Largest size for an exhaust 0.71 bore or?

A G Bell says 0.71 of bore being maximum and to leave 0.5mm between exhaust and transfers,

I've heard a few different numbers quoted, from different sources from motherland tuner friends over Facebook, UK national based tuners, I'm just wanting to see what others have achieved and maybe for how many hours run time on their cylinders, i stuck with a cautious 55% for my bike.

What do people think is the largest, or what is the largest people have widened their holes too, and what sort of space was left between the exhaust and transfer.

Re: Largest size for an exhaust 0.71 bore or?

Probably Fred /

1960s books written by Bell, Graham and others are great info but it’s info is mostly for larger displacement manual shift race bikes that can take high rpm narrow power bands,

It wasn’t written for feeble one speed thin case e50s that are very hard on the clutch, bell, main gear and has crank walk/flex with hi power at hi rpm, narrow powerbands

I wouldn’t wanna make the powerband possibly more narrow or higher rpm to get more power by increasing exhaust size and possibly snagging a ring doing so,

I’d rather have less max HP and a more tractable powerband if the bike is going to have any street use and reliability so I would say leave it where it is at 55

You could actually go pretty fast with better speed control with a lower rpm broad powerband e50,

Stock bottom end and crank, Unported 64cc kitted e50maxi with 15mm bing (90jet) and trick reed plate that makes good power but mostly from 5,000 RPM to a bit over 9,500 rpm,

It cost around 435$ USD to build (not counting the bike) in 2006-2007

And it still runs great years later with routine ring changes every 3-5k mi, piston with ring changes at 8k-10k mi, head temps are cool never goes over 330f, bottom end has over 23k mi on it and has never been rebuilt,

I rode it twice this week and it still put a smile on my face!

Stock clutch is tuned for around 5-5,500 rpm so any speed you’re at : 10, 20, 30, or 40 mph when you twist the throttle it pulls with no lag to 55-57mph making it great for late breaking into corners and blasting out of them and clutch slipping just perfect for hills,

Re: Largest size for an exhaust 0.71 bore or?

Adam Smith /

I agree with what you’re saying ken, at the moment I’m going to leave it alone as it is my daily transport!

But I’m joining a drag racing club who do speed attempts on 3 miles of beach they currently a record of 63mph for Puch maxi.

Once I’ve got a car and don’t need this daily anymore my goal it to go for that. Sand eats power though so I don’t doubt it will be tough.

Those Polini kits are really monsters.

Also I meant to put 5mm between exhaust and transfer not 0.5mm

Re: Largest size for an exhaust 0.71 bore or?

Dirty30 Dillon /

Ken's got 3 videos that are somehow a response to any question.

He was asking a technical question, not for opinions or advice on how to build his bike to ride in the Midwest.

If you want a Scientific approach, Alex Degnes' (2strokestuffing) cylinder examples and calcs are just about the most wild you can manage. Read back through some of Jbots threads too to see trial and error for aux exh ports and such.

In reality, a safe place to start is 2/3 bore and massage from there.

Re: Largest size for an exhaust 0.71 bore or?

Indeed - check out Alex Degnes' videos for goodly information on whiskey, fine cigars and everything you need to know (and a bit more) about building a lot of speed (at your own risk)

If you want to drag race my only suggestion (take it as a positive suggestion, please) is to get a couple of spare engines and start now, dont wait until whatever you want to fall into place so this, that and the other thing will be perfect so you can start. Time has a way of taking good dreams and turning them into ideas, but if you dont act on them, time will march on and they end up on your bucket list or even in your eulogy as something you always wanted to do. Time is indeed fleeting, just get to things you want to do NOW.

Re: Largest size for an exhaust 0.71 bore or?

Adam Smith /

I follow Alex's channel already he does share some great content!

I will get some reading in on jbots old threads.

It is somewhat of a pipedream for now, i'd want to case induct the engine also.

Thanks for the advice guys

Has anyone else got any figures they have ported their kits too?

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