fjbm wrote:I'll fit new valve springs which are intended for high lift cams. The only reason I'm thinking about normal lift cams is that I plan to use the std pistons I already have (with the small valve cuts).
I think that I need to change to the hepolite pistons with the large valve cuts to fit a Q420, right?
I'm also going to port the head to stage II/III.
What should I have in attention if I'm going to fit a high lift cam?
Cheers
Sounds like your aiming to build a 150 style hp engine based on a standard cast iron crank and rods with upgraded head. The Dave Bean catalogue gives a good description of this sort of development
Bottom end needs large cut outs in pistons as you describe. ARP rod bolts worthwhile if using above 6500 rpm max on occassion. But put in a good electronic cut out set at less than 7000rpm as the standard bottom end goes bang at around 7300 rpm and the engine if built well will be more than happy to rev to 8000 rpm. The rest can be left as standard but carefully balance everthing. Careful measurements also needed to confirm you get the 10.5 to 1 comp ratio you need.
Ported head and larger valves needed to get the true benefit of a higher lift cam. A cam like the QED 420 is a good one for this application and delivers the 150 hp target which is about the max you can get out of road compression 1600 twin cam at 7000 rpm. In the for sale section is a dyno curve of an engine in a plus 2 with this cam that produces this figure. Off set dowels or vernier adjustable sprockets requied to ensure you can time the cam correctly.
Setting up the valves for a high lift cam takes some attention to component detail to get it right. The high lift cams typically come with a 1.1 inch base circle versus 1.2 on the original Lotus cams. This keeps the nose of the cam small enough in radius so its still fits on the bucket and also provides more room for high lift springs when used with longer valve stems and thin steel tappet followers. You need to work through the detail of your head and cam measurements to ensure you get a correct set of components that work together. Some one like QED should be able to help you with component selection but you need to do some critical head measurments to make sure you get them right as most heads have been worked on lots by now and often a long way from standard in dimensions.
The high lift cams if cast iron should have a special long bolt fitted to ensure they dont break under the increased drive loads they require.
You also need to go to around 34 mm chokes in the carbs and rejet them to suit and a big bore exhaust header and tail pipe to match the breathing capacity of the head and cam. You also need an ignition advance curve to suit the engine. The UK non emissions sprint curve is pretty good
The link that Gary posted in the second post on this topic has some more detail on the head set up and measurment requirements with high lift cams that I previously posted.
cheers
Rohan