76 GT550 - Shaving the head?

[Coyote]

I know a common 'hop up' is shaving the head to raise compression. Some say .5mm, other say 1mm. Is there really much to gain doing this, or you asking for fine tuning and high heat problems? I came close to shaving .5mm off my last GT550, but in the end I opted not to do it. I saw it as just more money for a really minimal gain.
What's the real scoop on this 'hop up'? Who has done this? How much material was removed? Did it make any real 'seat of the pants' difference? I'm thinking there may be some draw back(s).
What's the general consensus, opinions? I'm at a point I where can easily get this done. I just want to know if it's worth it.

[Craig380]

I did a 20thou / 0.5mm skim on my 380 a while back. It perked up the mid-range (around 3,500 to 6,000) a little but made no difference at higher rpm. No drawbacks I noticed over several thousand miles, and I do tend to ride the bike fairly hard.

However, when I rebuilt the top end after re-ringing the pistons, I used a good used standard head -- largely because the skimmed head had thread repairs on a couple of the spark plug holes. I didn't notice the difference on the road.

It's probably worth a small skim of the head if you're doing any mild porting (raising the exhaust etc) to get back compression, but on a stock motor I found the the difference wasn't big.

[yeadon_m]

As Craig said, on its own, its probably not that big a deal for the upside, but Suzuki experts were heard to say do a 0.5mm skim in conjunction with mild porting, perhaps 3:3 pipes and bigger carbs! They'd fitted VM32s in place of the stock VM28s. The mag article said performance was stock at low RPM but distinct improvement through the rev range and quite noticeable at the top. I have no expertise - I'm just parroting what I read
Mike

[tz375]

As you know, I don't have a 550 but what we expect to see with raised compression is more power at all revs but a sharper cut off at the top end as the higher compression starts to get in its own way - so to speak.

If it adds say 10% everywhere, that's 10% of not a lot at low revs and 10% of that miserable 38hp at the top end and that you would probably feel.

If ports are raised, it's usually prudent to raise CR to prop the bottom end up.

The other side to the argument is that higher CR is not always a good thing if the gas is less than stellar or if timing walks. Ethanol has a naturally higher octane level than gasoline but modern E10 is a bit of an unstable soup mix that is not consistent.

The bottom line is probably that if the head are scratched or distorted, I'd machine a bit extra off while it's in the machine shop and if it's OK I probably wouldn't bother.

[Vintageman]

Coyote,

You want to try it? Simply get 3- T350 head gaskets as a low coat easy to back out of option. They are 1/2mm versus 1mm stock.

I think the GT550 is 6.8:1 now (not low really) and this may bring you over 7.0:1 my guess so watch for pinging... maybe need a little more fuel to cool it if it does for example

When I raised my Ex port 1.5mm to 38mm Ex height on my GT550 I bought a spare head and had it shaved 7/8 mm. I thought i was going to need to take 2mm off Ex for my goal was to get to 38mm Ex height. However only 1.5mm was needed to achieve my goal. I also run 3 into 3 pipes that boost very well tuned for street riding.

As stated before by others the mid range with those head was much stonger for sure. My butt did not notice any top end loss (even at 38mm Ex height the GT550 porting still mild). If sustained high end as Tz noted may have extra heat due to added pumping losses. I think the GT550 ram air design can shed this extra off OK

However, when the 3 into 3 pipes kicked in (and mine do at midrange ) I was nervous it may be pinging.
I had changed a few things at once and was nervous . So, i put the stock head back on until I sorted things out and had some time on my new top end.

I now plan to use T350 head gaskets with stock head (since I did shave some off Ex). Start here first for I don't want to worry about finding premium fuel and my mpg is up to 39-40 mpg verus 31 -32 that it was and I like this too.

Since the GT550 has a great midrange stock a little more emphasis on what is great about this bikes personality is even better say versus changing it into something else all together (rev monster ) me thinks... unless you have more than one GT550

Try what you say be careful.