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  #1  
Old 27-02-2008, 08:16 PM
grbrown grbrown is offline
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Cool 107 cu inch Mountain Motor!

OK, inflation has hit since I started my "106 inch Mountain Motor" string just a few weeks ago - in more ways than one! What started as an idea to bore out my stroked motor to 106 inches has transmogrified into a new engine and grown an inch. While I wait for it to be delivered I thought I would share some of the logic that went into my choice.

But first a bit of scene setting. Bill Rook, in his book 'How to Build a Harley-Davidson Torque Monster', quoted no less a figurehead than Don Branch in saying that the first thing anyone should do to tune a Twin Cam is to reduce the overall gearing. The reason is that in order to reduce noise emissions the factory have raised gearing to lower engine speed at the road speeds the noise tests are conducted. This may seem a long way from choosing an Evo motor, but stick with it! My wife has a recent Dyna and we have done just that - fitted a Screaming Eagle crankshaft sprocket and primary chain kit, 'not for road use', reducing the sprocket from 25T to 21T.

That sounds a lot, but the bike's theoretical top speed is reduced from an unbelievable 140mph at 6,000rpm to a more sensible 115mph, which it is more likely to achieve. Roll-on acceleration in top gear is quite remarkably improved, which should come as no surprise. Just the sort of thing a girl needs! (The only other mods are a carb jet kit, SE filter and SuperTrapp silencers.) Top gear now translates to around 19mph per 1,000rpm, while by contrast my Glide is geared for around 24mph per 1,000rpm. Keep those figures in mind, as torque relative to road speed is of interest!



I investigated various brands of engine before settling on S&S. These days it is quite easy to import a motor direct from the USA, although shipping, import duty and VAT will add approximately 25% to the US cost. Those same add-ons will apply to any UK sourced engine of course, but will be invisibly buried in the bottom line.

The reason I chose S&S is because they have a well established importer here in the UK, who is also an authorised warranty agent for the factory. I couldn’t find the same confidence for other brands and feared that if there are any problems they could be difficult to resolve across the 'pond'. I can knock on the door of my local Harley and S&S factory representatives, even shake them by the hand, but not any other engine makers that I am aware of.

I spent quite some time agonising over which of their models to chose. On offer for Evo lovers are a 96, 107, 113 and 124, the 107 being specially tuned for touring use, which for a Glide owner draws attention! I rejected the 96 simply because it is not much larger than my current 89inch and is expensive for relatively small gain. Choosing between the others was a little more difficult, until I reflected on the mods we have done to that Dyna.



And so to my graphs, using data from S&S literature, to compare the different models. I imagine every brand of after market engine manufacturer will have similar variations of performance across their ranges. The lines appear smoother than the published graphs simply because of the way I have transposed the data. Think of road speed when studying those torque curves! You will see that in all cases peak torque in top gear corresponds to road speeds most Harley owners rarely, if ever, achieve.

Now it is many years since I last ran engines on test beds, but my reading of these curves is that they correspond to full throttle performance. Plainly an engine will run happily at lower speeds than shown in these graphs, but they will not accept full throttle, or maximum engine load, below the levels shown. Consider the challenge of getting half a ton of loaded Glide underway from standstill! At low engine speeds the 113 gives no more torque that the 107, and will not pull full throttle until far further into the rev range.

Not surprisingly the 96 gives less torque, while the 124 is better than the others, but only at higher revs. In most cases peak torque corresponds to a road speed, with typical Glide gearing, of well over 100mph. Use of the later 6-speed overdrive gearboxes will drop all of these engines way down their torque curves, with questionable benefit, as will raising gearing on any model.

Most of my riding at present is done below 90mph for which the 113 gives the best peak torque, but at the cost of a much shorter rev range at normal road speeds than the 107. Interestingly all of them, except the 113, suffer a shallow dip in their torque curves just where it could be most usefully filled out. It would be interesting to know if anyone has managed to tune that out and how they did it.



The price is higher than other common brands, S&S being relatively expensive, but there are good reasons for selecting them, in addition to their UK support. Their engines are available with various optional ignition systems, their IST version being a highly sophisticated system that includes temperature and knock sensors. A particular attraction is that for a 107 fitted with this there is a three year warranty. I don’t believe any other engine maker offers anything quite like that!

Some brands of larger engine are fitted with manual decompressors, to ease cold starting, but the S&S has automatic electronic ones, activated by the starter circuit. They also make a dresser specific silencer in stainless steel that promises maximum power output, with subdued noise levels. So to the dedicated dresser owner S&S provide what I find to be an irresistibly attractive package. Mind a 'touring' version of their 124 would be great....

So in summary while the larger engines have their appeal the benefits of the 107 'touring special' win for me and my application. It has the added benefit of incurring relatively little knock-on costs, requiring only a beefed up clutch spring. It has the widest rev range of them all, from 1,500 to 6,250rpm, plus the benefits of being tuned to give smooth and quiet running, characteristics carefully dialled in by the S&S engineers to better suit it to extended touring use.

