Map Cycle 850cc 10.5/1 compression pistons

APRRSV

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Hi all,
I will be building an 850 motor from scratch. I will be using a used (well checked and refurbished) lower end, a new barrel and a Fullauto head.
I'd like opinions on the Map 8.5 vs 10.5 pistons. The bike will not be used for racing but I am looking for spirited performance.
Are the 10.5 pistons too much for a road bike?
Thanks,
Ed
 

Fast Eddie

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Can you post pics of the MAP pistons? I’d like to see if they’re flat topped.

This topic will always generate varied answers. I’d say you really need to think about your desired intent for the bike and the riding style it’ll be used for. The stock set up works very well. But if you do want “more spirited“ riding, this can be catered for.

I used 10.5:1 JS pistons in my 850. The piston crown height is raised in order to get the compression ratio up but they are still flat topped.

This has the added advantage of tightening up the squish band, and with a bit of careful measuring and adjusting you can get the squish gap to somewhere between .030”-.040”. At this gap, in my opinion, the squish works.

I’ve never had any issue with running the 10.5:1 CR on my 850 (and in in my 920 I’m running 11:1 without issue). I’m running a JS1 cam and Comnoz flowed head on my 850 and can confirm it transforms the performance (JS1 is a bit hotter than stock but still mild compared to a 2S or PW3 etc).

Having a working squish band allows for a higher CR without detonation. It’s especially handy on a road bike where you’re not using race fuel.

HOWEVER...

What cam are you using?

In general, higher CR is used in conjunction with sportier cams. CR ratio is usually measured as a ‘static CR’.

Then there is ‘effective CR’ (I think that’s the right phrase) which refers to the CR when the engine is running. Sportier cams with higher lift and duration tend to lose some CR due to this. Eg the Combat had high CR, but it also had a 2S cam.

What I’m getting at in a roundabout way is, 10.5:1 may be too much for use in conjunction with a road cam. It May be fine, but I don’t know cos I’ve never tried it. Hopefully someone who has will chime in.

Maybe consider a mildly more aggressive cam?
 
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gortnipper

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As Nigel says, what cam?

I run 9.5:1 with a Webcam 312a with a ported big valve FA head.

It is very spirited, but that combo moves all of the fun >4500 rpm.

A LOT of fun.
 
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I think a lot also depends on what fuel you have available. When I went for a CR or around 10.5:1, there were quite a few US people who proclaimed that I would not be able to find suitably high octane fuel. It seems that here in Europe we do have better fuel grades than in the US?
 

APRRSV

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Can you post pics of the MAP pistons? I’d like to see if they’re flat topped.

This topic will always generate varied answers. I’d say you really need to think about your desired intent for the bike and the riding style it’ll be used for. The stock set up works very well. But if you do want “more spirited“ riding, this can be catered for.

I used 10.5:1 JS pistons in my 850. The piston crown height is raised in order to get the compression ratio up but they are still flat topped.

This has the added advantage of tightening up the squish band, and with a bit of careful measuring and adjusting you can get the squish gap to somewhere between .030”-.040”. At this gap, in my opinion, the squish works.

I’ve never had any issue with running the 10.5:1 CR on my 850 (and in in my 920 I’m running 11:1 without issue). I’m running a JS1 cam and Comnoz flowed head on my 850 and can confirm it transforms the performance (JS1 is a bit hotter than stock but still mild compared to a 2S or PW3 etc).

Having a working squish band allows for a higher CR without detonation. It’s especially handy on a road bike where you’re not using race fuel.

HOWEVER...

What cam are you using?

In general, higher CR is used in conjunction with sportier cams. CR ratio is usually measured as a ‘static CR’.

Then there is ‘effective CR’ (I think that’s the right phrase) which refers to the CR when the engine is running. Sportier cams with higher lift and duration tend to lose some CR due to this. Eg the Combat had high CR, but it also had a 2S cam.

What I’m getting at in a roundabout way is, 10.5:1 may be too much for use in conjunction with a road cam. It May be fine, but I don’t know cos I’ve never tried it. Hopefully someone who has will chime in.

Maybe consider a mildly more aggressive cam?
Thanks Eddie for your detailed reply. This is a link to the MAP pistons. Flat top I believe:


I will be using the stock cam unless I'm persuaded otherwise. Again, I'm only interested in street riding and love low end torque.

