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Discussion Starter · #1 · (Edited)
:grin2: Hi, new guy with 1995 900SS - longtime mechanic. Been going through her and getting ready to get my motorcycle endorsement back. Took her for a ride yesterday around neighborhood to sort suspension and see how she behaves. When aggressively rolling in the throttle from low speed / RPM (~ 30MPH and 3K RPM) she ping'd a wee bit for very short duration. BTW for what this bike is - this thing friggin rips - far more power and acceleration than I was expecting. (Bought bike on a chance - not running when I got her and had been sitting for 4-5 years in a garage - chance paid off big time! Smoked like an industrial bug fogger for first 20 minutes - thought I was going to have to rebuild her - and then suddenly she sealed back up and been good ever since.)

So I did a hot compression check which was another reason for riding her - I got 185 PSI front and 182 PSI rear with a leak down of about 2% (I am aircraft mech and used my certified tooling) so very happy with those results.

However after scouring posts could not find any with a compression reading as high as mine?

Any guessing as to what lurks inside the top end?
 

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High compression pistons are a pretty common mod since stock rings are super expensive and easily broken.
 

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Discussion Starter · #3 ·
High compression pistons are a pretty common mod since stock rings are super expensive and easily broken.
Kool, me thinking the same thing. Any clue what the compression would be? Seems like 12-12.3:1 with rote mathematics.
 

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Kool, me thinking the same thing. Any clue what the compression would be? Seems like 12-12.3:1 with rote mathematics.
When I subtract the static 14.7 psi for atmosphere and calculate 170PSI/14.7PSI I get 11.5 - turns out one of the most popular options is by JE Pistons, also the least expensive option and is 11.5:1. Assuming no-one messed with the cam timing that is. It's probably not a 944 kit as I think those are usually limited to 10.5:1.

Of course, I'm no expert and my opinion should be taken with a grain of salt, etc. etc. etc.
 

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Discussion Starter · #5 ·
When I subtract the static 14.7 psi for atmosphere and calculate 170PSI/14.7PSI I get 11.5 - turns out one of the most popular options is by JE Pistons, also the least expensive option and is 11.5:1. Assuming no-one messed with the cam timing that is. It's probably not a 944 kit as I think those are usually limited to 10.5:1.

Of course, I'm no expert and my opinion should be taken with a grain of salt, etc. etc. etc.
Thanks! I appreciate your input! My compression was a tad more high than 170 tho - so I figure now between 12.2~12.5:1

Learning this bike as I go for sure. Playing catch-up with mods, etc, etc.
 

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Thanks! I appreciate your input! My compression was a tad more high than 170 tho - so I figure now between 12.2~12.5:1

Learning this bike as I go for sure. Playing catch-up with mods, etc, etc.
You need to remove static/ambient air pressure before calculating total pressure. Pressure in the cylinder (excluding other effects) at the beginning of the compression stroke is naturally ambient 14.7 PSI and will also be 14.7 psi at the end of the power stroke so you don't count that against the compression ratio.

Or have I been calculating this wrong my whole life? Boy, I'm starting to feel foolish.
 

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You need to remove static/ambient air pressure before calculating total pressure. Pressure in the cylinder (excluding other effects) at the beginning of the compression stroke is naturally ambient 14.7 PSI and will also be 14.7 psi at the end of the power stroke so you don't count that against the compression ratio.

Or have I been calculating this wrong my whole life? Boy, I'm starting to feel foolish.
I'd be happy for someone to run me thru the math - I'm pretty oblivious to this. :confused:
 

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Discussion Starter · #8 · (Edited)
You need to remove static/ambient air pressure before calculating total pressure. Pressure in the cylinder (excluding other effects) at the beginning of the compression stroke is naturally ambient 14.7 PSI and will also be 14.7 psi at the end of the power stroke so you don't count that against the compression ratio.

Or have I been calculating this wrong my whole life? Boy, I'm starting to feel foolish.
HUH? Remove the pressure that is already in the cylinder before compression? That would leave no air mixture for compression......So if cylinder is BDC the air mixture in the cylinder is 14.7:1 assuming all things equal (I do not want to calculate for my altitude) and is then compressed to ~185 PSI I would certainly think the introductory air mass is a part of the equation. So yes, we are taking ambient pressure and then squeezing it to its max mechanical value. In this case 185 PSI. If one is going to divide by ambient then the original charge of ambient is a part of the total equation because that ambient charge is already a portion of the compression stroke.

But again this is just an estimation - but I prefer to keep all things equal when estimating because that is my baseline. And 12.5:1 certainly seems plausible due to my new parts unlimited battery (2nd one) has a horrible time getting through that first compression stroke, but the AGM battery I have in it now does not even notice the first compression stroke and spins her up immediately. Coupled with power-pulse sound and extreme low speed power-pulse snatch leads me to believe a higher CR.

Also, the pressure will not be 14.7 at end of power stroke, the fuel air mix is still in an expansion phase and pressure is quite more than ambient which is why valve overlap is engineered-in to provide a slight suction of intake air in order to more fill the cylinder with charge mix. This could not happen if end of power stroke is same as ambient and would cancel any effects of engineering valve overlap.

:nerd:
 

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Do you have a bore scope in your tool box?

Even without one you are simply looking for a domed piston, any dome to speak of is an aftermarket and with many brands over the years compression could be anything. Likely as mentioned it is at least a hi comp with your readings so you should be running high octane until you figure out what the motor wants. If the bike WAS a former race bike know that it may need race fuel but if it was a street bike it will likely run on 91 if jetting and timing are right. Some even tolerate 89 but better safe than sorry.

