This past Summer I put the AFR 195's on the stroker and life's been great ever since.

Had her dyno tuned after I finished it, and the new dist curve works great; car runs like a scalded dog.

Now, months later, I put the AC compressor back on & charged the system. Yesterday was a hot 85* autumn day in San Diego and I drove it for the first time, with the AC on and the new AFR heads. As expected, the engine temp went to 200*F from the normal 185-190* range it always runs in (thanks to dual spal fans).

But doggone it now, I need to get her back on the dyno and recurved! At 200*F and part throttle, it pings worse than it did with cast iron heads. Before the AC was put on, I drove it in much hotter weather, and never heard the ping. But the engine temp even in 100* weather, was never more than 190* since the aluminum heads went on it.

Now, just a tad more engine temp and I have a huge ping prob.

 

Is the ping under load or just constant cruise speed down the freeway?

What is the part number of your spark plugs?

 

It's when driving around town and your'e giving it part throttle to speed up. Or going up hills at low freeway speed like 55-60

Plugs are Accel Shorties I think 576S is what we used every time I've put it on the dyno.

 

hot underhood, or cool air to the carb?

 

Hot

 

What is your total mechanical timing set at?
Do you have Vacuum Advance connected?

Make sure you have no more then 36deg total mechanical advance. You want that all in around 3000 rpms

If that is set, try disconnecting the vac advance and see if the problem goes away.

You don't want to reduce the timing too much, as that will cause cylinder temperatures to rise, which will not help your problem.

If you're running Vac my guess is that's the problem - if you look around, I think Lars published a vac advance can spec and you should get a vac advance can that produces less advance.

 

Remember this car was tuned on the dyno before the ac was redone.

I have 38* total, and it's all in by 2500RPM. Vacuum advance is connected and the can is adjustable. Ignition is a Mallory Billet distributor with 8.5mm wires and Accel Shorty plugs.

 

Back the timing down 2 degrees. The AFR heads don't need 38 degrees timing. They are a fast burn modern chamber design. Actually 34 degrees should be perfect. Leave all else alone and see if that cures your problem. If not go with one of the next heavier spring to get your all in timing back up to 3000 RPM or a little more. This will reduce the timing at cruise RPM a little. The AC unit adds additional load to the engine probably causing the ping. The 10 degrees extra temp is also a result of the additional load of the compressor.

 

That's the problem. When you tune on a dynojet you will always end up over advanced and lean. It's because the dyno has much less inertia than accelerating the car. That's unless you used a dyno that can actually put a real load on the engine like the Mustang dyno.

 

Exactly. Dyno tune is really a ’perfect condition’ situation and it's optimizing for WOT to achieve best power. It's not subjected to the variety of load/rpm/heat conditions as an engine in a car. I had the same thing happen on my stroker build - best power was 38Deg total.

Interestingly, my engine builder warned me if I ran the engine as-is from the dyno I would be over-advanced and RICH (not lean), because you're jetting for max airflow and no mufflers or exhaust restrictions. He recommended I lean the car out slightly and back the timing down to 36 and he ended up on the money.

You need to back down to 36 or 34 as 63 Mako suggests. 38 in a street car for sure will be a problem.

And, Dyno tune or no, the specifications of your Vacuum advance need to be dialed in because, I’m pretty sure, the car was not dynoed with Vac advance connected. And if it was, it was inoperable because you're running WOT all the time.

You could very well have the incorrect total advance (too much), or coming on too fast with the can. I don’t think you’d want to add more than 12 deg of advance off the vacuum canister, and you want it to come on as late as possible, just to give you good cruise mileage. If not, just disconnect for now because bad mileage is better the ping/detonation.

 

Thanks guys for the feedback. Yes, it is a DynoJet (I used to be service manager at this facility)

RE tuning - you're thinking of HP runs on the dyno. Yes, that certainly is going to be at WOT.

Dyno Tuning however is completely different - AF readings are taken at cruise, idle, and WOT. The tuning is done at cruise.

Whether or not that 'cruise mode' on the DynoJet is similar to actual cruise is a good question. But the car was certainly tuned for street driving, not only WOT. In fact the only time we did full throttle testing was to do before & after horsepower runs on the Dyno. It's putting a bit over 300 HP at the wheels in it's current setup.

This engine was set up when I built it, on the same dyno. After getting rid of the iron heads and installing AFR 195's the carb needed to be redone of course, but I dont' think we changed the dist curve on the dyno after the top end was done. I'd have to check my invoices.

It's cool outside today (15* cooler than yesterday) so it may not be a great day for backing off the timing and trying to make it ping. But one thing's for sure though: those T-Tops will be coming back off and the AC switch will be turned off since it's 72 degrees and partly cloudy this afternoon! Can't wait to get off work.

 

You have several items. 32-34 max advance on AFR fast burn heads.

I looked into the wrong heat range of plugs agle also. the 576 are the hottest heat range and the wrong thread and taper type

http://www.jegs.com/vs/ACCEL+PLUGS/111

Right from AFR web sight

(14mm x 3/4" gasket) no tapered seat style. For race applications Champion C59YC Autolite 3910 or 51, A/C R41 CXLS & NGK R5672A-9 is a good starting point. For street applications use AC FR3LS, AC 41629, or Champion RC9YC spark plugs or equivalent. Plug selection is of course dictated by many factors including rpm level, compression ratio and type of fuel. All the above plugs are a starting heat range, blowers or nitrous application usually 1 to 2 heat ranges colder. Spark plug gap should be determined by the ignition manufacturer.

 

Thanks for the correction! I was correct - 576S Accel were what I ~WAS USING!~ for the past several years on cast heads. But the Autolites are what I am using now - from the recommendation of AFR themselves given my engine application.

I am running best fuel I can here in CA which is Chevron 91. I will definitely bring the timing down though, and see if I can get rid of the hot ping without recurving/re-dynoing the engine.

It's certainly worth a try! And by the way thanks again for your help & direction on this.

 

this is a good artcle that will help you understand heat ranges. I have found AFR heads to recommend exactly what they need. Only N2O or a blower need plug type changed.

http://www.carcraft.com/howto/116_07...eat_range.html