Combustion chamber analysis after teardown

[Jefnes]

Background:
High specific output 4 cyl engine, 1.6L disp,
making ~340whp on 18 psi of turbocharged boost.

My specific question relates to combustion and detonation.

I tore down my current engine due to an oil pump failure and took the
opportunity to examine the pistons and the combustion chambers.
Why not since I needed to tear it down anyway.

I noticed that all four piston/combustion chambers are a nice golden
brown with exception of the area near the intake valves...I have
a small 'hot spot' that is evident due to the 'clean' piston appearance
and very slight scuffing of the cylinder walls in this area.
Plugs look good too. (can't tel much from plugs unless you shut off mid-pull)

This wear is bascially even on all four cylinders.

This could be (is) evidence of slight detonation but due to the rod bearing damage from the failed oil pump I cannot use them for discovery. Usually I see the rod bearings taking a 'pounding' if detonation was present.


Main question:
What is the cause of the intake side being the 'hot-spot', as this seems to go against traditional wisdom? (hot spot near exhaust valve)
Is the a/f mixture not truly homogeneous at high load/rpm? (fast burn and little time, just a hunch)

I am looking for a way to 'solve' this issue that doesn't involve the brute force method of just adding more fuel
or less timing. (easily done, but not elegant)

Any comments/suggestions would be helpful.

Thanks,
Jeffrey Atwood

 

[AboveRedline]

Many times a clean spot on a piston is due to fuel wash (or contact with the valve). Fuel wash on the intake side would make sense. Does the injector point directly at the spot?

 

[Jefnes]

I'm not exctly sure off hand...if the injector points directly at that spot.

Typical port injection where the injector is in the intake manifold runner, 1 per cylinder.

BTW this is on a Honda B-series engine.

The clean spot doesn't bother so much as the scuffing of the cylinder wall. There are no nicks or cuts per say,
you can feel the roughness,its just not shiney like the rest of the cylinder bore.

Jeff

 

[evelrod]

Stick with the "fuel wash" idea as that is most likely.
As to the "scuffing" ??? It is just possible but remotely so that the fuel is "cleaning" the top ring area and bore enough to cause localized galling. I said REMOTELY possible.
Is the scuffing in the same spot relative to the intake valve? To the clean area of the piston? How much time at rpm on this engine? You say an oil/bearing failure---couild bits of bearing material been thrown up into the bore area? You say "hot spot"---if that is so then the underside of the piston will show signs of it. I can think of many more scenarios but, bottom line, all this sounds fairly normal to me. ANY detonation in an engine with the power level you describe will show signs IMMEDIATLY!

Rod

 

[Jefnes]

I thought this a well, 'normal' engine wear.

yes all the, wear, and clean spots are all near
each other: intake valve side of chamber, just below intake valves. Imagine spraying 'cleaner' from the
intake valves towards the cyl wall, this is the pattern.
(lends creedence to fuel washdown)

I am running a/f in 12.0-12.3 neighboorhood.

The local galling sounds plausible.

The undersides of the pistons look brand new.

The oil pump exploded on a long high speed run,
my driver heard the 'pink' saw no pressure
and shut it down immediately. Quick enough that I
could re-use the crank bearings if I wanted.
Some rod bearing damage, but no spun bearings.
Crank, rods, cams all OK.

Thanks, Jeff

 

[ShaunT]

What oil are you using? Also, if you are running leaded race fuel, switch to Turbo Blue. Very similar to VP C16, but a lot more bang for the buck! It is still leaded, but has almost no Aromatic Hydrocarbon content. It is real easy to tune, and turbo's love it. Sun refining, parent company of Sunoco is the Manuf. IF you are running unleaded, try VP unleaded GT plus.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[schmidtj86]

I've recently read in Four Stroke Performance Tuning (A. G. Bell) that clean spots can sometimes be attributed to the flame being stopped, usually from combustion chamber shape (read page 241-2). If you have a high compression engine, the piston shape may be poor for flame propagation, and the flame stops, thus allowing unburned fuel to be there, and no carbon from a lack of flame. It may indicate that the flame is being forced from the combustion chamber and through a place where the piston is too close to the chamber, yet still has some volume, OR it is just the quench area of the chamber, OR your clearance in the quench area is big enough (too big) to get lots of unburnt fuel in there, but too tight for flame propagation. I'm just speculating though.

 

[ShaunT]

I see this in Big Block Chevy's all of the time. Aside from the small chambers in pro stock heads, most of the shelf style big block heads have big chambers which negate a need for a really big dome if you want big compression. It kills burn/ flame propogation as mentioned.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)

 

[moeflo]

This is similar to some of my observations, and questons.

On a customers, 683C.I.(426 style) nitrous-hemi: His observations, are a near central carbon pattern. If he is running on the lean side, the piston is clean around the bore, with the largest clean spot being in the intake valve-relief, nearest the bore. If he fattens it up, the carbon pattern, remains centralized, but moves outward, towards the bore. If he fattens it up to the point the carbon threatens to cover the intake relief, it burns the piston, in that area. When lean, this clean intake-relief spot, looks "cold". Like it just came out of the box.

It's all to common for pistons to give up, where the intake-relief is nearest the bore, wedge, or hemi. Is this just a function of that area being a heat-riser?

(these nitrous motors, typically makes more power, with more fuel, as nitrous is increased in relation. This lean pattern was used at a slick track, that wouldn't tolerate any extra power, down track.)

But, reading a piston-top has never inspired much self-confidence. Washed clean, or totally dry with no combustion. Seems one would appear hot, and the other cold, but, I'm never cocky about it.