Porting and Engine Tuning

Maintained by mods and admins. but contains all the answers to all those questions that get asked over and over and over . .
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Porting and Engine Tuning

Post by knackeredMk1 » 06 Jan 2011, 11:11

I'm going to start a new evolving section here for owners who are interested in porting and engine tuning.

The theories and work on here are entirely the views of the people who contribute. I, Admin and the contributers make no guarantee that the subjects and work on this thread/post will work on your engine and no guarantee that they will not adversely effect the way your engine works. Do not attempt to do anything that is in this post unless you are sure about what the purpose and methods used are, and that it is well within your capabilities.


Many of the workings below are to remove 'manufacturing defects' to bring the perfomance back to what Mr Honda originally specified. These may potentially increase the power of your engine, depending on the original condition, but not significantly alter the way that power is produced. If you want to seriously change the power characteristcs of your bike by porting it - take it to a professional.

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Transfer Port Matching

Post by knackeredMk1 » 06 Jan 2011, 11:49

The size and particularly the height of ports contribute to the character and the quantity of the power deliver of a 2 stroke engine. I propose that you do not change the shape and area of the transfer ports as this will significantly alter the power delivery. However -

Generally the quality of Honda castings is very good but where the aluminium casing of the barrel meets the cast iron liner there is often a missmatch of surfaces. Sometimes this missmatch is quite significant.

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The large elipse marks a ~1mm high ridge with the cast iron standing proud of the aluminium.

These intrusions significantly effect the passage of gasses and the engine would benefit from their removal. BUT don't change the shape and size of the ports. Try to produce a smooth flow across the iron/aluminium interface with the minimum of disruption to the iron.

I use this for porting work but there are many other similar tools that can be used.

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The final result should look something like this (Though there is some porosity in the cast iron casting in this instance) -

Image

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Cylinder Base Profiling

Post by knackeredMk1 » 06 Jan 2011, 21:23

There are two cylinder bases in this photo the rings mark where work has been done. Basically general profiling of the aluminium/iron boundry.

There is also some thinning of the tips of the seperation vanes between the front and back transfer ports. This is called 'knifing the ports' and is considered by some tuners to be unneccessary (including Stan Stephens). At this stage you could also work on smoothing where the crankcases meet the cylinder. This again is slightly contentious as the need to do this depends on the size of the missalignments and whether a restriction in this area has any limiting effect on the power production of the engine. Furthermore there is some disagreement as to whether it is necessary, or even desirable, to produce a polished surface within the ports.

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The cylinder on the right is a Mk1 and therefore has no hole in the shirt. The inlet area to the crank from the carb is tightened on the Mk2/3/AR to speed up the inlet gas flow and raise the low end torque so they needed to make this hole for some reason. I have seen a Mugen Mk3 barrel which is similar (the same as far as my eye could see) to an original Mk3 barrel apart from a section of ~6/7mm removed from the inlet skirt straight through the 'hole' and parallel to the crankcase/barrel interface.

There can also be a restriction caused by the cylinder base gasket. In this instance the gasket overhangs the port by >1mm at the edge where the gases are moving the fastest. This overhang is best removed with a rotating stone or grinding wheel not cutting.

Image

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Gaskets

Post by knackeredMk1 » 08 Jan 2011, 17:15

Gaskets can be use to fine tune the engine's power characteristics.

Standard gaskets are 0.2mm of head and 0.5mm for base. Changing the thickness can do several things -

Base:
Thicker base gasket reduces compression and raises the exhaust port height = Less bottom end and more overrev.
Thinner base gasket increases compression and lowers exhaust port and transfer port height = opposite of above.

Head:
Thinner head gasket increases compression = More low end power.
Thicker head gasket decreases compression = Less low end. Sometimes used to reduce the 'hit' of 'violent' engines (e.g.TM300 :? )

Athena makes CRM 250 gaskets but I don't think they make them in different thicknesses. You could certainly make your own base gaskets if you bought certain thicknesses of gasket paper and cut them out yourself. I have seen and measured a Mugen head gasket and found it to be no different from the standard Honda one. Remember that if you add sillicone or other gasket compound to the gasket on installation you are adding to its thickness.

Personally I can see no point in messing about with gasket changes on a CRM250 with one exception perhaps if you want to go racing. - Using a thinner base gasket to raise the compression and physically grinding the ports to keep their original heights or perhaps increase the exhaust port height slightly :?: .

