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Author Topic: M750 Motor. Polishing the Turd and Fuel Octane.  (Read 2360 times)
koko64
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« on: February 18, 2021, 02:24:00 PM »

Loving my M750. I really enjoy the character of the short stroke motor. At 61.5 x 88 mm it revs with an internal balance and harmony of crank/rod/piston mass that was lost with the more powerful 900 imo (I've owned four 900's which I also loved). That engine balance allows a light flywheel without the side effects that compromise a 900.

The well documented problem is the heads. This motor could be so much more. I have read the bikeboy article on hotting up a 750, poured over Doug Lofgrens 750 work and paid special attention to JD Hord's 750 SS project.

There is talk of our fuel octane range changing down here. Currently we have 91, 95 and 98 RON fuel. Ethanol is a niche fuel and a seperate market option, so no worries there. There is the possibility that 91 octane could be dropped so we are more to the EU standard.  There is also talk of removibg 95 octane leaving low and hi octane only. If the 91 goes then I might consider hi comp pistons and while I'm at it, 900 cams and valves and some porting. I would unshroud the valves if I were to use the big valves and use FBF pistons to return some comp. If the 95 goes I would different porting with stock comp. Or maybe only use the 900 intake valves only (iirc JD Hord did that). There are a few recipe options here. Even the stock pistons would need some machine work for clearance. This would all occur in a spare set of heads.
The direction of head development will be determined by the future of the fuel available. It's a job I can start on the bench and take my time with.
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« Reply #1 on: February 18, 2021, 02:31:37 PM »





« Last Edit: February 18, 2021, 02:33:35 PM by stopintime » Logged

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« Reply #2 on: February 18, 2021, 03:02:50 PM »

M695 heads and drop in high-comps.  Also, the later M800 heads w/7mm guides can be used in place of M695 heads.
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koko64
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« Reply #3 on: February 18, 2021, 06:22:57 PM »

M695 heads and drop in high-comps.  Also, the later M800 heads w/7mm guides can be used in place of M695 heads.
Thanks Stu.
Some questions.
Can I use the early 800 8mm heads? With 800 heads will I need to fit hi comp pistons to restore compression due to the larger combustion chamber? Can I keep the stock pistons with the 695 heads without reducing compression?
With the 695/800 heads will I have to machine the pistons to deepen and widen the valve pockets?
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koko64
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« Reply #4 on: February 18, 2021, 06:24:22 PM »







What work on the chamber? Welded up and ported? Looks good.
Pistal pistons?
« Last Edit: February 18, 2021, 06:44:26 PM by koko64 » Logged

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« Reply #5 on: February 18, 2021, 06:31:48 PM »

https://hordpower.com/gallery3/index.php/Dyno-Charts/album07/750ss
Hi comp 750 with stock vs 900 cams. I think the cams weren't advanced 6 deg ala bikeboy looking at the power curves.

https://hordpower.com/gallery3/var/albums/Dyno%20Charts/album07/795ssIgnition.jpg?m=1416607249
795 SS. Hi comp  big bore pistons, porting, 900 cams, 900 inlet valves, FCRs. Ignition tuning and race gas tests.
« Last Edit: February 18, 2021, 06:36:57 PM by koko64 » Logged

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« Reply #6 on: February 18, 2021, 06:42:41 PM »

An advantage using a 750 is no need to retard ignition and buy an Ignitech and no need for heavy duty engine studs. In fact it seems the motor likes the extra advance with premium fuel.


If I could find 695 heads cheap enough then maybe that's an option. Is the combustion chamber smaller than in the 800 heads and do stock 750 pistons require machining for the 695 valves and cams?
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« Reply #7 on: February 18, 2021, 09:06:50 PM »

Chambers (from what I remember) are about the same.  The high comps w/695 or 800 heads (either 8mm or 7mm stems) are a good ticket as the cams are more aggressive so you can run higher static compression.

