Exhaust For Lightly Tuned 750

ksoholm

ksoholm

Senior member
Location
Bellingham, WA, USA
Lads,

My SF2 project has 10.5:1 pistons, 5C cams (Megacycle), blueprinted heads. I've been through the posts, and there seems to be evidence that hi-comp pistons and bumpy cams like something freer flowing than the stock exhaust. The esteemed Piet says dyno testing showed the SFC 2-2 with X-crossover as being better all-around than the factory 2-1; others say the 2-1 is best. Is that about right? This won't be for sweet, gentle, rides through flowery scenery, but for tearing around chasing pals on fast Nortons, so noise is fine. I wear earplugs anyway :).

Thanks -
 
What are you looking for, noise or performance? SFC type 2/2 is probably good and a bit less noisy than the 2/1 which puts out 120 dB. Don't forget that in both cases, you need to junk the centre stand and that a Norton weighs 40 kg less. You need to be a good rider above all else.

Paul
 
Having both systems for my SFC I find the 2/1 is my preferred exhaust for hooning around. Bike goes way better.
I think it was Marnix who summed it up by saying the 2/2 makes the SFC go like a well tuned SF compared with the 2/1 that makes the SFC go like it should and I totally agree. Yes it's noisy but I don't commute on it and when I'm riding the SFC I'm hooning it.
I'd go with the 2/1 and if noise is a problem fit one of Malcolm Cox's baffled 2/1s
 
The best-sounding SF twin I ever heard was Steve Bs race bike, it would draw a crowd anytime it got started. It looked like 2 header pipes joined to some kind of huge collector under the engine. It was pretty loud but what a great sound.
 
That bike and its proper SFC brother reawakened my Laverda interest, my 3c had sat shed-bound for a couple of years, I was seriously Dirtbiking a lot at that time and after seeing both bikes and especially hearing them at the 2000 last Harrar of the Bathurst Bike racers got me dragging my 3c out of hibernation and rejoining the NSW Laverda Club and doing lots of club rides. Don't underestimate the power of watching bikes racing to get bums on seats. We went from club nights of 2 and 3 blokes to typically 10 and more after The Barry and The V6 came to Oz.
 
motoddrob said:
Having both systems for my SFC I find the 2/1 is my preferred exhaust for hooning around. Bike goes way better.
I think it was Marnix who summed it up by saying the 2/2 makes the SFC go like a well tuned SF compared with the 2/1 that makes the SFC go like it should and I totally agree. Yes it's noisy but I don't commute on it and when I'm riding the SFC I'm hooning it.
I'd go with the 2/1 and if noise is a problem fit one of Malcolm Cox's baffled 2/1s
Click to expand...
Thread was for a lightly tuned 750. Of course, the disc brake SFC runs much better on the 2/1.
If anything else than a lightly tuned 750, one would need to start with a much lightened crank and a close ration gearbox, then worry about exhaust.
A well ridden Norton will leave a 750 Laverda for dead.

Paul
 
Generally speaking, any tuned Lav750 will perform better with the SFC 2 into 1. Once you put a silencer in it, you instantly lose midrange power. In my experience, the best solution is to place an additional absorption damper at the end of the SFC megaphone.

I think Piet’s experience with the 2 into 2 silencer was based on a particular SFC (Georg Kühnert) that was intensively ported and with a modified combustion chamber shape too.

What worries me a bit are the 10,5 : 1 pistons. If you have real 10,5 compression then with an SF type head detonation is very likely to occur.
In case you have pistons from an Electronica SFC, those will only give a 10,5 compression in combination with an (all aluminium) Electronica cylinder head. If you mount them in combination with an SF head, you end up with less compression than with standard SF pistons.

Marnix
 
MarnixSFC said:
Generally speaking, any tuned Lav750 will perform better with the SFC 2 into 1. Once you put a silencer in it, you instantly lose midrange power. In my experience, the best solution is to place an additional absorption damper at the end of the SFC megaphone.

I think Piet’s experience with the 2 into 2 silencer was based on a particular SFC (Georg Kühnert) that was intensively ported and with a modified combustion chamber shape too.

What worries me a bit are the 10,5 : 1 pistons. If you have real 10,5 compression then with an SF type head detonation is very likely to occur.
In case you have pistons from an Electronica SFC, those will only give a 10,5 compression in combination with an (all aluminium) Electronica cylinder head. If you mount them in combination with an SF head, you end up with less compression than with standard SF pistons.

Marnix
Click to expand...
I should think that it depends on tuning Marnix. You might want more back pressure than the 2/1 can offer.
10.5/1 pistons other than Electronica shouldn't be a problem to find. I put some in an engine with a lightened crank, a 5C cam and an SF head decades ago, with no dire consequences. Sourced the pistons at Phil Todds I should think.

