Originally Posted by Racer
I wouldnt buy OC heads full stop. They are prone to detonation which is a worry for a lot of Clevo owners.
Providing its ok, the toploader sounds like a deal. I paid $750 alone just to have mine reconditioned. Any sort of shifter at all is worth $200.
The OC heads are OK when used with high compression pistons and high quality fuel. Otherwise, a flat top at low compression is about the only way to use an OC head. The key to making them work is highly polishing the chambers to a mirror finish and smoothing out the chamber so that there are only slippery smooth transitions. Also, polish and smooth the tops of the pistons. Absolutely no sharp edges or transitions.
You also need to watch the cam choice for an OC engine. Try to avoid a longer lobe separation angle cam. The F238 and F246 are good choices. You want 110* or narrower if possible.
Also, you will want to push your A/F ratio up to about 12.7:1. This will help decrease detonation at the expense of some power, unless you're pouring 96 octane fuel. With PULP, you will want to have about 31* of total timing, 13* initial with it all in by 2800 RPM. Another useful benefit is to use small diameter extractors with the 4V OC heads. Try the 1-3/4" primaries into 2-1/2" collectors through 2-1/4" tail pipes with an X-over tubes set up.
Over carb the engine slightly. Use at least a 750 DP, but an 800 is a better choice, with a good quality open plenum like a Torker with a 4-hole spacer. A 1-1/2" spacer seems to work the best in my experiences. Do not use a VS carb in a OC head combination. You want all the extra fuel the DP provides.
Prevent (at all costs) hot underhood air from entering the inlet. Directly duct cooler outside air directly into the air filter. Absolutely do not use an open filament air cleaner assembly.
Use heat shielding on your fuel lines.
Focus on porting the exhaust bowls/ports so that you get a really good transition from the chamber through the valve. What you're looking for is the best possible exit strategy. The quicker the exhaust gases get out through the port, the less heat you will see in the chamber. That's why you want to use small diameter extractors, to keep exhaust gas velocities high. The X-over will help to balance the sides so that each side works to help pull the other side.
You can build a very serious OC 4V headed Clevo. You have to really think about keeping the engine running cool by reducing underhood temps through the selection of smart materials. Avoid polished and chromed valve covers, inlet, water pump, etc. Get your heat dealt with and you won't have detonation problems. Use a standard bore block taken to a minimum overbore and certainly not greater than .030". Use as much (unpolished) aluminum as possible to take heat away. Use ceramic coated extractors.
If you've got OC heads, you can use them and make them work. The one thing that is true about them is that they are not even close to being as good as a set of AFD 2Vs or similar alloy heads.