rotoryota

Was the opening and closing specs the same on both the round and the square port? This is very interesting.

bumpstart

currently building up a 12a intended for boost
so have opened the exhaust timing out a fair bit to about 2 mm lower than 13bt stock port,, and rounded top 4 mm taller than 13bt

i positioned the template so as to put the flat bottom at a height
so as to remove the step left at the bottom of the port when the insert is removed



13b and 12a templates back to back



placed flat on the bench the 12a template is in fact a few mm lower than the 13b one
and the 13b one a few mm higher in close timing
the 13b one has a very slightly crowned lower edge,, while the 12a one is flat



stock s1/s2 rx7 12a housing




marked up, with 13bt stock port comparison

note how much later open the 12a stock is to the OEM 13bt
the factory's main move was to open the exhaust earlier for the turbo cars

which asks the questions,
is there any more gains to be had going down further on a 13bt ?
has the factory already reaped all the main wins from doing so ?
does opening the exhaust even earlier cost too muck power stroke ?
or is this loss offset by better turbo spool ?
at which point does this cost more than it gains ?

i didnt gamble much,, and hence only down 2mm over a 13bt ,, though its significantly lower than a stock 12a

however , another 13bt is on the cards soon
and i may try a littler earlier open on these
to compare with the progressive moves in the other direction that i have already made






1st crude cut ( unfinished ) comparison








a few lined up on the bench,, with stock 12a and stock 13b comparisons
note some of these are variations from other builders, i have only ported the pair pictured above



exhaust port view

cewrx7r1

iTrader: (2)

This one is almost exactly like the Racing Beat exhaust template, mainly opens sooner with almost stock closing.

Carlos Iglesias

iTrader: (6)

Personal experience with the hazards of insufficient bevel...
https://www.rx7club.com/showpost.php...&postcount=198

GoodfellaFD3S

iTrader: (213)

A nice gradual bevel on the closing is certainly essential..... i've seen some beat-up and worn apex seals from lack of one, but never an outright failure from it.

On FD rotor housings, in general we don't go crazy with the exhaust porting. We spend a fair amount of time on them, but don't go huge. For a sub-500 rwhp street car that's expecting some longevity, I feel it keeps velocity up (and thus spool) by not following the 'bigger is better' mantra.

2a+RoN

I've had extremely good luck with my re-sleeved large exhaust ports, but that's just me..

Black Knight RX7 FC3S

iTrader: (7)

is there a difference between a square opening and a tapered opening?

The square would let more exhaust gas to flow out once the apex seals is open to the port?
So I am guessing the tapered opening would do the same, but to a lesser extent, but it will keep the velocity up high?

AnthonyNYC

iTrader: (3)

Wow, thanks for sharing, I never saw something like this.

djseven

iTrader: (56)

That damage was caused more by the uneveness of your porting than the bevel. While I bevel every port I do to try and mimick the factory bevel, I have torn down 20+ engine without bevels that showed no signs of irregular wear. It has a lot to do with the shape of the port and how it closes to how significant the bevel actually is. Pettit hasnt beveled their closing edges ever that I am aware of and I have seen no issues from their builds, however if you look at how they design their ports there really isnt a need for the bevel.

To clarify, I RECOMMEND BEVELING THE EXHAUST PORT, just wanted to say it isnt always necessary and your above pics look like the damage was caused by how uneven you ported the closing edge of the exhaust port.