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detonation problems

detonation problems

detonation problems

We have a race engine that is confusing us all.
It has 10.8 comp runs on avgas with total ignition of 34 deg
this engine is wining some races & up the pointy end in all the others. The major concerne is that it is showing signs of detonation on the plugs,eg glossy beads, black pepper spots & the occasional what looks like tiny balls of aluminium on the porcelean near the electrode. We have tried less timing, richer mixtures etc but the condition still cotinues to bug us.We have tried different fuel mixes, different advance curves colder plugs etc etc etc with no success. As a final resort we dismantled the engine to find no erosion of pistons & no visible damage to the head . The chambers are all showing signs of good burn through the chambers to the far end of the quench area.The piston tops have a good burn pattern across to the far end of the quench area except for a small arc of clean aluminium about 3mm in width but with no erosion. It looks similar to fuel wash but theres no scoring of the bores in this area.The ring lands show signs of pitting due to detonation.Could this be the area where we are getting the tiny spots of metal from.Any clues on this detonation would be more than helpfull.We gave it a run on a wheel dyno before we dismantled it & from 5000rpm on the h/p curve dipped up & down along a basically straight line.We could not correct this on the dyno no matter what we tried.

RE: detonation problems

Pitting from above the top piston ring is a classic sign of detonation, and I am certain that is where your aluminum on the plugs is coming from.  

There are several items that can cause or contribute to pre-ignition detonation.  They include anything that creates higher than "normal" cylinder temperature or pressure. Advanced ignition timing, rich fuel mixture, high jacket water temperature, high boost pressure (from any form of supercharging), high compression ratio (tends to increase with increasing cylinder deposits), a too-high of valve lift (allows more air in, thus creating higher pressure during compression), etc.  

A hot spot in the combustion chamber, such as from carbon deposits or a sharp edge, can also cause pre-ignition.  Are your heads fully polished?

My old Ford pick-up engine used to "ping" excessively whenever going up hill or under load.  The problem went completely away after I replaced the cast iron intake manifold with an aluminum one.  I think the iron held too much heat and caused the incoming air to be at too high of a temperature.  I can now run almost any fuel and ignition setting, within reason.

I hope this gives you a few ideas for other things to check or change.  Good luck!

RE: detonation problems

What style engine are you running? Wedge type chamber benefits from running the piston as close to the head as possible (without making contact)in lowering the detonation threshold. I have had good success with reverse flow cooling technique in SB Chevrolets and mopars.Can run 11:1 compression on 92 octane cast iron head. With a good after market rod I have run .030 piston to head clearance. You can probably squeeze this # down a bit but its a pretty good ballpark.-----Phil

RE: detonation problems

To kpuddarby
We have polished chambers & have checked for burs ,sharp edges etc before assembly & on stripdown- nothing there?
Comp is low for avgas, tried different advance curves & fuel mixtures- to scared to try to-lean with this problem though?
Valve lift is by no means excessive,but we do have great intake flow right through the lift range.What is the easiest way to get accurate cylinder pressures at revs to test for higher than normal comp?
The intake manifold is aluminium it gets a lot of heat from the extractors which are ceramic coated & heat wrapped, it gets hot.

RE: detonation problems

Do you have access to an engine dynamometer?  I think most of those shops have spark plugs that are fitted with pressure &/or temperature probes.  They can also determine your exact air/fuel ratio at all operating levels.

I have not been involved with race engines, so some of my thinking on fuel ratio changes may be off.  My current experience is with large lean-burn gaseous-fueled engines.  With these engines, a richer than normal mixture causes higher heat and pressure, thus promoting detonation.  If memory serves, liquid-fueled stoichiometric engines run cooler when operated on the rich side. This is due to not having enough air for full combustion of the fuel, and the excess fuel acts to cool the charge as it evaporates.

In reviewing detonation theory from Waukesha Engine, I see that they define detonation as "the autoignition of the end gas after spark ignition has occurred..."  The end gas is "...that part of the air/fuel charge that has not yet been consumed by the normal flame-front reation."  The detonation sound you hear is created when the two flame fronts collide.  I think the aluminum gets pulled from the piston after the collapse of the pressure wave in a similar fashion as other forms of cavitation.  This is a bit different from pre-ignition detonation which I think is usually caused by a hot-spot in the combustion chamber.

Interestingly, the Waukesha table of detonation promoters and reducers says that the stoichiometric air/fuel mix is a promoter, while either lean or rich mixtures are reducers, assuming the engine is not being overloaded.  I think this is because at the stoichiometric ratio, you can make the maximum amount of heat in the cylinder.   You can actually make more power with a slightly rich mixture, because you provide enough fuel to utilize all of the air that your engine is capable of taking in.

Maybe it gets back to high cylinder inlet charge temperatures.  Can you do some ducting work to get colder air to the throttle or to cool the intake manifold?  Can you cool the fuel prior to the carburetor or injector?  Have you tried a lower temperature thermostat?  One of the old hot-rodding tricks was to go about 20°F colder than stock (promotes a cooler & denser inlet charge).  I wouldn't go below about 150-160°F, because you need to keep the engine parts at their thermally expanded size.  

If you don't have a thermostat, you could actually have too much coolant flow.  The coolant passes through so fast that it doesn't adequately absorb the heat from the engine, and it may not release the heat as well in the radiator.  You could experiment with various sizes of orifice plates at the engine outlet.  Conversely, the coolant flow may not be adequate to start with.  In this case, you could have too much restriction or the pump may not be adequate for the job.  The radiator may not be adequate if the flow rate is ok.  Make sure it is clean inside and out, or try to fit in a larger one.

Hope I have been some help.  Let us know if you find the cure.  I'll post again if I run across any other ideas or potential solutions.

