases you can exceed these numbers slightly but they are a good rule of thumb. You didn't state what your timing figures are. On a small block Chevy you should be running an initial timing of around 10 to 12 degrees with a max of around 34 to 36 degrees advance. Take a look at the sticky regarding timing and compare it to what you currently have. Changing distributors will not eliminate pinging. If your distributor is functioning properly the brand name will make no difference.
My bet is that your compression is too high and you are going to have to compromise the timing or use an octane booster.
Good luck.

 

Mostly agree w/ vettefixr
IMHO flattop pistons probably not the best choice here ... dished would've been better but a bit late for change now.
Dunno exactly which cam "CS270H" is ... but a larger cam may bleed off more compression.
Another distributor might be easier to curve/tune ... but your's can be recurved/tuned too.
Most octane boosters don't do much ... try mixing some true race gas. Many tracks have gas w/ about 110 octane available; a rare few will allow you to pump direct into street car ... most require pump into a jug.
However, your goal should be to tune it to run on readily-available gas.

 

I ran your DCR with the cam you used and it is at 8.83. Thats just too high for pump gas with a decent timing curve.
You are either going to have to drop you compression some or do something about the cam at the very least.
I calculated the DCR assuming the cam is installed with the intake centerline at 106, which is how its ground. If you retard the cam 4 degrees from where it is to a 110 center, then your DCR goes down to 8.57 which might work, but no guarantees.

 

probably gonna get flamed for this but isnt a 600 cfm a little small for that engine?? i had a pro engine builder build my 383 with 10.5:1 cr and a comp cam 280 magnum cam and he said to use a 750 cfm. irun stricly 93 octane and have never had pinging
I am no expert but this builder built motors for guys like big daddy Don G. back in the 70`s

 

Yes the Edelbrock 600cfm carb is a little small. I already had the performer RPM manifold and carb. They both were taken from a wreck new rebuild. It is all about how much money you will want to spend. I can always buy a larger carb if needed. If someone thinks that a 750cfm would do the trick, I would be willing to invest the money. My Mechanic seems to think that a stronger firing distr might do the trick.

 

You screwed yourself with too small of a cam. I did it to myself years ago with a Crane 272 and 10.7 with iron heads. I always had to keep the timing down and a bottle of moroso 104 octane boost ready to dump in. After a year or two of fighting it I swapped out the cam with the motor in the car.

The jump to the Crane 278 fixed the ping problem and I was happy for about 10 years. I run two motors now with high compression 383 with 11.2 and 434 with 11.8 C/R. They both have many thousands of ping free miles on super unleaded.

The "Dynamic Compression Ratio" is what matters. Not the static.

The 600 carb is plenty big enough for a 383.

 

My mistake, the compression is 9.7 to 1. Not 10.7 to 1. A friend recommended the MSD 85551 dist with a box. Any thoughts.

 

9.7 and that cam with aluminum heads should not ping, I ran that cam with 10:1 and iron heads no problem. Changing ignition I doubt will solve the problem. You have a basic setup problem somewhere. Start by disconnecting the vacuum advance and plugging it. Set the mechanical timing to be 34deg in by 3000RPM and I don't see any reason why the motor should ping

 

has this been since new cam installed? if so was it lined up properly when installed with the timing chain?

 

I am running an iron head 383 with the xe274 and I was also getting a ping under certain load conditions. I had to work with the timing. Overall, turns out I built what I believe is a bad combo. I should have used more cam to bleed of compression. Anyone want to chime in on piston dwell time at TDC with the longer stroke? The way I figure it, the longer stroke increases the TDC piston dwell time and increases the odds of having compression related issues such as pinging.

 

If your compression is in fact 9.7:1 you should be OK as far as compression ratio goes. I would suspect that your distributor is advancing too fast or too much. This can be cured by using a limiting bushing (often left out during a rebuild) and heavier springs.

I tried to find your cam on Comp's website but they didn't list the specs. It's also possible that your cam is gound on a narrow lobe center along the order of 108~110 degrees. This can sometimes cause a problem in running compression because it doesn't have a lot of overlap. In many cases you can go to a larger cam if you pick one with lobe centers ground at 112~114 degrees. This allows a bleed off of compression at lower engine speeds and a better idle.

I would look at the dizzy first and make sure your mechanical and vacuum advance are both working correctly. Also make sure that the vacuum advance doesn't pull in more than 10 degrees advance because this can sometimes be a problem with stock distributor vacuum advances. You can check Lar's paper for the right vacuum can number. A vacuum advance that pulls 10 degrees advance will end up giving you a total advance of 44 to 46 degrees at light throttle when set up correctly. Many stock vacuum cans will give 18~20 degrees advance which would put you at 52~54 degrees advanced which is way too high on a high performance engine. Check this first and see what your curve looks like.

Good luck.