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Old September 9th, 2018, 12:21 PM   #1
thumper64
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porting the 250 head

I'm going to be taking the head off my bike, I have a tick coming from the top end and the CCT cleaning and valve adjustment didn't fix it, so I found another head and timing chain tensioner as well as guide on eBay reasonable.

I don't expect a surge of power, but from what I've been reading, if you bump the compression and timing and maybe advance the intake cam a tooth, it could liven the bike up a bit more and add a little mid range and top end. I've already stripped what weight I can, put pod filters and a 2-1 Area P exhaust on and rejetted it. The carb tuning doesn't scare me and I've built a couple car engines, so I think I can tackle this. I have a GSXR 750, I'm just tinkering with the 250 because it's less expensive if I ruin a head, which I expect, so I'm buying a couple used ones.

Where do I work the head? I've read to leave the exhaust alone, but cleaning up casting lines and sharp edges where the bolt and runner meet shouldn't hurt. It looks like the intake side gets a little work widening the floor of the short side radius and the bowl hangs over the valve seat a bit, probably from core shift, so that could be cleaned up, though I'm not sure if I should leave a little bump in the bowl, necking down just a bit, right below the seat. Do you sand down the short side radius just a bit, or at all? That should help flow at higher RPMs. It looks like the heads I've seen have all been left with just a sandpaper finish, no polishing. For a street bike, I don't want to ruin the low end, but that's part of why I want to advance the timing a bit and bump the compression a little, even if I need to run 89 or 91 octane. I just can't find anything over 91 reliably if I'm somewhere unfamiliar.

I know the engine is about 12.4:1 compression in stock form. If I deshroud the valves, how much volume will I be able to remove again by removing the base gasket? It's 62mm wide for the bore, but how thick is the gasket? Or is there any point on this head besides just eliminating hot spots? I don't have a complete engine to assemble and clay, so I'm not sure how far I can go without P2V clearance issues. I can mill the head at work and modify cam gears to be able to adjust the valve timing back in spec.
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Old September 9th, 2018, 01:40 PM   #2
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What type of riding are you doing?
Where do you want improvement in powerband?

Pre-gen is already optimised for top-end power; it makes max HP pretty close to redline. All that's left is higher-lift cams with adjustable gears. One-tooth advance is too large and rough of adjustment, you'll lose all gains for all mods combined and more.

What is done in many cases like this is to narrow down the ports to improve mid-range torque. Latex is poured into port and pulled out to model its shape, then extra metal is welded in to narrow ports. Then blended in with grinding and polishing.

In end, you'll have something similar to new-gen head.
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Old September 9th, 2018, 02:19 PM   #3
thumper64
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It's just a street bike, to and from work, grocery store, a little fun ripping around when I find a good road on my path.
I'd like to improve mid and top end, but porting sounds like it will hurt bottom end a bit, so that's why I already plan to increase compression and timing a bit.
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Old September 9th, 2018, 03:15 PM   #4
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Old September 9th, 2018, 03:31 PM   #5
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One of the things I like about my 250 is its use of regular gasoline. I'd hate to have to use premium just for a very small HP gain.
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Old September 13th, 2018, 04:27 PM   #6
thumper64
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With it taking just over 4 gallons when I fill up, and premium being about 30 cents more a gallon, another $1.30 or so to fill it isn't a big deal to me. My car and other bike both take premium also. Whether I were to put an ex500 engine on or ride my other bike, I'm still going to pay more for fuel. If you expect to make more power off a motor, you're generally going to be upping compression and timing, and sometimes that means needing a little higher octane to prevent preignition. It's generally been a good bike, but I have a tick from the left cylinder that won't go away with adjusting valves or the cam chain tensioner, so I'm digging in farther and pulling the head off. While I have it off, I just figure I'll just go through another head to throw on and while I'm at it, why not just clean up the ports a bit and see if there's any free power to be gained?

I'm not expecting giant gains, but in essence it should accomplish the same end goal as putting a higher lift cam in. It'll shift the power band up a bit, but with 1st gear being as low as it is, I'm not really concerned. I'm doing this to practice a little before attempting a head for my car. If I ruin a head or two for this bike, they're $30-50 shipped for a used one off eBay and if it's a total failure of an idea then I'll have a stock head to toss back on. Hopefully I can learn a little and make my mistakes on this rather than my car's cylinder head that costs 10x as much.
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Old September 13th, 2018, 07:06 PM   #7
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You can’t move the timing a tooth. Just a couple deg.
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Old September 13th, 2018, 10:01 PM   #8
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Head porting is combination of CFD science and black-magic. Must do it incrementally in baby-steps in conjunction with flow-bench and dyno. That way it won't be too far gone if you make a tiny mistake. Can make flow-bench with automobile MAF-sensor.

It is extremely unlikely that any mods you make to pre-gen head will carry over to auto's head.
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