Increasing compression Ace 100

[Modracer70]

What's the easy/least expensive way to increase compression on an Ace 100 engine? On my almost daily rider I put a high compression head on and have about 125psi. It's was a low dome/super rat piston with super rat head, that combo had about 100psi, both with stock head gasket. Concerned about piston to head clearance with milling head/thinner gaskets, how much clearance is needed? Can you make a high dome piston, super rat head combo work?
Thanks, Dave

[Zyx]

Seems to me there is a tech article on the website warning against mixing piston and head styles. Have you thought about stuffing the crank?

[Zyx]

Here's what it says in the tech article "Know the Difference in Your Pistons" --

"Bad things can happen if you install a type 1 (high dome) piston in an engine designed for the type 2 (low dome) piston. Compression will be excessively high and engine life will suffer. Excessive compression leads to excessive engine heat and detonation. Both are engine killers. Do NOT install a high dome piston in a “low dome” engine."

[thrownchain]

Seems to me both of your starting psi readings are a bit low. both readings are about 35% less than they should be.

[Zyx]

Past discussions seem to place normal pressures with matching head and piston at 135 - 145 not to exceed 190, and that at least one person who had a rat head on an Ace 100 cylinder/piston got 195, which is too high. See the discussion in the Old Forum Archive near the top of the first page of entries.

Note, when checking compression, open throttle fully for all checks. Test cold with open throttle, and if the numbers seem low, spray a bit of oil in the spark hole and test again. If the numbers climb significantly, either your rings were not well seated first time around, or are worn to the point of leaking pressure. Oil in the bore tends to seal ring leaks, which is why the pressure will climb. You don't have to use a lot of oil. Paul has suggested WD40, which is okay, but is not a lubricant per se and is light bodied. When I was service manager for a Honda shop way back when, I always used a squirt of motor oil. Excess is pumped out after the first stroke or two anyway, but just a squirt will do. If the numbers don't really change between "dry" and "oiled" the rings are fine, the pressure is what it is, and if it is way too low, you have other issues, such as a leaking head gasket. At kicking speeds, bottom end leaks don't really change the results because pressure is made from the top of the exhaust port to the top of the stroke, and air is moving too slowly for any ram effect from the transfers and mass momentum. Pressures rise when running, which is why having excessive high pressure while doing a static test suggests dangerous pressures while running.

If adding oil does not change your pressure test, pull the piston and rings, check skirt clearance and ring gap. If out of spec, do a top end overhaul.

Here's a tip from back in the high money bike shop days: the first thing we ALWAYS did on a running customer bike in for ANY reason was to check compression. If it failed that test, we recommended a top end overhaul before doing anything else. While I didn't like doing that because I am cheap by nature, that is what Honda recommended and it is how the shop made big bucks. If the customer refused to do the overhaul, and insisted on just doing a tune up, the shop would not guarantee the results, which was disingenuous, but it was the way it was done. There is truth, however, in the idea that if the compression is lower than spec, the darned thing won't run right no matter what else you do.

[Modracer70]

Maybe I need to try another gauge, bike runs great, starts first kick 95%'of the time, just always looking for a little bit more, can't stop messing with things.

Thanks, Dave

[Zyx]

I have to admit that a bike has to be pretty far out of spec before you start getting starting and idle issues. My old race engine is 10/1000 out and still runs okay, starts reliably, and idles without a lot of fuss. If I didn't know how it ran when fresh, I would not notice much of a problem now, all such things being a matter of degree, not really a pass/fail sort of thing. What I notice more than anything is piston rattle.

Question would be 1) did you check with the throttle open (closed throttle drops pressures more than you think and could drop a 135 to a 115 or less for that reason only), 2) did you try a bit of oil to see if it changed the result (if not, give it another try -- spray some lube in, kick it through four or five times to get the oil moved around, and check again). Differences in result even using the same equipment can vary from tech to tech due to differences in technique. Once you are quite comfortable that you are getting reasonably correct results in your pressure test, and if then the numbers are way below what they should be, I would consider a top end overhaul, after I verified the skirt clearance and ring gap on what I have to either identify that there is an excess clearance issue or that there is not. If the piston is nice and tight, and the ring (or rings) are gapped right, not stuck or broken, not chipped or whatever, then I would start to look elsewhere. Check the cylinder bore to see if it is scored. Check the head gasket for signs of blow-by. Check the head to verify that it is square at the gasket surface and not warped. Check cylinder stud torque to make sure that all studs are properly torqued. Make sure that the spark plug hole is not cracked (this I have seen from time to time). Make sure you have the right head gasket and not a thicker one from somewhere, i.e., could have been from a different model with a different gasket thickness -- one of the more experienced Hodaka techs could say whether Hodaka used different thicknesses on different but similar models. For that matter, make sure your base gasket is not overly thick. If you or someone else has made a gasket from sheet material, it could be that the material used is way too thick.

Sooner or later you are going to find out what is causing low readings, and yes, I suppose the gauge could be the culprit. Try someone else's if you have access, of if yours uses hoses and adapters, check for air leaks, blockage in the hose, or debris in the gauge port. Maybe it is something really simple, and there is no problem at all.

[Zyx]

Cruising through my 1972 Clymer's shop manual, covering Hodaka 90's and 100's from 1964 to 1972, the Ace 100 and 100B compression is shown as 80 to 150 psi. That's a pretty broad range. Curiously, the 100 MX Super Rat of the day is shown as only 85 psi assuming the piston and ring are broken in for at least two hours. Thinking about your question, the higher compression you have, the more critical timing becomes, and the greater the need for high octane fuel up to a point. At the upper range of pressure I would expect you would want to run either what passes today as premium gas, or race gas, just to keep the flame spread rate under control, and changes in altitude and barometric pressure start to make more demand to get timing and jetting right, even if it takes changes from day to day at the same place because of changes in pressure altitude. For the casual sportsman, such things are not a lot of fun, so if you are not racing to win, I would not try to push the envelope on pressure. You are only talking about a couple of horsepower anyway, depending on what else you may have done to the bike. Mild porting can achieve more, I think, than pushing up the internal pressure, and gives you more flexibility in targeting changes aimed at either torque or HP. There are several reputable porting guru's out there who can improve your performance without making the bike picky on timing and fuel.

There are several tech articles on the SH website, I think written By Harry Taylor, regarding pressures, race gas, jetting and so on. Read them all, and you start to get a sense of what works, and what just makes things hard to nail down.

[junker2k]

If it is a stock ace 100, they came with about 8 to 1 compression. The head could be shaved a few 1000's to raise
the compression, or put a super rat head on it, that is a bout 12.5 to 1 compression. But raising the compression
also adds more heat to the motor. At 125lbs this should be a good starting and running bike. When the compression
goes above 160lbs then it is time for race gas and a motor that will not last as long. Below 90lbs the become hard
to start and don't run as well.
Jack K