Causes of detonation?

-
MopaR&D

MopaR&D

Nerd Member
Joined
Jan 4, 2008
Messages
5,527
Reaction score
2,825
Location
Augusta, GA
I got my Carter AFB 625 cfm installed on my 318 in my '70 Duster and I'm having some weird pinging issues. Before I continue it's an A904, 2.76:1 open 7-1/4" car and the 318 has stock bottom end with reman'd stock #302 heads, stock exhaust manifolds and single exhaust, Lunati Voodoo 256/262 cam, Edelbrock Performer intake and of course the Carter AFB, all running at 6500 ft. elevation. I also use 85-octane gas.

What's happening is when I accelerate with just the primaries it's completely normal, but when I get into the secondaries it pulls harder up to about 3700 RPM then starts to "lay down". At the same time I can hear some light pinging coming from the engine and I usually back off before 4300 RPM. At first I thought the secondaries were too lean, so I drilled the jets (.086" size) out to .094" but the pinging is still there (pulls a little harder though, haha). I'm going to try checking the total advance this weekend but I'm pretty sure it's not more than 36* (of course I'm not sure what RPM that's at though).

One thing I've been wondering, can you get detonation from an overly restricted exhaust? The stock system can't be over 2" ID (on a single pipe, THE HORROR!) and has multiple dents in it along with a very quiet Sears "Muzzler" muffler (what the car came with, the name kind of scares me). Could excessive backpressure be causing this detonation? I'm 95% sure it's the reason why it won't make power over 4000 RPM even with the cam and 4-bbl.
 
Yeah, that's a good place to start......I read what you posted, then my mouth dropped open. Performer intake, AFB, aftermarket Voodoo cam & single exhaust? Whoa daddy......

Headers please.....2 1/2" duals please. BTW, those 302 heads probably bumped the compression over 9 or 9.5:1 at least, I'm thinking. Yeah, start with the exhaust.
 
I don't think I'm going to worry about headers now as I don't have the money for good ones. Besides I'll be giving up 10 HP at the most on this mild engine. I'll have to try to convince my dad now to get me dual exhaust so I don't blow up my engine. LOL!!!
 
What is the timing curve? Who did the heads? What do the plugs look like?
 
Hedman headers are less than $200 as I recall, just FYi so you have info to work with. I went with stock duals after the flowmasters, and 2.5 from the headers to the flowmasters.
 
Detonation is caused by premature ignition of the fuel air mixture in the cylinder. The cheapest fixes are retarding ignition timing or going to higher octane gasoline. You won't be getting detonation from an overly restrictive exhaust.

I have the 302 heads on a 68 318 in the Demon. With the stock heads, it was rated at 9.2:1. (Should be the same for your 70 model engine.) The 302 head has fewer cc, which bumps compression up higher. I'm estimating the engine has just a little over 9.3:1. I had a ping problem on 87 octane, but cleared it up with 89. The engine is too green to do a real hard run on it. Consequently, I can't say how it will behave over 3000 rpm.
 
MOPEkidD-3 said:
I also use 85-octane gas.
Click to expand...

Oh yeah, I forgot to mention......you've gotta get some 92 octane. A cooler thermostat could help too.

MOPEkidD-3 said:
I don't think I'm going to worry about headers now as I don't have the money for good ones. Besides I'll be giving up 10 HP at the most on this mild engine.
Click to expand...

I seriously doubt that.....I'm thinking more like 35 or even 40 @ higher RPM's. Get some cheapies for now.
 
cudavert said:
Oh yeah, I forgot to mention......you've gotta get some 92 octane. A cooler thermostat could help too.



I seriously doubt that.....I'm thinking more like 35 or even 40 @ higher RPM's. Get some cheapies for now.
Click to expand...

I don't think higher octane should be necessary; the stock rated compression of my 318 was 8.7:1, so with the 302 heads I doubt it's over 9:1. This shouldn't require premium gas, especially with my thinner air. I'm going to check the timing first and if it doesn't need to be changed I think I'll just take it easy on the gas pedal until I get dual exhaust.

