Battling Detonation After Piston Upgrade

[Shadoobie]

I moved this post from Engine when i realized the question is more suited to this category. Hello everyone, I'm about 3500 miles into the new engine and I'm having a battle with my most feared enemy; detonation I've been pulling the timing back, but now I'd really like to try and deal with it once and for all. As a reminder, my build is as follows:
-1979 2L block, 1mm overbore
-10.9/1 Persan domed pistons
-rebuilt head to stock spec, lightly milled
-early style narrow lobe cams
-stock electronic ignition, new stock icm with standard plugs and wires.
-32/36 DFEV with factory jets
-stock automatic (boo, I know)

As it sits I can run 8 degrees timing with premium (93) gas and an octane booster with no problems, but I'd like to try and eliminate my worries without having to pull the head and install an extra thick head gasket. I'm working on a performance head to drop on in the future for this purpose, but I'm not quite ready yet. My first thoughts are to install a high performance coil and wires with colder plugs. Seeing as I'm just barely on the edge, I'm hoping this should put me in the safe zone at 10 degrees. Any thoughts?

[BEEK]

you should be looking for 34-36 degrees of total advance. Not really worrying about where it is at idle. Putting a thicker head gasket on the engine will kill the performance. The squish area, the area where the flat part of the piston and the flat part of the combustion chamber, this is a very critical area for power. installing a thicker head gasket defeats this area. Is the intake air going to the engine cool? or is it underhood temperature? that will help detonation.. Also what is your air / fuel ratio running, If you are lean that will also cause issues.

[baltobernie]

Between the domed pistons and head milling, that's a lot of compression for stock cams. Higher-performance pistons are usually paired with higher-performance (more overlap) cams. DO you have any cylinder pressure numbers?

[blazingspider]

Definitely try a set of plugs one heat range colder. That's the cheapest and easiest potential solution. As others have stated, getting colder air to the intake and installing performance cams with more overlap will also help. Using a distributor that allows you to control the amount of advance and when it begins in the rpm range is another option for you. Good luck!

[AndyVAS]

I see "stock jets" in the carburetor. It is rare for the factory jetting to be correct. Have you checked your air / fuel ratio? If adjusting the carburetor is a bit much for you then find a local British car place or somewhere that does old VW. Those guys tend to have a stock of jets and such on hand and can quickly dial in your carburetor.

[Shadoobie]

you should be looking for 34-36 degrees of total advance. Not really worrying about where it is at idle. Putting a thicker head gasket on the engine will kill the performance. The squish area, the area where the flat part of the piston and the flat part of the combustion chamber, this is a very critical area for power. installing a thicker head gasket defeats this area. Is the intake air going to the engine cool? or is it underhood temperature? that will help detonation.. Also what is your air / fuel ratio running, If you are lean that will also cause issues.

I am currently running Midwest-Bayless's "lunchbox" air cleaner as the 32/36 won't accept the stock one without extensive modification. I have, however, run a snorkel to from the grill to the box. That has improved things a bit, and the car runs a couple degrees cooler on the road as a result. I unfortunately don't have an AFR gauge, but I'm strongly suspecting a lean condition. The plugs are a light brown; not quite white, but lighter than I would like. I unfortunately don't have the cash to drop on a $250 gauge at the moment though, so I may just have to buy a kit and size up as needed.

[Shadoobie]

Between the domed pistons and head milling, that's a lot of compression for stock cams. Higher-performance pistons are usually paired with higher-performance (more overlap) cams. DO you have any cylinder pressure numbers?

I agree, in an ideal situation I would have done cams first. Unfortunately I was forced to rebuild the engine recently and I decided I might as well upgrade while I was there. However, the cams I do have are from a 1608 engine which are 22/66 as compared to the 5/33 2L cams, so they're a little bit better than just dead stock. I'm thinking about having my stock 2L cams reground to replace these, so I'm exploring my options there. I ran a compression test a couple weeks ago and got a consistent 210psi in all cylinders, if that's what you mean.

[Shadoobie]

Definitely try a set of plugs one heat range colder. That's the cheapest and easiest potential solution. As others have stated, getting colder air to the intake and installing performance cams with more overlap will also help. Using a distributor that allows you to control the amount of advance and when it begins in the rpm range is another option for you. Good luck!

I ordered Autoricambi's NGK Iridium IX plugs yesterday with that same thought, and I'll let you know how they work for me when they get here. A selectable curve distributor is definitely something I'm looking at in the long term, but as I'm about to transfer to a University in the fall that's not really an option right now.

[Shadoobie]

I see "stock jets" in the carburetor. It is rare for the factory jetting to be correct. Have you checked your air / fuel ratio? If adjusting the carburetor is a bit much for you then find a local British car place or somewhere that does old VW. Those guys tend to have a stock of jets and such on hand and can quickly dial in your carburetor.

I was actually running one size below on the secondary for my stock build, but I sized up to base a couple days ago and it seems to have helped a bit. As I told Beek, I don't really have the funds to dish out for an AFR meter, but I've called every mechanic and tuner in town and I've found nobody.. I guess that's the price of living in the middle of nowhere, Alabama. I called a classic Euro motor shop in Birmingham a few minutes ago and I'm hoping they'll be able to do a proper tune for less than the cost of a jet kit and a meter.

[baltobernie]

I ran a compression test a couple weeks ago and got a consistent 210psi in all cylinders, if that's what you mean.

That's quite a bit IMO. You may have to run a little rich or a little less advance, in addition to colder plugs.

[TX82FIAT]

while I agree that a thicker gasket robs performance. What is your compression ratio? 210 PSI seems over the top to me. Maybe a thicker gasket is needed if block was milled at some point or domes are a little too much? Seems like a lot of PSI to me..

[Shadoobie]

Hey guys, just a little update. I've installed a set of colder plugs and things have gotten much better. The only time I'm experiencing any detonation is a high gear flat out pull or under heavy (85%+) acceleration. However, I'm noticing even more so a pretty big flat spot when I get into the secondary. My plugs aren't showing me running especially lean for the most part, but I have a strong feeling that I'm running lean on my secondary which is contributing in major part to my problem. By this Saturday I should hopefully have a performance coil and wires and can go about rejetting the carb. I'm pretty hopeful! I'll post back once I get everything installed.