Decision time on 900ss engine mods...standard bore, high comp, or 944 kit?

[Rudemouthsky]

So, my ride is almost ready to pick up...spring maintenance, 5.5" wheel upgrade, and new shoes mounted and balanced. My 900ss engine has the top end dismantled...paint is going to be drying soon...

How far should I go with the engine work? I haven't had a fast bike in over 20 years..an '84 Sabre. I love my 750 but c'mon man the thing won't even gas wheelie in 1st gear. The longer stroke of the 900 alone should keep me grinning for quite a few summers, but I don't want to waste this opportunity.

I have a fresh set of FCR41's, dynacoils, open airbox, and a race exhaust. I kind of like the idea of keeping the engine stock bc I ride the bike every day weather permits, I'd like to be able to use 87 octane gas, not have any timing gremlins to sort out, etc. I have a talented mechanic friend at my disposal who built Audis, Porsches, Japanese and British bikes for a living for quite a few years. And I'd really hate to squander this opportunity while my engine is on the bench.

I guess to get to the point, what I'm asking is this; how much different of a beast is a stock 900 compared to a hopped up one? I'm at an impasse here..

 

[koko64]

Throwing ideas around.

Hi compression pistons really wake up these motors, so ignition mods and more expensive fuel come with that mod. But you want to run cheap fuel since its a commuter which is fair enough.
Unfortnately, new rings can cost more then a whole piston kit. I dont know what the local bargains are just for rings. Good engine studs if yours are the crappy stainless ones. Dont risk it, I found out the hard way .

Ok, maybe keep stock compression and ignition, but port and gas flow the heads, have the cams dialed and advanced ala Brad Black with adjustable pulleys, try a light flywheel (maybe, but it will be smoother with the fat flywheel when toolin around town), and run a 44mm inlet valve with porting to match. The restrictions to this motor are in the heads. If you go the porting route, consider an enlarged airbox. Maybe some may consider porting a low comp motor a waste of time. I dont know who has done it and what the gains were. Maybe porting, valve job aimed at a smooth, tractable type of power rather than the higher comp explode out of corners power. Worth a discussion with an expert.

They are a great street motor. The carb and coil upgrades are very worthwhile. A nice dyno tune for the carbs maybe?

Money put into brake and suspension mods may serve you even better.

[brad black]

compression is one part of the power puzzle, and undoubtedly the best way to wake these motors up.  but i understand the std fuel thing completely.  the fuel decision is the first to make.  if you want to go low, run std comp.

if it needs rings you will pay more for std rings than for je pistons.  std top ring is a dykes, so there is no aftermarket alt that i know of for the 900.  altho, i'm not sure if there is negative implictions to running a flat ring in a dykes ring groove.  wiseco/je make (or supply anyway) plenty of rings for nikasil bores, xc and xh series maybe.

maybe leave the base gasket out.  plenty of people have done this over the years, and altho by my calcs it makes the squish very tight at the outer edge, it's only the outer edge where contact can occur as the angle on the piston crown is shallower than the angle of the chamber.  there's some commentry on this here: http://bradthebikeboy.blogspot.com.au/2012/05/setting-squish-and-cam-timing.html

carrying on from the above, doing the cam timing will help it, esp if it is otherwise std.

porting might not help as much as it would with more comp, but it will still help if it is done well.  altho i'd possibly not bother, it gets expensive quite quickly.  ask doug lofgren if he'd take your money happily knowing what you want to achieve, that will give you an answer.

fcr will help a std comp motor equally as well.

comp is the big issue.  my 750 motor loves 98 fuel (our fuel is rated differently to yours, we have 91, 96 and 98), but is ok on 96 and it allegedly has around 12:1.  the price difference locally these days is fairly big from 91 to 96 tho.

std comp is easiest, and you already have those pistons.

if you go std comp spend money elsewhere that you would otherwise have spent on the better fuel, etc.  light flywheel and clutch, etc.

[koko64]

It will be interesting to see what the final engine combination will be and how it performs with a low comp motor as a basis.
Besides being a practical commuter, it sounds like a recipe for a grand touring motor that can run on any fuel available in the countryside.
 

[ducatiz]

We used to flycut the chambers in VWs to drop compression for street use.  Not much, just a few mm.

I've got a pair of V2 heads here (on my desk)... Seems you could cut a few mm from the sides of the chamber to make it a bit rounder to drop compression some.  Then use the high comp pistons and dial it back based on calculations with the chamber volume...  crazy?

[ducatiz]

...also.... Ignitech TCI can accept input for temp and trim/retard spark so you can run higher comps...

(use a thermocouple and run the input to the servo input and use it to trim advance)

probably not as reliable on an air/oil cooled engine, but something to explore..

[koko64]

We used to flycut the chambers in VWs to drop compression for street use.  Not much, just a few mm.

I've got a pair of V2 heads here (on my desk)... Seems you could cut a few mm from the sides of the chamber to make it a bit rounder to drop compression some.  Then use the high comp pistons and dial it back based on calculations with the chamber volume...  crazy?

Interesting. As you say, and unshroud the valves, maybe using a 44mm inlet valve, reshaping the combustion chamber to work with the chosen porting.

