2007 Sherco 250 - What Compression Ratio?

[2011gasser]

Does anyone know what the factory compression ratio is on a 2007 Sherco 2.5?  Or better yet, what an ideal target would be on premium gas?  I've searched high and low, and haven't found it anywhere.

I've had the bike for several years, and while it's ran just fine, it's always felt a little 'flat' to me.  I decided to freshen everything up this winter.  I discovered that it had two base gaskets (unknown if from the factory or not) and a whopping 2.3mm of squish!  That likely explains some of the sluggishness!

So even after swapping in a single new base gasket, it's still measuring north of 1.8mm.  I know from previous experience on a 250 2T enduro bike that this is still way too much, and getting it down to ~1.0 will make a big difference.  I'm going to machine down the head and/or cylinder, but I have absolutely no idea what compression ratio I should be targeting.

Any advice is greatly appreciated!

[Glayne]

150lbs is standard for most bikes certainly for my Gasser but gauges vary. If you have less than 150 you may be looking at at least rings.

[2011gasser]

2 hours ago, Glayne said:

150lbs is standard for most bikes certainly for my Gasser but gauges vary. If you have less than 150 you may be looking at at least rings.

I appreciate the input.

I'm specifically looking for the compression ratio however.  As in, (swept volume + chamber volume) / chamber volume.

I've already removed 0.5 mm of deck height by removing the additional base gasket, and potentially removing another ~0.8mm to get the squish where it should be.  That equates to around 1.5-2.5 more points of compression.  a.k.a a LOT. 

So I need to remove some volume from the dome in the head (chamber volume) to bring the compression back down.  The problem is, I have no clue what it should be / how much dome material to remove. 

The only data I've found is from fairly recent Beta and GasGas press releases.  They have the 250s / 300s in the 9.0:1 - 11.0:1 range (which is still a fairly large range), but since their port timing, piston dome, head shape, etc. are likely different from the Sherco, they're not really comparable. 

Case in point:  the compression ratio on my 250 2T enduro bike is nearly 14:1, but it runs perfectly fine on 93 pump gas.  Reason being that the port timing is so much later that the dynamic compression ratio is actually much lower.  This isn't the case on a trials bike, as the port timing is very early, and thus you can't run nearly as much compression.  The question, of course, is how much can / should be run on this particular bike?

[trailmeisterjoe]

The 2004 Beta REV 250 I have states 12.5 to 1 somewhere in the manual. A bugger to kick that left side mini lever too. Later on the evo's went to 9.5 to 1 as a reference.

Joe

[stpauls]

Most people in the use 99 octane in their bikes, not 95 or even 93.  Higher octane = smoother running, quicker response and greater power.  Use a much higher octane fuel than 93 and don't worry too much about squish bands.   I'm guessing that you are not based in Great Britain?  If higher octane fuel is not available in your area, then consider using an octane booster or even Avgas.

[2011gasser]

11 hours ago, trailmeisterjoe said:

The 2004 Beta REV 250 I have states 12.5 to 1 somewhere in the manual. A bugger to kick that left side mini lever too. Later on the evo's went to 9.5 to 1 as a reference.

Joe

That sounds high for a trials bike, but again, it's certainly possible depending on port timing and other factors.  Which highlights the reason I'd rather not take a guess at what my bike should be.  If it's 'supposed' to be 12.5:1 and I make it 10:1, it'll likely still be as sluggish as before.  And if it's 'supposed' to be 9:1 and I make it 11:1, it'll probably need race gas to prevent detonation. 

For reference, the 2023 Betas are shown as 10.6:1 for the 300 and 9:1 for the 250.  It's a bit unusual because smaller displacements are typically run with more compression, not less, but it sounds like Beta is specifically tailoring the 250s as a more beginner friendly option.  So that may be why it's less than the 300.  https://www.dirtrider.com/story/dirt-bikes/beta-trials-bikes-first-look-2023/

The 2023 GasGas is reported at 10.4:1 for the 300 and 11:1 for the 250.  https://ultimatemotorcycling.com/2022/07/13/2023-gasgas-txt-trial-lineup-first-look-11-fast-facts-61-photos/

But that's pretty much all the concrete data I've found regarding compression ratios in trials bikes.  Considering how readily available this info is for enduro bikes that I spend most of my time on, I've found this quite surprising. 