There was a thread a while back about headers that quoted an article from Mopar Muscle. In the article they found that on a mild engine, such as a 300-hp 360, there was only about 13 more HP found going from "restrictive" stock 318 2-bbl. exhaust manifolds to Hooker Comp headers. At this HP level it's not really worth the expense and hours of frustration for installation.
 
I run the same cam in my 318 with 302 heads but have a bit more compression and headers. mine likes about 34* total and I run 89 octane. I would say for your setup I would pull the timing back a couple of degrees (all in by about 3k) and run a mid grade gas. I would say the lack of power is the restricted exhaust but nothing to do with the pinging. its simple enough to find your problem by just pulling a couple degrees out and put some good gas in. cheaper than shelling money for the exhaust and you could be causing damage to the motor driving it around like that. remember on a scale of 1-10 audible ping is about a 7 so.... try simple and free first.
 
Pinging is not the same as detonation. Pinging is caused by heat in the chambers, or lower octane. I asked for timing curve info and the condition of the heads because oil coming down guides can also make an engine ping even on higher octance fuel. What are your cylinder pressures?
 
moper said:
Pinging is not the same as detonation. Pinging is caused by heat in the chambers, or lower octane. I asked for timing curve info and the condition of the heads because oil coming down guides can also make an engine ping even on higher octance fuel. What are your cylinder pressures?
Click to expand...

True about the possibility of oil causing it.....but I always considered pinging and detonation as the same thing, whatever the causes.

Knocking (also called knock, detonation, spark knock or pinging) in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. The fuel-air charge is meant to be ignited by the spark plug only, and at a precise time in the piston's stroke cycle. The peak of the combustion process no longer occurs at the optimum moment for the four-stroke cycle. The shock wave creates the characteristic metallic "pinging" sound, and cylinder pressure increases dramatically. Effects of engine knocking range from inconsequential to completely destructive. It should not be confused with pre-ignition (or preignition), as they are two separate events.

http://en.wikipedia.org/wiki/Engine_knocking
 
That ones a rather shallow definition. But they are seperate issues. Pinging you wil always hear. Detonation is sometimes silent. Pinging is caused primarilly by hot spots lighting the air/fuel mix rather than the spark. This could be carbon, points, edges, or the casting. It's a domino effect too. Once it starts, until the heat is removed it will continue. Ping makes heat-heat makes hot spots hotter-more ping. Octane helps control the tendency for fuel to ignite.. And any oil will drop the octane instantly and drastically.
 
I'm thinking that you have not checked your distributors curve and have to run a lot of initial timing to get it to idle but that pushes your total timing way over what you really need. Tell us what your initial and timing total are.

Moper is right about oil causing problems too, whether it be from valve guides or ring wear.

BTW, did you use the recommended valve springs from Lunati for that cam? Were they checked when in installed for bind and proper installed height?
 
ramcharger said:
I'm thinking that you have not checked your distributors curve and have to run a lot of initial timing to get it to idle but that pushes your total timing way over what you really need. Tell us what your initial and timing total are.

Moper is right about oil causing problems too, whether it be from valve guides or ring wear.

BTW, did you use the recommended valve springs from Lunati for that cam? Were they checked when in installed for bind and proper installed height?
Click to expand...

ramcharger nailed it on the head. Turns out the remanufactured electronic distributor has a very wide mechanical advance curve and is set up for engines running close to TDC initial timing (I guess for emissions). With my dial-back timing light and the initial set around 10* I ended up with almost 40* total around 3400 RPM. Put the initial back around 5* and the pinging is gone. After I get exhaust I think I'll have my distributor recurved by FBO.
 
You can do it yourself for about $40 if you have no mig. Might be $10 if you have one...
 
You must log in or register to reply here.
-

Back
Top