The works Dakar Elefants ran quite low compression for obvious reasons, but were said to have made good power in the 80s. I wonder what they did to those heads.

[ducatiz]

Interesting. As you say, and unshroud the valves, maybe using a 44mm inlet valve, reshaping the combustion chamber to work with the chosen porting.

The works Dakar Elefants ran quite low compression for obvious reasons, but were said to have made good power in the 80s. I wonder what they did to those heads.

The first Dakar Elefant engines were pretty much a standard*** early 750 lump (88x61.5), and allegedly gave around 70hp, but who knows what they did with the heads -- no idea if that's at the crank or rwhp either.

The later Elefants with the 944cc engines made around 80hp (again, no idea if that's crank or rwhp).  Seems to me they probably focused more on lightening and balancing the internal bits rather than exotic changes to the heads....?

***with gobs of oil coolers.  they used the later 750 style case with full flow coolers.  i think the 750 Dakar had about a full liter of oil cooler capacity spread across 2 coolers.

[koko64]

Hey Tiz, I really dunno, I thought you might. 
I think they also ran an 850ish motor too, 750 stroke, 94mm bore.

I noticed MBP have a standard piston/bore option for a 904 build, but with the fancy big valve, short manifold heads. They claim about 90hp for that. Pricey. Rudey, have you checked out the MBP site?

[Rudemouthsky]

Hey Tiz, I really dunno, I thought you might. 
I think they also ran an 850ish motor too, 750 stroke, 94mm bore.

I noticed MBP have a standard piston/bore option for a 904 build, but with the fancy big valve, short manifold heads. They claim about 90hp for that. Pricey. Rudey, have you checked out the MBP site?

No I haven't but I will now.

That's multiple smart ppls now, specifically saying; "high comp really wakes up these motors." I feel I can't ignore that...espec when just new rings are gonna set me back what the JE kit will.

What concerns me about the ignititech is a comment Chris K once made- he stopped carrying them bc carby customers were having issues and ignitech wouldn't support them. So it seems like the jury is still out on these things.

[ducatiz]

What concerns me about the ignititech is a comment Chris K once made- he stopped carrying them bc carby customers were having issues and ignitech wouldn't support them. So it seems like the jury is still out on these things.

Chris is a smart guy.  He had some issues with them and (I think) he got frustrated with them.  They only provide email support and sometimes their english isn't perfect.

That being said, I've been running mine for several years, I have three different versions and I've gotten a handle on them.  They work well.  The issues Chris was having (IIRC) was due to using non resistor plugs with them, which shorts them.  If you use the R plugs, then there are no issues.  Also, he had one of the older versions, they've come out with 3 updates since then. 

[koko64]

If you are definate about using 87 gas, stay with the lower compression option . If you need to use the cheap gas, dont let us talk you out of it.
One thing about light flywheels, they give more zip, but some find them annoying in slow city traffic.
Getting the cams dialed will smooth things. Its nice to have cam timing the same on each cylinder! Then there is playing with the timing (see Brad's site bikeboy.org). Brad dialed the cams on my bike and I immediately noticed it was smoother and more responsive. This requires adjustable pulleys.

Talk to your buddy about the options discussed, interested to hear what he thinks.
Good luck.

[Düb Lüv]

I'm glad I was lucky enough to buy Chris's last ignitech

[Rudemouthsky]

Hey Koko;

AFAYK, How well does the baseline tuning of the FCR41's behave with the standard JE piston upgrade? Well right out of the box?

If I spring for the ignitech, will I still need to retard timing a couple degrees?

Thanks for all your help in this thread. I think I'd be nuts not to do this mod.

[koko64]

Definately adjust the Ignitech map for hi comp pistons. Max advance is set back (retarded) 2-3 degrees and people map a gentler advance curve. With hi comp, you will be committing to premium fuel, how high an octane Im not sure, I would check with Ducatiz because I'm in Australia and our fuel and octane rating are different.

My FCRs standard jetting was for a stock motor according to the literature that came with it. US bikes may jet close enough for my jetting baseline to be a reference. Again check with Tiz, ducpainter and others.
My bike has porting and an enlarged modified airbox so my needle and slow jets a a little richer. Without porting the stock open airbox will suffice.

160-165 main jets is a common range from talking to forum members.
Needle positions 3-5 also get mentioned.
Slow fuel jet is the tricky one. The carbs come with 60s which is too rich unless you open up the adjustable slow air jet. The SAJ is set 1 1/2 turns as delivered and may have to be opened up to 2 turns to balance the overly rich slow fuel jet (for Aussie fuel anyway). I have heard of people using slow fuel jets as low as 50 .
Your buddy will have to help you there.

Check Patrick Burns Keihin FCR tuning guide.

Make sure you get advice from guys like Tiz to allow for your local fuels. Locals can adjust my jetting recommendations. If I lived in Alaska I would leave the 60 slow fuel jet or it would never start! I feel the different fuels have a greater impact on jetting on the slow fuel circuits. I'll defer to any local knowledge.

Without porting and fancy airbox mods a local recommended setting could well be close enough. Forum members may also have good ignition maps for you.