Cams road test PW3 vs JS2

Ken, Peel's engine is Maney 920 and standard stroke with shaved Combat head. I thought you were able to keep the chambers smallish 43-44 ml and CR close to 10.5 with JSM pistons and some gasket thickness fudging keeping squish at most .045".

hobot said:

Ken, Peel's engine is Maney 920 and standard stroke with shaved Combat head. I thought you were able to keep the chambers smallish 43-44 ml and CR close to 10.5 with JSM pistons and some gasket thickness fudging keeping squish at most .045".

Click to expand...

Hobot, Jim S suggests adjusting the CR with different head gaskets, and on a 920, if you've got 43-44cc combustion chamber volume, you're going to totally sacrifice your squish band if you want circa 10:1 CR.
The figures that you suggest will put you way over 11:1.
Reggie (poster on here) has a Maney 920, its 9.8:1 CR and is running a squish band tight enough to work (so est .045-.050), his piston were dished by approx .075 to achieve this.
Your JS pistons are only approx .175 thick at the crowns, so that's clearly not an option for you.

One thing about squish is that it is also quench, so even though the CR goes up to get a proper squish clearance, the tendency toward detonation is reduced by the quench effect. I was recently reading about hotrodded 351 Cleveland Ford owners having detonation problems on premium pump gas running 11 to one and even 12 to one CR and hot cams. They solve the detonation while maintaining the same CR or even higher by switching to squish heads, which Ford did make for the 4 bbl version of that engine.. At the same time they saw big increases in power output with the squish/quench, especially in midrange.
This is all fine for in theory with a Commando, however where that magic number is for ideal squish clearance and maximum CR without detonation for various configurations is something that can only be learned by trial and error.

One other factor which can eliminate detonation problems is a double curve ignition controlled by vacumn. When detonation is generally at its worst is on a hot day, bike loaded and pulling a hill in top gear. In this situation the intake vacumn drops so this type of ignition automatically drops down onto the less advanced curve until vacumn increases as rpm climbs back up or throttle is shut down (hill levels off).
Some of us are running Vincents at around 10.5 to one using this ignition, no detonation problems, some have done very high miles without problems. The igntion is Harley Screaming Eagle EI ignition ealtered to work on the Vincent.

I wonder if any such ignition exists for a Commando?

Glen

Thanx for the bing bang boom reviewing Fastie and wornout. I've read reports of hemi's with and w/o squish bands working well in Norton's and others, as well as failures. I don't know how Peel with Norris D cam will work when the going gets heated but I know I'm pressing limits and luck. Peel is meant for long term daily use - after she is has some shoot outs with elite moderns and some land speed and drag tests. Realistically Peel must exceed 100 ftlb and 130 hp to not get caught back up with in 1/2 mile opens after turns. I will try to do it on pump gas but if not then hi octane for the contests then de-tune for real life. Peel has a few tricks up her skirt though, waste injection and digi ignition with vacuum/boost sensors for ign. curve switchers plus plasma blasting non spark combustion assist that uses the tiny electrical spark as conduction pathway for golf ball size bursts. Combustion surfaces coatings too. Online calculators imply mid 7's CR with the late intake valve closing for starts and lugging around low rpm use. Peel 750 head bored out for 920 pistons gives 3/8" ish wide squish bands so might be enough to add an effective Singh Groove or two after I observe where the carbon collects on chamber and piston to guide best position and aim angle of groove. I'm spending money this season on data logging equip that may be able to share around for fun and education. Notice what a bit of humidity does... which also eats up plugs but easy enough to change regularly in a Norton. Peel has higher voltage coil than this one uses...

http://www.youtube.com/watch?v=Z6QgG7iORgM

Fast Eddie said:

One of the things that stopped me going for the 920 option on my build (also using JS long rods and light pistons) is thus: with flat topped pistons, set to utilise the squish band, the CR is just too high for street use.
I know some say the squish band is not important on a Norton, and I have no Dyno proof to the contrary, but, especially when opened out to 81mm, there is a very nice squish area that, in my opinion, is sacrilege to ignore!
I thought about machining a shallow 'dish' into the piston crown to lower the CR and maintain the squish, but there just isn't adequate spare metal in the JS piston crowns (I have since learned that this is exactly what Maney does with his pistons, but they're only suitable for stock length rods).
So, being faced with a long rod-light piston 850 OR a short rod 'heavy piston' 920, after a lot of deliberation, I went for the 850 option.
It seems to me there is a market for slightly modified pistons suitable for long rod 920s. Not sure quite how big that market is though!

Click to expand...

