Getting it in and out. That's what it's all about. Super six carburetor, or small four barrel, mild camshaft, headers or modded opened up exhaust and bigger exhaust pipe. Milled head and or block for more compression. It's like any other engine, really with the exception that if you only do one thing, they payoff is much, much less, simply because you're working with less in comparison to a V8. Less bore size. Less bore numbers. Less cylinder head all the way around. Less port volume. Less valve sizes. Depending on how far you want to go, you can have the head ported and larger valves installed. That's getting pretty costly, but will add even more. Even an air cleaner designed to straighten and smooth the incoming air can make a difference. All of these things together will make a pretty big difference with a slant six.
It's entirely possible with smart modifications, to add a solid 100 horse power from the stock highest rating of 145 and remain fairly streetable. Difficult naturally aspirated, but not impossible. Here is the latest cylinder head I am working on I just recently got back from my head porter. Completely ported, polished and modified for 318 valves. He was able to get from 99 CFM on the exhaust to over 150.That's an improvement I'll really be able to feel. As I said, it's costly. You just have to decide "how far" you want to go.
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