Thread: Group K BB
12-03-2005, 11:48 PM #1
Group K BB
I am trying to decide between the group k bb kit $1980 and Lowes Porting to a 1300 $2410. Will I see any difference between these kits on the water? Is Lowes worth the extra $$$. Help!
12-04-2005, 01:01 AM #2
I would say Montana all the way. IF you are going to do it, may as well go all the way rather than getting something and then thinking "god, i wish i had gone all the way".
Sometimes the feeling of not going all the way is worse than being where you were before doing anything at all. But i guess it depends on what you are willing to spend to get it. I would go with Lowell for sure if you have the funds to do it at this time.
12-04-2005, 10:28 AM #3
12-04-2005, 01:16 PM #4
- Join Date
- May 2005
Lowell did my porting and I am happy with it. But on the other hand there are some fast GroupK BB ski's out there also. Regardless of it, all of the 87mm PIstons are coming from GroupK only, also get GroupK pump shaft support sleeve for the inner bearings races!!!!
12-04-2005, 02:14 PM #5
Can group K reach 77?
Do you think Group K with the proper tuning can reach or exceed 77mph.
12-04-2005, 02:21 PM #6also get GroupK pump shaft support sleeve for the inner bearings races!!!!
12-04-2005, 02:36 PM #7
Mark, this is on Group K's site about the pump bearing:
The 1390 makes tons of torque, and so, puts a much heavier load on the pump bearings. The stock GPR pump has two large pump bearings that support the drive-shaft. Unfortunately the pump is designed in a way that delivers “all” the impeller thrust load to only the rear bearing in the pump. For our 1390s, we offer a chromoly “thrush bushing” that fits between the two bearings to allow the thrust load to be shared by both of the bearings in the pump case, and greatly increase bearing life
12-04-2005, 03:04 PM #8
12-04-2005, 03:14 PM #9
Yeah, I'd read that. But I've never lost a bearing in many years. I've never lost one in a gpr pump. Usually the cheesy ass vanes blow out first.
12-05-2005, 09:07 PM #10
- Join Date
- Nov 2005
- Fort Mohave Arizona
One of the on-going perceptions about our 1390 kit (on the Riva site that I was disallowed access to) is that our 1390 kit was configured in a way to be "less" than the best we could make it.... Nothing could be farther from the truth. However, it was never our goal to create a kit that sacrificed many qualities of user friendliness for a few extra rpm... as others might do.
At Group K, I personally port "every" cylinder, and I personally conduct all the prototyping and on-water data gather for our kits. As someone who has been prototyping and porting 2cycles for over 30years, I know how to build things that are very close to "the edge". Making radical port layouts that seriously harm low range power and long term piston/ring wear does not require a genius... It just requires an unlimited budget, and an unlimited willingness to buy new parts to keep an "on the edge" setup fresh.
As many aftermarketers have discovered ... when you make a speed claim ... many folks have the full expectation of getting that entire measure of speed .... forever....with Toyota style longevity. While everyone claims to "want" the big numbers... no one "wants" to hear about the expensive maintainance that might come with a "radical" setup.
At a certain point, one of the best tactics to increase power is to configure specifications that make various parts"very" perishable.... and require frequent replacement and/or maintainance. We encountered this phenomenon in a big way with stand-up racers during our Kaw 750SXi development a few years ago. Given that, we included the following text on our web document for our 750 mods.... it pretty well sums up the realities of 2cycle race setups. It goes:
So You Want Exactly What MacClugage Has - There is a perception that the engine modifications used by Chris MacClugage are some secret specs that are not spoken of so that Chris can maintain some competitive advantage … nothing could be farther from the truth. However there are plenty of reasons why you may not want “exactly” what Chris has. The following is a brief explanation of some of these reasons.
Since Chris is an exceptional riding talent, he can control an SXi at speeds where others cannot. To access those high speeds, the race team techs utilize an exhaust port whose timing is very high, and whose roof shape is very squared off (to obtain maximum exhaust port area for that height). While this large squared off exhaust port offers exceptional high rpm power abilities, it is “very” abusive to the piston and piston rings. In fact the abuse is so great that the effective piston and ring life is about 5 to 10% of normal. Since this operating time is just beyond the recommended break in period for most pistons, the break in period must be done away with. To do so, the cylinders must be set up at a clearance that would ordinarily be considered excessive. This excessive clearance then results in slightly greater piston rocking, and thereby even more accelerated ring wear and piston skirt collapse. This added wear reduces the “optimum performance” piston/ring service even more. While all this equates to a very short piston/ring operating interval, it is easily long enough for Chris to complete a full day’s event. Unfortunately after several full days of this kind of operation, the cylinder bores experience heavy wear from constantly being operated with very loose fit pistons. Since the bores are at the maximum legal bore already, there is no overboring. To maintain optimum output, the cylinder is replaced with another maximum bore ported cylinder after every third or fourth outing.
At the same time, this same high exhaust port timing causes a loss in low-end power. In addition there is the compression loss from running such a loose clearance pistons. To recover that compression, and low-end power, the compression ratio is increased to well over 200psi cranking pressure. As the engine is run constantly at it’s very high peak rpm against this kind of compression, the connecting rods and bearings are subjected to extreme loads that no recreational engine would ever experience. After a few racing hours, these extreme loads can be great enough to slightly stretch the connecting rods, and/or cause fractures of a needle-bearing cages. Constructing heavier rods is not sensible because it would increase reciprocating and rotating weight … and the bearings are going to have to be replaced in short order anyway. Given that any kind of crank or rod failure can damage so many other engine components, the crankshaft assemblies are frequently replaced, more out of precaution rather than failure.
Understandably, not many racers are willing to bear the costs and considerable time to perform this kind of regular maintenance. At the same time, most aftermarket shops (Group K included) are a bit coy about selling machinery that is so maintenance intensive.
In truth, Chris’s SXi 750 is capable of going faster than anyone (but Chris) is capable of riding. The other truth is that by choosing specifications that are just a tick away from the specs that Chris runs, the projected longevity of all moving parts will increase by huge margins. The Group K 800cc Hammer kit is our interpretation of how close we can get to the output of Chris’s SXi without involving the same level of maintenance as Chris’s SXi.
If there was a big market for building high maintainance 80+ mph GPRs.... Fercho would be a wealthy guy.... however Fercho (like others) understands how easy it is to create a "perishable" platform that is fast... but not really sell-able. For now, Group K is happy to sell the base 1390 kit with our realistic claims. We have many customers who have done their own tweeking to their boats to net speeds of 75-77 from their 1390...and I applaud their efforts. I love big numbers as much as the next guy .... but I don't love them enough to use specs that result in a short fused, highly perishable setup....In my opinion, it's just bad business to do so.
Respectfully, Harry Klemm
btw...we love Fercho
Originally Posted by ABBOTT
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