Porting Tolerance

[dangerousd]

I'm not talking about my patience tolerance level here.... I've got plenty of that

Just want to know what kind of tolerance you try to hold between cylinders on both intake, exhaust, boost, etc. I know a couple thou. either way isn't going to hurt, but how much is too much. .005, .010?? Like to hear it in both degrees and deck height dimensions.

I'm getting started on porting my '98 XR6 and want to know what the limit is. At what point do you say, OH CHIT, there's a new sleeve

If you get a little too happy w/ the file on one port, I assume it's best to raise the others to match?

[rob@umt]

By the time you have taken .010 too much with a FILE you won't be too happy at all. You have too much metal to try and do it all with a file. By the time you get the ports right with the sleeves in the block you will have wished a thousand times you just bought new sleeves that were already right.

[Raceman]

Rob, say what???????????? You must've found a cheaper place to get 6 sleeves installed than I'm aware of.

Dangerous, where did you get your port specs? I think the answer to your question would lie in how radical your port map is. Have you already cut one too far? If not, just use the specs you're gonna have as your absolute max and take your time. Too little is always better than too much when it comes to whacking ports. Some would disagree, but if I went a tad high with one cyl, I wouldn't raise the rest of them to it, especially if you're borderline high anyway.

[JFN]

Dangerousd;

Research done by GP Blair and his guys at QUB have found that a difference of .020" or 1 deg is a significant amount. They tested a cyl that had the two main transfers mismatched by .020" against a cyl that had them both the same and found that there was no increase in top end power but matching the two ports did fatten the torque curve due to better cyl scavanging.

If your doing the port work yourself you should be able to match all the ports to within .oo5". If not, get someone else to do it.

JFN

[dangerousd]

Thanks JFN, that's exactly the info I was looking for. Nothing has been cut too much (yet ), but it's good to know at what point you can actually tell a difference in performance. .005 should be pretty easy to maintain.

For now I'm just using what T-REX has told me of 1.550 exhaust and 2.150 intake/boost. US1 also offered to send me port maps to get me in the 240-250 HP range. Not sure, but they might just be the stock 260 maps. Since John is in the middle of moving and it may be a little while, I'm using the one's T-REX has told me. They should put me in the same HP range I'm looking for.

Hey Rob, I should have said burr, not file. Once I get close w/ the burr, I have a file for the die grinder to clean up the cut just a bit.

[PaulO]

I have done some machine work in my past (mostly 4 stroke auto and motorcycle stuff) and I would really like to know how you would accurately measure port size and placement within a cylinder particularly when you have chamfers?
PaulO

[Superdave]

Type in PORT MAP and you will find several port maps. One is from John for a 2.5@330hp. Another is for APB rules. I tried to link to them and tried to attach them from my computer, but I could'nt get them to work. If you email me I will send my copies if you want.
Dave
PS. Be carefull with the exhaust ports. You go too high and you will break a ring. Mine is at 1.550 and it's slow coming up, but it screams on top!