Turbocharged cylinder head question?

[James P Grosso]

Eric, I was bench racing with a buddy about cylinder heads for a turbocharged engine build. He was looking to buy some inexpensive aluminum heads, and he wanted to know which flowed the best. I tried to explain that the best “flowing” (and cheapest) head may not be the correct choice for a turbocharged engine because it most likely has narrow valve seat margins to increase airflow, and being a cheap head, likely has iron valve seats. I argued that both the valve job and valve seats (and maybe even the valves too) might be fine for a normally aspirated engine, but will not transfer heat away from the valves very well, and that it will likely limit boost pressure due to detonation from the hot cylinder chamber/exhaust valves. I recommended he get the heads bare and have better valve seats installed with a good seat margin to transfer heat from the valves better, and that any small loss in cylinder head flow could be overcome by allowing for higher boost pressure because of the better thermal control. I’m not saying he needs Copper-Beryllium seats, but at least some powered metal seats or Dura-Bond “Killer Bee” PM seats?
Also, with the new valve seats, he can have the machinist make sure the press-fit is correct. Who knows how tight the press fit is (valve seats) on the cheap off-brand heads?

Now that you have a turbo car, what are your thoughts?

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[dan]

I am not eric, but i know a few things about forced induction.

Under normal aspiration you have your natural 14.7 PSI of atmospheric pressure at sea level trying to get into your engine, if you are trying to increase power under natural asperation quite commonly people try to increase static compression and reduce the size of the combustion chamber volume by going with smaller combustion chambers or domed pistons or both, while at the same time promoting good air flow with large valves, free flowing ported and polished intake and exhaust runners, as well as changing the valve timing to promote better air flow at higher RPMs to increase power.

With forced induction however things are as you stated different, the air going into the engine is exceeding or greatly exceeding the 14.7PSI at sea level of atmospheric pressure, and you have a new set of challenges to meet, with forced induction quite commonly to keep the octane requirements lower people reduce compression and compensate by adding more boost, and with forced induction because you are cramming more air and fuel in rather than just compressing it tighter you are dealing with a lot more heat! especually if you are running alcohol or E.85 so you can play with more boost.

Good valve seats and valves as you stated are a must! because the exhaust valves are going to be opening and letting in much higher temperature gasses out during the exhaust stroke, as a result if the valves and valve seats aren’t up for the job, the valve seats can soften or even crack if they are crap valve seats, and you can loose a lot of power due to leaking compression! also good valve timing can help, with forced induction quite frequently a change in valve timing for a little more oreanted towards forced induction can help with a turbo, also good head gaskets, or even a decent head that can support the increased flow and volume of air, as well as hold up under the abuse is very important, so you don’t end up with blown head gaskets.

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