[tooski]

The info I have read on the forums indicate that no way a cam with the specs of the HC-02 should be causing detonation at 9.77. Unless it was poorly installed. Which it was. By me. At the time (mid 80's) I had no idea about cam degreeing. Turns out it ended up 4° retarded. I am thinking that would significantly affect the DCR leading to detonation.
I can see the engine not being happy with the smaller cam even if it was installed correctly. Hence the head change to lower the compression. I think 8.1 to 8.4 is a bit low.
As Jay posted I believe I will need to go to a custom piston (Autotec) because of compression distance. Why put those nice new forged pistons on old cast rods? So, new full floating forged rods would also be in order.

["QUICK-SILVER"]

Originally Posted by tooski

The info I have read on the forums indicate that no way a cam with the specs of the HC-02 should be causing detonation at 9.77. Unless it was poorly installed. Which it was. By me. At the time (mid 80's) I had no idea about cam degreeing. Turns out it ended up 4° retarded. I am thinking that would significantly affect the DCR leading to detonation.

Backed up 4° would have less effective stroke. Advancing is what builds higher cylinder pressure.

Need to get it opened up and see what's going on.

I suspect the detonation problem you had was from the little ridges on them 8VR's heating up and acting like a glow plug. Higher cylinder pressure means higher cylinder temperature. And your 9.77 SCR combo was just right for heating them 8VR's up.

Clay

[Jay S]

I would also agree with you that the 9.77 should have been fine with the HC-02. But there are a couple wild cards there. The 8VR replacement pistons are really hard to predict. Sometimes they have a terrible quench and can be .040 down in the deck. Then I think either the crown of the piston runs hot between the inner and outer valve reliefs and/or the extra contours create hot spots and end gas issues. Some engines will do ok with those pistons with good compression, most seem to be with OEM pistons that had a decent quench and a OEM cam. The replacement ones with the shorter height seem to have all kinds of problems unless the deck is cut like your engine.

Another thing that might have worked against it was the cam being retarded that much. Whenever I degree a cam in I assume it is running 2 degrees retarded from were it is set at. So 4 retarded is getting closer to 6 dynamically. That is not a big deal on a little cam with a small amount of overlap. But I think the bigger cams start heating up the air charge significantly when they are ran retarded. IMO, that is one of the reasons that bigger cams loose performance faster when timing chains stretch. But your timing chain could have easily stretched those 4 degrees to if you drove it much, it could have started out dead on. I would still tend to believe it was more to do with the 8vr pistons.

I am all for more compression, a good piston, rods and the summit 2801 or 2802. I have never ran the HC-02, but always though it looked like a great performance street cam, unusual to see issues with it. Jay

[Formulajones]

There are 2 ways of thinking when it comes to cam install.

Most think that retarding the camshaft is a good idea to ward off detonation, and advancing increases detonation.

It's actually more involved than that, and I think Jay eluded to it. Advancing the cam can in fact help to fight off detonation by allowing some of the hot gasses to escape the combustion process. I've been experimenting with this theory thanks to Paul on the last few cam installs I've done and it works exceptionally well. The key is to make sure the intake valve is open about .030" higher than the exhaust on the overlap stroke at TDC. That's when you know you have enough advance. The Voodoo cams especially respond to this install position. Keeping in mind it's more difficult to accomplish this with cams that have a large amount of LSA.