Blowdown
- Thread starter Saturn 5
- Start date
alvin l nunley
Site Supporter
In 1973, I discovered Dr. Blair of Belfast University. His formula uses exhaust port opening in degrees and the RPM where the engine reaches peak torque. Blowdown has no part in the calculation.
When I built Mac 91's, I used 85° ATDC with 25° blowdown. It worked real well, (Northern California Reed Open Champion amongst other wins) but if more or less would have been better, I have no way of knowing.
Pete_Muller
Moderator
Saturn,
As you can probably imagine, there is no single/simple answer. If you are working with a particular exhaust pipe (where the angles of the cones, and the distance between the front (divergent) and rear (convergent) cones are "fixed", that is one conversation. Another more involved way to look at the issue would be how the angles and relationships might need to change, depending on blowdown (in other word: a change in exhaust pipe could be in order).
If you look specifically at the distance of the rear cone to the piston/port face, increasing blowdown probably means lengthening the exhaust pipe -- and if you do the math, you will find that it may mean lengthening it significantly with a moderate increase in blowdown.
The reason for this is: Think of the distance to the rear cone being a function of the amount of time (or we can even say "degrees rotation") from exhaust *opening* to transfer *closing*. If blowdown is increased (by raising the exhaust and/or lowering the transfer ports), then the amount of degrees of crank rotation (exhaust opening to transfer closing) has increased. This is somewhat "independent" of how high the exhaust port is, since the sound wave that travels down/back in the pipe does not begin until the exhaust opens (regardless of how high or low it is).
Of course once again: if you are working with a particular exhaust pipe, lengthening the flex to put the convergent cone in the correct position may result in the front/divergent cone being too far away from the piston (exhaust port-face).
There is no clear simple answer -- but the short answer would be: if an engine is modified to increase blowdown, and you are working with a particular exhaust pipe, then the first thing to try would likely be lengthening the pipe. Do the math and you'll get a rough idea how much.
PM
As you can probably imagine, there is no single/simple answer. If you are working with a particular exhaust pipe (where the angles of the cones, and the distance between the front (divergent) and rear (convergent) cones are "fixed", that is one conversation. Another more involved way to look at the issue would be how the angles and relationships might need to change, depending on blowdown (in other word: a change in exhaust pipe could be in order).
If you look specifically at the distance of the rear cone to the piston/port face, increasing blowdown probably means lengthening the exhaust pipe -- and if you do the math, you will find that it may mean lengthening it significantly with a moderate increase in blowdown.
The reason for this is: Think of the distance to the rear cone being a function of the amount of time (or we can even say "degrees rotation") from exhaust *opening* to transfer *closing*. If blowdown is increased (by raising the exhaust and/or lowering the transfer ports), then the amount of degrees of crank rotation (exhaust opening to transfer closing) has increased. This is somewhat "independent" of how high the exhaust port is, since the sound wave that travels down/back in the pipe does not begin until the exhaust opens (regardless of how high or low it is).
Of course once again: if you are working with a particular exhaust pipe, lengthening the flex to put the convergent cone in the correct position may result in the front/divergent cone being too far away from the piston (exhaust port-face).
There is no clear simple answer -- but the short answer would be: if an engine is modified to increase blowdown, and you are working with a particular exhaust pipe, then the first thing to try would likely be lengthening the pipe. Do the math and you'll get a rough idea how much.
PM
alvin l nunley
Site Supporter
Increase the blowdown, i.e. raise the exhaust port, would mean a longer pipe, except that, raising the exhaust port, shortens the design, also raises the peak HP RPM, which means an even shorter pipe. That also means a shorter header.
In 1974, I had 3 pipes, designed by Bob Lawley, using the Dr. Blair formulas. Number 1#was for the open K 88,#2 was for the stock McCullough 91 and a #3 was for the Mac 49. They all worked really well for the times. All 3 pipes had different headers, different overall lengths, and different section lengths. Dr. Blair's formula was quite extensive.
Pete_Muller
Moderator
Moved a copy to my Dropbox folder so anyone can grab it: https://www.dropbox.com/s/085qe9hyanlxohs/Slippy_exhaust.pdf?dl=0
PM
alvin l nunley
Site Supporter
I once built an adjustable Bullet pipe. As an experiment, I made an arrangement using a piston ring to seal the sliding cone. Nobody could pull it in the pressure was so great. Some of the problem was with the cable/cone pulling device, the leverage ratio was too great.
Thanx , slippy info is rare.
Pete_Muller
Moderator
Yea.... I glanced at the article before I sent the link, and saw that I stated I did that "a bit over 20 years ago"... which made me laugh. I actually did that well over 40 years ago! hahaha! I think I posted that article online in the mid or late 90's. Ouch.... time flies!