4 stroke compression ratio / rpm question

[icedeocampo]

i have yamaha 115cc 4t engine with (variator transmition)

bore: 50mm
stroke: 57.9
compression ratio: 8.8:1

---

so installed a 59mm cylinder kit.

i get good power on low rpm but my rpm does not go up... i did some computation and my compression ratio is near 12:1, i DO NOT have pinging sound.

now here are my questions:

1. does the compression ratio limit my rpm?

or

is there any other thing that limits my setup from reving higher?

 

[ruud.]

a high speed knock is better to detect with a detonation sensor. Maybe your intake is not good enough, your porting + valves.

 

[icedeocampo]

hmm.. maybe the stock cam is suited for a smaller engine displacement... perhaps a cam can help me?

 

[Mexicano]

You will need a bigger carburetor and a sport/ racing cam to get more rpm's, also lightening your fly wheel (stator) would help.

 

[Astrolopitec]

You've gone from 115 to 160cc. Almost a 50% volumetric increase. If you have'nt touched anything on the carb. You are guarantied to be running way too rich.
Best would be a bigger carb. But failing that. You need to down jet.

Juan

 

[icedeocampo]

hmm... i'l do some plug reading again, last time i checked it was light brown though...

i'll try to swap in a smaller jet, if needed...

will a sport/racing cam have a "leaning" effect?

 

[Big B NRG]

hmm... i'l do some plug reading again, last time i checked it was light brown though...

i'll try to swap in a smaller jet, if needed...

will a sport/racing cam have a "leaning" effect?

Yes, it will change the powerband of your engine to higher rpm. And then it will drive better.

How much RPM are you doing now?

 

[icedeocampo]

around 8k

 

[50cczip racer]

? have you tried lighter and heavier rollers its quite possible your peak power will now be at a different rpm to where it was before

 

[icedeocampo]

i have installed a free flow exhaust system... grr noisy! did some porting and polishing in the cylinder head, installed a higher gear ratio... adjusted weights, and the weird thing is that the power i get from my new setup is very near to that of a stock 115cc engine (variator system and gearing were modified).

we went on a 100km trip to a beach. when we're going up hill, my bigger engine gives me the edge at steep climbs i get to 90-100km/h, but on long downhill runs, if the stock engined friends of mine duck, they'll be able to over take me, they run at around 140km/h, but when I duck, I will again over take them so I maybe running at 140+km/h, but my gauge is maxed out.

on those long downhill streches (around 4-5km), i should've been able to out lead them comfortably, since i have almost 50% more displacement, but that was not the case.

 

[Mexicano]

Icedeocampo: Bigger carb and Malossi Cam

 

[icedeocampo]

malossi does not have a cam for my engine, so i'll have to settle for a thailand made cam, we do have a cam cloning machine hehehe... if i can get my hands on a cam, i'd be able to produce lots of its replica.



hmm... so what you're all trying to point out here is that the compression ratio does not have anything to do with the low rpm im getting?

 

[tiba_karotsu]

the bigger the motor the lower the peak rpm, the longer the stroke it also lowers peak rpm.

try CMR street cam for Mio Bob of Bobscooters use it on his Mio & i have tested his scooter it runs real nice for a 4stroke.

 

[icedeocampo]

tiba,

so the compression ratio does not have anything to do with the rpm?

 

[tiba_karotsu]

yes that's correct compression ratio has nothing to do with rpm it's the engine lay-out that determines rev capability.

a SOHC 2V with a bore/stroke of 58,5 x 57,9 isn't what you call a high rev engine it is more of a torque motor suited for lower rpms the most rpm you can get out of it maybe around 9500 to 10000 before the limiter cuts in.

take a cylinder out of say a CBR600RR which has a compression ratio of 12,0:1, it has DOHC & 4V per cylinder & has a bore/stroke of 67mm x 42.5mm, note how big the bore is compared to the stroke, this is what you call an oversquare motor & it's designed to reach & exceed 15,000rpm when tuned for racing, also the bigger piston allows bigger valves to be put in the head for better high rpm breathing.

if you want your Mio to be a screamer you must change the layout of the motor, install a 4V head, add a bigger bore & decrease the stroke, install a larger carb, use a performance cam, & change the exhaust pipe that matches the flow of the 4V head. after you've finished with these mods you can expect your engine to rev around 12500 rpm.

100cc Comer go kart engines have 15:1 CR but they rev to 19,000+ rpm, 50cc GP bikes of the 60's & 70's have 20:1 CR & they rev to 20,000+ rpm, the RC211V has 14:1 CR but it revs to 16,000rpm. it's in the layout my friend.

btw kumusta ka na? di na kita nakkita nagpost sa MCP, wala ba balita kung kelan may drag?

 

[icedeocampo]

ic... so it's the design...

As of the moment i have two options, i have what appears to be a perfect 4valve head for and a 2 big valve setup, which would be better?

aside from the additional valve area, what else is the advantage of a 4valve head to 2 valve head?


OT:

im ok, kinda busy that's all... got hired as a full time faculty sa ateneo ... so i have my work cut out for me.

my line is computer science

oh well must save up... i still am in touch with my scooter addict friends, i'll keep you posted as soon as the drag sched reaches me.

 

[icedeocampo]

hmm no more comments?

 

[Eight_Ball]

i agree with Tiba. At least, you need a bigger carb and a free flow exhaust. If you get a cam it would be even better. If you don´t find a cam, i can tell you where to send your stocker for a regrind. That way you´ll get quite some more power from your actual setup.

When you get the new carb, port head´s intake to carb´s size, port your manifold too if needed. If your intake valve´s boss protudes into the intake tract, cut it. Polish your exhaust port, don´t cut it´s valve boss.

This way i think you´ll get all the power you can get with the setup you have.

Tiba´s advice is even better, a racing head with a race cam, a shorter rod and a bigger cylinder. But you need to buy that new parts.