2F engine knocks under load. Questions. (1 Viewer)

[DocB]

I have a 77 FJ40, 2F rebuilt; has higher than normal compression (175 psi); desmogged, vacuum advance toyota distributor. It knocks only under uphill load but otherwise runs great.

Question 1: Timing is set where "Dot" is seen just above "Hashmark", like 1 mm. Does advancing the timing make the Dot go up or down with respect to the Hashmark?

Question 2: Does too advanced timing cause "Knock"? What about too retarded timing; can this cause knock?

Question 3: Does higher octane fuel fix knock and why?

Question 4: At factory specs 7 deg BTDC, is the dot even with the hashmark?

Question 5: Does turning the distributor clockwise advance or retard timing?

Question 6: With a higher compression engine, should the timing be advanced or retarded? Should higher octane fuel be used?

Question 7: Do you advance or retard the timing when driving at higher altitudes? (I live at 4000 feet).

I plugged the vacuum advance port and the engine still knocks under load.

 

[Poser]

making the ball go outside the lower part of the window, is advancing the timing...


Ball on pointer is supposedly 7deg.


Too much advance can make the engine ping..



If you let up off the throttle, does the ' knock ' go away?


What fuel are you using?


Higher-octane fuel resists detonation, (combustion well before top dead center)

 

[DocB]

Yes when I let off the throttle my Ping goes away. Using 90 octane, the highest readily available here.

 

[Pin_Head]

Question 1: Timing is set where "Dot" is seen just above "Hashmark", like 1 mm. Does advancing the timing make the Dot go up or down with respect to the Hashmark?

The more the dot goes CCW, the more advanced it is.

Question 2: Does too advanced timing cause "Knock"? What about too retarded timing; can this cause knock?

.

Too advanced: Knock. Too retarded: lack of power, overheating of exhaust manifold.

Question 3: Does higher octane fuel fix knock and why?

.

Yes, it is harder to ignite.

Question 4: At factory specs 7 deg BTDC, is the dot even with the hashmark?

Question 5: Does turning the distributor clockwise advance or retard timing?
.

Yes. Turning the distributor body CW retards the timing.

Question 6: With a higher compression engine, should the timing be advanced or retarded? Should higher octane fuel be used?
.

Retarding the tming should reduce knock. The higher the compression, the more susceptible to knock. HIgher octane fuel may eliminate knock.

Question 7: Do you advance or retard the timing when driving at higher altitudes? (I live at 4000 feet).

I plugged the vacuum advance port and the engine still knocks under load.

Engines are less sensitive to knock at higher altitude because the air is thinner and there is less air to compress.

You might try backing off the advance a couple of degrees and see if it goes away. Also try 91 octane fuel. The compression numbers seem high. Maybe due to carbon loading of the combustion chamber? This can contribute to knock.

 

[too tall]

how many miles are on the motor? Is it a knock or a ping? Main bearings?

 

[DocB]

Engine only has 9k miles after professional rebuild. I do not know the details of what was done internally, but for sure new pistons, bored, crank ground; generally should be a good rebuild. It has good power and exhaust smells good, no smoke etc. Runs smooth and cool except for knock under load. I dont know the difference between a knock and a ping. I suspect the engine has higher compression due to head shaving, piston to chamber size, etc and (I hope) not to carbon loading or build up; I paid a lot for this engine. Maybe also the timing gear is worn and the Hashmark does not represent 7 degrees BTDC exactly on my motor? Lots of possibilities.

 

[Poser]

Main bearings?

How would a main knock?

 

[tonkota]

When I started reading this post, I was fearing the worst type of knock. You know, that deep down hard knock from a rod?

Fiddle with the timming till it "just" quits knocking. If you retard it too far, it will start to back fire some too. I would say that the timming needs to be a little different from stock since your engine isn't stock any longer.

Just glad to hear that it isn't a rod...

 

[DocB]

Tonkota: no, it does not seem to be a deep down hard knock, but rather a ping sound only under load; Im pretty sure its just a timing issue. Maybe like you said since my motor no longer has factory spec compression (higher after rebuild), my timing needs to be different or I will have to add octane booster. Maybe I'll have more power than most and I can say I have a "Hotrodded 2F" at the cruiser meetings.

