Thoughts on the "slant plug head"

I thought I better start my own thread on the slant plug (high compression) head. By no means am I complete with this yet, so the writing will appear blogish.

I want to emphasize from the start, for $22 (delivered) this is not a plug and play bolt on, despite the web appearance of such. I think for the price it's a bargain. It looks like it has a nice squish band with a very compact combustion chamber. This must be the "advantage" vs. a regular hemispherical head - although interestingly I think a hemi head with a smaller chamber and squish band might really be the kat's meow. The slant head is smaller, and looks cool with plug laying at an angle. The visual appearance of the head is a little rough. Not complaining, but the mating surface did not look perfect.

Bolt on? Of course, that is exactly what I did. Took the old head off, threw a new gasket on, torqued the slant head on to 15 ft. lbs, screwed in a BP5HS. Bike fired right up, but I could really feel the clutch slip and the raised compression. Took off, but it seemed rough, clutch would slip when I changed gears - but there was a feeling of increased power and all the power, the power band, graphically looking at the power vs. rpm, seemed to come on at some much lower RPM and above that RPM there was very much a flattening and a drop off of power from the peak. The clutch slip was getting worse and very irritating.

Rode back to the shop. Messed with the clutch way too much. (separate thread)

I changed to a cooler plug without the extended tip (B6HS)

Retorqued the head. Rode for awhile. Felt better. Then I notice the head is leaking right in front (engine NEVER had a head gasket leak before). So I take it apart, aluminum paint a new gasket, reassemble and torque to 17 ft. lbs. Rides OK with the exception of clutch issues again. Only notice a slight throttle issue, so I move the e-clip up a notch. Makes a nice difference, seems to be dialed in now. Will check the plug soon.

So while messing with clutch again at the end of the day yesterday I look at the front of the head/gasket seal - it is goozing BIG TIME. So tonight I will pop the head again and check it for flatness as I should have done twice before!! Arggh.

I noticed when I first got my slant cyl head I did notice the head did not have a perfectly flat mating surface for the head gasket but neither did my factory one... any way 5 min of sanding I got a flat surface and with a new gasket and properly torqued head nuts, i dont have any head gasket leaks. As for the clutch I noticed once it is correctly adjusted, even without the clutch mod mentioned in the previous thread, when not abused I rode 600 miles before I beat it real bad one day which glazed the pads...

Has anyone tried turning down the stock head?

It's a 90 degree head should be able to make a adapter and chuck it up in a lathe and cut it down.
and reshape the chamber.

used to do it with B&S flat heads years ago.

bmc_az

I installed mine today and I can't say it was a huge difference in power, but it did give a little more bottom end. The main thing it did for me was smooth out the motor. It just purrs from idle right up to redline. My old head gave the motor quite a bit of vibration and "chug" to it. (couldn't think of a better word for it.) I did have to use 2 head gaskets to get the clearance needed to run.

I just wish the shipping was quicker. It took almost 2 weeks to get here. I know it was slowed by the holiday, but still....

In conclusion, I would say it's worth the $22 that I paid. The motor seems happier and a little more peppier now.

Seriosuly, dont buy the head if you dont have the equiptment to remachine it lol.

My head was pretty much garbage, the bosses/holes where the bolts sit were counterbored in a drill press, not milled. The spark plug hole was counter bored too! FFS how stupid are these chinese.

So first things first. I re machined all chinese "machined" surfaces and that fixed that.

Now to the combustion chamaber. The squish band was terrible and would only work if the head was shimmed up. This then leaves a huge squish clearance than would make performance worse! and also raise fuel consumption! The squish band has to gapped to the minimal amount possible so a smaller percentage of the fuel/air mix is there and thus more is in the chamber where it burns.

Also the chinese squish bands blending radius was nothing! A 90 degree edge was it. this is not accetable as the thin blur on this edge would heat up rapidly and promote detontation.

So, I turned myself a whole new head on the lathe - I pretty much brought a casting... not a bolt on part. So now, after all the hard work I have got 65/35 squish band head gapped to 1.5mm (I'm closing this slowly, its just that under no load revs who knows was the piston max hieght with all the bearing play and con rod stretch) and wit ha blending radius of 1mm - oh yeah and all squish/combustion chamber surfaces polished; of course...

The result? Awesome. Ive bumped up compressiong to 12:1 (uncorrected - corrected its about 7.3:1 - like most motocross 2 strokes) and it pulls like a tractor up the hills. This is all using a modified stock exhaust can as a muffler and a expansion chamber (thats tuned for all out speed 6000RPM +, so infact that chamber caused me to lose some torque on the hills but meh). Oh yea the bikes engine had been balanced/trued, port matched transfers/intake/exhaust, my own home made straight intake manafold and last but not least - dual boost ports (took me around 5 mins to do on the mill with a 12mm end mill, just cut the port than blend the roof angle to 45* using a engineers scraper).

