Cam, detonation

[Aussi440]

Guys, haven't posted in a while, here's my question, my car is a 68 charger, 4 speed 323 gear, stock 440, 375 horse resto cam, 9.5 compression, 160 cranking, stock carb, exhaust etc, 452 iron heads. I have 40k miles on the motor drive it all the time, and have never been able to run much timing in it without detonation. I can't get much above 28-29 degrees total. Stock distributor with Petronix igniter, I have tried changing springs to bring timing in later, and the knock will change in rpm range but still can't get much timing.  Should I be able to run 36 degrees total on 93 gas with this compression and cam or am I fooling myself?  Cam duration is 208/221 115 LSA. Only thing I haven't checked is to see if the harmonic balancer somehow spun and my timing marks are off.  I'd like to be able to tune it like it should be without running 100 octane or mixing some in.  Any ideas? Thanks. Ryan

[heyoldguy]

That is appearently the combination from hell. My father built a similar 440 for his truck and I got it after his death. There was no way I could keep it from detonating. I fear the combination of the 452 heads, 9.5:1 compression and intake duration that short is just lethal on pump gas. When the 452 heads came out, the factory was using 7.5-8.5:1 compression in their engines. I finally took the heads off and made the shortblock a dyno mule for testing cams and cylinder heads. No detonation now but we are using much bigger cams or better combustion chambers in our cylinder heads.

[Aussi440]

Hey old guy, is there a cam you would recommend that would fix the problem? Would I need to change valve springs etc? I have the speed pro pistons with 4 reliefs so I should be okay on clearance for decent size cam. 

[firefighter3931]

Sometimes the fuel curve is an issue so before tearing into it i would get some A/F readings before swapping hard parts. I've been able to make 160 psi (cranking) run on pump premium.... but gas quality can vary significantly from region to region 

Does it detonate only under heavy load ? 


Ron

[Aussi440]

Ron, it detonates when I get into the pedal yes, if I am just putt putting around its fine, it doesn't detonate as much when the weather is cool, obviously when it's hotter out its worse.  I have the stock carter avs on it.  Plugs don't show a lean condition, doesn't run too hot.

[Kern Dog]

With all of what I went through last year trying to quell the detonation in my 493, I am surprised to read that any engine knocks with only 160 cranking psi. I will not go against any experts advice though because I am still trying to learn as much as i can about it all.
lots of possible variables that i would check:
*Timing marks, as you said can be wrong. Get a "piston stop" and verify where true TDC really is.
*Borrow another compression tester from someone and see how the numbers compare. I had one here that was defective, giving me false numbers. Remember to remove all the spark plugs so the engine moves without compression building in any cylinder except the one you are checking. This is important! Testing with the other 7 plugs in place always gives artificially lower numbers that may fool you into thinking your numbers are low enough to work on pump gasoline.
* An air/fuel guage is a helpful tool to determine if you are running the proper fuel ratio. Recently I've been tuning my car using one made by AEM. Funny thing is, I used to think I was running clean if I didn't see black smoke or smell fuel. The guage tells the story so much better. Currently I am still too rich but the car smells really clean! Your eyes and nose will never be as accurate as the guage. If you do go this route, the goal is to be around 14.7 at idle in gear, the same at cruise speeds and around 13.0 while at wide open throttle.

[Kern Dog]

Keep us informed on the progress. Every experience you relay may help someone with similar problems in the future.

[fy469rtse]

First thing is to get rid of the stock distributor, try ice ignitions very similar to the firecore that Ron works with, I would look at better flowing heads such as the edelbrock s , try throwing in high octane and see how it performs
You need to get the curve on the distributor right to match the engine, this is your first area to work on
Sorry Ron , it's a local product and lots of guys with experience to help him

[firefighter3931]

First thing is to get rid of the stock distributor, try ice ignitions very similar to the firecore that Ron works with, I would look at better flowing heads such as the edelbrock s , try throwing in high octane and see how it performs
You need to get the curve on the distributor right to match the engine, this is your first area to work on
Sorry Ron , it's a local product and lots of guys with experience to help him

No offense taken 

Seems very pricy for what you get....twice the price as Firecore for a MSD look alike distributor with HEI terminals using an oversized GM cap 



Ron

[firefighter3931]

Ron, it detonates when I get into the pedal yes, if I am just putt putting around its fine, it doesn't detonate as much when the weather is cool, obviously when it's hotter out its worse.  I have the stock carter avs on it.  Plugs don't show a lean condition, doesn't run too hot.

Did you block the heat crossover on the intake manifold ? I'm still thinking that A/F is a potential problem and if it were mine I'd want to see what those numbers look like before making any decisions and condemning the cam. 

