125 Wombat

[WoodsWarrior]

Good day. Need a little help on a 1973 Wombat , If I use a Combat Wombat head do gain anything ? or is better to cut the stock head for higher compression ratio , I ride at 5000 t0 6000 ft. in tight tress , Or help to get more bottom end . Thank you : :

[olddogs]

If you ride tight woods the money may be better spent on a reed valve setup. Any boost in performance with just the head swap would be minimal. Also if you are doing a lot of brake sliding in tight woods, a higher compression ratio may make the engine easier to stall during quick brake stabs.

[WoodsWarrior]

Thank you

[Bullfrog]

Well POOF! My reply from earlier today seems to have disappeared into the ethers. So I'll try again.

I am convinced that bolting on a Combat Wombat head will result in excessive compression - and that is not a good thing. Do not bump compression higher than about 150psi unless you plan on running high octane race fuel blends all the time.

A reed valve would be the way to go to achieve improved performance - especially on the "bottom end". The improved precision of fuel/air mix at lower RPM works wonders for "grunt".

Ed

[matt glascock]

I will concur with real world proof in the form of a podium finish a couple weeks ago - 50+ open (hare scrambles) behind a KTM300SX-F and a YZ450F on a reeded CW. I got murdered on the few speed sections and uphills, but in the tight woods sections comprising the majority of the course. I made it all back. Great woods weapon. Reeds are the best bang for your buck with suspension upgrades a close second if you are putting together a competition scooter.

PS- CW back in action Oct 31st - hare scrambles

[JPark]

The density of air at sea level is .075 and at 5000 ft is .0625 which is pretty significant. The 94 has a 7.2:1 CR and the CW is 8.0:1 . You could always take along a tester and compare readings at sea level - assuming you ever get down that low with bike at hand - and at your riding area. As has been said, 150 psi is the tipping point on generic gasoline but at altitude you may be much lower than that.

As I pointed out in recent thread, what you get as a base reading is only as good as the tester replicating the actual - spark plug in - combustion chamber volume because that volume is so small. That's why having a sea level base reading to work from is the only way to be sure.

However...... the port timing of a CW barrel is 180 degrees and the 94 is 162. This is a huge difference and if you use that head on a 94 barrel the number could be way higher than 8:1. This is because the swept volume above the port is smaller with only 25mm of stroke vs about 31mm on the 94. That much difference could put you in deep trouble. So shaving a stock head would be the only option and I'd only do that if you have a spare head to mess with.

You're never going to recover the power loss from the density loss with altitude, but you may get better response by compensating.

[WoodsWarrior]

Thank you all !