Re: Funny shaped & crazy rough head

That 8.6:1 looks like a totally reasonable number, until you factor in the boost the pipe will be making.

Re: Funny shaped & crazy rough head

Lia the Raven /

Exactly. If the pipe achieves 76% or better trapping efficiency, it'll knock with premium gas. I'm totally making up theories here, but I figure it needs to be expanded by at least 1.5cc to be streetable, but by no more than 2.5cc.

Re: Funny shaped & crazy rough head

no way you're going to want to be like 8-9 CC at a minimum, thats hoping that you have an auto retarding ignition.

is this going on the A55, do you have a trigger ignition, jog box?

my recommendation would be to shoot for 9 cc dome with a 35% squish at 1.8 mm clearance factoring in the head gasket.

Re: Funny shaped & crazy rough head

Lia the Raven /

The head is already 8cc, I subtracted 3cc to account for the piston crown in my compression calculations. So I was thinking 9.5-10.5cc, inclusive of bowl & squish, exclusive of piston crown. Do you mean that just the bowl should be 9cc? I imagine it might have problems idling at that point, but you're the guru.

I still need to measure the old piston, or better yet actually bore the case and assemble it before I can give concrete measurements.

Yes, it's going on the A55 with stock retarding ignition & external trigger. I've been looking into other ignition options, but I don't want to give up wattage. I've even considered putting in a 6-pole scooter stator to get more wattage, if I could make that work...

Speaking of ignition, I was working on this guy's Honda Cub the other day and saw that it had retarding points... I thought that was awesome! y'know, if you wanted a performance bike that can survive an EMP. Never know when you might need that kind of thing.

Re: Funny shaped & crazy rough head

Lia the Raven /

Re: Funny shaped & crazy rough head

the C70 are actually advancing points, because on a 4 stroke you want them to advance, not retard.

you can use the JOG CDI box, leaving the A55 ignition the same. You'll definitely want to use some kind of auto-retard ignition, plus the stock A55 box cuts out at 9000-ish rpm.

Yes, your math is correct, i read that quickly and mis-interpreted what you were saying. At least 9-10 total CC volume in the head.

you have to remember the pipe supercharging is the issue here. you're dealing with a supercharged engine so you want the compression to be closer to recommendations for supercharged not naturally aspirated. 10:1 nominal is a good starting point for 'max' but you're probably going to want to be closer to 9, 9.5:1

You figure displacement is going to be pretty much the same for kits @ around 70cc, other factors like pipe and porting have way more impact than +/- 5cc of displacement, so i go more off of 'rule of thumb' and also what kind of chamber you can get within the exisiting casting.

if the casting is too shallow to get the desired volume, i'll massage the bowl and squish area to get more volume. If it is deep enough i'll go for a tighter squish and take the compression lower with a bigger dome because ultimately that will be a better performing head.

all the books and stuff are a good starting place but there are so many variables present that the guys designing race bikes or designing from scratch don't have to deal with, that it ends up being mostly guess-and-check more than science.

Re: Funny shaped & crazy rough head

Lia the Raven /

Cool. I know I get overly mathematical on account of my lack of experience, sorry about that.

I totally get the need to allow room for the pipe to supercharge. I was a little concerned about the trapped going below 6:1. After some interneting I've found apparently some outboards idle on as little as 4.4:1, although I imagine those boats are running on like 85 octane. We only have 89 here in ethanol-free.

I was able to mic the old piston today, so I redid my calculations and came up with completely different numbers this time that suggest the head should be countersunk. However, I am severely sleep deprived so I'll run them a third time tomorrow when I'm more awake.

Wish I had the time to just bore and assemble the thing but it's both crunch time at university AND passover is tomorrow, so..... hmm, maybe i'll celebrate by wrenching. Now that I'm stuck riding a scooter and writing reports all day, wrenching on mopeds is like recreation instead of work!

