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#345317 - 11/27/10 3:13 am Low compression pistons  
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peter berry Offline
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peter berry  Offline
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Ontario , Canada
1966 TR6 Engine rebuild Want to build a nice mild engine what are the lowest compression pistons available . Will I notice any difference . Thanks Peter
1976 T140
1966 TR6 project

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#345347 - 11/27/10 12:37 pm Re: Low compression pistons [Re: peter berry]  
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Mike667-neighbor of the beast Offline
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Boston -
http://www.vintagemotorcyclepistons.co.uk/

believe down to 7.5 - i have noticed that the difference between 9.0 and 7.5 is marginal w/ a stock motor. higher a bit more torque/top end - lower less parts falling off and a generally easier running motor. I prefer about 8 - 8.5 (max) in my motors


73 850 Commando
75 Ducati 750gt
78 Laverda 1200
97 Ducati 900 SS
61 BSA A10
62 Triumph T110
#345356 - 11/27/10 2:21 pm Re: Low compression pistons [Re: peter berry]  
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Stuart Online content
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Scotland
Hi Peter,

Originally Posted By: peter berry
Will I notice any difference

I should co-co.

The Cardinal (originally a police-spec. T160 for the Saudis and 8.25:1 c.r.) is noticeably different to a standard triple (9.5:1 c.r.). Moreover, as the build quality of triple engines was ... ah ... variable, the c.r. difference exacerbates any quality issues.

Otoh, it's well-known that, for example, Roy Peplow - famous British off-road rider for Triumph - ran 9:1 pistons in the normally-7.5:1 twins he raced for Meriden.

Imho, unless you just want to disguise the symptoms of build-quality shortcuts and subsequent lack of attention to fuelling and timing, you'll end up with a nicer bike if you eliminate the causes of problems as you build and then set up. 2c

Hth.

Regards,

#345370 - 11/27/10 3:45 pm Re: Low compression pistons [Re: peter berry]  
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btour Online content
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We are not talking about a triple here are we?

My mind boggles at the possiblites to static this thread with my new book in hand.

Highest CR to run would be determined by the fuel available to run. And in this day and age, that means consistency, or the worst possible, gas to "get off the barge" first. In my experience, 9.0 will give some detonation is some infrequent batches of fuel. (mostly in the off seasons of spring and fall, here in my neck of the woods). So one just needs to be aware of that and guard against that, by keeping the rpm's even higher so that the big D does not have time to happen, when one has one of those batches in.

Irving mentions that matching the shape of the top of the combustion chamber is as important as the CR itself, as far as performance goes. See if you can match that.

So I guess one could use lower crown compression pistons, if they were a closer match, and then lower the barrels by running with gasket or some other means, and if the clearance were safe with valves, theorically one could have the same performance with lower dome pistons. I guess.

Conversely if one can improve swirl and or tumble, by any of many means, including shaping ports, valve angles, intake angles, etc., one can more easily run higher dome pistons. I wonder if anyone has tried reshaping the head's combustion chamber shape, so that the outside area was less and the chamber around plug deeper, while making valve pockets in the piston tops. Thus one has a proper squish.

It was discovered that having the proper 'burn' rate to put it simply was more effective than just having higher CR per se., in producing usable power.

Fascinating topic. What was the question? Would you notice a reduction in power, with lower dome pistons? I would venture a guess and say, yes unless you did other things to compensate.

To give Jaye at Britech a plug. I remember him expounding on this topic once. Near as I can remember, what he said is he has lower dome pistons, ready to go, which will give the same power, as they have squish. I can not remember whether or not one has to re-work the shape in the head or not. You might want to give him a call and ask this question. smile half way to laughing more like :chuckle

Last edited by btour; 11/27/10 3:52 pm.

