310 dome / compression issues

[hontrx265r]

This bike runs ok but I'm always tinkering and just can't leave stuff alone. When I first assembled the engine (kit was already on the bike). I glanced over everything but didn't really do a whole lot of measuring just wanted to see it run. esr310 cylinder bore/piston were brand new. Put together as normal. Well today I decided to check the squish clearance and came up with huge numbers so I threw a compression gauge on it and it was pretty low. The squish clearance was 1.6mm I of course popped the head to see if it looked like the cylinder needed to be decked but it looked good piston coming up just flush with the cylinder deck at tdc. I started checking some numbers first pulling the dome it is a pump dome which is fine but still huge clearance. What I realized after measuring is the dome edge is far larger then the bore size 72mm so when measuring the squish its already up into the taper of the squish band. Its like its ready for its largest bore right off the start. So my question to a builder or someone who knows is should the dome technically be sized for the bore every time for optimum performance. What is being lost by it being this way because it seems ridiculous. My plan now is to have a dome custom cut but again if I have one cut and the bore is stock how is the dome setup, for std bore or larger?

[rsss396]

There are lots of ways to setup motors, some builders like very tight squish clearance and narrow squish bands widths
others will run the squish clearance wider with wider squish bands. The extra width of the dome is like you said cut for future increased bore sizes, if not done probably 80% or more of ESR customers would never know to cut the dome bigger and would just run it and have problems, the same with the dome profiles they sell most are a compromise between performance and reliability, if they did not then to many people would have issues from detonation caused from gas not high enough in octane, it's the same thing all the OEM does to there motors.

with the head you have now you will probably be safe adding a good amount of ign timing to the motor to compensate for a slower squish velocity caused by the large squish you have in your dome.

[hontrx265r]

I can understand that. Its not really that big of a deal, its just a detail I never noticed at a glance. I didn't think about the ignition timing though.

[udontknowme]

its hard to say if it runs worse with the head diameter bigger than the bore. but to me it makes sense to have both diameters the same. like rsss said its probly cut like that on purpose because if they didnt then the average dude would bore the cylinder and the head diameter would be smaller and i could see the piston hitting the small lip on the band.

[rsss396]

The squish velocity of a head which is influenced by rpm,squish clearance, squish band width, squish band angle in relation to piston crown and compression work closely with the ignition timing at low rpms when squish velocity is low and allot of residual gases are still left in the combustion chamber ignition timing needs to be advanced as the motor increases in rpm and squish velocity increases, port velocity increase and the pipe effect increases ignition timing must be retarded.
A good design for a woods bike will have a wide squish band compared with a high rpm drag style head with narrower squish band.

[Jerry Hall]

This bike runs ok but I'm always tinkering and just can't leave stuff alone. When I first assembled the engine (kit was already on the bike). I glanced over everything but didn't really do a whole lot of measuring just wanted to see it run. esr310 cylinder bore/piston were brand new. Put together as normal. Well today I decided to check the squish clearance and came up with huge numbers so I threw a compression gauge on it and it was pretty low. The squish clearance was 1.6mm I of course popped the head to see if it looked like the cylinder needed to be decked but it looked good piston coming up just flush with the cylinder deck at tdc. I started checking some numbers first pulling the dome it is a pump dome which is fine but still huge clearance. What I realized after measuring is the dome edge is far larger then the bore size 72mm so when measuring the squish its already up into the taper of the squish band. Its like its ready for its largest bore right off the start. So my question to a builder or someone who knows is should the dome technically be sized for the bore every time for optimum performance. What is being lost by it being this way because it seems ridiculous. My plan now is to have a dome custom cut but again if I have one cut and the bore is stock how is the dome setup, for std bore or larger?

Mail order products have to be made so that the a guy without any experience can bolt it together and it will run.   My experience has been that guys will mix and match all kinds of parts trying to save money, not follow the engine builders recipe and then blame the engine builder for the engine not running properly or very long.  

It is best to have an reputable engine builder do the original build and then someone with experience can maintain the engine if they pay close attention to details.  

It is very difficult to predict some of the stupid things that happen when you send an engine kit and a set of instructions on how to assemble and tune it.  It seems everyone has a friend that use to race and knows everything and will not follow the engine builders step by step instructions.  



The head should match the bore.  If the head diameter was made to match the bore on the Hondas, there are not any provisions other than using the head studs from Honda or any of the aftermarket cylinders to hold proper alignment between the two during assembly.  Some engine manufactures provide dowels, or a flange on top of the cylinder to provide precise alignment of the bore and the head.

The head diameter being larger than the bore does not hurt performance measurably, but it can affect the heads ability to help control detonation. An engine that has detonation issues caused by sloppy or production type tolerances such as these, may not be able to be tuned for optimum performance without detonation raising it's ugly head.

[hontrx265r]

Understandable jerry I appreciate you posting, I do know esr is a somewhat cookie cutter setup. It was on the bike when I got it so i have been attempting to work with it.  I know it will never be a top level performer but I would like to get it to an acceptable level of enjoyment, without pumping tons of money into it. I have managed to get it tuned as is to run farily well, but there are a few things I think that can be overcome without going overboard and starting all over at the base. Obviously virtually every R out there is recycled in some way, so it is pretty common many want to work with what they have.

[hontrx265r]

Now that I'm at work and have some more accurate tools to measure with I'm actually about .069" squish at 72mm bore. The head is cut to the edge of the squish band at 74mm. The actual squish at 74mm is .048". That is a difference of .021" which seems excessive.

[rsss396]

how wide is the squish band

[hontrx265r]

To approx the center of the radius where it transitions into the center .425"