Re: My Suzi T500 Project (exhausting!)
Heads are a whole other situation. In a squish head we want maximum turbulence with a respectable MSV. As the squished gases pass the sharp edge into the bowl it creates turbulence that helps atomization and that's what we are after.
Knife edges are another issue again. Gas flow separates as it passes a knife edge, where it flows around a bullet nose. That's for example why people are often told not to knife edge the transfer port divider.
When I say low rev motor, were are talking GT's. A GT has a 64mm stroke so with a peak mean piston velocity of say 3500, that equates to 8500 revs max. So if we are looking for a nice meaty powerband, we want to focus on the 6-7k are rather than looking for peak power at 8500 and ignoring losses lower down.
That's what I mean by low revs. A kart motor by comparison is living between say 11,000 and 14,000 and they used to go much higher than that.
Header length is important up to a point, but what dictates tuned speed are gas temperature, exhaust duration and overall length. With the same pipe, if we want to move power up or down the rev range, the belly section is the place to do it. Bimotion calls for a header length of 7 to 9 times the exhaust port diameter. That works out to roughly 300 - 400 mm. A little less is OK but much longer and I'd expect drag to start to rise.
With section changes there are a number of factors at play. Larger angles create stronger pulses, but over a narrower rev range. Larger angles also tend to result in flow separation which in a 100-120mm pipe is really not too much of an issue, but could be a problem at the tail pipe entry where a venturi is a better way to merge. Changes in angle are usually limited to around 3-3.5 degrees in the divergent cones and 12 degrees or more in the baffle cone.
The thing I try to keep in mind is that almost any pipe will flow enough gas. What I need to focus on are the sound (pressure) waves moving up and down the pipe. Pressure waves are traveling at the speed of sound where the gas is moving at less than half that speed most of the time. Gas speed changes with engine speed but pressure waves change only with temperature. There is some reversion of the actual gas, but mainly it's about the pressure waves which often travel in the opposite direction to the gas.
Let's say we want to change our existing design to something shorter for more top end. We might use the same pipe with a smaller tailpipe to try to push temperatures up, or we could lean it out at the top end or retard the ignition to get more heat into the pipe. More advance at lower revs (if the motor can take it) and less at the top end might be enough to stretch a powerband.
Or we go with a shorter pipe. That means a shorter belly and that's not practical, so we have to increase the cone angles which makes for a larger diameter belly or a longer one of the same diameter - or some mix of both. If we keep the same belly diameter and go with larger included angles, that allows us to shorten the whole pipe.
If we got it right, power will go up at the top end and we will lose in the mid range.
You are right to focus on transfer ports. A GT has insufficient transfer area. At a target of say 8bar BMEP at say 7750 rpm, we have enough exhaust area to support 70 crank HP but only enough transfer area theoretically to support 56 crank HP. As revs go up, that situation gets worse, so we need to keep revs down and to make the exhaust work better to empty the cylinder and pull gas up through the transfers. At 7500 transfers gases flow nicely but at 8000 gas velocity drops, the temperature peak from exhaust rises by over 400 degrees in the top of the transfer port compared to the 7500 rpm effect. The same happens with transfer port gas velocity and pressure. At 7500 works like a charm, at 8000, not so much. Looking at the pressure, temperature and velocity traces at both engine speeds it's clear that the transfers are in crisis at 8000 and the exhaust is out of synch.
For a race motor with good CR 6 speed trans, the package of ports and pipe may well be very different. That's phase 2
In the real world though, all we can do is to try something and see if it works. If it does, great and if not, well it's back to the drawing board.
Heads are a whole other situation. In a squish head we want maximum turbulence with a respectable MSV. As the squished gases pass the sharp edge into the bowl it creates turbulence that helps atomization and that's what we are after.
Knife edges are another issue again. Gas flow separates as it passes a knife edge, where it flows around a bullet nose. That's for example why people are often told not to knife edge the transfer port divider.
When I say low rev motor, were are talking GT's. A GT has a 64mm stroke so with a peak mean piston velocity of say 3500, that equates to 8500 revs max. So if we are looking for a nice meaty powerband, we want to focus on the 6-7k are rather than looking for peak power at 8500 and ignoring losses lower down.
That's what I mean by low revs. A kart motor by comparison is living between say 11,000 and 14,000 and they used to go much higher than that.
Header length is important up to a point, but what dictates tuned speed are gas temperature, exhaust duration and overall length. With the same pipe, if we want to move power up or down the rev range, the belly section is the place to do it. Bimotion calls for a header length of 7 to 9 times the exhaust port diameter. That works out to roughly 300 - 400 mm. A little less is OK but much longer and I'd expect drag to start to rise.
With section changes there are a number of factors at play. Larger angles create stronger pulses, but over a narrower rev range. Larger angles also tend to result in flow separation which in a 100-120mm pipe is really not too much of an issue, but could be a problem at the tail pipe entry where a venturi is a better way to merge. Changes in angle are usually limited to around 3-3.5 degrees in the divergent cones and 12 degrees or more in the baffle cone.
The thing I try to keep in mind is that almost any pipe will flow enough gas. What I need to focus on are the sound (pressure) waves moving up and down the pipe. Pressure waves are traveling at the speed of sound where the gas is moving at less than half that speed most of the time. Gas speed changes with engine speed but pressure waves change only with temperature. There is some reversion of the actual gas, but mainly it's about the pressure waves which often travel in the opposite direction to the gas.
Let's say we want to change our existing design to something shorter for more top end. We might use the same pipe with a smaller tailpipe to try to push temperatures up, or we could lean it out at the top end or retard the ignition to get more heat into the pipe. More advance at lower revs (if the motor can take it) and less at the top end might be enough to stretch a powerband.
Or we go with a shorter pipe. That means a shorter belly and that's not practical, so we have to increase the cone angles which makes for a larger diameter belly or a longer one of the same diameter - or some mix of both. If we keep the same belly diameter and go with larger included angles, that allows us to shorten the whole pipe.
If we got it right, power will go up at the top end and we will lose in the mid range.
You are right to focus on transfer ports. A GT has insufficient transfer area. At a target of say 8bar BMEP at say 7750 rpm, we have enough exhaust area to support 70 crank HP but only enough transfer area theoretically to support 56 crank HP. As revs go up, that situation gets worse, so we need to keep revs down and to make the exhaust work better to empty the cylinder and pull gas up through the transfers. At 7500 transfers gases flow nicely but at 8000 gas velocity drops, the temperature peak from exhaust rises by over 400 degrees in the top of the transfer port compared to the 7500 rpm effect. The same happens with transfer port gas velocity and pressure. At 7500 works like a charm, at 8000, not so much. Looking at the pressure, temperature and velocity traces at both engine speeds it's clear that the transfers are in crisis at 8000 and the exhaust is out of synch.
For a race motor with good CR 6 speed trans, the package of ports and pipe may well be very different. That's phase 2
In the real world though, all we can do is to try something and see if it works. If it does, great and if not, well it's back to the drawing board.