Extra base gaskets to widen rev range?

[Perthbiker]

I've been thinking about adding an extra base gasket on my RSE 70cc. By my reckoning, it should lengthen the exhaust and transfer port timing. My concern is that it will shorten the inlet timing but that may be lessened by working the bottom of the inlet port or the bottom of the piston. Longer port timing means more time to move air into and out of the motor especially at high revs. Have I got all that right?

Compression in the combustion chamber and crankcase will both be slightly reduced but that should be mitigated by the RSE high comp head and reed kit that I have on the motor.

I'm after a slightly wider rev range than I have right now which peaks at just past 6,000rpm. If that can be stretched to 7,000 or 7,500rpm I'll be very happy.

Other extras on the bike are the SBP pipe and a CDI that supports retarded ignition at high revs. Bike pulls really well right now especially in the low to mid range but I would be happy to sacrifice a bit of that to gain the extra 1,000rpm.

Any pointers from the porting authorities out there?

 

[Perthbiker]

The more I think about this the mnore I reckon that I may be on the right track. I'll definately work the bottom of the piston skirt on the inlet side by a 1mm or so to make up for the barrel moving up by the thickness of a gasket. Also looking at widening the exhaust port to increase flow.

I'll take a careful look at the transfer ports both at the cylinder lining but also at the crankcase end to make sure there are not obstructions and that all is smooth to maximise gas flow.

Recognising that the gas mixture that flows through the motor needs to be treated as having inertia which needs to be considered carefully at high revs is key to making extra power by getting the motor to breathe better and rev higher. Need to open the relevant ports earlier to make up for inertia of the gas flows at high revs.

 

[Perthbiker]

Note to self. Don't mess with the piston....

I trimmed some of the skirt off at the inlet port and some of the top of the piston at the exhaust port, both in the interests of extended port timing. Results were disappointing with an overall drop in power and a seemingly higher running temperature. The running hotter issue could be me hanging on the throttle more hoping to get back the old grunt of the motor.

I've ordered a new piston and hope to restore peace inside the engine again soon.

 

[marksie]

I have a Super Hawk R80 that i have been experimenting with some mods. This is what I have : port timing is exhaust port 27mm from top of barrel to top of port, transfer ports from top of barrel 31.5mm and intake port 59 from top of barrel to bottom of port. I have taken 5mm from bottom of piston skirt on intake and drilled 8 holes on intake side of piston at 5.5 diametre. I have used Jaguars site as a guide for these settings. I have taken .3mm from top of head to 'up' compression. I run a 12" header pipe with SBP expansion pipe and a CDI that supports retarded ignition at high revs. Also run a 19mm carb with a 75 jet. Also have a reed valve intake made by arrow and soon to test the rock solid reed. My bike makes good power from 4,800 to 8,500 rpm, still looking to make more power below 3,500.
Hope these figures give you something to compare to your settings.
Cheers
Marksie

 

[Deleted member 12676]

yeah dude ya messed up. raising the cylinder means you are increasing the intake duration already without having to take off any of the piston skirt.
raising or lowering the cylinder via base gaskets never works out good because it throws off the duration between the exhaust and transfers too much.
if ya want more top end then just dremel the exhaust port a bit higher. while there you should widen all ports without touching the front edges of the transfers.

 

[Deleted member 12676]

Marksie, a 12" header sounds too short. lengthen it to the sweet spot and you will gain both low and top end power. Start off by adding 3". let me know how it goes.