If it runs fine with some race gas then you'll know it's octane related and not the a/f ratio....easy enough test. 


Ron

[Aussi440]

Thanks for all the back and for guys, looks like my first tests should be with race gas.  A 50/50 mix should be good enough right? I can get 100 locally.  Since I still have the old carter avs on there if I do need to richen it up, where would I get jets and rods? Do the new kits for edelbrock carbs work? Is the only real way to check the a/f ratio to weld in the o2 sensor and run the gauge? I have a local race shop that can do it but boy it's expensive.

[Aussi440]

Ron, I did block the crossover, event though the distributor is older, wouldn't having the Petronix in there provide good spark?  I have the timing coming in total around 2500, and the curve is pretty smooth.

[firefighter3931]

A 50/50 with 100 should be plenty for testing purposes....if it still detonates then you'll know it's an issue with the a/f ratio. A wideband O2 guage or a visit to the chassis dyno will tell you all you need to know.  

Another possibility is the fuel pump. If you're not maintaining a solid 6psi under acceleration that will certainly aggravate the problem. Low float level will also make it go lean. Just throwing out things to look at and check.  

I don't use Carter and from what I've heard the tuning parts are hard to come by.  

Several companies sell a wideband with guage...I have an Innovate LM-1 and love it. I've lent it out a few times to friends and it's allways an eye opener for them when they plug it in and start seeing how off the fuel curve really is despite no evidence on the plugs. It's an excellent tool and eliminates a lot of guesswork.  


Ron

[firefighter3931]

Ron, I did block the crossover, event though the distributor is older, wouldn't having the Petronix in there provide good spark?  I have the timing coming in total around 2500, and the curve is pretty smooth.

Not a fan of Pertronix.....they are hit or miss.  I don't think the issue is the distributor firing at this point but you could try slowing the curve down with some heavier springs to see if that helps. Try bringing the total in at 3000 to see if that helps. 

In theory, you should be able to use 16* initial & 36* total with 160psi cranking if all is right.



Ron

[don duick]

your heads are open chamber with no quench area, also I used a carter AVS on my 440 and it ran lean these 2 are not good for detonation. google "quench" interesting read. Your 208 cam is about standard the R/T cam is about 224 inlet and 236 exhaust. Larger cams need more timing  16 - 18 initial and 36 total.
my 440 has about 10.3 compression and a comp 224/236 cam with aluminium 440 source heads. MP distributor and msd 6a box 750 holley vac sec initial timing is 18 and 36 total is in at 2500 rpm vac advance disconnected.  the piston pin height is 2.067 that means the pistons are .030 down the hole and using a .020 gasket this gives a .050 quench area. I tried 98 octane and 91 octane and not much difference between the 2. A carby expert told me carbies do not atomise high octane fuel very well as it is too dense. sometimes I get a bad  batch of fuel which will make it run a bit rough. any way  I do not have a detonation problem with this combo.
before I rebuilt this motor my previous combo was 9 to comp with iron heads had total of 36 timing and no detonation. I did for a while have my total at 40 without knowing about it and it did detonate. I would check your balancer as you suggested first.

[fy469rtse]

Aussie440, yes the new edelbrock s are carters, Don't throw your metering rods out, try and reuse if possible the old carters have three steps in the rods, much better to tune with , no mid flat spots, or less chance with steps in rod
Find the coresponding edelbrock carby size to yours and order the jet kit ,
Ron didn't realise that much difference in distributors, $$$
Buy the firecore, online tuition from Ron , and if you get stuck he will be right over, he might have to leave the barbecue and his sudden liking for Australian beer at my place, but what's a man to do when duty calls

[Mebsuta]

This is weird.  I have that same cam in 383 4 spd RR.  It has 902/213 heads (one of each) and cranks 150 psi.  It will run on 87 octane with about 15 BTDC initial and 40 total.  Years ago I had to use 91-93 octane, but all the gas has ethanol in it now, which helps in that respect.   

[69wannabe]

In my opinion I would put a good firecore distributor in there and get some for sure timing on it. You get a good solid ignition system on it and see how it runs and you will be able to set your initial and total timing to suit your needs and make sure you got a good performance carter street pump on there that holds a solid 6 1/2 psi to your carb so you will know what you got there at least. The small duration of your cam may be a factor in some of the pinging thats going on since bigger cams seem to bleed off some of the cylinder pressure but i'm not sure of how much verses a slightly bigger cam since you don't want to lose any driveability from too big a cam tho... Maybe some cooler heat range plugs would help too. So many different conditions contribute to detonation so but keep picking at it and it will eventually get figured out.