Oh yeah - if your slant head didnt seem to do much when stock it could have been like the one I brought turned out to be, a stock compression head with a angle spark plug hole - some slant heads actually raise compression while some just pretty much reposition the spark plug.

Popped the head off again. Not even close to being flat. I am surprised it ran at all! And I did feel higher compression - too weird. Took some elbow grease to get it flat.

Don't have time to put any more into it right now. My duties call. I'm a little behind....or a big a$$...take your pick.....let me rephrase that......

im glad i didnt get a slant head.
i have all the power i want with my regular one.
my bike takes off like a rocket and zooms down the street at full power.
i could drag a couple people down the street behind my bike(i wouldnt do that thou).

DIYMark said:

Seriosuly, dont buy the head if you dont have the equiptment to remachine it lol.

My head was pretty much garbage, the bosses/holes where the bolts sit were counterbored in a drill press, not milled. The spark plug hole was counter bored too! FFS how stupid are these chinese.

So first things first. I re machined all chinese "machined" surfaces and that fixed that.

Now to the combustion chamaber. The squish band was terrible and would only work if the head was shimmed up. This then leaves a huge squish clearance than would make performance worse! and also raise fuel consumption! The squish band has to gapped to the minimal amount possible so a smaller percentage of the fuel/air mix is there and thus more is in the chamber where it burns.

Also the chinese squish bands blending radius was nothing! A 90 degree edge was it. this is not accetable as the thin blur on this edge would heat up rapidly and promote detontation.

So, I turned myself a whole new head on the lathe - I pretty much brought a casting... not a bolt on part. So now, after all the hard work I have got 65/35 squish band head gapped to 1.5mm (I'm closing this slowly, its just that under no load revs who knows was the piston max hieght with all the bearing play and con rod stretch) and wit ha blending radius of 1mm - oh yeah and all squish/combustion chamber surfaces polished; of course...

The result? Awesome. Ive bumped up compressiong to 12:1 (uncorrected - corrected its about 7.3:1 - like most motocross 2 strokes) and it pulls like a tractor up the hills. This is all using a modified stock exhaust can as a muffler and a expansion chamber (thats tuned for all out speed 6000RPM +, so infact that chamber caused me to lose some torque on the hills but meh). Oh yea the bikes engine had been balanced/trued, port matched transfers/intake/exhaust, my own home made straight intake manafold and last but not least - dual boost ports (took me around 5 mins to do on the mill with a 12mm end mill, just cut the port than blend the roof angle to 45* using a engineers scraper).

Oh yeah - if your slant head didnt seem to do much when stock it could have been like the one I brought turned out to be, a stock compression head with a angle spark plug hole - some slant heads actually raise compression while some just pretty much reposition the spark plug.

Click to expand...

Good post Mark!

Guys just as a matter of reference; the cylinder head should have an angled area that is 3 degress off angle to a line tangent to the piston dome from a point on the piston diameter and another point 10mm inward from the radius of the piston. It is critical that this is correct for any real benefit to occur. Ideally the piston should come within .030" of this area in the cylinder head, (know as squish band).

As Mark pointed out...It is a good idea to sneak up on the piston clearance.

The first graphic below is a cut-away of the engine split right down the center.

The second is a close-up of the ideal cylinder head geometry for a 70cc.

Jim

Spot on, thats it.

Also what happend to the intake Jim? At TDC its only 35% open?

Yeah I know,

I think a lot of this may have been intentional to keep the power output down.

Some of it is simply sloppy machine work that is typical of a mass produced product where cost is the only driving factor.

The Chinese are quite capable of making quality engines.

Jim

Good stuff guys. Thanks for the input. Just my intuition would tell me a hemispherical head with sparky in the middle is the best bet. Too bad cheezy stock doesn't have at least a reasonable approximation of a squish band,

But - I'm still in the $22 range!

You can machine a stock head in such a way where you will get a 6.9mm squish band (50/50 for a 4mm bore) BUT its a bit of a gamble...if you get any casting void near the squish blending radius the head will leak because there the aluminium is only like 1.5mm thick after machining and its highly likely the void will be larger than that.

For any of my next builds Im going for a stock head thats machined to give a squish band and a long reach plug to get combustion to happen in the dead middle of the combustion chamber. Slant heads are ok ONCE you remachined them and then still the fuel/air is ignited from the side...