RE: detonation problems

smokey 44211

Sorry i haven't replied sooner the engine is ohv,cast iron head,D shaped chamber- wedge type:
The reverse cooling is a great idea & worth following up on
The piston to head clearance is .038" -billet rods.
What are your thoughts on raising the comp to create better turbulance?
We tried snub nose plugs(shorter porcelean) this seems to have helped a little.
At least now we can see a difference with mixture changes.
Have you tried ceramic coated piston tops, if so your thoughts on this. I have been told that it helps detonation but it doesn't last for to long?

RE: detonation problems

Let me begin by suggesting some checks on some things that you might have overlooked. Verify TDC. especially if you are running any offset in the wrist pin. (TDC is when wrist pin, crank pin, and main journal centers form a line.) Incorrectly cut key way in crank and/or hamonic balancer would give a false indication. I've also seen outer ring on on balacer move out of phase. Tight clearance between top of piston and head helps promote chaos (turbulence) within the chamber at tdc. This is desirable for raising the detonation threshold (Ricardo effect). I think you could safely take .010 off the block to gain in this area. Small change in compression ratio would be insignificant. Reverse flow cooling technique has two advantages. Most V style engines do not circulate coolant though both banks of the engine at the same time. One side is being cooled while the other is heating up due to the coolant deadheading at the thermostate. The other is by introducing the coolant into the head first lowers the exhaust valve temp. and thus the detonation threshold for a given fuel quality and pressure. If you do decide to go this route let me know. I can give you some suggestions for rerouting the internal coolant path. I have used Swain ceramic and friction coatings on internal parts. To date I haven't experienced any flaking. Power gains are small but measurable. Big gain in oil temp. reduction. Every little bit helps. One final thought. I have seen some V8 cranks that were somehow ground that the crankpins were not 90deg from one another. Might be worth a look. Report back you findings when they become available.-----Phil

RE: detonation problems

To Kpuddarby:
Thanks for your input it is appreciated & i will keep you informed
I would like any info you can find to help sort this problem

THANKS AGAIN-------willeng

RE: detonation problems


As you could imagine ive checked, checked again, re-checked, checked to make sure i re-checked properly & so on as we all do with problems.
I agree every little bit helps & it's all these little bit's that turn into real numbers in the end.
I will try the reverse cooling you suggested in the New year.
One question:The running temp of the engine we thought is not to bad it runs 185deg in clear air to 195-198deg behind cars the temp sender is in the thermostat housing do you think reverse cooling will help even more.---your thoughts on this would be appreciated.
We reworked the head (porting) for more power this has helped the intake manifold temp greatly to our surprise, it's gone from leave your skin behind hot to almost cool if not cool to touch. We are getting great airflow. The last run was a hot day & the temp difference of the intake manifold is amazing!!!
It still shows the same det signs though,there is a very slight improvement, it shows a considerable gain in the power dept due to the port work but were still not getting it all I'm sure like you said it's all the refinements & all the little bit's.
Thanks for you advice keep it up.

RE: detonation problems

I have found a reference that says too hot of a spark plug can promote detonation, but I think you said you have tried colder plugs, so maybe not much new there?  Also, if you have a misfiring cylinder, the others try to compensate and become overworked.  Since you seem to be getting good power, this may not be an isue, either.

Can you hear the "pinging?"  My reference also says detonation is promoted at lower engine speeds.  I was wondering if you could tell when it was occurring, such as when you try to accelerate out of a corner and the rpm's might be lower (if you didn't downshift).

Your coolant temperatures sound ok for a street engine, and I don't have racing experience.  You might check with the piston and/or other parts suppliers to see if you can go colder with the thermostat.  They should know whether or not the components will be expanded to proper size at, say, 170° in the clear air.

Good luck & Happy Holidays.

RE: detonation problems

Hey Hows it going?
There is one thing I have not heard anyone talk about is your cam. What manufacturer makes it?  What is the lobe lift and duration and what is your duration @ .050 lift? What is your overlap? Is It a split pattern cam? Did you install it with the markers straight up? Chances are if you did you are already 4-6 degrees advanced already. Comp Cams grinds this much advance in there cams straight out of the box.  
Is your chain an el cheapo dead roller? This can cause issues also,because you are advancing the ignition timing for the motor running retarded, because of slugginess. You motor will have great top end(to a degree as it will out perform your cam specs). I have seen dead double pin timing chains stretch within 20 miles. If you use a true roller bearing type chain, it won't stretch as a result of tighter tolerances due to the fact that the rollers take more of the load and not the pins on the chain. Just make sure you soak the chain in oil for a while before installing it,for that is the one thing that will cause a true roller to stretch.lack of break in oil.
  Last but not least cam timing? Did you degree your cam? I do not care if it is a stocker rebuild I always degree the cam. Back in the super stocker days they used to make cheater bump sticks that increased cylinder pressure horrendously. All the measurements asked above can have a profound effect on cylinder pressure and performance. If  you do not know how to degree a cam Comp Cams Lazer Cams, Crane cams and Hot Rod have Tech Articles on degreeing cams

RE: detonation problems

Sorry i haven't replied, i haven't looked at this post for a while.
To answer a few questions, yes the cam is phased in & rechecked, changed etc on dyno as well.
I wouldn't like to specify cam specs unfortunately--custom ground.
The timing chain we have checked & phased in cam again once run- regular check.
smokey's suggestion of decreasing piston to head clearance has helped a little theres no doubt this has helped, it's still showing signs but to a lesser extent. Thanks
to smokey

Personally i would like to hear more about those cheater bump sticks?.


RE: detonation problems

Thanks to
kpuddarby & smokey44211 I have found some of your information helpful & this has lead me to different things that show promise.  Thanks again. Keep it up.

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