 

[Poser]

You *could* have the dist. a tooth off, and have the truck way advanced already...

 

[Mark W]

"Pinging" or preignition is the precurser to detonation. It starts lightly (sounds sorta like bbs in a coffee can) and get worse (louder hollower sounding) under heavier load. "Knock" is usually looseness in the rotating assembly and usually doesn't change other than getting faster. And it generally doesn't come and go under load. Generally.
Preignition/detonation is caused by the fuel air mixture igniting on it's own from the heat of compression (added to the heat of the combustion chamber) before the spark ignites it, or from an explosive reaction as soon as the sprak triggers is instead of a controlled burn. Basically there is too much heat at the moment of ignition for the volitality of the fuel used. It of course gets worst when the engine is hotter and it is a self increasing situation because it also generates a lot more heat in the combustion chamber.

Your timing mark can not change or vary regardless of wear. Timing (cam) gears don't (and can't) wear enough to cause any sort of change in relationships.

175psi cranking pressure is good and healthy, but not overly high for a rig with bumped compression. Milling the head just enough to true it up during a rebuild will not boost it that much. But cutting the heck out of it purposely to increase compression can.

Eeven with raised compression you should be able to run the rig on the factory timing mark (7 DBTDC) or up to 13-14 DBTDC.

It is not really the static timing causing your problems, but the combination of this, the centrifigul advance and the vacuum advance which is providing more total advance that your engine wants to see.. Most (ALL?) US 2F engines in '77 had a vacuum retard dist. I have seen advance retard modules for '77s sold as aftermarket replacments. What year dist do you have. What carb are you running to get carb vacuum?

You may need to recurve your advance. A cooler spark plug can help too.

Hiogher octane fuel ignites a little less readily and it burns slower. Because of this it is less prone to self ignition and it is less likely to explode instead of burning. This means it is happier to wait for the spark plug to light it and to push on the piston instead of smacking it hard as it approaches TDC (BTW the rattling that you here under light preignition is the rings bouncing around in the grooves and hitting the edges of the grooves and the cylinder walls).


Mark...

 

[DocB]

Mark I'm not sure exactly what distributor I am running; I was told it is Vac Advance; it is Toyota and the vacuum port is on the passenger side and faces up. I think I have confirmed it is Vac Advance in the past with an advance timing light. My carb is a stock 77 rebuilt to "desmog' specs. Not sure what this means exactly. The vacuum hose to the distributor is coming off the manifold not the carb; this may give waaaay too much vacuum and therefore too much advance. This is why I plugged it first when chasing down my Ping; but even with no advance this engine is still pinging. Should the hose be going to the carb?
I have on the shelf a rebuilt Toyota carb from specter built to Non USA specs; also a new Non USA Vac Advance Distributor with the little octane selector thingy; also a new old stock 1F coil with internal ballast resistor and no igniter requirement; all on the shelf right now. I am thinking of putting on all this Non USA stuff and no igniter just to simplify things. What do you think?

 

[65swb45]

I'm gonna take a big step back from the timing issue to ask about your cooling system. Do you know the real world temp of your coolant? Do you have anything other than the factory temp guage? Can you get a radiator thermometer. They are available at most air conditioning supply houses. Harbor Freight sells them too. Less than $10 and accurate to within 1 degree.

Even bone stock motors will ping when they get too hot. Your 77 motor obviously is gonna generate more heat just because of it's compression ratio. You prolly also have a clutch driven fan, as opposed to the older cruisers that are direct drive. Mark W wrote an EXCELLENT article a few years back in the TT about how a less than optimum clutch fan can plague an otherwise good motor. He even had a PN for the fluid it takes to re-fill the clutch if you don't want to spend the big bucks on a new Toyota fan. IIRC, he recommended staying away from the aftermarkets. I have heard this elsewhere too.

You shouldn't have to retard your timing from it's present position.

I had a similar situation when I first rebuilt my F and ended up with 180psi. A factory Toyota 8 blade, direct-driven fan cured my problems!