Bob, Lifetime bike: '71 T120R, bought in '71 at Ken Heanes, England.
#345388 - 11/27/10 5:42 pm Re: Low compression pistons [Re: peter berry]  
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Irish Swede Online content
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Triumph made lower compression pistons for the triples that were sold in countries with lower quality gas. A friend rebuilt his '69 T150 with a set of these pistons and noticed little loss of performance, but a lot less problems with today's gasoline.

I think lower compression equals longer engine life. I agree
with Mike667 that 8:1 or 8.5:1 compression is high enough for the use we give our old bkes.

#345432 - 11/28/10 12:39 am Re: Low compression pistons [Re: peter berry]  
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HawaiianTiger Online content
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With 7.5:1 you'll see a fairly dramatic reduction in overall power especially on top end. You might even see an increase in low end power though, depends on the cams etc. I like 8.5:1 because I think its the best compromise for a rider given the condition of today's fuel. You can still get a pinging pig if you don't do a good job of it.
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Flying Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
#345434 - 11/28/10 12:56 am Re: Low compression pistons [Re: peter berry]  
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TJD Offline
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O.k. Can I tag onto this thread by asking what ignition timing would you use with 8.5:1s as I am running them at the moment. I,m using a Boyer igniton and have set the timing using a strobe to the standard 38 setting. Does the forum suggest a slight retarding of the timing from that setting?
Thanks.

#345443 - 11/28/10 2:25 am Re: Low compression pistons [Re: TJD]  
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Pete R - R.I.P. Offline
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Higher compression,among other things (like better mixture distribution and atomization,increased swirl and squish,twin-plug heads,removing sharp edges and obstructions from pistons and heads,and especially higher burn-rate fuel),causes faster combustion and usually requires less advance.

All other things being equal,you may get away with one degree more advance than someone using 9:1 pistons.In practice,you're most likely better leaving your timing at 38 degrees as long as you get no "pinking" under load.Things have changed since 1965 or so,especially fuel.

8.25:1 compression should be able to cope with most of the "good" fuel available today,not standard unleaded.
9.5:1 is stretching the friendship,even with good fuel.

What do you get with lower compression?
*Reduced power output,not in direct proportion but equal to about 1/2 that proportion.40% lower compresssion = about 20% less power.
*Increased fuel consumption.
*Slightly more engine heat for a given power output.(More of the fuel's energy is converted to heat,less is converted to useful energy at the crankshaft)

Last edited by Pete R; 11/28/10 2:28 am. Reason: paragraphing
#345557 - 11/28/10 10:46 pm Re: Low compression pistons [Re: peter berry]  
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750 Tracker Offline
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Vermont
Many of the problems blaming "bad fuel" as the villain, can be traced to the ancient ignitions these bikes were originally built with. Boyer improved the performance, but it fires both plugs at the same time, thus reducing efficientcy of the system. Also, some qualified mechanics have tested Boyer advance curves and discovered they are not consistant and can sometimes advance too early. The latest Tri Spark systems use much more advanced electronics (firing each plug individually )and accurate curves. Before re-engineering an engine, try correcting the 40 year old ignition design with modern components including coils.

#345561 - 11/29/10 12:06 am Re: Low compression pistons [Re: peter berry]  
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RHall-HCV Offline
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As i have said before on here i am safely running 10:1 comp ratio on 93 pump gas with a Boyer at 35 degrees. Squish is machined matched at .032. You will definitely see improvements in performance and fuel econ when you get a flatter top piston, but you will lose low end power that the compression help make. I would run the stock style JCC or hepolite pistons and try to match the squish to around .040. Just be sure to chech the taper on the pistons to choose the right clearence for them. HTH


ROB HALL
HCV MOTORSPORTS
#345592 - 11/29/10 3:44 am Re: Low compression pistons [Re: RHall-HCV]  
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peter berry Offline
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Ontario , Canada
Thanks guys , really helpful do plan on updating electrics jury still out on piston
Peter
76 T140
66 TR6 project

#346232 - 12/03/10 10:58 am Re: Low compression pistons [Re: peter berry]  
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Stuart Online content
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Hi Peter,

Originally Posted By: Irish Swede
Triumph made lower compression pistons for the triples that were sold in countries with lower quality gas.

