Map Cycle 850cc 10.5/1 compression pistons

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I did the calculations when switching from a thick copper head gasket to a thin composite type. My recollection is that the thin gasket was 20 thou thinner than the thick gasket and bumped compression by 1/2 point.
This agrees with the JS Motorsport info. He suggests milling 40 thou from the head will increase Cr. one point, as in from 8.5 to 9.5
So .020" off the head combined with an .020" thinner gasket should do it.

Glen
 
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Unfortunately there is some misinformation on the MAP website. They claim they have the "lightest, commercially available, vintage, British bike, piston kit on the market" - but that is not true.

JSM offers both forged pistons and billet pistons. The Billet JS 4032 low expansion alloy med C.R. 73mm piston weighs only 175 grams and they are the lightest. By comparison the MAP long rod 73mm med C.R. piston is 183 grams.

The JS billet high C.R. 73mm piston is 180 grams. By comparison the MAP long rod 73mm high C.R. piston is 193 grams.

The stock rod MAP 73mm 4032 alloy med C.R. piston is 228 grams.

The JSM billet 850 pistons are lighter than the MAP pistons as well.
 
Combustion conditions are affected by the balance between three factors - comp. ratio, ignition advance and fuel to air ratio. If you change one factor, you need to adjust the other two. As you go up in comp. ratio, it is normal to retard the ignition. But in the end you have to get the carburation right. In my experience getting the best out of a race cam does not need an increase in comp. ratio. The cam does exactly the same thing at any comp. ratio. It moves the opening and closing points of the valves which determine where the power band is located in the usable rev. range. A race cam usually moves the power band up the rev range and makes it more distinct , unless only the lift has been increased.
In your situation, I would raise the comp.ratio but retard the ignition a couple of degrees, then jet to suit.
 
The thing about light pistons is that at the top and bottom of every stroke, the pistons stop and reverse direction. Inertia has a very big effect. If you fit lighter pistons into a motor and change nothing else, the effect is very noticeable. I did it once with a Triumph 650 motor - it became much faster. However the pistons I used were flat top out of a 350cc Gold Star BSA, and i had re-shaoed the crowns to fit the head and give some squish. Triumph 650s never had that.
The other thing about light pistons is they move the balance factor in the right direction.
 
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Al, a race cam usually has more duration than a road cam. This means the combustion chamber looses charge, lowering its effective compression ratio.

Put low comp pistons with a race cam and the cam is most certainly not going to give the same results. If it did, I think the top tuning outfits in the world might have worked it out by now...
 
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Fast Eddie said:
Al, a race cam usually has more duration than a road cam. This means the combustion chamber looses charge, lowering its effective compression ratio.

Put low comp pistons with a race cam and the cam is most certainly not going to same results. If it did, I think the top tuning outfits in the world might have worked it out by now...
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Silly Eddie :eek:

Just lower the compression and run it on methanol til it coughs

Then put it in your shed and talk about for the next 30 years

It'll never break, and as time goes by it'll get to be able to outperform MotoGP bikes while still on the original perished tyres :p
 
Seeley920 said:
Silly Eddie :eek:

Just lower the compression and run it on methanol til it coughs

Then put it in your shed and talk about for the next 30 years

It'll never break, and as time goes by it'll get to be able to outperform MotoGP bikes while still on the original perished tyres :p
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Rather a sad turn to the conversation really. Seems to be designed to hurt rather than inform.
 
Seeley920 said:
Silly Eddie :eek:

Just lower the compression and run it on methanol til it coughs

Then put it in your shed and talk about for the next 30 years

It'll never break, and as time goes by it'll get to be able to outperform MotoGP bikes while still on the original perished tyres :p
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There are lot's of old wives' tales :
You need high comp. to get the best out of methanol.
If you've got a torquey motor, you don't need a close ratio gearbox.
You need high comp. to get the best out of a race cam.

What is an independent variable ?
 
With high comp. you use more fuel to get the combustion temperatures right, so you get more power. But more fuel means less air and it is the expansion of the air in the mixture which gives you power. The gains are never what you might expect.
 
Al, where are you going with this?

In general terms, within the various limits of the components and the package at hand, higher compression ratio engines generally give more power. It’s not an old wives tale at all !

Of course, go too high and you’re very quickly into the law of diminishing returns and increasing negatives, but below that threshold, no.

