Originally Posted by bobl

These heads have the heart shaped combustion chambers. I've built and dynod several engines very similar to this. HP is around 450 at 5500 rpm(but I don't run quite as large a cam) They will detonate with very much timing. Because of the chamber design, combustion occurs much faster, thus requiring less lead time for ignition. This information has been documented many times. Pull the timing back and give it a try. BTW I own Full Throttle Marine and build marine engines for a living(so you know my background).

Originally Posted by Ghostrider

I definately have the Vortec heads (10141279...96-up...oval..CLOSED.."Vortec 7400", 100cc chambers) Technically speaking, now 105cc.

I had them openened slightly to allow better flow and I had the oval ports matched to the Felpro gaskets. I then bought the RPM Air-Gap manifold (PN 7561 - Oval Ports) and had it CNC Flowed and port matched as well. .I had the heads resurfaced / shaved 0.020" to 55Ra which will also help with the comp ratio. The combination of head chamber size and my pistons allow for about 10.5:1 Comp. They run the standard valve size (2.065" I & 1.72" E) with the large oval ports and a better exhaust port profile (which Merlin and Dart use on their heads now too.) Reason you don't see many marine engines with them (IMO) us that too many people think that all oval ports are too small and they 'must' run Rect ports to make any power in a boat! Truth is, the Vortec heads flow better than all the other ovals and the exhaust port profile makes them a far more efficient head! Only downfall is I'd like a set of 2.19" intake and 1.88" exhaust Iconel valves in there to open them up!

I'm interested to learn more about the 'fast burn heads' thoughts though from Bobl. I hadn't heard that before and it is well worth considering. So if I set the timing to 8 deg initial withthe 20 deg curve (for 28 total) and give that a go it should be safer? Or will it possibly make more power?

Cheers all. Good discussion!

Originally Posted by bobl

These heads have the heart shaped combustion chambers. I've built and dynod several engines very similar to this. HP is around 450 at 5500 rpm(but I don't run quite as large a cam) They will detonate with very much timing. Because of the chamber design, combustion occurs much faster, thus requiring less lead time for ignition. This information has been documented many times. Pull the timing back and give it a try. BTW I own Full Throttle Marine and build marine engines for a living(so you know my background).

Originally Posted by ezstriper

yes, the intake ports on the 781' 049 are much larger than heads you are running...I have a pair of 781's we ran on a mild race engine...

Originally Posted by Ghostrider

Thanks for that mate, that clears up that issue I'd say.
This engine was bascially put together to run about 500hp using the bits I had (block, crank and heads.) I thought the heads seemed like a good idea and figured I'd build the remainder of the combo to best suit them by maximising combustion through comp ratio and smoothing out the ports on the intake and manifold. I think the valves are a limiter to anything much more. I did as was advised with the bowls and chamferred the edges and the tip of the heart to limit edge glow and hot spots, I went with a finer Ra on the surface to help seal the increased comp. I have a Crane 741 cam I was planning on testing but I think now that these heads won't allow it to perform well enough!

Time to look for decent set of Big Valve heads? I'd love to find a set of big oval ports so with big valves already in them so I can keep my current intake combo! Are the 088s the best?

Thanks again all. Funny how a Carb discussion invariably leads back to the root cause of most issues: Timing and Flow!