COMPRESSION RATIOS

[Trojan]

Firstly,HAPPY CHRISTMAS to all,now the question,if its been covered before my apologies, what I would like to know,is whats the main difference between a compression of 11.5:1 and a 8.5:1 obviously one is harder to turn over but need to know which one is suited for long journeys, M Ways and general riding and is the other purely for sprint etc, hoping some can advise,cheers ........

[Adam_Winstone]

The lower comp is better suited to today's poor fuels and will keep the motor running cooler for reliable touring, therefore, the 5.8:1 is what you want.

If you have a seriously ported race kit then the 11.5:1 might suit your motor... BUT run very hot and cause you to seize or hole a piston on a standard or mild tune motor.

Adam

[Trojan]

Cheers for that Adam much appreciated,set up is a Rapido 225 Race so needed to make sure I fit the right head with compression ratio...thanks again....

[Adam_Winstone]

Rapido Race does have a pretty high exhaust port, which drops the corrected compression ratio (compression cannot start until the piston closes the tall/high port), so it all depends what you want the final motor to do. If you're saying that you want a Race ported kit for touring, then I'd question whether you are buying the right kit, let alone the right head for it. The Rapido Race is a pretty good kit, for various applications, but you will need to build the engine with gearing, pipe, carb, ignition timing, etc. to suit, and if you're building it to rev (as the porting is designed to do) then you may even need to consider a higher comp head.

Your best bet is to ask this question of AF Rayspeed, then go with their recommendation. I say this as they designed/sell the Rapido Race and they sell heads of different ratios to suit... so if anyone should know which ratio suits best kit, it should be them.

One thing to watch out for, which makes talking compression ratios difficult, is that most places that quote what the ratio is do not get it anywhere near right! Those that actually understand how to measure geometric compression ratios commonly find that the actual ratios are nothing like those quoted, which is normally the result of people forgetting to consider the additional volume of the squish band, simply measuring the head volume alone. All of this gets very frustrating and is really beyond what most people should expect to consider when buying 'off the shelf' parts.

Your best bet is to struggle to get through to AF on the phone or to send them an email asking them the question, then go with their recommendation.

Adam

[Chris in Margate]

You are going to have to explain for me "corrected" compression ratio. Surely compression ratio is cylinder + head volume over head volume.
I always measure with water the CCs of the cylinder head with spark plug in. My 225 (almost 229...) with raised exhaust port and nicely ported is a tad over 8.5:1

Regards
Chris

[Muppet]

corected compresion uses volume trapped above closed ex port

[Chris in Margate]

OK... got that so by how much does this differ to quoted compression ratios ? I'm always willing to learn.

[Adam_Winstone]

It matters a massive amount and is why you will hole a piston if you take a head off one barrel and put it on another, or lose a massive amount of power if you do it in reverse...

As compression cannot start until the exhaust port closes, trapping the charge in the combustion chamber (note that expansions differ slightly because of a return pulse from the secondary cone), a barrel with a tall exhaust port stays open longer, allowing some of the incoming charge to escape out of the cylinder and into the u-bend or exhaust. This means that not all charge is in the cylinder to be compressed, therefore, the 'corrected' compression ratio is lower. Compression ratio, like most things 2-stroke, is a compromise and certain compression ratios are more efficient for use at certain rev ranges (e.g. a higher comp ratio will give more grunt at low RPM but can then cause a motor to run flat at higher RPM as the motor has to fight compression every time it tries to reach TDC) so you need to go with a comp ratio that suits other factors too (e.g. pipe operating RPM), rather than exhaust port height alone.

If you have a low geometric compression head and fit it to a kit with long exhaust duration, with a 'box pipe' such as a clubman, then you can find that the corrected ratio is very low by the time that some of the charge has escaped into the u-bend/exhaust, which causes the motor to lack power throughout the rev range.

If you take that self same low compression head and fit it to a cylinder with a low exhaust duration, the piston quickly seals the exhaust port off and traps more of the charge in the combustion chamber for ignition... the bigger the charge, the bigger the bang and the bigger the push, all leading to a motor that works more efficiently for the RPM that it is designed to work at.

Now consider the higher geometric ratio head on these same 2 motors...

On the ported cylinder, the exhaust port is always going to let some of the charge escape into the exhaust so reducing the compression ratio. In order to bring this back up to the point that the balance is once again in harmony, raising the geometric ratio (by skimming, etc.) will ensure that the smaller % trapped is correct for the smaller head volume. This brings back much of the low to mid RPM range power that would otherwise be lost, yet should be suitable to allow the motor to still spin freely at higher RPM. Getting the balance right is not easy.

Now fit the high geometric ratio head to the standard cylinder or mildly tuned cylinder and you can easily find that the exhaust port closes very early and traps ALL of the incoming mixture in the combustion chamber... great for efficiency UNLESS you have reduced the combustion chamber too far, pushing compression ratios up to the point that you now have too much pressure trapped in the combustion chamber. This pressure can very easily become too high and lead to the trapped charge suffering pre-ignition, pushing up running temperatures and either holing the piston or resulting in seizure.

Unfortunately, fuel quality has changed over the decades and RON / octane rating is only part of the problem that we now face for these old vehicles of ours, with the ever increasing % of ethanol in fuel adding to our woes. This change in fuel has typically meant that we are having to do 2 things to keep running temperatures down; reduce compression ratios, retard ignition timing.

The calculation/formula for correct compression ratio is available on many websites or in most 2-stroke tuning manuals. You then need to compare this to a target figure, which seems to vary from source to source, application to application, where corrected ratios will still be higher for competition machines than they are for reliable road use. Eden does has a function/calculator on his Lambretta Images website that allows you to plug in your data and it will work out both the geometric and corrected ratios for you.... NB: not that I'm taking any responsibility for the accuracy of his site's results!!!

I hope that the ramblings above make things a little clearer (!).

Adam

PS - Building a motor and measuring the trapped volume at TDC is still the best way to work out the combustion chamber volume (combined head and squish), which can then be used for calculating geo and corrected ratios. I cringe every time I see someone measuring the volume of the combustion chamber alone, without any consideration of the volume held in the squish area! Without considering all of the trapped volume (combustion chamber of head AND squish area) then the resultant ratios are miles out! Measuring actual assembled volume (as described in previous post) accuracy is, without doubt, the best way to measure combustion chamber volume.

[Muppet]

OK... got that so by how much does this differ to quoted compression ratios ? I'm always willing to learn.

depends on port height higher port equal lower corrected; don't ignore geometric ratios; if y think about it compression can start before the port shuts cus ya get a plus from a decent expansion chamber wen on the pipe it adds positive pressure before the port shuts; different pipes equal different pressures at different rpm

[Chris in Margate]

I understand the tuning theory and the effect that raising exhaust ports causes, back pressure, pulses, exhaust characteristics fully (as much as I need and want).
My question is: how do I measure and accurately establish my new "corrected" compression ratio.
Stroke 58mm x Bore 70.8mm = 228.433 ccs.
Head volume =30.4ccs
Bore + Head = 258.433/Head 30.4 = 8.51
Compression ratio =8.51:1

Exhaust port raised by a notional 1mm.

How will you now calculate "corrected" CR.

For the record I had never previously considered this and go along with what you are BOTH saying and my sums there are most definitely not to score points. Quite the opposite. I am intrigued and interested to understand this better.

Regards
Chris