The first low-comp. pistons for triples were made by NVT at Small Heath in either late 1975 or early 1976 specifically for the team in Saudi setting up the triples for the police and military and training the indiginous mechanics. The earliest pistons were made by turning normal 9.5:1 pistons in a lathe but, because that could change the shape of the piston skirt, Hepolite subsequently cast 8.25:1 pistons. Some of the latter probably made it out on to the world market after NVT were finally wound-up in April 1976, plus many triples intended for Saudi were never exported, being resold in GB, and some of those were rebuilt with 9.5:1 pistons soon after.

Originally Posted By: 750 Tracker
Boyer improved the performance, but it fires both plugs at the same time, thus reducing efficientcy of the system.

That isn't the reason for reduced efficiency in an ignition fired by a Boyer-Bransdsen e.i. A B-B e.i. firing two 6V coils on a twin or three 4V coils on a triple works perfectly well unless there are other issues.

Originally Posted By: 750 Tracker
The latest Tri Spark systems use much more advanced electronics (firing each plug individually )

Trispark for a triple fires the plugs individually, Trispark Classic Twin fires the plugs together.

Hth.

Regards,

#346256 - 12/03/10 2:03 pm Re: Low compression pistons [Re: peter berry]  
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John Healy Online content
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Quote:
Before re-engineering an engine, try correcting the 40 year old ignition design with modern components including coils.


This is really strange, because I have a copy of a letter where Brian Jones claims that after extensive testing both on the dyno, on the street and at the test track at Mira, including a session where the bike maintained over 100 mph for an hour, that the riders couldn't see any difference between the conventional points system and our beloved Lucas Rita... This was during the period where they were testing using samples of US blended no-lead fuel provided by Mira.

Isn't it strange that the Modern coil that is in a lot of cars today that are sensitive to emissions and combustion efficiency fires two plugs at the same time...

Read Mr. Hall's comments above! There is more to combustion efficiency than which ignition system you use and how many coils it fires.


#346278 - 12/03/10 4:15 pm Re: Low compression pistons [Re: peter berry]  
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btour Online content
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Okay. This is turning out to be very interesting. I agree Mr. Hall. And I mentioned these other factors in a post above, which I understand only from my reading of theory, and not by praxis. Such factors being, squish, tumble, swirl, etc. Things that effect, the orderly speed of the flame front, to prevent the unwanted "explosion", that is detonation. When it comes to ignition, no one has yet mentioned adding another source of spark, i.e. another plug to each cylinder.

However, from empircal evidence, I do wonder about Mr. Hall's statement that he is ALWAYS, and in every case successful, at running those pistons on pump gas without any pinging, ever. I say this because from my experience the vagaries of pump gas are high. While I can agree that in 99.5 percent of time, success in there. But there are times when that pump gas, is just not up to snuff. The blend is way off.

I can see no other reason that would explain it. All the other factors being the same. Same engine, same tuning, same rider, same roads, etc. The only variables, being the pump gas that is available at the time of a major change in atmospherics, Spring and Fall.

One would have to ride, every day of the season, in my region (which is all I have for comparison) to experience this effect.

So I would have to ask, if Mr. Hall, rides and buys a tank of gas every day of the season. But then again, I see he lives in a different region, and the barges supplying the "pump gas", (the first to get off the barge that day) most likely are entirely different that mine are.


Bob, Lifetime bike: '71 T120R, bought in '71 at Ken Heanes, England.
#346722 - 12/06/10 2:48 am Re: Low compression pistons [Re: peter berry]  
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RHall-HCV Offline
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Charlotte NC
The quality of fuel is definitely not as good as earlier fuel, and i can tell a difference in different brands and blends. Shell and Hess seem to be the best around here. I also (when i remember to take it with me) mix up lead substitute with some castor Premix to add to the tank. Does either really help?? dont know for sure, i havnt seen failures due to not running with it but its just a little peace of mind.