 

9 hours ago, stpauls said:

Most people in the use 99 octane in their bikes, not 95 or even 93.  Higher octane = smoother running, quicker response and greater power.  Use a much higher octane fuel than 93 and don't worry too much about squish bands.   I'm guessing that you are not based in Great Britain?  If higher octane fuel is not available in your area, then consider using an octane booster or even Avgas.

Yes, I'm in the USA.  If I understand correctly, our pump premium / 93 octane is equivalent to your 98/99.  I've never met anyone running higher than 93 on a trials bike, as the bikes still make plenty of power at factory compression ratios / on pump gas. 

Not fixing the squish isn't an option.  Removing 0.5mm of squish clearance on my enduro bike made a substantial improvement.  Considering that this bike needs >1.0mm of squish reduction, and that fixing the sluggishness of this bike was my primary reason for tearing it down, I'll definitely be bringing it down to appropriate levels. 

But, I'd rather not fix one issue (squish) and create another (compression ratio).  It'll be easy to address, if I only knew what it 'should' be. 

[feetupfun]

6 hours ago, 2011gasser said:

But that's pretty much all the concrete data I've found regarding compression ratios in trials bikes.  Considering how readily available this info is for enduro bikes that I spend most of my time on, I've found this quite surprising.

But, I'd rather not fix one issue (squish) and create another (compression ratio).  It'll be easy to address, if I only knew what it 'should' be. 

Yes <1.0mm squish is a good thing. I've recently modified a trials motor to achieve this, and at the same time, copied the combustion chamber shape from a modern KTM. It goes well, however I didn't increase the compression ratio because it was perfect already.

A good person to ask with experience in what you are seeking is Paul Arnott from The Hell Team in Sydney.

 

[trailmeisterjoe]

9 hours ago, 2011gasser said:

That sounds high for a trials bike, but again, it's certainly possible depending on port timing and other factors.  Which highlights the reason I'd rather not take a guess at what my bike should be.  If it's 'supposed' to be 12.5:1 and I make it 10:1, it'll likely still be as sluggish as before.  And if it's 'supposed' to be 9:1 and I make it 11:1, it'll probably need race gas to prevent detonation. 

For reference, the 2023 Betas are shown as 10.6:1 for the 300 and 9:1 for the 250.  It's a bit unusual because smaller displacements are typically run with more compression, not less, but it sounds like Beta is specifically tailoring the 250s as a more beginner friendly option.  So that may be why it's less than the 300.  https://www.dirtrider.com/story/dirt-bikes/beta-trials-bikes-first-look-2023/

The 2023 GasGas is reported at 10.4:1 for the 300 and 11:1 for the 250.  https://ultimatemotorcycling.com/2022/07/13/2023-gasgas-txt-trial-lineup-first-look-11-fast-facts-61-photos/

But that's pretty much all the concrete data I've found regarding compression ratios in trials bikes.  Considering how readily available this info is for enduro bikes that I spend most of my time on, I've found this quite surprising. 

 

Yes, I'm in the USA.  If I understand correctly, our pump premium / 93 octane is equivalent to your 98/99.  I've never met anyone running higher than 93 on a trials bike, as the bikes still make plenty of power at factory compression ratios / on pump gas. 

Not fixing the squish isn't an option.  Removing 0.5mm of squish clearance on my enduro bike made a substantial improvement.  Considering that this bike needs >1.0mm of squish reduction, and that fixing the sluggishness of this bike was my primary reason for tearing it down, I'll definitely be bringing it down to appropriate levels. 

But, I'd rather not fix one issue (squish) and create another (compression ratio).  It'll be easy to address, if I only knew what it 'should' be. 

Looks like 12.0 to 1 and like I said it's a real bugger to kick. May be one reason Beta greatly reduced it in later years.

https://bikez.com/motorcycles/beta_rev_3_2004.php

Joe