Compression Ratio for new build big bore motor - Long term with new fuel - 12:1 ??

[Diebythereaper]

Hi guys,

With our ever changing fuels and more coming I am currently doing a big bore Gsxr motor for a road project and want to know what is the best comp ratio to be aiming for?

I know in the past we could do 12.5:1 and run good fuel and it will be fine but I don't have that option - Where I am the only choice is what is in the pump (currently E5 and prob E10 on the way) and no high octane unleaded or any of that.

Does 12:1 sound like a good goal for regular petrol or should I go lower? I honestly don't know

thanks

[Gixer1460]

My 1186 kitted GSXR used to pink before unleaded was even a thing! (showing my age) in summer and going up hills. Wasn't bad enough to cause damage, just let you know it wasn't happy at that time! You can run whatever CR you like provided you can control fuel and spark better than std. OEM! With more accurate fuelling you minimise lean hot spots causing detonation and accurate adjustment of ign. timing particularly around the mid range / peak torque areas stops det. occurring. Simplest way is EFI or at least mappable Ignition . . . . . . . . . without this i'd pull it back to 10.5 - 11:1 IMO.

[Diebythereaper]

I'll be on CV carbs for road use and running a dyna 2000 ignition as I've never had a problem with them personally.

[Gixer1460]

15 hours ago, Diebythereaper said:

I'll be on CV carbs for road use and running a dyna 2000 ignition as I've never had a problem with them personally.

Same spec as was on mine - 1186 kit was Wiseco and I think 12.5:1. Did make 144rwhp so it was good! IMO unless you have the 2000i Dyna you are stuck with their 'stock' ign. curves! At least with the 2000i you can attempt to knock some timing out in the midrange if knock happens but is still not optimal. At least using E10 you can actually run a bit more timing to recover the losses due to its lower energy content LOL!

[Diebythereaper]

I'm going to go with 11.5:1 I think and set the ignition timing at 36degrees and that should do forever more I hope?!

If anyone has any thoughts about that let me know please

[clivegto]

Have a look at ignitech, @MeanBean49can get a discount on them if you ask nicely 

[dupersunc]

5 minutes ago, clivegto said:

Have a look at ignitech, @MeanBean49can get a discount on them if you ask nicely 

This. Dont waste your money on dynatech. Its last century's tech at a premium price .

You can set the ignitech to what ever advance you want.

[Sandman]

On 10/29/2021 at 5:01 PM, Diebythereaper said:

 

I know in the past we could do 12.5:1 and run good fuel and it will be fine but I don't have that option - Where I am the only choice is what is in the pump (currently E5 and prob E10 on the way) and no high octane unleaded or any of that.

Does 12:1 sound like a good goal for regular petrol or should I go lower? I honestly don't know

Why are you reducing the CR?

In terms of its octane rating, ethanol has a rating of 113. Fuels with a higher octane rating reduce engine knocking and perform better. Also, almost all gasoline in the US contains 10 percent ethanol. When you mix 10 percent 113 octane ethanol with 85 octane gasoline it increases the octane two points to the normal 87 octane most consumers use. So the higher the ethanol content, the higher the octane. The octane rating for E15 (15% ethanol) is 88 octane and E85 (85% ethanol) is 108 octane.

The use of ethanol blends in conventional gasoline vehicles is restricted to low mixtures, as ethanol is corrosive and can degrade some of the materials in the engine and fuel system. Also, the engine has to be adjusted for a higher compressive ratio as compared to a pure gasoline engine to take advantage of ethanol's higher oxygen content, thus allowing an improvement in fuel efficiency.

 

 

 

[Diebythereaper]

Now that's a well thought out reply Sandman and thank you! I am concerned about detonation with our ever changing fuel situation which will continue to change, from what you are saying them I will be fine with whatever I do.

I mentioned the Dyna deliberately as I knew people would say Ignitech, thing is I have a Dyna on the shelf for this build for years and they are fine. I don't need to chase every last hp out of the engine I just want something I can work on and use myself.

[Gixer1460]

@Sandman  a well reasoned argument although in the UK at least, not strictly accurate! Our E10 is mandated to be of minimum 95 octane ie. the same as previous E5 so even if the 5% extra ethanol made 2/10th's of f**k all difference to the overall octane rating, the inclusion of 10% ethanol by volume (ethanol having about a 1/3rd less energy v's petrol) produces a 'blend' with maybe 5% less overall energy content, so you have to burn more to get the same 'work energy' out. Yes, E85 is fabulous in a boosted engine, running daft boost numbers to take advantage of the high octane but using nearly 2:1 quantities, and in a country (UK) that has very few actual outlets that have E85 is a moot point + if you run out and the only garage you can find has E10 - you are having a pretty slow ride home staying out of the boost LOL!

[cnap504]

Pump premium gas in the U.S. is rated  mostly 93 octane.  Modern bikes with sophisticated ignitions (knock sensors, throttle position recognition, gear related curves) do get by with today’s fuel and relatively high comp ratios.  The digital ignitions give the spark just enough lead to keep things fun without detonation.  I’d opt for a half point less compression and a couple/few degrees less advance if working with a mechanical or electronic predetermined advance 

Edited November 2, 2021 by cnap504