.png "Powered by vBulletin")
This thread is really only applicable to the 8 valve 400s that were available outside the US. And I suppose I should apologize in advance - always a dubious move - for not having been more scientific and all that. And no pictures.
I haven't personally owned a Twin Swirl 400 all that long; about 10k and a year [6000 miles], but the bike is original bone stock so representative of what they were to begin with. I did spend some time years ago with a friend's 'X', but it was so shot that it was hard to tell what it was like originally. My 425 seemed lie a better bet.
One of the first things that struck me was that the motor seemed a bit rough and over advanced in the 4000 - 4500 rpm range. 5K was smoother and 6k is like an electric bike. I don't recall the exact specs but the 2 valve motors had 35 degrees at 3500 and the specs for the 4 valve are 40 degrees at 4000. 10 degrees seems universal at idle.
Without getting into a religious discussion about whether high octane gas burns slower than regular - I think they start out about the same but the high octane gets less exponential at the end - it seems rather odd that a fixed 40 degrees would be optimum at 4000 and still manage to be able to burn in time at 8000. You would think that 20 degrees at 4000 would be the equivalent timing. But throttle opening restrictions of the CV carbs and cam overlap and all those gas flow factors mean that the combustion characteristics below the 6000 to 10,000 power band aren't optimal and burning is slower.
Today's motors have the possibility of any sort of advance you want to program, plus knock sensor overrides. This makes a mechanical advance seem primitive, which it is. As I see it, Suzuki took the same old advance that ran the points and just gave it a touch more throw and had it open at 4000 because that's what the motor could tolerate. Probably the same old springs and weights and such. It worked.
I experimented a bit with gasolines and found that the motor tended to knock a lot WO in the 4000 - 4500 range unless it had 91 octane. 87 was hopeless, although it would sorta work up high. 89 was fine but still had the 4K knock problem when you wound it open when it was hot. I've run enough engines to know when a motor feels advanced beyond its needs. So I decided to take a spare advance mechanism I had and modify it to give 40 degrees by about 4700. In theory this would give about 30-35 in the bad spot which is more like a two valve, and hopefully make the power band more linear, with maybe a bit less vibration.
The advance weight weighs 8 grams, but the bulk of that is around the pivot and not contributing much centrifugal force. I seat of the pants estimated that a half gram off the business end would probably be about right so I ground that much off the flat of the end so the stop would still fall in the right place. The weight is 3.5mm thick and it hit target at 3mm, starting at about the halfway point out from the pivot.
The backup plan was that I could always fall back on the original setup if I didn't like it. The test ride I used has a nice long 'dyno hill' on it where you can whack a 400 wide open in 5th or 6th and do some analysis. From the vantage point of my ears and butt - the bottom line - I think it is hitting full advance at 4800 now and that means it must be low mid thirties at 4000 which actually seems to produce equivalent power but without the harshness. In fact, the whole low range from 3K up seems more linear and of course there wouldn't be any changes anywhere else. Gas mileage hasn't changed, that I can tell, but it shouldn't.
And the harsh feeling and extra vibration at the 4K point is gone. It's not a big deal, but I can't see why I'd want to go back. Sure, on 91 octane there's no serious ping stock, but if you ever run into a situation where the gas isn't as wonderful as the headline number you'll be better off.
I haven't personally owned a Twin Swirl 400 all that long; about 10k and a year [6000 miles], but the bike is original bone stock so representative of what they were to begin with. I did spend some time years ago with a friend's 'X', but it was so shot that it was hard to tell what it was like originally. My 425 seemed lie a better bet.
One of the first things that struck me was that the motor seemed a bit rough and over advanced in the 4000 - 4500 rpm range. 5K was smoother and 6k is like an electric bike. I don't recall the exact specs but the 2 valve motors had 35 degrees at 3500 and the specs for the 4 valve are 40 degrees at 4000. 10 degrees seems universal at idle.
Without getting into a religious discussion about whether high octane gas burns slower than regular - I think they start out about the same but the high octane gets less exponential at the end - it seems rather odd that a fixed 40 degrees would be optimum at 4000 and still manage to be able to burn in time at 8000. You would think that 20 degrees at 4000 would be the equivalent timing. But throttle opening restrictions of the CV carbs and cam overlap and all those gas flow factors mean that the combustion characteristics below the 6000 to 10,000 power band aren't optimal and burning is slower.
Today's motors have the possibility of any sort of advance you want to program, plus knock sensor overrides. This makes a mechanical advance seem primitive, which it is. As I see it, Suzuki took the same old advance that ran the points and just gave it a touch more throw and had it open at 4000 because that's what the motor could tolerate. Probably the same old springs and weights and such. It worked.
I experimented a bit with gasolines and found that the motor tended to knock a lot WO in the 4000 - 4500 range unless it had 91 octane. 87 was hopeless, although it would sorta work up high. 89 was fine but still had the 4K knock problem when you wound it open when it was hot. I've run enough engines to know when a motor feels advanced beyond its needs. So I decided to take a spare advance mechanism I had and modify it to give 40 degrees by about 4700. In theory this would give about 30-35 in the bad spot which is more like a two valve, and hopefully make the power band more linear, with maybe a bit less vibration.
The advance weight weighs 8 grams, but the bulk of that is around the pivot and not contributing much centrifugal force. I seat of the pants estimated that a half gram off the business end would probably be about right so I ground that much off the flat of the end so the stop would still fall in the right place. The weight is 3.5mm thick and it hit target at 3mm, starting at about the halfway point out from the pivot.
The backup plan was that I could always fall back on the original setup if I didn't like it. The test ride I used has a nice long 'dyno hill' on it where you can whack a 400 wide open in 5th or 6th and do some analysis. From the vantage point of my ears and butt - the bottom line - I think it is hitting full advance at 4800 now and that means it must be low mid thirties at 4000 which actually seems to produce equivalent power but without the harshness. In fact, the whole low range from 3K up seems more linear and of course there wouldn't be any changes anywhere else. Gas mileage hasn't changed, that I can tell, but it shouldn't.
And the harsh feeling and extra vibration at the 4K point is gone. It's not a big deal, but I can't see why I'd want to go back. Sure, on 91 octane there's no serious ping stock, but if you ever run into a situation where the gas isn't as wonderful as the headline number you'll be better off.
Comment