OMC 56 Oval port motor port specs

[phillnjack]

Today I measured port specs on my oval port spare block. I got some very surprising results and measured it loads of times to make sure of accuracy.
What I found might help some people who are looking to modify their oval port, and or save them stripping one down to see what is in there or not .
I am trying to see what needs to be done to get a lot more power from my motor, I think the results I have found today just might help me
see what I can and cannot do to get it revving a lot higher without going too mad.
I myself only want to get it up near the 7,000 rpm band for the odd short blast down the river .

I measured all the ports on my 1995 oval port 56 motor. Cylinder block part number 0436908
I have read on a few places about how much smaller the oval exhaust is compared to the bridgeport exhaust. !!!....err thats is not exactly true if we are talking of area of the exhaust ...
Maybe some blocks are different ? I dont know and can only say what is here is exactly what I have in front of me.
The standard fishing motor oval exhaust is WIDER than the sst60 spec for APBA, the other ports are also slightly wider. ?
here is the results
sst60 exhaust width of each port is 1.075 x 2 = 2.150..........oval exhaust width is 2.185039 ( 30.5 thou wider )
sst60 Boost port max width is 1.255...................................oval port boost port is 1.29921. ( 44 though wider )
sst60 Transfer port max width is 1.5800........................... oval port Transfer width 1.633858. ( 55 thou wider )
Some of the heights of the ports on SST60 are higher by a fair bit , but there is room to take any or all of ports to the same height.
Sst60 exhaust port height is min 1.450 and max height 1.520...........oval port height is 1.500 standard ( lower by 50 thou )
sst60 boost port height min.....1.860 and max height 1.950..............oval port boost port height 1.88976 ( within sst60 spec )
sst60 transfer port height min. 1.825 . and max height 1.890...........oval port transfer port height 1.948819 ( lower by 104 thou)
Not sure if the sst60 measurement for transfer top is with or without chamfer ???
exhaust and transfer port roofs are up a fair bit higher than the sleeves, so are capable of being taken up to sst60 spec.

Going by these measurements, with a bit of work on the exhaust and a touch on the transfer port height, the oval port would be able to breath a lot better.

ALL the ports behind the sleeves need to be cleaned up a lot and the sleeves themselves have some terrible edges on some of the tops of the ports that are
more like a serrated knife edge than a proper nice clean port, they definitely could do with cleaning up.
OMC did not take any care when assembling the 56 oval port motors, no attention paid to detail at all.
Sleeves look like they have been put in without them being finished off.
The exhaust outlet to afaptor from the bottom of the block itself needs to be cleaned up inside as far as possible to get rid of some very bad casting
marks, this can be done with a small flexible shaft and small wire brushes/wheels etc, not an easy task but it would pay off big time to let the gases flow better.
with the use of a tiny endoscope ( very cheap on ebay with 6 built in lights) a lot can be done to the exhaust as in polish it up to flow better.
for those who might be interested, I have made a sort of port map drawing to show the oval port versus the SST60 spec for APBA rules..
( nothing like true map or scale) just to show the port sizes and heights. sst60 specs in the blue boxes.

.

[phillnjack]

So now my thinking is to raise the oval exhaust port up by around 30thou and the transfer ports up by around the same.
I would leave the boost alone as that is ok going by the specs.
this I think would give an extra boost of power up the range without going to the point of losing power down the bottom and mid range.
It probably would not be enough for an out and out racer, but could put the oval port up in the 6700 to 6900 bracket and still have the
power down the bottom that the sst60 does not have.

To me this would allow the big props to get the desired pull away that the oval already has, and " could " give that extra bit of high rpm it lacks.

this along with a cut head to bump compression from 145 to 160/165 psi could be exactly what I am after. ( might be a disaster)
strong motor that will pull a 23 prop all the way to 6700 rpm..

I have not seen much about moving the transfer roofs up ? is this a bad idea or good idea ? what does taking those transfers up actually do ?

anyone ?