To head mod, or not to head mod. That is the question!

[Studboy]

Alright, so here is the scenario.

I just purchased a 2002 YZ250.  After playing with the jetting a little bit I REALLY like the way the bike performs now. (MODS: V-Force 2, Doma Pipe&Silencer.)

I am at approx. 4,000 ft and I usually run premium pump but occasionally when I feel like getting serious :) I run some race gas. (I change my jetting of course.)

So my question is:

1.  Would I benefit from getting my head modified for pump premium just because of my altitude?  And will this kill my overrev?

2. I also have a VHM Head (The one with interchangable domes.) With a dome that specs at:  Piston Height 2.80 mm  Volume 20.60 cc and Squish 1.50mm.   I haven't tried this out yet.  I can also get other inserts made up if I want to.

What is this going to do for me? (I assume I'll need to run race gas.)  I don't know much about the whole squishband/head thing so any information pertaining would be helpfull!

Thanks a lot!  :worship:

 

[bclapham]

hmmmmm.

i switched to race gas and was obsessed with modding the head. unfortunately, i talk english english, but the machine shop guy spoke american english. i had 1.8-2.0mm squish (dont think the bore is fully square) (set by eric on my porting job), i asked for 0.5mm to be milled off, but the bloke took off 0.8mm. i went to a bigger main to stop pinging but it was a tad spoogy (C12+10%MTBE, btw) but the bike would just not rev out as it would before (it revved out quite well, despite the low to mid porting).

so in the end, i had to order a thicker base gasket 0.8mm vs 0.5mm stock, but i am yet to try the bike, so ive kinda reversed the porting (eg base milling) but i am happy to get a bit of top back anyway!

moral of the story. be sure you want it, since it can open a whole can of worms!

cheers

BC

 

[Dragmeistr]

My bike isnt a YZ, but a 03 CR250, and I just installed the VHM head with the MX insert yesterday and took it for a test ride. I can honestly say that I was not happy. I had done some of the mods you discussed earlier, pipe, v-force, etc, and was pretty happy with how the bike ran. The head seemed to kill what all the other mods had helped. If anyone can offer suggestions on what to do from here, or what to try I would appreciate it. So for my .02, I would say stay away unless you have a tuner at your side that knows exactly how to fix what the head takes away.

 

[Rcannon]

Stud, I know we lose combustion pressure at 4000 feet. I wonder if it would be ok to mill the head enough to get the loss back?

Then again, if your running at the dunes a lot, the added heat might not be the thing to do!

 

[motometal]

clap, is there any way to add a thin head gasket on top of the standard one or do those have an o-ringed head?

 

[bclapham]

o-ring. that said, I am REALLY happy to get a tad of topend back, rode it yesterday, best the motor has ever run!:thumb:

 

[Studboy]

So, that said, would it be OK to remove enough material to get the compression back to what it would be at sea level? Or would I still be losing a noticeable amount of overrev?

 

[motometal]

isnt' the reduction of overrev from a head mod a byproduct of the increased compression? and if so, shouldn't your modded bike run similar to a stock bike at sea level?

just speculating...

 

[Studboy]

OK, last night I put my VHM Head on the MX dome (1.50 squish, 20.60 cc volume) Anyways, compared to the stock head it looks to have a wider squishband and less volume. I mixed up some fresh C12 today and took it out for a ride for about 2-Hours. I have mixed feelings. The powerband comes on smoother and lower in the RPM's which is kind of nice, it really doesn't feel any more powerful. I lost a noticeable amount of overrev, but I doesn't "fall on it's face" at high RPM's like my KX265 did. Like I said before, I think the bike at least "FEELS" more powerful with the stock head, but a trip to the sand dunes may be in order before that can be determined for sure. Also I didn't tweak with the jetting very much, the needle especially still felt rich.
Overall, I'd say don't waste your money with the VHM head unless you have a real use for the interchangeable domes. I was lucky enough to have one with the bike when I bought it. :) :) :)