Very angry!!! Please help

[Steve Best]

The price they charge is very little in the porting world, I'd guess they deliver a gain?
I'd worry about the lack of chamfering on the port edges.
If a person doesn't know anything about porting, they might not know about chamfering too.

Porting doesn't have to be beautiful to work, and I have seen a lot of beautiful porting that didn't work.

 

[gary55]

This is what I know about the gt 80 as opposed to a modified non reed valve engine.
The first pic. is a engine I built non reed valve.
work I did.
The non reed engine had the transfers knifed at the bottom, jetted for altitude, Transfer windows cleaned of casting, and knifed at the windows, transfers matched to case, intake port lowered appx. 2mm and rounded horiz. to 24mm. Exhaust rounded to 24 mm, lowered 1mm, cyl. decked .030, after market performance head listed as 6cc really 8cc. Compression 125 lbs.

The second is The gt80 with reed valve 3 port cyl..
Work I did.
No jetting, Port windows cleaned and knifed. Most of the casting was already removed, and the bottom of transfers were already knifed well. Matched transfers to case, piston ramped at exhaust and transfers, no intake or exhaust mods. no decking of cyl. The compression was 90 lbs.

Oddly enough both engines run between 41 and 42 mph. If I had decked the gt 80 it would probably have done better.

 

[gary55]

Andres I forgot to add in reply #6 If you are only adding a gt 80 cyl. and reed valve, but you are using the stock slant plug head you don't need to deck the cyl.. If you do your comp. will be way to high.

 

[Steve Best]

Gary, I found my Grubee 66cc peaked out at just over 40mph but that it was an rpm related thing.
Do you suppose that both your engines were peaking out on rpm?
Could you feel a power difference between them? Say, on hillclimbing?

I notice a typo where you said "exhaust lowered 1mm".
I am guessing you meant "exhaust raised 1mm".

Steve, with more questions than answers...

 

[CrazyDan]

Yeah it's all about rpm, gear reduction and tire size. Doesn't matter what engine size, rider weight or whether it's 2 or 4 stroke. Equal rpm with the same gear reduction and tire size will have equal speed.

 

[Steve Best]

Yeah it's all about rpm, gear reduction and tire size. Doesn't matter what engine size, rider weight or whether it's 2 or 4 stroke. Equal rpm with the same gear reduction and tire size will have equal speed.

I don't think I can agree with that.
My Grubee started pretty slow at about 25-30mph and I worked it up to 40mph with porting.
You can even lose speed by gearing up, if the power curve does not match the load curve.
I don't have a lot of motorized bike experience with this but I do have a lot of motorcycle, truck and car gearing experience.

Different engines work at different rpm. A Briggs likes 3500rpm, the China Girls 6000.
What is constant is the exponential increase in resistance to speed as we go beyond 30mph.
This tends to cause a phenomimom where even if you double the HP, you only gain a few MPH.
The 40mph wall.

 

[CrazyDan]

But a China girl and a Briggs running at 3500 rpm each with the same gear ratio and rim size will match speed. What weight, hill grade, losses through your drivetrain and wind resistance will prevent would be achieving your max rpm if not geared correctly. A 26" tire spinning at 200 rpm will always translate to 15.5mph regardless what engine is being used and power input.

 

[gary55]

Gary, I found my Grubee 66cc peaked out at just over 40mph but that it was an rpm related thing.
Do you suppose that both your engines were peaking out on rpm?
Could you feel a power difference between them? Say, on hillclimbing?

I notice a typo where you said "exhaust lowered 1mm".
I am guessing you meant "exhaust raised 1mm".

Steve, with more questions than answers...

This bike belongs to a friend of mine. I built it, But my only rides on it was with a new un broken in engines, so I didn't flog it or push it hard up hill. The info was provided from him. he is a pretty straight shooter.

I don't think either engine reached there potential rpm as john only weighs about 130 soaked and he uses a 41 tooth sprocket. It was probably either the head configuration, type of expansion chamber, my mods. or a combination of these not allowing it to peak out.

It wasn't a typo. I take a little out of the bottom and sides of exhaust for volume. I leave the top alone other than clean up so it keeps the comp. as long as it can for better low end and hills. Also the speeds are not gps. they are cats eye velo 7 that I programed. I took a pic. before I switched engines And I am off a bit. This is with non reed engine. I'm going over to his house tomorrow to get him to wind it out with the new engine just to make sure I'm not talking out my ass with this comparison.

 

[gary55]

Yeah it's all about rpm, gear reduction and tire size. Doesn't matter what engine size, rider weight or whether it's 2 or 4 stroke. Equal rpm with the same gear reduction and tire size will have equal speed.

It can't be all about rpm gearing and tire size alone. If you have a vw engine and a cummins diesel geared the same and the same size wheels, then you load them each with 15,000 lb. trailor and run them up identical inclines one will not be able to reach its potential because of engine type, engine size, and load.

 

[CrazyDan]

Of course, but I'm just saying if both are reaching the exact same rpm with the same gear ratios and tire size then speed will match...