I could make such dished pistons as you describe but they would be special order and you would have to order 4 of them because there is not enough demand to stock them. They would also cost more like my domed pistons.

hobot said:

Ken, Peel's engine is Maney 920 and standard stroke with shaved Combat head. I thought you were able to keep the chambers smallish 43-44 ml and CR close to 10.5 with JSM pistons and some gasket thickness fudging keeping squish at most .045".

Click to expand...

Sorry, Steve, but that's not quite right. Your head has 45.8 cc combustion chamber volume, slightly larger than the 45.0 cc for the one I mentioned above, but that's not enough difference to get it down to 10.5:1. If you recall, we originally started out with a Cosworth piston (old Maney kit before he switched to JE) for your 920, that would have given a CR of 12:1. We were planning on machining the top of the piston to bring the CR down. When you switched to the JS kit that changed. The JS piston has .070" less deck height than the Cosworth did, which brings the CR down around 10:1, but also increases the squish clearance by the same amount. These numbers are all with a .040" head gasket. I won't have exact numbers until I have the engine together and measure the actual height of the piston above the cylinder. At that point, we can juggle the CR to suit you with gasket thickness, but I doubt that you can get the lower CR and still have a squish clearance in the .040" to .045" range I usually go for. I should have the actual measurements soon, and you will be able to choose where you really want to go with the CR.

I should point out that there are a whole lot of stock Nortons running around quite well with large squish clearances from the factory. Recall also that the factory short stroke 750 had a full hemi combustion chamber with no squish area at all, and made a pretty effective race engine.

Ken

Ok Ken thanks for showing off your good memory with lots of engines you deal with. Long distance engine romances are hard to keep track of. Peel is experimental in a lot of ways and squish effect ain't the whole story in hemi anti-detonation power. If no valve clash with .020" gasket then that's what I'd like to start off with. I think the counter sunk wire will be needed to try to seal the expansive pressures regardless. Just need brake hoses ordered for last major parts missing then can start wiring and final finishing assembly this spring. If no joy with current pistons and anti-detonation features then JMS dished will have to be spend on. De sided to slice off Drouin lower chin for fouling and may it not work out so want basic hot rod 920 and just forget about spanking moderns in the longer opens. Yet if does work out as planned OH LA LA.

I am still under the impression that the Commando engine is superior to the Triumph of the same capacity because of the Norton's squish band and it's longer stroke. I suggest it depends on whether you set the motor up for more midrange power and use higher overall gearing. If you don't do all the expensive stuff to a commando engine, the sensible rev limit stays at 6,500 RPM. My friend held the championship in the up to '62 historic road race class for several years with a Triton 750 - the cylinder head was Triumph, however no other part on it was made by the factory, and even the frame was a featherbed replica. He spent more on his historic bike than keeping his son on a superbike ($40,000 per year). In the end he could not keep beating the Nortons.

jseng1 said:

Fast Eddie said:

One of the things that stopped me going for the 920 option on my build (also using JS long rods and light pistons) is thus: with flat topped pistons, set to utilise the squish band, the CR is just too high for street use.
I know some say the squish band is not important on a Norton, and I have no Dyno proof to the contrary, but, especially when opened out to 81mm, there is a very nice squish area that, in my opinion, is sacrilege to ignore!
I thought about machining a shallow 'dish' into the piston crown to lower the CR and maintain the squish, but there just isn't adequate spare metal in the JS piston crowns (I have since learned that this is exactly what Maney does with his pistons, but they're only suitable for stock length rods).
So, being faced with a long rod-light piston 850 OR a short rod 'heavy piston' 920, after a lot of deliberation, I went for the 850 option.
It seems to me there is a market for slightly modified pistons suitable for long rod 920s. Not sure quite how big that market is though!

Click to expand...

I could make such dished pistons as you describe but they would be special order and you would have to order 4 of them because there is not enough demand to stock them. They would also cost more like my domed pistons.

Click to expand...

Oh... you never mentioned this option when I enquired about dished 920 pistons Jim. That's a big shame, as I would have gone for it.

Fast Eddie said:

jseng1 said:

Fast Eddie said:

One of the things that stopped me going for the 920 option on my build (also using JS long rods and light pistons) is thus: with flat topped pistons, set to utilise the squish band, the CR is just too high for street use.
I know some say the squish band is not important on a Norton, and I have no Dyno proof to the contrary, but, especially when opened out to 81mm, there is a very nice squish area that, in my opinion, is sacrilege to ignore!
I thought about machining a shallow 'dish' into the piston crown to lower the CR and maintain the squish, but there just isn't adequate spare metal in the JS piston crowns (I have since learned that this is exactly what Maney does with his pistons, but they're only suitable for stock length rods).
So, being faced with a long rod-light piston 850 OR a short rod 'heavy piston' 920, after a lot of deliberation, I went for the 850 option.
It seems to me there is a market for slightly modified pistons suitable for long rod 920s. Not sure quite how big that market is though!

