exhaust port timings

[rosscla]

if that barrel in my pic was used with a 58mm stroke the crown would be sitting much higher at bdc. it has a 62mm stroke so ive lowered the port quite a lot!

But that's because you've stroked it and knew you were tuning the port to match the stroke. If you're working a standard stroke, you wouldn't do that.

[hendy]

What I can't understand is why he has cut the exhaust bottom so much lower than the transfer bottom. 5mm! You don't measure it, you can see it!

Seriously, when I installed it I had no idea about any of this, but it's as plain as day now.

In fairness I have the whole engine and can see the piston in situ, whereas he just had bare casings so it wouldn't have been as obvious.

[Adam_Winstone]

^... yeah, the extended barrel that I said was tuned to the wrong, standard length, measurements was a case of tuning according to figures on a sheet of paper, rather than building the engine and double checking... a case of the old 'measure twice, cut once' would have been a worthy undertaking. If you don't jig it up then I can see how mistakes could be made.

Adam

[hendy]

I went to see him last night as a new barrel has arrived.

I don't have a photo, so a description will have to suffice:

In the centre of the bottom of the exhaust port, there is a 'notch', the bottom of which is 62mm(ish) below the top of the barrel.

Consequently by the time the bottom of the exhaust port is cleaned up it will be the same as the one I already have. To be honest I can completely understand how someone would make this msitake unless they had the complete engine to work with.

[shocky]

Im not tuning expert but there are some very basic checks you do before even getting a grinder out and one of them is checking the lower edge of the exhaust against the tranfer port if its miles out you dont even start making dust as you are litrally wasting your time or a customers money if your a tuner

[hendy]

The chap who tuned my barrel is now onto his third attempt (it's costing him a lot of time and money now and I'm a softy at heart).

He has sent me the attached photo to show his latest attempt. Is it good enough? Remember that this is a £160 barrel and piston, not a £600 fancy kit.

In my mind I have two choices now: either accept what it is and live with it for a while or get my money back and put it toward a GT186 kit.

If I'm honest I wish I'd just bought the GT186 in the first place, but that's in the past now.



The only things stopping me from demanding my money back are the fact that I feel sorry for the guy and my lack of knowledge about how much of a problem this is.

Will it really blow up and kill my children, or is it just less use at WOT for a full tank?

I'll be speaking with him in the morning, so I'd like as many (informed) opinions as possible before then.

cheers

[scooterslag]

The chap who tuned my barrel is now onto his third attempt (it's costing him a lot of time and money now and I'm a softy at heart).

He has sent me the attached photo to show his latest attempt. Is it good enough? Remember that this is a £160 barrel and piston, not a £600 fancy kit.

In my mind I have two choices now: either accept what it is and live with it for a while or get my money back and put it toward a GT186 kit.

If I'm honest I wish I'd just bought the GT186 in the first place, but that's in the past now.



The only things stopping me from demanding my money back are the fact that I feel sorry for the guy and my lack of knowledge about how much of a problem this is.

Will it really blow up and kill my children, or is it just less use at WOT for a full tank?

I'll be speaking with him in the morning, so I'd like as many (informed) opinions as possible before then.

cheers

Is this bloke a actual tuner/dealer or just a keen amateur? if he's a dealer then he should not be buggering things up on his second attempt, as for his third? if its covered the top ring ok- I'd run it but then I'm just a keen amateur

[hendy]

Is this bloke a actual tuner/dealer or just a keen amateur? if he's a dealer then he should not be buggering things up on his second attempt, as for his third? if its covered the top ring ok- I'd run it but then I'm just a keen amateur

Sort of half way between the two. He runs a small business as a hobby that earns him a little cash. He used to do it full time apparently, but from what I hear he was struggling; it isn't easy running your own business.

However, that's (sort of) beside the point. Is it going to run reasonably well and is it going to be reasonably reliable?

If it fails on either of these criteria then I want my money back.

Personally I can't see that performance would be significantly compromised as the port opens fully but would be more turbulent at bdc. I imagine that this is insignificant as the flow would be disturbed until the piston was exactly at bdc (or a few degrees either side) even if the crown was perfectly flush at bdc.

[hendy]

Actually, it's more significant than I'd guessed.

From 10degrees before bdc and 10 degrees after the piston only moves 0.9mm (with a 58mm crank) so it is approximately fully open for 20 degrees blowing spent charge through a turbulent orifice when a well made barrel would be blowing through an almost perfectly smooth one.

Is this a big deal or has all the charge left anyway and this period is unimportant?

[hendy]

I've solved the problem.

Got a refund and spoke to Richard Taylor of gt186 fame.

I'll be ordering my kit next week after he's checked out my crankcase porting.

I feel happier now.