Hopefully you will find the graphs of interest, especially if you are in a quandary choosing which will suit your application best!

Added later, link to better graph:

http://www.harley-davidson-hangout.c...ze/big/cat/500
Attached Images
File Type: jpg S&S Graph e.jpg (16.1 KB, 2294 views)

Last edited by grbrown; 18-05-2008 at 11:47 AM. Reason: Link to graph added.
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  #2  
Old 28-02-2008, 02:26 AM
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Strawhead Strawhead is offline
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Re: 107 cu inch Mountain Motor!

A very interesting summary.

I would be interested to see how your 89 kit did in comparison.

Funnily enough I thought that having the larger stroke would be better for the torque values but the torque curves do not really back that up.

let us know how the 107 runs and if the benefits were worth the large expenditure. the torque curve certainly looks very nice but will the extra lb's be all that noticeable on a full dresser

very interested to hear your views
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Old 28-02-2008, 09:12 AM
grbrown grbrown is offline
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Re: 107 cu inch Mountain Motor!

Thanks for the feedback. I have given up for the time being trying to make that graph more legible! It looses something being miniaturised and uploaded.

Unfortunately I never had my bike in its current form dyno tested! I was assured it would give around 85-90 bhp and lb/ft torque by RMD when it was built and a Harley fitter later reckoned on similar numbers, on the strength of tuning and riding it.

Neither the previous nor current carbs (Edelbrock and Holley respectively) were well suited to the engine, so it has never performed to its optimum. It was the prospect of buying a further carb (Mikuni) that initiated this whole exercise - perhaps a bigger bore, etc!

The 'old' motor is 3 1/2 inch bore, 4 5/8 inch stroke, whereas the 107 is 4 inch x 4 1/4 inch, so quite different proportions. I reckon I have stretched the current engine close to its limit of performance and it will be interesting to use a large engine that has been thoroughly developed, rather than my 'prototype'.
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Old 28-02-2008, 12:09 PM
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Steppenwulf Steppenwulf is offline
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Re: 107 cu inch Mountain Motor!

Nice post and interesting reading especially to learn that TC are over geared to meet noise regs. Definately food for thought. Looking forward to hear how you get on, all the best with it.
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  #5  
Old 28-02-2008, 05:11 PM
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Hawkeye Hawkeye is offline
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Re: 107 cu inch Mountain Motor!

This make for interesting reading, i also thought that an over square motor was good for torque. I suppose it goes hand in hand with BHP really.

The idea of lowering the gearing seems interesting but doing that lowers all the gears making first gear extremely low surely. First gear would pull you up a brick wall if you had the grip.

On my Sportster i geared up as the engine revved quite hard on motorways. I went up three teeth i think on the gearbox sprocket, made for great motorway cruising but forst gear in traffic was a bit tall. Bit like the old Gold Stars, 70mph in first.
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Old 28-02-2008, 06:44 PM
grbrown grbrown is offline
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Re: 107 cu inch Mountain Motor!

Quote:
Originally Posted by Steppenwulf View Post
Nice post and interesting reading especially to learn that TC are over geared to meet noise regs. Definately food for thought. Looking forward to hear how you get on, all the best with it.
I think Harleys have suffered from over gearing for quite some time. I reckon our Dyna is just great with its lowered gearing - not as bad as one might think. My wife does not tear around on it, just wants the thing to go when asked, which will often be in the wrong gear!

I'm waiting for a delivery date for the 107 at present, hoping to have it installed by the end of March.
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Old 14-04-2008, 12:20 PM
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Re: 107 cu inch Mountain Motor!

Graham, any news on that engine yet?
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Old 14-04-2008, 06:18 PM
grbrown grbrown is offline
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Re: 107 cu inch Mountain Motor!

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Originally Posted by simonDeuce View Post
Graham, any news on that engine yet?
Hi Simon,

It is waiting for me at Matts and I plan to deliver the bike this coming Saturday! Still waiting for its S&S SPO mufflers to be delivered.

I strained my back a few weeks ago doing some winter servicing on the same bike, like a twit, which held up getting it road-worthy, although it and I are both OK now.

We have a long drive on Saturday, so hoping for some decent weather.
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  #9  
Old 14-04-2008, 07:51 PM
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Re: 107 cu inch Mountain Motor!

Sorry to hear about your back. Hope it sorts itself out soon. I know what a bad back can be like!

Fingers crossed for decent weather on Saturday for you.
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  #10  
Old 19-04-2008, 11:05 PM
grbrown grbrown is offline
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Re: 107 cu inch Mountain Motor!

Simon,

After a truly filthy journey, Glide delivered to Matt today. The salt on the roads at present, despite the heavy rain is gruesome. New 107 patted and petted, due to be fitted in a few days time. More news as it breaks!

Graham.
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