 

[rgentry]

Previous post says that the vacuum advance was hooked to manifold
vacuum and not carb vacuum. In my experience (limited), this is a
no-no. Total advance should be a sum of:

1. Static starting point (7 degrees before top dead center or DBTDC)
2. Mechanical advance which is related (proportional?) to engine RPM
3. Vacuum advance which is related to the load on the engine and
the throttle setting

If you hook the vacuum up to the manifold, I think you are likely to
corrupt the input #3 above.

I am interested in what someone wrote earlier. Is it true that the
distributor could be in the wrong spot if he is able to get the BB in
the window? I figured that if he was one tooth ahead he could never
get it retarded (clockwise) enough to even see the BB.

 

[rgentry]

One more lesson from a bonehead (me). Once I lost the lockwasher that holds
the points onto the distributor plate. I went ahead and installed the points
and hooked everything back up. Before I fired up the engine, I noticed that
I could no longer vary the advance with the little knob on the side of the dissy.
Turns out that the screw on the distributor plate had traveled too far down into
the distributor and had locked the distributor at one timing setting. Wonder
what would have happened if I had cranked my engine?

 

[DocB]

Just remembered that in the fall I put in Bosch spark plugs for some reason. Did a search on this forum; some have found they "run hot." Hopefully this is my problem. My local Toy Dealer had stock ND plugs; I will get them and put them in tomorrow and hopefully solve my self induced problem.

 

[JasonH]

When you remove the EGR from the system, you can introduce pinging when you are using the stock distributor.
EGR introduces exhaust gas into the combustion process. This slows down the combustion process, therefore the stock timing at partial throttle is more advanced to compensate for it.
Mine used to ping after desmogging. I sent my distributor to Jim Chenoweth who rebuilt and recurved it for me. No more pinging.

 

[informationjunky]

Hey! Not to steal the thread, but nice to finally notice ya on this board! You have helped me out on the LCML several times!

 

[Mark W]

The vacuum advance has to be hooked up to ported carb vacuum, not manifold vacuum. The two work opposite to each other.

Removing the EGR system can make an engine more prone to preignition/detonation. But not always. I have done a few high compression desmogged hopped up engines and not had a problem with this. Others have had other experience. It may be because I always switch cams for longer duration ones which reduces cylinder filling efficeincy at low RPMs. Maybe.
Recurving the dist is often a good thing so that more initial advance can be run and less centifigul.

The heat range of the spark plug makes more difference than the brand. All brands make hotter and colder plugs.

 

[Steve C]

Here are a few of the underlying issues. In an engine, one of the main goals is to get as much energy out of the fuel as possible. A particular engine is limited in its efficiency by its compression ratio. Also, the spark is fired in advance of top dead center to allow the fuel to be burned early so that the gas is at an optimum pressure to get the most energy out of it as the piston goes down.


The unfortunate thing is that the air fuel mixture can start combusting before the flame front from the spark gets to it. This requires the use of lower compression and sub-optimal spark timing just to eliminate pinging. The lower compression and retarded spark result in lower efficiencies for the engine, but at least it dosn't blow up.

Octane rating is a measure of a fuels ability to resist combustion due to increased pressure/temperature. So, when the piston is going up, the air temperature and pressure increase. A higher octane fuel can resist burning at higher pressure/temperature.

here's the progression of a ping.

1. intake stroke.
2. compression stroke. piston going up. pressure and temperature going up. Somewhere before top dead center the spark plug fires, and fuel starts burning. Because of the burning of the ignited fuel causes an increase of heat and pressure, the unignited fuel experiences higher pressure/temperature. If the pressure/temperature of the unignited fuel gets to high, it can start a separate source of ignition wich somehow results in a ping.

Does too advanced timing cause "Knock"? What about too retarded timing; can this cause knock?
yes, because the spark is ignited earlier, the pressure in the intake stroke due to combustion (which adds to the pressure due to compression) occurs sooner resulting in increased peak pressure/temperature. If that gets too hot for the octane of the fuel, PING.

Question 6: With a higher compression engine, should the timing be advanced or retarded? Should higher octane fuel be used?
Higher compression requires less advance to optimze efficency because of faster speed of combustion, however it will generally need higher octane fuel or retarded spark timing to avoid pinging.

Question 7: Do you advance or retard the timing when driving at higher altitudes? (I live at 4000 feet).
because the lower density slows speed of combustion, more advance is needed for maximum efficiency.