Only good part of having an offset spark plug is that you can orient it so that the intake side has the spark plug to distribute heat evenly.

Heres the million dollar question...

You have 2 heads, Same compression, one is hemispherical and reaches all the way to the cylinder wall (so as wide as bore), but the other has a 50/50 squish band (but still hemi shape C.Chamber)...what makes more torque?!?!

I think the hemi will because theres more surface for the combustion to push against where as in the squish head theres less (only 50% of the bore).

You see, squish bands are only to get way with high compression with out pinging - and once you get to that compression, these engines are disposible - so why not back off compression (to a corrected value of say ~6:1 - that equates to 10.5:1 uncorrected). Then you can use a hemi head with out detontation - it should make more torque...shouldnt it? What are everyone elses opinions?

PS - When running a fixed gear setup on a MB you need highest average torque, so raising your exhaust port and giving your self a longer exhaust open duration (plus blow down time) is great if you want all out speed - however, if you ever want to go up a hill again you need to work on widening the intake and exhaust and increasing compression. If any one was wondering...the port timing on these engines is actually pretty good for there use - to make torque.

The port open durations are spot on for mid range power and the fact that no one balances their engine make these port timings ok for top end as on these engines top end is at around 7000rpm. All the 2 stroke tuning books are pretty much for racing where top end is defined as +12,000rpm. So if you want a "perforamce" mod you can actually notice (ie faster up a hill, better acceleration, wheelies) widen your exhaust to 32mm and intake to 30mm and try to get the head machined at a machine shop. Better flowing carbs and exhausts are good too but they are generally the most expensive mods.

This is all way above my head. My thoughts with this slant head is opposite of what most others have said.

I feel as though the head caused me to lose some lowend and gain a fair amount on the high end. I also feel as though it smoothed out the power band. I def. gained around 3-4 mph on the topend. Mine is not flat either, and I do not plan on fixing that. Ive gotten it sealed. The idle is alot more smooth as well.

Thats my opinion on the head.

actually Bob - that can be an effect of the squish band, so what you say is real enough to me.

I got more low end pure from the (fleeting) higher compression.

I'm too stinking busy to slap the head back on!

OK I put the glass flattened head back on. Runs better.

Guys,

Without an entire series of modifications these engines do not, and will not, have a powerband. They were not manufactured to have a powerband. Even if you're big; nail the throttle on an 80cc 2-cycle motocrosser and the word "powerband" will be explained in full detail. Now go to a 125cc or even a 250...and hang on it's going to be a ride!

With the exception of a tuned pipe, there are currently no proper performance upgrade parts. In order for any cylinder head to be of much benefit it needs to be located concentric to the piston & cylinder, (Machined & Dowel pinned).

I know, I know...I have both heads; and yes the head with the butchered squish band does have a smaller chamber which does raise the compression...but it is far from correct in terms of geometry.

Jim

Of course they have a power band. All engines have a power band. Now are these $50 engines optimized for a power band, NO of course not.

By crudely raising the compression ratio, home porting the engine, messing with the carb and installing a tuned pipe, you can increase and shift the power band.

Stock...out of the box...these engines do not have a powerband!

Every one I have installed has had a flat torque/power curve from 3,000-7,000rpm.

Yes the power gradually increases with rpm...this is the definition of horsepower...A power band?, NO!

If there is a china girl that has a noticeable power band out-of-the-box, I would like to know about it! I have engines from just about everyone...no powerband.

Jim

Of course a HT has a powerband - if any ONLY if it is fitted with an expansion chamber.

And Jim, the reason why a motocrosser has a more noticable powerband is due to exhaust powervalves (increasing exhaust port area just in time for power band).

How about you try this - ride a 60cc motocrosser with the powervalve cables cut and only in 3rd gear?! HT "powerband" much?

All engines have powerbands. 2 strokes and 4 strokes. Just because the HT is piece of junk and you can't feel it doesn't mean it doesn't have a powerband. It has a weak ass flat powerband and you certainly can not feel it come "on the (stock) pipe".

The power band refers to the range of operating speeds under which the engine is able to operate efficiently. A typical gasoline automotive engine is capable of operating at a speed of between around 750 and 6000 RPM, but the engine's power band would be more limited. The engine would typically not generate maximum torque until higher operating speeds of perhaps 2500 RPM, after such, the torque drops off. The peak power (horsepower) might be closer to 5000 RPM. Such an engine would be said to have a "power band" of 2500-5000 RPM (another example would be from torque peak to redline: 2500-6000 RPM).

This can be applied to any engine and establishes a reliable quantification of the above notion "the engine is able to operate efficiently".

Click to expand...

Power band - Wikipedia, the free encyclopedia