Alrighty, so out here in AZ 91 octane is the highest available at the pump and have noticed that my car detonates quite a bit at WOT, wasn't so much an issue when I lived in TX where 93 is everywhere. Aside from backing off on the timing, any other tips you suggest I try in order to maintain as much performance and avoid detonation? 11.0:1 CR apparently doesn't like 91 octane

 

Unless you can locate a pump with 93 octane fuel...or add some anti-knock (octane booster) to your fuel, backing the timing off a bit is all you can do. If you want to address it more permanently, you could put some thicker head gaskets on it. Or, you could put some aluminum heads on it and do away with the detonation issue (maintain your C.R. and still use 91 octane fuel).

 

Keeping the engine as cool as possible will help. So coolant temps down as much as possible with a good cooling system.

Cold air intake system instead of under hood intake air on an open filter. Keep intake temps as low as possible.
Blocking the crossover heat is one way to drop this down but requires removing the intake manifold.

richen up the mixture.

Run a colder range spark plug.

Putting on a thicker head gasket could actually make things worse or no better and you just lost CR. Getting a good squish distance of .040" is very important for combustion turbulence which reduces detonation.
So a thinner head gasket may be the direction you would want to go. Piston top shape will make a big difference. If it's a dish piston with 4 valve cut outs that is about the worst case for detonation prevention since you cannot get a good squish distance for decent quench or turbulance.
Quench transfers heat from the combustion chamber to the cooling system via the head.

Make sure your plugs do not have any sharp edges on them. Any sharp protrusion is a good starting place for undesired ignition.

If you're burning oil, eliminate it. Oil contamination to the combustion promotes detonation.

That should get you started.

 

If your L46 is factory stock, it should run just fine on 91.

 

those octaine boosters don't work. best thing you can do is back off timing a little.

 

Same problem here. Stock 70 350 with 11:1. Distributor curve and vacuum advance appear to be dead on stock. Octane boosters have very little effect. I have made the mixture more rich and dropped the timing back 4 degrees. Still getting some above about 2200rpm, fine at slower speeds in all gears. I have the correct head gaskets, and I was careful to radius all sharp edges in the combustion chamber and piston domes. Car runs perfectly except for this. Very troublesome.

 

Perhaps a vacuum advance can that provides less advance? I remember reading something about this some time ago. Initial advance and centrifugal advance were OK, but part throttle and midrange were giving some spark knock. Changing the advance can fixed it.

 

Vacuum advance can affects timing when at idle and cruising. At WOT, it does nothing. OP's trouble is pinging during hard acceleration (little to no vacuum in that mode).

 

What is stock initial timing on that engine? If it is 10* or less I would try advancing the initial timing, but modifying the curve so it does not reach total timing until the same point it does now, and not to exceed 34* to 36* or maybe even 32* total.

Are you getting your vac advance from a manifold source?

Port vacuum and low initial timing advance numbers make the engine idle hotter making the intake charge and everything else hotter, promoting detonation.

 

snow cooler water / methanol injection. is a possibility.

 

Like you I have battled with timing, idle and engine knock with my L46. I did follow Lars papers on vacuum can selection and setting the timing. I followed his directions and it made a huge difference to all my issues. Lars instruction I set the distributor spring set to give "all in" mechanical advance of 36 degrees @ 2500 rpm. I still had engine knock in first and second gear so per Lars paper I backed off the timing by 2 degrees. This was better but still some knocking so I backed off another 2 degrees. I was uncertain about continuing this procedure so I stopped there. It seemed like the knocking occurred between 1500 and 2000 rpm so I speculated that the distributor centrifugal advance was coming in too soon when combined with the vacuum advance. So to bring in the centrifugal advance further up the rpm curve I went to stiffer springs. I'm almost there so I'm going to run it for a while and see. I don't know it this is correct or not but it has significantly helped. If you have not read Lars papers ... it is good information. I did also change my plugs from R45 to the cooler running R43. I like the idea of blocking the exhaust crossover that was previously mentioned. In desperation I did try octane booster ... jury is still out on that.

 

Note that the total timing for the high compression L46 and LT-1 was lower than the other small blocks. Stock timing was 26° @ 5000 RPM. You can come in sooner than 5000 RPM, but I think you would be wise to back off the timing and come closer to the stock value than 36°.

 

I had a YZ250 2 stroke bike that had a real problem with detonation. I tried everything I could think of to get it to stop. Running the highest octane I could get at the pump, colder plugs, enrichened the mixture until it was drooling gas, nothing would stop the detonation.
So I took the top end apart and had the head shaved down to reduce the squish. Shaved it down to .035".
BAM! success, the detonation was gone. I was even able to reduce the octane back to 91.
I know this doesn't help you guys, but it does illustrate the importance of combustion turbulance and it's significance to proper burning of the fuel.

If I couldn't get detonation to stop and I didn't want to pull the engine I might also try installing an intake with an oil splash baffle with the crossover blocked.
The hot oil in the lifter valley can add a lot of heat to the intake manifold which then adds it to you intake charge.
If it would fit I would go with the air gap intake. Not all of our cars will fit the air gap intake. It seems to be on a case by case basis as to whether it will fit.

I've also wondered if your detonation is occuring in the area of greatest cam efficiency. In other words the period in which the engine is taking in the most air per intake event. What if you were to restrict the intake air. This would make the intake event less efficient and reduce the cylinder pressure and may stop the detonation since cylinder pressure cannot get high enough to generate it.
It might have to be done with a plate under the carb with smaller holes so that the draw on the carb is less, I'm not sure. It would of course reduce power output of the engine, but you're already doing that with timing and enrichening the fuel, so it may be worth a shot. Then you may be able to advance the timing back and set the mixture properly.
Just a thought. I've never tried it myself.

 

Before shaving cylinder heads and replacing components, I would strongly consider revisiting your timing curve. Engineers designed these systems that worked fine when new, what has changed?

I recommend setting the timing back to factory specifications and check for detonation. If no detonation, then you can try and advance your timing for more performance if you wish.

 

Thanks for the info, I'll be digging into the timing again this weekend and see where it's at now and follow the steps above.

 

BINGO !!

snow performance...use factory washer fluid container..inject in the blue off the shelf washer (approx 40% methanol)...

works like a charm

 

Do the obvious and logical before yet another $50K frame off.

 

what temperature is it running? a cooler thermostat may help a lot.

 

I agree with LeMans Pete. I am not sure I have the distributor curve where it should be yet. My guess is, the mechanical advance is too quick or too much total and that is the problem. I will revisit this soon, as I have a couple of longer trips planned.

 

I also have L46, here is what I did:
165 F thermostat.
8 degrees BTDC 650RPM, 18 black stopper ring, no vacuum, total 26BTDC (MSD 8571)
Cold spark plugs.
Shell classic fuel (EU 99 octane/US 93 or 94 octane)