blown engine

[minter]

hi there,

building a blown rover 3.5
interested in a set of big valve heads a guy is selling.
here are the measurements,

valves, inlet = 41.11mm exhaust = 35.45mm
ports, inlet = 25.50mm by 41.34mm exhaust = 25.50mm by 37.40mm
bulleted guides
30cc combustion chambers.

would these heads be to big? shrouding problems?

cheers guys.

[kiwicar]

Provided the valves fit down the bore I don't think they will be "too big" in that a blown engine will be less likley to heve flow reversal in the ports. however really you need to consider it as a whole package. A head optomised for a NA high compression engine with lots of cam duration has different requirments in port flow to one for a short duration and lower compression, this in turn differs to a head for e supercharged or turbocharged engine. Without gong into too much detail for a blown engine (particulally a turbo) you would bevelope exhaust flow by fitting bigger valves and flowing the port, even moving the valve location in the bore in preferance to inlet flow, Ideally ending up with inlet to exhaust flow ratio in the region of 80% exhaust to inlet, with a normally aspirated HC long duration head you would do the reverse ending up with a flow ratio about 65% to 70% exhaust to inlet flow. Now on a low boost moderatly tuned engine it probably doesn't matter so much as you can add exhaust duration on the cam (and maybe some lift with higher ratio rockers) but really you need to know where you are starting from by getting some flow figures for the heads. If you are going to buy heads without knowing how they flow, I don't think you should be worrying about shrouding by the bores .
Best regards
Mike