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Compression Ratios and Skimming

Post by knackeredMk1 » 08 Jan 2011, 17:56

High compression engines are talked about all the time as high compression gives the fastest and most complete burn of the petrol/air mixture. However you cannot raise the compression ratio above a certain point or the fuel/air mixture will self ignite like a Diesel engine. This usually happens some time before the piston has reached Top Dead Center and causes catestrophic engine damage.

A quote for Eric Gorr 'Basic 2 Stroke Tuning' - 'Cylinder heads can be reshaped to change the power band. Generally speaking, a cylinder head with a small diameter and deep combustion chamber, and a wide squish band (60% of the bore area). Combined with a compression ratio of 9 to 1 is ideally suited for low to mid range power. A cylinder head with a wide shallow chamber and a narrow squish band (35-45% of bore area) and a compression ratio of 8 to 1, is ideally suited for high rpm power.'

From that it might be deduced that the desirable compression ratio for a CRM might be close to 9:1. However stock CRMs have different compression ratios - Mk1 = 6.2, Mk2/3 = 6.4, AR = 6.7. BUT with the exception of the AR all engines have the facility to be bored out when worn and without work done to the head at the same time the compression ratio will rise; Though apparently not to pre-ignition levels if not done as most people don't bother. Mugen heads raise AR compression ratios to 8:1.

Having read several articles about Japanese CRM tuning there seems to be a consensus that skimming (taking a thin layer of metal from the gasket surface) the head by 0.25-0.3mm adds significantly to the low end power without making the engine unreliable. 0.5mm has been done but only on Mk3 heads.

Be aware that in raising the compression ratio it is prudent to use higher octane (RON rating) fuel. Higher octane rating does not mean more power but more resistance to pre-ignition. Be aware that a previous owner may have skimmed the head already :? . Also you are now making your engine work harder so it will wear more rapidly.

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Exhaust Ports

Post by knackeredMk1 » 10 Jan 2011, 17:32

Basic principles:

If you have a low top edge to the exhaust port you have a low endy engine and if you have a higher port top edge then the engine will rev higher. If you have a high exhaust port your engine will rev highly and produce most power at high revs but the higher the port gets the less low end power you have. This is the principle of the Powervalve which varies port height according to engine RPM to produce a good spread of power.

See the illustration below. The powercurves on the right correspond with the fixed exhaust ports heights on the left. The unmarked curve is the best of all worlds when the PV is moving according to RPM. (And no the piston doesn't hit the PV as in reality it is slightly recessed into the cylinder wall :roll: ).

Image

The larger the port area the more gas that can flow out and the more power you can get (depending on other considerations like how much gas you can get into the engine...). Exhaust width is obviously limited by the width a piston ring can be reliably suspended across. A CR250 has a double exhaust port with a bridge down the middle to support the piston ring and is thus able to flow more gas than a CRM250 and produce more power.

I have done some rubbings of CRM exhaust ports to illustrate differences (+/- 0.5mm maybe more :!: . Measurements taken from the outside of the dark pen). The measure on the top of the ports is the distance from the top of the port to the top of the cylinder.

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The one below is my home porting effort on a Mk2 cylinder (As in above photo of base work). Proved to be reliable, more powerful (by the seat of several people's pants) and with more overrev. I had to adjust the Powervalve actuating arm slightly anticlockwise so as to achieve peak opening but I haven't noticed any loss of low end 'tractorbility'. The base of the port was not altered.

Image

Remember that these are FLAT drawings of a horizontally CURVED surface.

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Reboring

Post by knackeredMk1 » 10 Jan 2011, 20:16

This is an area that frequently gets overlooked. I.E. the fact that when you rebore the cylinder you get a larger capacity and this larger capacity produces more power.

Initial capacity of a CRM Mk1/2/3 is 246cc (66mm bore/72mm stroke) if you bore this out to its maximum bore (68mm) you get 262cc. This is 16cc or 6.5% bigger so in theory you should get ~6% more power = about 1.8bhp at the wheel (given that a standard CRM produces ~ 30bhp at the wheel or 2.4bhp if you use Honda's 40bhp).

Worth considering :?: - probably the same as an aftermarket pipe and silencer but more across the range :? . 1/3rd of the cost. Much, much cheaper than a Mugen ECU with more than twice the gain :!: .