Click to expand...

I could make such dished pistons as you describe but they would be special order and you would have to order 4 of them because there is not enough demand to stock them. They would also cost more like my domed pistons.

Click to expand...

Oh... you never mentioned this option when I enquired about dished 920 pistons Jim. That's a big shame, as I would have gone for it.

Click to expand...

I don't have everything at once. I add as time goes on. Somethings work out and others don't and often I am just too slammed to get anything done and can't take on any extra work. But I have made some special order pistons on occasion when time permits.

How do dished top pistons compare, combustion and bhp wise with pistons that don't come quite as high up the bore at TDC. ?

In general a flat top piston has better flame travel so less detonation prone so dished pistons are only used as lesser evil to lower CR in some cases.

Rohan said:

How do dished top pistons compare, combustion and bhp wise with pistons that don't come quite as high up the bore at TDC. ?

Click to expand...

I think the main thing is that the squish band should be about 30 thou inch. The comp.ratio is a different problem.

Re: Cams road test PW3 vs JS

Rohan said:

How do dished top pistons compare, combustion and bhp wise with pistons that don't come quite as high up the bore at TDC. ?

Click to expand...

I don't know. But my thoughts are that as we are only talking about a dish depth of .075 inch or there about, the effect of the dish itself will be undetectable. But the benefit of getting a tight squish band AND ideal CR will be very much worth having.
In response to your Vincent info, I also ran a 1330cc Vin at 10:1 CR with no pinking at all on pump fuel, but it had squish heads!

acotrel said:

Rohan said:

How do dished top pistons compare, combustion and bhp wise with pistons that don't come quite as high up the bore at TDC. ?

Click to expand...

I think the main thing is that the squish band should be about 30 thou inch. The comp.ratio is a different problem.

Click to expand...

Not for a Commando, Alan. .030" is too close for most Commando builds. The minimum squish clearance depends on several factors, including whether you are using alloy, steel, or titanium rods, what the bore size and piston clearance are (the piston does rock, you know), piston skirt length and design, crankshaft design (stock, billet, heavy, light), and most importantly, how hard the engine gets flogged! For a standard 750 I've gone as low as .035" in race engines, but generally try for .040". I've torn down an 878 cc race engine with billet crankshaft and titanium rods set up at .020" squish, and the pistons were hitting the heads so hard it broke the crankcases. The top ring groove had been hammered so hard it seized the piston. I've recently heard from Jim Comstock that he has had to set the squish as high as .055" on some big bore, short piston builds to keep from hitting the head. Commando engines are definitely not in the "one size fits all" category.

Ken

Hello Ken,

Care to comment further on how much of a factor aluminum versus cast iron barrels factor into quench band clearance?

Makes sense that , aside from the other factors mentioned, a cast iron barrel /aluminium rod motor would need the greatest cold squish clearance, cast iron barrel /steel rod somewhat less, aluminium barrel /steel rod even less, an aluminium barrel and ti rod lower yet, due to the very low thermal exp. coefficient for Ti. I am currently building a non Norton motor with aluminium barrels and steel rods, squish clearance is designed for. 035" +- .005" on that one, hope it works!

Glen

Agreed though I am not sure temperature is as much a factor with connecting rods as is stress. Titanium has 1/3 the modulus (thus three times the springiness) of steel for a given stress. I think connecting rod influence on allowable quench band clearances has more to do with the strain resulting from the stress than thermal coefficient of expansion.

I hold that in the case of 360" twins cranks supported only on the ends, its the hi rpm crank shaft 'jump rope' shape bending strain with pistons thrust a bit higher at TDC that require enough squish to avoid a smash. Al rods are the best for extreme pressure combustion loads as act as shock absorbers but steel is better for hi rpm as less springy. Ken you have hard logic points to ignore so maybe Peel should just go with .040" gasket and hold rpm to 8000 max for dyno and track tests only. Signh grooves are said to stop working if gap is over .055".

Ok now get back to degree-ing testing of the JS2 please.

I've never really thought about adjusting the squish thickness, I only know what is sometimes used on a 500cc manx, and obviously a commando engine is much more sloppy. My 850 motor has had the bare minimum done to it, and from experience I know it is fast enough for the short circuit especially with a good gearbox, and on methanol. I've ridden quite a few old racing bikes, and this one is the nicest. It inspires a lot of confidence, except for the potential big bang which is always there waiting to strike at the first sign of carelessness. When I get rich again, I'll be buying Jim Schmidt's rods and light 12 to one comp pistons.