750 cams are really weak to work with the 9.0:1 CR.  Static compression with just a head swap will remain around 9.0:1.  However, because of the more aggressive cam timing, your dynamic compression ratio will suffer.  That’s where the 11:1 pistons come into play...
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« Reply #8 on: February 18, 2021, 11:07:32 PM »

Thanks for the info.waytogo
A few things to consider.coffee.
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« Reply #9 on: February 21, 2021, 02:04:26 AM »

Read some articles that say our federal govt is keeping our fuel range the same until at least 2027. It will take time to upgrade our refineries to catch up with other OECD countries to produce lower sulphur gasoline at 10 ppm. Our 91 (87) is 150 ppm and our 95 (89/91?) and 98 (93-94?) are at 50 ppm, so we have a way to go.
I got time to find a bargain on hi comp pistons and heads!

I have been intrigued by how Cagiva got 75-80hp out of their 750 and 823ish Elefants in the Dakar while keeping low compression due to the fuel available. I know they had some help from Honda with fancy magnesium carbs, etc.

I'll focus on the heads and look to use up the parts I have laying around (if no one buys them).
Cheers.

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« Reply #10 on: February 21, 2021, 03:05:24 AM »

If you’re cagey...

M695 engine is factory compression ratio at: 10.5:1 with a stroke of 57.2mm and it shares the M750’s 88mm bore.  It was also (at the time) the highest HP/CC air cooled Ducati twin ever mass produced.

M750 stroke is 61.5mm, a 4.3mm or 7.5% increase.  Since the M695 pistons have a higher pin height due to a shorter stroke and the same length connecting rods, you could (if one was cagey) space up each barrel 2.15mm (1/2 the stroke) and run M695 pistons.  The increased stroke would raise the 10.5:1 CR up to 11.3:1.

The raised barrels would space out the intake manifolds which would cause the carbs to be slightly rotated, but maybe that’s not a big deal...

Cam timing would need to be reset also as there would be a greater distance between pullies & maybe the exhaust would be more difficult to fit (2.15mm offset but maybe not as that’s not a great deal.)

Besides doing the maths, I haven’t tried this but since high compression 750 pistons were hard to get a while ago, it got me thinking...

YMMV, good luck, and either I’m sorry or You’re welcome.
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« Reply #11 on: February 21, 2021, 03:40:23 AM »

Forgot to add:  The high pressure oil feed to the vertical head will require its own 2.15mm spacer above the O-ring.  Also may need 2.15mm longer oil feed tubes between the case & barrel or another spacer.

So, in this country: 2.15mm = 0.0846”.  I can get 0.085” sheet aluminum and planned on using a water or laser cut shop to make up the spacers based on the cylinder base gaskets.
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« Reply #12 on: February 21, 2021, 04:50:36 AM »

Wow, interesting. It's amazing what happens when one's mind gets on a roll  laughingdp.

Also interested to see the piston to valve clearance on stock 750 pistons with 695 heads.

FBF still sell hi comp 750 pistons. I wonder if they still have the half price Monday sale?

I'm actually starting to lean towards using "floor sweepings" parts for these mods. I do have perfectly good 900 V cams and valves laying around. The bike aleady has MX type FCR39's and Staintunes. Maybe I can find some cheap 750 heads (who wants them any way?) and start with the 900 big valve and cam conversion and some porting. I can stay cagey on the pistons in the meantime. Grin
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« Reply #13 on: February 21, 2021, 10:57:00 AM »

Interesting stuff Stu.

That's exactly what Kevin Cameron and I did on my S2 Kwak to raise the ports, except I think it was more than .085. Being a 2 smoker it was really simple. No belts, cams, or other hardware to complicate things. Yes, it was almost a half century ago when Kevin was a regular guy. Tongue He sure knew his Kwak's, however.

Turned that little 350 into a light switch. waytogo
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koko64
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« Reply #14 on: March 11, 2021, 11:33:05 AM »

The only 88mm pistons available are from FBF US$581 or Kamna €600. Then shipping and local taxes (10%). The stock pistons will still require machining of the valve pockets if using 900 cams and valves.
« Last Edit: March 11, 2021, 04:43:08 PM by koko64 » Logged

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