The Helios Project (Aprilia RS 125 Rotax 122 Biaggi Race Replica)

Calum

@irongamer727 That's savagely highly.

The stock DT is 8:1 lol.

The higher the CR, in general, the more extremely the powerband and the less torque.

I'd be very surprised if the somewhat powerless Athena kit is 13:1. You can have too much which has adverse affects.

I'd be more inclined to say the Athena is a lot lower, as it's generally a more torquey setup.

The higher the CR the more...unreliable an engine becomes. A lot of stresses on the bottom end and components you see.

Irongamer727

@calum http://www.rrd-preparation.com/en/kit-high-engine-motor-bike-125cc-2-time/671-kit-170cc-athena-moteur-125cc-derbi-gpr-yamaha-tdr-dt-tzr-2-temps.html

Here's where I got the info.

Irongamer727

http://www.athenaparts.com/eng/kits/view/7483/cylinder-kit-with-head-170-cc

The data sheet says 13:2:1. No clue if that's the same as 13:1

Calum

@irongamer727 I don't know. But 13:1 is very high

DTR+NSR

Those compression ratio's are both probably un corrected values, hence so high. The 'true' or corrected CR is worked out from the stroke once the piston has covered the ports, when your truly compressing the mixture. Not from bdc.

Calum

@dtr-nsr Yes that would make sense.

The Aprilia barrel's transfer ports, like the KTM, are incredibly low in the barrel. The Athena barrels are relatively high by comparison.

Jens Eskildsen

Generally, higher CR gives more torque, and a better powerband, not the other way around. Otherwise they would surely reduce the CR on endurobikes compared to their MX rivals, to give them broader powerband, but they dont...

"A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency"

This pic from the book "basic 2 stroke tuning":

Calum

Forged Piston Fitted.

alt text

Fitted it all nice and snug.

Put the engine away, amazingly (luckily) found one of the circlips had fallen out onto the bench.

Thankfully I found that, not sure how that happened. But that's back in now!

Of course I have bought titanium head bolts for this engine. But I was too eager to bolt all the pieces together and they haven't arrived yet.

But I will fit them when they arrive.

Irongamer727

@calum Oh my thats beautiful!
Do you know how big the difference between the piston diameter and the cylinder diameter is? Or did the piston arrive with the cylinder perhaps? I've always been uncertain how big the difference should be when fitting a new piston.

Calum

@irongamer727 Yeah it probably says in the spec sheet it comes with. I didn't read it. You can tell the piston to cylinder clearances in this engine are large. However I woupd expect that from forged components due to their thermal expansion coefficient.

For reliability I have knocked the compression back, this piston only utilises a single ring. Less frictional losses and bore wear

Irongamer727

@calum thicker basegasket?

Calum

@irongamer727 exactly that.

Irongamer727

I guess you purchased the cylinderkit on PJME. I mean that was a pretty good price for it!

I know you have bought the special cylinderhead and got it ported but still, I thought the cylinder would be like £600 or something.

The polini big bore was a bit more. I guess you reckon you can get more power from your kit, but how?

Calum

@irongamer727 So I didn't get this barrel ported. I just said that the porting work is damn good, albeit some minro flaws.

The last RS in my garage used the Polini big bore, before the 140 kit was released.

https://www.youtube.com/watch?v=GobIl4e-mU8&t=2s&list=PL66C1264C7D6205BB&index=3

Very quick bike, but the Polini eventually seized after very little miles and most of it was being ridden softly.

It was my brothers bike, but mi casa, su casa as the Spanish say.

Big bores are hard on engines. It's not the first time I've said it, and it won't be the last, in my experience the Athena was pretty poor for performance. Reliable mind you.

Irongamer727

@calum so the polini isn't very reliable then?
I've heared the same. I'm happy with the Athena though. Strong, not very fast but comfortable.

Calum

@irongamer727 Good torque gains from the Athena, if that what you're after. And, unlike the Polini, doesn't affect reliability.

I can only comment on my own experiences, and that may have been down to incorrect jetting. As this wasn't my bike I wasn't at liberty to jet it as I saw fit.

The reason I went with the 140 kit, is that I am not interested in more CC's, I want it to rev to the moon and the fact that the person who designed this designed the original. It was also (as you say) cheap. Just £40 more than having my old barrel replated and ported with a new piston. Something different as well, I've not experienced this cylinder before.

I'm excited Still £2k of bits left to buy for this bike yet.

Irongamer727

@calum it's going to be interesting to hear how she runs!

Calum

@irongamer727 Yeah sure, I expect it to be...disappointing lol. If I bought a rag around RS and bolted on a cylinder kit then yeah, fair enough. But having been building it for nearly 3 years, the expectations are too grand.