Interesting Video on Quench and Compression

[Dan1950]

 

[jseng1]

Quench is great but there's no room for in on a Norton unless you go to a large bore with the same valve sizes. Crowding the valves on the sides with a bathtub inhibits flow and reduces top end HP. Reshaping the piston for a squish usually adds weight and then your motor blows up. These things make it difficult to improve the Norton combustion chamber.

 

[maaseyracer]

Very interesting I had a discussion with a friend on this subject yesterday. His takeaway is that modern fuel gets a bad rap and it is better than the fuels of yester-year and largely misunderstood. His standpoint was that modern fuels burn faster so you can run the same compressions, however you would need to retard the timing 17 to 18 percent to compensate for the faster burn rate. This would be the equivalent of timing your norton to 25 or 26 degrees on 91 octane.

 

[Triton Thrasher]

you need to retard the timing 17 to 18 percent

Frankly, that sounds highly unlikely.

 

[Dan1950]

Quench is great but there's no room for in on a Norton unless you go to a large bore with the same valve sizes. Crowding the valves on the sides with a bathtub inhibits flow and reduces top end HP. Reshaping the piston for a squish usually adds weight and then your motor blows up. These things make it difficult to improve the Norton combustion chamber.

Although the Norton combustion chamber is not a "bathtub" and subsequently do not exhibit as much quench, IME it will still benefit from the small quench area that is present. The area in question will stil have reduced carbon deposits and eliminate hot spots thus reducing the likelihood of detonation.

 

[maaseyracer]

Frankly, that sounds highly unlikely.

I think i remember Jim Comstock posting a while back that he had bikes running well at 26 degrees which would be 17 to 18 percent from the factory 30 degrees.

 

[Schwany]

What I'd like to know is when are you going to finish that 850 motor Dan?

I watched a couple minutes of the video, then remembered I needed to clean out my gutters. 200 foot tall Douglas firs shed a lot of needles. Seemed more important to get that done while the sun is shining. I'll have to finish watching it later. I did like the sounds he made for detonation and run on. Spot on

Nortons are not water cooled like hot rod V8 cars are, which is sort of why I decided to clean my gutters.

 

[Triton Thrasher]

I think i remember Jim Comstock posting a while back that he had bikes running well at 26 degrees which would be 17 to 18 percent from the factory 30 degrees.

And that was a standard bike, retarded 17 percent for “modern fuel?”

 

[maaseyracer]

And that was a standard bike, retarded 17 percent for “modern fuel?”

The video subject to this thread is about high compression, pump gas, and driving on the street. It does not have much to do with stock or standard engines.

 

[Triton Thrasher]

The video subject to this thread is about high compression, pump gas, and driving on the street. It does not have much to do with stock or standard engines.

Ok, does Mr Comstock say that any engine which ran well at 30 degree on old fuel, need 25 degrees (17% less advance) to run on modern fuel, with no other changes?

All I’m saying is 17% less advance sounds a bit drastic for a change of fuel.

 

[jseng1]

For quench to work you need .040 to .045" squish band clearance. None of the stock Nortons have that. It requires high compression pistons and racing fuel. The 750 squish band is probably too narrow to be effective but the 850 and bigger bores have enough squish band width.

 

[Dan1950]

For quench to work you need .040 to .045" squish band clearance. None of the stock Nortons have that. It requires high compression pistons and racing fuel. The 750 squish band is probably too narrow to be effective but the 850 and bigger bores have enough squish band width.

I think I have the . 040"-.045" clearance covered. 850 .020" over. 77.5mm bore.

 

[acotrel]

If I race, I always use methanol fuel because it has unlimited octane rating and heat problems are ver rare. However I could get similar results if I used Elf racing fuel , which is of consistent quality. Petrol has about 1.2 times the calorific value of methanol.
Combustion conditions depend on about 4 variables - fuel octane ratio, compression ratio, fuel/air ratio and ignition advance.
The four factors must be optimised for three things - best power, reliability and fuel consumption. If you change one factor, you must adjust at least one other.
A 350cc Manx Norton usually has a squish-band right around the combustion chamber, in a Commando, the squish band is around the rear of the combustion chamber.
My friend has a 350 Manx, which was brought to Australia by Jack Ahearn. It was actually a works bike in the early 60s. There are two squish bands in the cylinder head combustion chamber. The one which was higher-up has been welded- up and remachined tp remove it. Norton were obviously experimenting. I very much doubt that the squish band in the Commando engine would not be an optimum prescribed by development. The rest of the motor is excellent when raced with the right gearbox.
A lot of guys on this forum have raised compression ratios and appear to have gained more power. Doing that without changing anything else , has the same effect as leaning-off the jetting or increasing the ignition advance,without raising the comp. ratio. You get more power, but at what cost ?
Two-srtroke racing motors are tuned near the absolute limit. The critical compenent is the ignition system. Sometimes cylinder heads are reshaped, but it is rare that compreesion ratios are increased. The jetting of the carbs is used to compensate for the differences in the other three variables.
With a four-stroke racing motor, it is common to retard the ignition when using very high compression, however I suspect that for one given compression ratio, there are several combinations of the other variables which will give almost exactly the same result.
With two-stroke motors, when detonation occurs, it is usually in the squish-band.

The engine in the Vanwall racing car, was based on the Manx Norton. The technology has to come from somewhere. And the best racing car drivers in the old days usually started by racing motorcycles. Dave Roper was the only American to ever win a race on the IOM. He did not ride a Harley.

Brabham, Rosemeyer, Nuvolari, Hailwood, Surtees. - all motorcyclists.

 

[acotrel]

I think I have the . 040"-.045" clearance covered. 850 .020" over. 77.5mm bore.

If you look at a used 12 to 1 comp Triumph 650 piston, there is always coke on the side of the crown which is away from the spark plug. What does that tell you ? Nortons are always better. I had Triumphs for years, the best was when I fitted 350 BSA Gold Star pistons into a 650 and fitted the crowns closely to the sides of the combustion chambers. I
It makes a difference.