Raising Compression for e85 Help?

I

IanT720

New Member
Hey guys I have a '80 Yamaha XS850 Triple. Its a clean bike, I've made it more cafe-ish and dig it. But I want more power! (As always). I love the bike, so saying I should buy a rocket is un-productive. I want to run e85. Its plentiful by me, and I know its awesome from my Turbo car. My goal is 100hp from 75 stock. I want it to stay a daily ride, but have that extra power. I think the solution is e85, with its high octane and cooling property's :)

Stock compression is 9:2:1, I want 13:1 or so. Along with a higher ignition advance, better carbs, and a cam grind, it should be there.

Issue... Apparently the highest compression I can go is 11:5:1... Because of piston valve reliefs?

How can I raise compression??? Any Ideas? You don't really see the benefits of e85 until you get 12:1 at least without forced induction.

This is purely a think tank, I will not be rebuilding her for a while. Let me here what you guys think!!! :D
 
Combination of milling the head and/or cylinder jugs along with custom pistons will do the job. You can probably get the pistons made for about $600-$700. Head work will be in the $200-$300 range. Bigger cams might be an option, too.
 
Cams do not raise compression ratio. Timing,lift and duration can alter the max psi reading a little but compression ratio is a static measurement of cylinder volume vs. combustion chamber volume.

There are 2 ways to increase compression ratio.

1. Increase calculated cylinder volume

2. Decrease calculated combustion chamber volume.

Remembering that piston dome/dish cc is calculated as part of the combustion chamber, this tells us that a higher dome will reduce combustion chamber volume and thereby increase CR, but as mentioned this will only be practical until there is no room for the valves to open and close without hitting the piston.

Another way to reduce chamber volume is as mentioned by milling the head, where we run again into valve clearance and even timing issues

This is where the custom pistons come in. They will design a piston that has a higher dome but deeper reliefs for the valves. BUT, if you are going to have custom pistons made, there is really not usually a reason to also mill the head, it would be redundant to do so and you may also reduce flow due to slightly altering combustion chamber shape and design.

Squish is a whole other discussion, one that you will have to have if you want ridiculously high cr relative to an old air cooled engine.

Another way to increase compression ratio is to add material to each combustion chamber. This is expert territory.

Increasing cylinder volume on an xs850 is pretty simple. just have it bored out 3mm hunt for high compression xs1100 pistons.

There is a reason Wiseco doesn't even make a 12:1 piston for some of the older air cooled engines. It can cause reliability issues. 13:1 can beat the bottom end to death and it will likely run hot no matter what you do. That being said, I do think they used to, if not still do, make a 12.25:1 piston for the xs1100. If so, you're in business

The xs850 is kind of a pig, have you dropped all the excess weight yet? It makes a huge difference on them.

I had one with a kerker on it, was one of my favorite sounding bikes ever.

An 896 kit (stock CR xs11 pistons) and some hot cams would get you a good 10hp and it would stay simple and reliable.
 
You got some good points, I just never realized how much I interference this engine has. I looked all over the place but the xs11 pistons are only 10:5:1. Which is obviously an increase... Hmm guess I will have to think of what I want to actually do. Maybe a supercharger ;) haha. I stripped off the easy stuff on my bike, it's still heavy but I like having a solid bike for highway commuting. Maybe your right, a bigger bore, higher CR, and a cam grind is the way to go. I just kinda wanted to do something crazy and play with e85. Hmm.... Well thanks for the replies guys. I'm still very curious to know more, and I will keep reading this thread!
 
datadavid said:
Im pretty happy with my 896cc kit..
Click to expand...
David, did you do anything else to raise compression ratio on your engine?

What fuel are you using in your bike?

Any head work? Cams? Valvetrain upgrades?
 
No its basically stock, wanted something i could do a lot of troublefree miles with.
Cams and timing is stock, all i did was polish intake ports slightly.
I guess i could try advancing timing a bit, go up 20% in jetting and fill her up with e85 to see if it runs ok?
Im guessing here, but with bigger pumping volume and less overlap dynamic compression would go up a bit?
 
Btw I love the sound of the Triple. Main reason for wanting to keep the bike! I basically made my own Kerker. I used a Triumph header and fabricated the rest. So it's a 3-2-1. Sounds awesome!!! But yeah... I'd love to do a turbo!!! Especially when the Wastegate opens... Oh my god... Be so knarly!!!
 
The E85 will not give you any "Awesome" power.. it will just increase the fuel consumtion by a third..
If you jet it good you get aproxx 5% increase i power..
you have to go relative extream w an old aircooled triple to take advantage of the octane range of 94-96 (R+M)/2.
The xs850 is not a good engine to increase compression rate on...not a turbo or compressor happy engine either...

many parts in old machinery do not like the ethanol..
 
We typically ran 12.8:1 compression on our VW drag bugs, which we also drove on the street. They did fine for reliability. We raced with 110 octane and drove street with the highest octane available.
 
Running 13:1 using e85 is about as hard as running 10:1 on pump gas. But maybe you guys are right, my bike just isn't suited for it. Still should get a nice boost with the bigger xs11 pistons, along with the CR bump. With a cam grind I'm sure I will be happy. I'd love to convert to slide carbs... As I said my exhaust setup is great, so no need to worry about that.
 
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Ian, you obviously know what you are doing so that's a big help.

lots of good ideas and info.

if the e85 is much less money, you could still run a big bore kit with 11.5:1 and although that's less compression than you could run, you will still have around the same performance as you would if you had a big bore kit with maybe 10.0:1 with normal gas.

yes, you could get a turbo and it would work . . the obvious problem is determining the point of yield of your parts BEFORE you reach it, lol.

there is a chinese turbo that is around $150.00 that would work really well but as you may know, you should run a cam that is designed for turbos to get the optimum perf out of it and you will likely have a litte turbo lag.

as far as changing the timing goes, MSD has a programmable unit that stores two totally separate timing programs so you can change the timing between programs with just the push of a button.

the tubos work best with the carb after the turbo but that can be a pita to set up on a bike but it can be done and is much easier to jet if you have fuel injection.

pistons made from 4032 aluminum are not very forgiving to detonation so using 2618 material is a much safer route for this app but you have to run a slighter bigger clearance with them which may cause a little piston slap noise from time to time, especially when the engine is cold.


RHB31 VZ21 CHINESE TURBO

These can be found on evilbay.

http://www.ecotrons.com/products/small_engine_rhb31_vz21_turbocharger/
 
Ethanol does actually have an Octane number as such because it is higher than the scale, but it has much reduced lower heat value than the average Gasoline and runs a much richer A:F as you probably already know. Bottom line is a theoretical improvement in power of around 5% IIRC on stock CR and of course the advantage is that it can stand MUCH higher CR.

You can, as someone suggested earlier, wed up the head as we did on one of our big bore CB175 motors but after removing valve seats and welding it up and machining it and fitting new seats and guides and getting pistons machined and working out squish bands etc etc, I can say that I am not going to repeat that in a hurry. :)

I recall reading tests on a WRX a few years ago where they optimized it for E85 but I don't recall the final % improvement or how much they cranked up the boost, but it was significant. For the street, maybe not worth the effort.

Sounds like XS11 pistons plus maybe a little machining to improve squish might be a solid enough improvement.
 
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