ROB HALL
HCV MOTORSPORTS
#346771 - 12/06/10 2:23 pm Re: Low compression pistons [Re: RHall-HCV]  
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kurt fischer Online content
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RHall,

How much Castor pre-mix, or what ratio, or a few drops?

I have several quarts of NOS Castrol R30 (in screw-top plastic bottles) -- does it have a shelf life?

Thanks,

Last edited by kurt fischer; 12/06/10 2:24 pm.

Kurt
1968-70-71 Triumph TR6R Bitsa - 1969 Triumph TR6R - 1971 BSA A65L - 1973 Triumph TR5T
2005 Ducati MTS 1000S DS - 2012 Ducati Hyper 796 - 2014 Kawasaki ZX1000
#346800 - 12/06/10 5:48 pm Re: Low compression pistons [Re: peter berry]  
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RHall-HCV Offline
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for racing i do about 2oz per 5 gallons. For the street just a few drops to a tank. Helps lubricate the carb parts since the bean oil will blend with alcohol that is common in modern fuels. The old R30 will be fine in the fuel but I dont know if i would use as engine oil.


ROB HALL
HCV MOTORSPORTS
#346811 - 12/06/10 7:44 pm Re: Low compression pistons [Re: peter berry]  
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btour Online content
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Rob,

How much of the lead additive do you use on your street bike? And how do avoid a culminative effect wherby you may have too much and incur lead fouling?

Working on rather a long tome of a post.


Bob, Lifetime bike: '71 T120R, bought in '71 at Ken Heanes, England.
#346879 - 12/07/10 3:30 am Re: Low compression pistons [Re: peter berry]  
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RHall-HCV Offline
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Charlotte NC
I mix up the lead substitute and castor oil 50/50 and put it in a flask so it fits in my jacket nice. Havnt been pulled over and had to explain whats in the flask yet. I put about a capful per tank, so about 1/4-3/8oz per tank. I havnt fouled plugs on this amount yet but then again im always tuning on things and change plugs frequently to do plug chops.


ROB HALL
HCV MOTORSPORTS
#347148 - 12/08/10 7:37 pm Re: Low compression pistons [Re: peter berry]  
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btour Online content
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Hi Rob,

Thank you for the informative discussion. I do not know if we have hijacked this thread or not. I assume not because the only reason I can think of for Peter wanting a milder engine, and those lower CR piston, was to avoid detonation. And we are discussing how to do that without resorting to lower CR pistons.

But just is case, I make this assumption in error, I am going to start a detour thread on this topic, since I have prepared a rather lengthy post.

So I invite you to join in this one. And of course Peter if this is his concern, and anyone else interested.

I am just going to post one more thing, to defend my harping on modern gas.

From Vintage Bike, 2001-4 Enjoying the Past, by Kevin Cameron, pgs. 22-25

Originally Posted By: Kevin Cameron


Another problem of Vintage air-cooled engines in the modern environment is detonation. Pump gasoline today is lower in octane number (detonation resistance) than what the U.S. Army Airforce Corps used in its aircraft engines in 1936. sixty-seven years ago. As a result, if you want decent pump gas performance from your vintage ride, you're stuck




To be cont... Maybe John has a link to this article on his Tioc Site. Or should I type in a lot of this? John? Do you care about the copyright?

Last edited by btour; 12/08/10 7:45 pm.

Bob, Lifetime bike: '71 T120R, bought in '71 at Ken Heanes, England.
#347258 - 12/09/10 1:34 pm Re: Low compression pistons [Re: btour]  
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peter berry Offline
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My intial enquiry was just to build a mid engine , but dont worry about a new thread Im following this with great interest Peter


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