Re: Funny shaped & crazy rough head

It would probably be quicker to just carve the head out some, put it on the bike and compression test it. Keep doing that until you get the number you want. 125psi is what most geared 2 strokes come out to. At that pressure (approx 8.5:1) you can get the bike started reliably, less pressure it gets more hard to start. 140 psi is a good number. I myself shoot for 185 psi and can run with large hard hitting pipes but 92 octane is available here. Alcohol in gas can be a good thing. It suppresses detonation, is an oxygenator, and cools better by means of latent heat of evaporation

Re: Funny shaped & crazy rough head

except that she is sending the head halfway across the world to me for the 'and check' part of 'guess and check'

my blueprinting procedure is pretty simple

first i start with a stock bottom end if possible, if not possible i will either put a junker stock crank in the cases for a dry fit or worst case i'll assemble it very carefully. with tomos i like to pull the crank into the bearings, but it varies for each engine.

i put the piston, wrist pin bearing, and pin on the crank, and put on the cylinder with no base gasket. Then i measure how far the ports are from the piston crown, usually starting with the bottom of the exhaust port at BDC. Sometimes if a kit is really messed up it takes a bit of porting/massaging past that.

then i put on a base gasket that will space it out correctly, and check how far the piston is sitting below deck at TDC. If the piston is above deck you gotta start over and add more base spacing and port down the exhaust.... suck!

That deck height measurement is where i start for my head calculations. I draw the profile of the head and piston and use solidworks to model the volume. I adjust the squish angle, spacing, and bowl shape to get the numbers where i like them.

Then i machine the head, put it on and check with plastigauge, sometimes i will have to adjust from there but usually they come out pretty much dead on.

I seriously do not get anal about this stuff. I make sure the squish taper follows the piston well and that the plastigague doesn't have any wierd tight spots but other than that you're pretty boxed in just by the casting, piston shape, and trying to get a good compression volume so it doesn't really matter what the numbers say.

and yeah, don't worry at all about trapped CR, i've never once in my life even bothered to calculate that. CR in general is just a ballpark figure, i machine heads for 8-11 cc's of volume depending on how aggressive of a setup its going on- motobecane AV10 47mm with a zooker pipe and TM24- gonna be as much volume as i can get even if its running 200 degrees of exhaust timing.

Re: Funny shaped & crazy rough head

I find most heads I machine to basically comewading deep in the detonatin zone. Like they were cast as cruel jokes. The amount of fucks not given about these is impressive.

Re: Funny shaped & crazy rough head

Lia the Raven /

Graham Motzing Wrote:


> no way you're going to want to be like 8-9 CC at a

> minimum, thats hoping that you have an auto

> retarding ignition.


> is this going on the A55, do you have a trigger

> ignition, jog box?


> my recommendation would be to shoot for 9 cc dome

> with a 35% squish at 1.8 mm clearance factoring in

> the head gasket.

Waitaminute, 9cc sounds good but 1.8mm? That seems awful wide. Most places I see recommend 0.7-0.8mm, which is what I was running on the last kit. Yes I seized but that was after 10,000 miles and an air leak resulting from me stupidly using non-heat-resistant epoxy to extend the sealing surface AND spinning over 8000 RPM downhill without pulling the choke.

I've heard of even tighter but only for high-precision small-bore engines. My understanding is that if the squish is too wide, it fails to absorb enough head from the mixture to prevent detonation. What's your reasoning?

Re: Funny shaped & crazy rough head

eh yeah 1.8 is pretty wide, i guess 1.5 is probably more accurate. depends on the setup and geometry. You also have to remember that its only measured at the edge before the sqush band begins to diverge, so its really not that important of a number.

you figure by the time the piston expands, rod expands and all the bearing tolerances pull out, they add .2-.3 (when you measure with solder you are compressing all those clearances in the other direction) then you add however much inaccuracy you have from gasket compression, etc. also factor that the piston can rock in the bore and give you .2 mm side-to-side difference.

even with a perfectly setup, 100% dialed build where i've had control over all the variables, i won't go any closer than 1.0, i think given your goals for reliability and longevity over top performance you should stick to that 1.5 mm mark.

the difference between 0.8 mm and 1.5 mm is an incremental decrease in heat transfer, perhaps an incremental increase in power or ability to tune for higher temps... the risk on the other hand is very high for increased chance of contact or something shifting, even a slight tap on the edge of the piston can crimp the ring land shut and pinch a ring which is almost certain failure. IMO the risk isn't worth the incremental benefit.

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account