Basics of Boosting Engine Power

[technomancer]

sounds good. and yeah, i know all about persevering over a chunk of metal that needs to be sanded or filed.
thanks for the info.

i need to get another comp gauge now though.

 

[Fletch]

On my newest build (HT 66cc) I sanded down the raised circular lip on the head so that the mating surface was completely flat. I compared a new head with a stock head gasket, and that raised lip seems to be roughly the same size. So the way I figure it is I remove a head gasket worth of material from the head, which equals not using a head gasket and not removing any material (make sense?). The only difference is that more heat is being transferred to the head because of the flush mating surface.

I don't have a tool to measure compression (would like to get one), but I was overheating going up a 9 grade hill for about 1/4 mile (with shift kit). There are multiple factors at play such as plug heat range and oil ratio, but I ended up adding an extra head gasket to in effect bring the slant head back to stock size.

I've had two bikes overheat and seize up on that hill when I removed the head gasket or material. In retrospect I was using the either the stock Chinese plug which is pretty hot (equal to a NGK B4HS), or the extended tip NGK BP6HS which is 2 heat ranges cooler but increases compression further because of the protruding tip.

I guess what I'm trying to say is that in my experience so far (minus trying a cooler plug), removing a head gasket from a 66cc HT slant head will cause it to overheat with extreme load (huge hills) or long full throttle runs. I don't know much about motorcycles or high compression heads, but am I correct that they are meant for short distances of WOT, and NOT for climbing up very steep hills?

I just ordered some NGK B7HS plugs which are the coolest I will have tried. Hopefully this should help some. Also because I have a short billet intake, the carb was heating up and the gas bubbling (boiling)- hard to believe I know, but it's happened to someone else (search). A regular intake manifold would keep the carb cooler, but the ported short intake has little area for heat displacement.

Also, what octane gas are you supposed to use in an increased compression engine, and at what level (for example #91 if your compression is over X)? I like the idea of increased compression but I need this engine to last.

One last thing... I have read that these engines aren't built to tolerate increased compression (bearings, imbalanced crank, etc.). Even if you get it running well heat-wise... won't it cut the life of the engine?

 

[Fletch]

The only reason that I mill my head is not to increase the compression ratio, but to make a good head to gasket surface. In doing so this increases the compression ratio , so much the better. I NEVER put a engine together on a bike out of the box. I haven't found ONE head that didn't need to be milled.

Al.Fisherman- How much are you taking off? Are you leaving some of the raised portion, or milling the head completely flat? Are you using just one head gasket still?

I was thinking that maybe I removed some of the already barely existent squish by making it flat like I did.

 

[technomancer]

Balls. i just realized what compression testing on my bike motor entails for me.
basically a few good pedals in gear should do it, right?

stupid city streets...

 

[Deleted member 12676]

Overheating going up hills is just due to the stock CDI which advances the spark timing too much. I really believe it was designed for a four stroke engine but was put on the Grubee to lessen its power so it can be allowed into all countries. If you don't correct that problem then just jet the carb richer.
I've been running 135psi on my engine quite a while now with only the top connecting rod bearing (wrist pin bearing) changed to a slightly better one. I don't think 135psi is really considered "high" compression, just higher. There's guys running much higher than that.

 

[Fletch]

Overheating going up hills is just due to the stock CDI which advances the spark timing too much. I really believe it was designed for a four stroke engine but was put on the Grubee to lessen its power so it can be allowed into all countries. If you don't correct that problem then just jet the carb richer.
I've been running 135psi on my engine quite a while now with only the top connecting rod bearing (wrist pin bearing) changed to a slightly better one. I don't think 135psi is really considered "high" compression, just higher. There's guys running much higher than that.

Thanks... I measured compression on my bike at 25psi yesterday. I'd been using the NGK extended tip which raises compression some unknown amount too. That's with 2 head gaskets because I sanded the head down. If I remove one of the head gaskets I don't think I'll be able to start it because it's a shift kit and you have to pedal with the clutch engaged.

How do you correct the cdi... replace it? With my other bike I couldn't jet it richer without it messing up performance. I don't know if anyone else is climbing 2000 ft on their bike regularly, but it takes its toll. I'm running a cooler plug now.

I was under the impression that the first bearing to go was the crank bearing? Is it the connecting rod? Where did you get a better connecting rod bearing? The bearing that came with this latest engine I'm installing looks different than the ones I've seen before. The bearings are the same length, but they look slightly thicker, and the the total size of the "barrel" is longer. I wish I had a pic. I don't know if this is a better bearing or not?

 

[Deleted member 12676]

I hope you meant to write 125psi instead of 25psi.
Yes, replace it with a modified motorcycle CDI which is not as advanced in timing and retards the timing after 3600rpm like normal 2 stroke CDI's do. I'll message you more info.
No, the weakest bearing is the small wrist pin bearing, then the bigger rod bearing, then the crank bearings. Yeah it's better to have the wider upper rod bearing since the load is spread out more on the rollers. Any of the replacement bearings are better than the stock one.

 

[Fletch]

I hope you meant to write 125psi instead of 25psi.
Yes, replace it with a modified motorcycle CDI which is not as advanced in timing and retards the timing after 3600rpm like normal 2 stroke CDI's do. I'll message you more info.
No, the weakest bearing is the small wrist pin bearing, then the bigger rod bearing, then the crank bearings. Yeah it's better to have the wider upper rod bearing since the load is spread out more on the rollers. Any of the replacement bearings are better than the stock one.

Thanks jaguar, I've actually bookmarked your page in the past but forgot about it. I have a 66cc though. Unfortunately $ is tight now so some of this stuff will have to wait. I just posted a thread asking for opinions on what to do while I have the bottom end opened to clean out some shavings. I'm running an 18mm Mikuni carb on a ported engine (don't know exact measurement off the top off head). I lowered the intake 2-3mm, and raised the exhaust to somewhere around 1.07 from top of cylinder, as well as widened it to like 26mm. I have a SBP expansion chamber that has some dents in it from falling off and being run over! I'm eager to see how it runs (new build). Looking at your site just now it looks like if I experience too much vibration, the easiest way is to add/lower weight to the wrist pin? (meant to reply to PM...too many tabs open!)

 

[back2theborstal]

25psi seems very low to me. i havent done a true reading yet on my "65.9"cc sd stinger. but with just a couple slow peddles mine was at about 80 psi. i had my bike in the basement and i didnt have any room to really peddle the bike. im sure when outside and if i get going, it would be at about 100-115 psi. and thats just a stock engine with about 20 miles on it.

 

[Deleted member 12676]

Yes, the piston/wrist-pin is heavier than those of a 48cc engine and the crank is balanced for the 48cc (somewhat). You can just drill a bigger hole in the wrist pin to see if that takes care of most of the vibration.