The pressures you have are consistent with hi-compression pistons be they a big bore or just drop-ins. Look at the cylinders for a clue as a big bore needs to be bored and plated so if the cylinders do not match the patina of the heads and cases or are a non-oem color they may be bored. Either way it is a plus to you as it should mean they changed the chrome cylinder studs and this is a very good thing. If you have the chrome studs then it was probably done early in the bikes life before anyone knew they were an issue, if that's the case you can wait to change them when needed and at that point you will answer the hi-comp vs big bore question.
 

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14.7 psi is 29.92" inches of merc. @ sea level @ 59deg. F, is standard day. There will be a difference at an elevation of 6900'.

Uli-
 

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Look into your ignition system as well. With high compression, the timing has to be retarded a few degrees or you will have pinging even on 94AKI. I have 11:1 pistons in mine, and on the stock ignition boxes it was definitely very unhappy if I rolled on too much below 4k.

If this bike has more than 12:1, I bet the ignition has been modified previously, otherwise I can't imagine it running very well on pump gas at all. There are a couple different ways this could have been done, either using aftermarket ignition boxes, or by physically shaving the lump on the flywheel that provides the timing pulse. I would look into that, and maybe attach a timing light and get some measurements.

Brad's website is a good source of information and he is active on here as well:

BikeBoy.org - Ducati 2V Carb Model Ignition – Reducing advance, Ignitech TCIP4 and stuff
 

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Discussion Starter · #12 ·
Do you have a bore scope in your tool box?

Even without one you are simply looking for a domed piston, any dome to speak of is an aftermarket and with many brands over the years compression could be anything. Likely as mentioned it is at least a hi comp with your readings so you should be running high octane until you figure out what the motor wants. If the bike WAS a former race bike know that it may need race fuel but if it was a street bike it will likely run on 91 if jetting and timing are right. Some even tolerate 89 but better safe than sorry.

The pressures you have are consistent with hi-compression pistons be they a big bore or just drop-ins. Look at the cylinders for a clue as a big bore needs to be bored and plated so if the cylinders do not match the patina of the heads and cases or are a non-oem color they may be bored. Either way it is a plus to you as it should mean they changed the chrome cylinder studs and this is a very good thing. If you have the chrome studs then it was probably done early in the bikes life before anyone knew they were an issue, if that's the case you can wait to change them when needed and at that point you will answer the hi-comp vs big bore question.
Hi, Appreciate you response! Borescope I know..I did have one. Loaned it to a fellow mech and he broke it......And yes I agree with you 100% and I am running the highest octane I can find in Kansas...ahem....91......This state , I tell you....No chrome studs, nice black oxide ones - look new and cylinders are a different patina like you mentioned.....stock Kokusan tho - Where is best place to buy which best upgrade?
 

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Discussion Starter · #13 ·
Look into your ignition system as well. With high compression, the timing has to be retarded a few degrees or you will have pinging even on 94AKI. I have 11:1 pistons in mine, and on the stock ignition boxes it was definitely very unhappy if I rolled on too much below 4k.

If this bike has more than 12:1, I bet the ignition has been modified previously, otherwise I can't imagine it running very well on pump gas at all. There are a couple different ways this could have been done, either using aftermarket ignition boxes, or by physically shaving the lump on the flywheel that provides the timing pulse. I would look into that, and maybe attach a timing light and get some measurements.

Brad's website is a good source of information and he is active on here as well:

BikeBoy.org - Ducati 2V Carb Model Ignition – Reducing advance, Ignitech TCIP4 and stuff
Oh man thank you so much for this post! Seriously appreciate it! I know the left case side has been off as there is sealant squeeze-out but the stock Kokusan boxes are still up top. I have been reading about how a better ignition really helps things.....Where to buy which best ignition upgrade?
 

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While I haven't done it yet, it seems the best ignition upgrade is an Ignitech, and the best place to buy it is from Lliam at Fast Bike Gear in New Zealand.
 

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I am a fan of the ignitech unit as well.

That said I have a number of carbys that run just fine with the kokusan boxes so I have a hard time putting on the ignitech that is setting on top of my tool box. If you are having issues with hi test and the jetting is right then I would pull the left side cover and check that the timing has been adjusted. Some find that idle is a problem doing it this way But that has never been an issue for me on any of our builds, I have not noticed a big change on the ones we modified the lump either. Others have so there you are.

The ignitech unit is a good unit and does give you some more options to play with such as a TPS or a adjustable rev limit.
 

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I got my Ignitech from the manufacturer direct in the Czech republic. Electronic for motorbikes - IgniTech P?elou?

It's not exactly a modern online-shop, as you have to pretty well email them what you want, and they respond with an invoice. But once that is done they ship very fast. I had my box in hand here in Canada within 5 business days. Installation is plug and play, it ships with a harness that plugs straight into the OEM harness on the bike.

Then you also need a USB to serial bus adapter cable ($15 on amazon), unless you have an old laptop or PC with a R232 serial port on it still. The ignitech software only runs on windows by the way.


On the website I linked above, Brad talks about a certain delay that creeps into the timing settings at high RPM - this has apparently been resolved in the newer units I am told, so what you map is what you get. And yes it is a big improvement with the high compression, as I said in my other thread. The biggest advantage would be that you can mess with the curve to make it fit whatever other mods your motor has. However, since it still has to get its timing pulse off the flywheel, you have to check if that has been modified first.
 
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