Paul
 
I currently have 8 750 twins machines with big valve engines, all in different state of tune with 4/S, 2/C, 5/C, 6/C and special grinds, so have pretty good comparison.
They all love the 2 into 1.
Some have 2 into 2 megaphones (5000 batch SFC and the Segoni with 1972 works 2 into 2 meggas) but they don’t make these engines perform better. I only keep them on for originality’s sake.

Apart from the Elettronica, the highest compression pistons I use are SFC 74 type pistons (photo) which give 9,8 : 1 compression, and which is pretty much the limit in combination with cast iron skulls. I know that Phil did some additional skimming of heads, but don't know what that gave in terms of reaching the 10,5 compression ratio you mention.

Marnix
 

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MarnixSFC said:
I currently have 8 750 twins machines with big valve engines, all in different state of tune with 4/S, 2/C, 5/C, 6/C and special grinds, so have pretty good comparison.
They all love the 2 into 1.
Some have 2 into 2 megaphones (5000 batch SFC and the Segoni with 1972 works 2 into 2 meggas) but they don’t make these engines perform better. I only keep them on for originality’s sake.

Apart from the Elettronica, the highest compression pistons I use are SFC 74 type pistons (photo) which give 9,8 : 1 compression, which is pretty much the limit in combination with cast iron skulls. I know that Phil did some additional skimming of heads, but don't know what that gave in terms of reaching the 10,5 compression ratio you mention.

Marnix
Click to expand...
Those are very lumpy cams with important overlap you have in those engines Marnix. It seems pretty futile discussing exhausts when one doesn't know what engine we're talking about.

As for the pistons I put in the race bike I built, I honestly don't know the compression ration. I asked for high compression pistons and threw them in the engine. I'm citing from memory, it was probably a good 25 years ago.

Paul
 
MarnixSFC said:
Generally speaking, any tuned Lav750 will perform better with the SFC 2 into 1. Once you put a silencer in it, you instantly lose midrange power. In my experience, the best solution is to place an additional absorption damper at the end of the SFC megaphone.

I think Piet’s experience with the 2 into 2 silencer was based on a particular SFC (Georg Kühnert) that was intensively ported and with a modified combustion chamber shape too.

What worries me a bit are the 10,5 : 1 pistons. If you have real 10,5 compression then with an SF type head detonation is very likely to occur.
In case you have pistons from an Electronica SFC, those will only give a 10,5 compression in combination with an (all aluminium) Electronica cylinder head. If you mount them in combination with an SF head, you end up with less compression than with standard SF pistons.

Marnix
Click to expand...
Was simply a ported Electronica head Marnix, can't remember any combustion chamber alteration, valves remained stock. Don't think Georg would have been willing to have that head welded, I waved the magic wand over the valve seats... ;) Apart from a DMC ignition and the oiling/filter system, the remainder of the engine was stock. The beast pushed out a very healthy 85hp with over 80Nm, still on 750cm³. Can't remember the revs, but the porting was overall very sucessful!

Our findings were established on the Moto Witt dyno. Engine breathed best on 5C cams and stock 2-2 exhaust with X-tube. 6C gave only a modest increase at the very top of the rev range and only with the 2-1, the 2-2 would strangle them. 5C made more torque low down and provided the more pleasurable riding experience. Imho, the 6C and 2-1 are track only, whereas the 5C and 2-2 are equally at home on the open road. Both are a right bitch in traffic, especially when coupled with the CR gearbox. I rode that bike through rush hour Birmingham/UK once (on the way home from Baskerville, '92(?) ), a truely distressing experience, and I was a lot fitter then!

Experiments with modern exhaust cans hooked to the stock headers resulted in only very marginal improvements.

piet
 
Interesting to read Piet's assessment of 5C cams plus stock road exhaust with X-over as breathing well. My 74 SFC runs well with that system and doesn't seem unduly strangled at high revs. Sure, the 2-1 makes a nice noise but I don't think it provides a significant improvement to the point that I wouldn't use the road system on track. Maybe just as well since the open 2-1 registers 118dBA compared with the 2-2 at 98dBA! (tested at 5500rpm per ACU requirement)
 
I had to use the 2/2 for a track day at Mallory once due to noise restriction. Think the cutoff was 105db.
From my experience the 2/1 wakens the SFC up a little, as well as the neighbores!!!
I felt really bad once when parked in my sleepy town, a old couple were window glazing in the local gallery right next to where I was paerked and when I started the SFC they must have jumped a foot and gave me the filthiest look.
 
motoddrob said:
I had to use the 2/2 for a track day at Mallory once due to noise restriction. Think the cutoff was 105db.
From my experience the 2/1 wakens the SFC up a little, as well as the neighbores!!!
I felt really bad once when parked in my sleepy town, a old couple were window glazing in the local gallery right next to where I was paerked and when I started the SFC they must have jumped a foot and gave me the filthiest look.
Click to expand...
Good on them for still window glazing at their age! They must have had good backs, most glaziers have fucked backs by the time they are forty,
 
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