Knock

[C.Costine]

i am having a little trouble getting comfortable with knock. This is a nearly stock '67 1600 with a little head milling, a little decking, .050 overbore and domed pistons. I have been running 93 pump gas, and have taken three degrees off the timing, but it is hard to avoid just a little knock now and then. I want to install a sensor kit so that with all the sounds of driving with the top down I can get dialed in to the amount of octane booster that I need to add. Has anyone installed a kit that has worked well for them? I am not looking to get into any elaborate electronic controls just a sensor and indicator lights.

[keith0alan]

When you milled the head did you check combustion chamber volumes and the squish area? With matched combustion chambers and a good squish area you can run massive spark advance. Depending on how the head was milled you may have one or two tight chambers.

[C.Costine]

Keith, are you saying that one or two chambers may be significantly smaller than the others? And if this were the case I don't see the significance. Only one piston needs to get a hole knocked in it.

[redroadster]

Knock on acceleration? or constant
You cannot hear all detonation and all is bad
Retard timing if the compression is in range , to see if it looses the knock
Really Carboned up valves increase it
Take a cold compression test
The problem with the highest octane gas is it's high price few buy it ...and it can be a yr old
a QT hear in 02 Bought old gas cheap then put it in the 91 Oct tank same price as reg , our Kia dealer filled up , many cars running bad , techs putting on parts I smelled the gas knew what it was .

[Gregs672000]

As an owner who has run an engine with 11.7 to 1 compression on the street, there are many things that can cause knock or ping (they are different).
As Red said, do a compression test to see how many lbs of pressure it has. Before dropping compression down to 10.3 or so, mine was doing about 220lbs with some leaking valves; now 205 with sealing valves, so that will give you some range idea.
If you have a dial back timing light, see what total advance you're running to make sure it's ok (36-38 stock). Make sure the mechanical and vacuum advance etc are all working normally (check advance with vac connected and disconnected for mechanical). Vacuum advance is designed to improve economy and driveability under low load, but drops timing under load... if anything is sticky it's changing your timing. Consider an ignition upgrade, like an MSD box that may offer some timing retard or adjustment, or give yourself more ability to target specific areas in the timing map by using a 123 programmable dizzy, or convert to a crank fired ignition like I did (involved process). Ignition timing makes a lot of difference in power output, much more than air/fuel ratios, so pulling out an excessive amount under ALL conditions will ultimately reduce performance... address any air/fuel and octane issues first IMHO if compression isn't stupid high like mine was for the street. Is the engine running on the hotter side? If so, try to cool it better, maybe with a 160 stat; route cool air to the carbs to reduce intake temps; use a coated header and/or add a heat shielding material to the header to keep heat away from the carbs (Heat Shield products, more expensive than typical wrap but won't hurt the header when wrapped correctly). What kind of air/fuel ratios are you seeing (if you have a wideband meter; if not consider getting or borrowing one, or READ YOUR PLUGS)? A richer mix will help a lot but that may be a little more guess work with an SU, easier with a Weber style side draft to target. If you know you're going lean at specific rpms (like midrange for examle), someone like Keith may be able to recommend an SU fuel needle change. A lean condition under load will knock and puts your engine at risk even with octane booster. Finally, most octane boosters are garbage or stupid expensive... Boostaine however is the real deal and is surprisingly economical and easy to use to boost octane several NUMBERS, not "points", and a single can will last several tank fills when mixed appropriately. Available on Amazon.
Hope this helps, for clarification ask away! The easiest answer at this point if everything is operating normally is BOOSTAINE!

[Gregs672000]

BTW, I researched using a knock sensor, but I came to the conclusion that we don't know the frequencies we would be targeting as knock unlike engineers who spent a lot of time figuring that out for the engine design they were working on, so not so easy.

[redroadster]

Ya sure it's not rod bearing knock ?

[keith0alan]

Keith, are you saying that one or two chambers may be significantly smaller than the others? And if this were the case I don't see the significance. Only one piston needs to get a hole knocked in it.

Normally these heads warp down on the ends. If you mill it flat, chamber one and four end up small or they get progressively smaller from one end to the other. The small chambers will take less ignition advance. If you match the combustion chamber volumes then you are not limited by the smallest chamber. You are correct, only one chamber needs to ping to be a problem. Having one ping while the others want more advance is just counterproductive. The squish band area is very effective if done properly. Once again this changes when the head is milled. On the race car motor I matched the volumes and carefully matched the squish bands to the pistons. We run and obscene amount of advance on pump gas with no pinging.

[todd lorber]

As stated above, knock is more likely when heat builds up. Do you have a shroud around your fan? If not, this is a simple way to enhance your cooling system by making sure the air flows THROUGH the radiator instead of around it. We're just about to hit the hot season here, and without the shroud my car would do some light pinging, and dieseling at shut off.

[C.Costine]

Yes, it has the shroud. It comes up to 198 degrees and stays pretty much right there. I have checked compression but I don't recall what it was. It was, IIRC, a little high as expected. I will also check the vacuum advance function again. I can remeber checking it but I don't recall the results. Thanks.