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Cylinder Heads

Post by knackeredMk1 » 15 Jan 2011, 15:17

A quote for Eric Gorr 'Basic 2 Stroke Tuning' (Also above) - 'Cylinder heads can be reshaped to change the power band. Generally speaking, a cylinder head with a small diameter and deep combustion chamber, and a wide squish band (60% of the bore area). Combined with a compression ratio of 9 to 1 is ideally suited for low to mid range power. A cylinder head with a wide shallow chamber and a narrow squish band (35-45% of bore area) and a compression ratio of 8 to 1, is ideally suited for high rpm power.'

As far as I can tell (measuring cylinder head dimensions is not easy), the profile of a Mk1 and AR heads are the same or very similar. The AR is much smoother but the shape is the same apart from where the spark plug sits, which is much tidier on the AR. The squish band on the Mk3 is larger than the Mk1/AR which in turn is larger than the Mk2 (which has 2 domes in the head). This must be better for torque at low/mid range from what I have been reading :? .

The squish band on Mk3 as near as I can tell = 65% of bore area. AR/Mk1 = 54% . Mk2 = 47%. The compression ratio on the original bore is Mk1 = 6.2, Mk2/3 = 6.4, AR = 6.7.

An important element is the depth of the squish band. This varies between models and the number if rebores. CR250s apparently perform best when the squish band height is ~ 0.75mm. On my rebored CRM with a Mk3 head the squish band is ~2.7mm :!: . Making the squish band tighter and increasing the compression ratio makes the engine more lively and responsive at low/mid rpm.

Mk1, Mk2, & AR.

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Mk3.

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AR Mugen (very, very similar to a skimmed Mk3 :? )

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Generally Japanese tuners skim heads by 0.25-0.5mm depending on the model.

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Intake & Reeds

Post by knackeredMk1 » 02 Feb 2011, 17:31

Three principles:

1). The larger the amount that can flow through the intake the more power is possible.

2). The smoother the passage of the intake the more flow can be achieved.

3). The higher the speed of the gases through the intake the more power is possible.

So you would think that a big smooth intake would be best. Well for peak power this is true but big intakes cause low speed intake at low RPM and low throttle openings. So the CRM has quite a narrow intake to achieve higher intake speeds earlier. Particularly the Mk2/3/AR which have a significantly 'smaller tube' for the gases to pass through than the Mk1. This is part of the reason why they produce more torque.

There are things that you can change:

1). Restrictions in the flow. Sometimes when the intake manifolds are moulded there is significant flashing left -

Image

This can be removed with a rotary stone or wet & dry .....

2). New reeds, particularly dual stage reeds, increase the amount of gases that get through the reed block at lower RPM and lower throttle openings. Generally more power through the range but more noticable low/mid.

Boyesen Dual Stage reeds (Boyesen code - 633)

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3). New intake. Boyesen Rad Valve or vForce induction systems improve power and throttle response. vForce induction system requires some modification of the original inlet rubber (instructions provided with the system I believe).

Boyesen Rad Valve (Boyesen Code - RAD02SJ)

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4). Vanes on the inside of the carb can reduce turbulence at part throttle openings and thus smooth and speed up the flow of gases. Scaryfast, Boyesen and others make these but not for the standard CRM carb.

PowerNow & PowerNow Plus

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5). Using an aftermarket air filter and cage. Will improve flow a little.

Original v Twinair Powerflow

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6). Removal of the sliding baffle in the airbox will increase air flow.

Some of the above changes need alterations to the jetting to match.

7). If you have a Mk1 change the inlet rubber and Nylon insert for the reed cage to a later version.

The Mk2/3/AR version of the inlet rubber incorporates the Nylon insert part and makes for a smoother gas flow. Later models also have a smaller cross-sectional area so the intake gases travel faster leading to more low/mid range torque.


Some things that you really shouldn't change:

1). Removing the air box leads to big jetting problems.

2). Removing the air filter is so bad it probably doesn't need mentioning :roll: .

3). Removing the rubber extension on the intake manifold that goes into the reed cage because you know better than all the induction engineers in Honda.

4). Removing the reed stops on the reed cage unless specifically told to by your aftermarket reed manufacturer.

5). Using an air filter without air filter oil.


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