Ed
 

APRRSV

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I think a lot also depends on what fuel you have available. When I went for a CR or around 10.5:1, there were quite a few US people who proclaimed that I would not be able to find suitably high octane fuel. It seems that here in Europe we do have better fuel grades than in the US?
Thanks Steve. I'm in the US so I'm limited to 93 octane 10% ethanol. I do know that I can get higher octane non-ethanol but that is not generally available. I would need to buy it in small quantities at in flated prices.

Ed
 
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with you in the US I would stick with the 8.5 pistons. with a stock cam and as a all round rider you will not have to worry about trying to find 92 + octain fuel . I have a combat and they were rated at 10-1 when i built it I cut .020 off the pistons and still have the 2 S cam. I still have to have 91 + octain fuel, I have gotten away with a 50/50 tank of 87 a few years ago during a gas shortage but could not do a full tank below the 91. you will like it better a stock cam or a web cam 12 grind from comnoz and 8.5 as it will make a great all round runner from 2500 to red line.
 

Fast Eddie

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Ed, if you ‘love low end torque’ and if you struggle to find high octane fuel, I’d be inclined to agree with Bill.
 
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here in the US with 10.5 pistons I am afraid he would be buying race fuel as even 10-1 is bad enough with our fuel. that is why with a combat i machine the pistons to drop it a little.
 

Fast Eddie

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It should be doable Bill (if desired).

According to this web site, your 93 octane (which Ed can get) is the same as our 98 (I use either 99 or 97 depending on the garage I use).


But, I guess that wouldn‘t be of much use if you were miles from home and all the only local garage had was 91 octane...
 
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most of what i see in my parts is 91 and sometime 92 but as you said some places dont have the high test so you are stuck with 87 and maybe 89.
I still would not try a 10.5 engine build as a street bike.

It should be doable Bill (if desired).

According to this web site, your 93 octane (which Ed can get) is the same as our 98 (I use either 99 or 97 depending on the garage I use).


But, I guess that wouldn‘t be of much use if you were miles from home and all the only local garage had was 91 octane...
 

maylar

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What is the compression on a 72 Combat motor? They are quite tractable on the street. Though my buddy says his will ping badly on anything less than 93 octane. Fortunately, in this area that's readily available.
 

Fast Eddie

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What is the compression on a 72 Combat motor? They are quite tractable on the street. Though my buddy says his will ping badly on anything less than 93 octane. Fortunately, in this area that's readily available.
Combats were 10:1.

Your buddy seems to confirm the thoughts above, that the OP would need to use 93 if he went for the higher CR.
 
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You might find that these 8.5 s are calculated for a 40 thou thick copper head gasket. If so, with the thinnest composite gasket used instead you will have about 9 to one CR.
There is also the 5 thou? copper wire head seal offered by JS eng.
That would boost Cr to something around 9.5 to one.
Nigel's bike obviously works at 10.5, but might not like the 87 octane regular fuel one occasionally is forced to use out on the backroads of Montana, for example.
If using a bike like Nigel's in those rural areas, one could carry some octane boost.

Glen
 
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oldbeezer

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I believe it is possible to use a "thin" head gasket and get perhaps a 1/2 compression boost. That would make it about 9 to 1.
 

zefer

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Why not look into JS Motorsports rotating assembly ? I had the same goal as you...I went with Jim's Stage 1 kit...JS Kit included Carillo rods,light pistons,gapless rings his BSA grind cam and followers...all lightened and balanced to work properly, all of his gaskets and without a doubt it is the smoothest free reving Norton I have ever experienced ( I should mention that it is 40 over and the head was "breathed on" by Jim Comstock ) and all the engineering has been done for you...seriously it is a proven kit....just my 2 cents worth...good luck with your build and keep us informed
 
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I would run the lower compression pistons, and not think too much about it.

That head will probably show some spirit, but you can get a touch more spirit if you get the crank lightened some and the entire rotating assembly balanced. Motor will be more responsive and smoother running than an engine with a stock lower end. Well, that's how it worked on my Atlas turned hybrid Combat.

I don't know what the consensus here is on lightening up a crank. I did it in 1988 and didn't ask anyone if it was a good idea. I just figured it would be good for my style of riding at the time.

Edit: If I were to freshen up my motor today, I would use the JS Motorsports parts. Almost ordered everything to do it recently, but my old scooter runs pretty good even after sitting in the garage covered up and neglected for 30 years.
 
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