Compression ratios in modern engines are high, because with knock sensors and good combustion chamber design, they can avoid a lot of the negatives. But engine makers know that is the best way to get the best out of engines, best power, best fuel consumption. Period.

With our old bikes, of course, you ain‘t gonna turn a plodder into a racer just by bunging in high comp pistons. But as I’ve argued before, if your building an engine with a package of head work, race cam, correctly designed pipe, etc, then usually, a decent CR is a part of that package. Well, it is if you wanna go fast.
 
bsaboss said:
Rather a sad turn to the conversation really. Seems to be designed to hurt rather than inform.
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True.
But sometimes people just get frustrated by posters who continue to post the same miss informed personal opinions.
 
worntorn said:
I did the calculations when switching from a thick copper head gasket to a thin composite type. My recollection is that the thin gasket was 20 thou thinner than the thick gasket and bumped compression by 1/2 point.
This agrees with the JS Motorsport info. He suggests milling 40 thou from the head will increase Cr. one point, as in from 8.5 to 9.5
So .020" off the head combined with an .020" thinner gasket should do it.

Glen
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Thanks Glen for your input, the only useful information received. I have no intention to change to a race cam, or big bore options, I have a stock bore engine that could do with a little wake up, nothing else. There is a certain benefit from raising compression at least 1 - that is all I asked about.
 
Basically, on a stock 828cc motor, 1mm of the swept volume = 4.66cc approx.

So, rough rule of thumb type thinking would say that .020” is worth approx 2.33cc.

IF (that’s a capital IF) your motor is at 8.5:1 with MAP pistons and stock gaskets, then taking 2.33cc out of the equation with a .020” thinner gasket would make it 8.83:1 I think.

I said ‘capital IF’ because aftermarket piston makers are often only ‘approximate’ at best with their claims plus you don’t know if your head or barrel or crankcase mouth has ever been skimmed or valve seats cut, etc. So, if you wanna know what’s what, you really need to measure your combustion chamber volume with the new pistons fitted and work things out from there.

I measured my 850 before stripping it down and it was 7.6:1...
 
I've measured things with a disassembled MK3 850 and got 8.5 to one, stock bore, same as the big White book claims. On that one you can see the position that the top ring reached on the bore, so it was easy enough to measure to get TDC piston height +-5 thou
I got roughly the same results with my running MK3 , but I cannot recall if that was with the thin gasket or the copper gasket.

Glen
 
Maybe mine had thicker baskets or sumthin?

It did have a base gasket, I thought it shouldn’t (1974 850). The parts book I have shows no base gasket, but AN do show one. Anyone know which is correct?

Also, I recall a recent thread suggesting that AN had increased the thickness of their head gaskets, so it’s easy to get caught out.
 
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Also from that thread, Comnoz informed that piston height on stock bikes varies a bit.

Glen
 
If you increase the comp.ratio, it is normal to increase the size of your jets or retard the ignition or both. Sometimes you end up back where you started from. I run methanol fuel at standard comp. and get good power. My brother runs an 880 JAP at 16 to 1 comp. and it really flies. But when you do that, you have to live with the consequences of fragility and unreliability.
Jim Schmidt sells 12 to 1 comp. light pistons and long rods. The piston crowns are probably domed. If you look at 12 to 1 comp. pistons out of any race Triumph 650, you will always see they have coke on the side of the crown which is away from the spark plug.. On a Triumph 650 motor, it is possible to twin-plug the cylinder head.
 
I just measured the gaskets on this all original 10K mile MK3 - base gasket 20 thou, composite head gasket (after use) 33 thou. RH4 head.
I also measured a few new copper gaskets on hand they are 40 thou, and a new composite gasket is 35 thou.

Wondering about not using the base-gasket - (-20 thou) and with a new composite head gasket, which means for a 40 thou change for 1 point of compression I would have to machine 20 thou off the head.
Would no base gasket adversely affect the cam/lifter geometry?
 
bushman said:
Wondering about not using the base-gasket - (-20 thou) and with a new composite head gasket, which means for a 40 thou change for 1 point of compression I would have to machine 20 thou off the head.
Would no base gasket adversely affect the cam/lifter geometry?
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The mk 2 and mk2a 850´s come originally with no base gaskets so no problem.
I went the Map billet pistons and steel rod route .and am more than happy with the out come.
 
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