Recommendations, boring, ballancing

[GregT]

If you need to drop compression one way would be to machine a recess in the piston crown. You'd leave about a 6mm wide ledge around the edge to make the squish work but gain chamber volume with the recess.
It's not uncommon to use a recess like this and looking at the depth of your valve pockets it should be quite safe.
The alternative of adding volume to the head space is actually quite difficult with the 4v heads - they're a very good shape to start with.
Remember what I said early on in this build...these heads have a rep for cracking...so don't overdo the compression.

Following you through this is good mental exercise for me as i'm doing race motors most of the time - but currently mainly 2V and 2 strokes....

[Mekanix]

Well, over 2mm is way too large piston to head clearance, IMO. It will ruin the squish band and hurt performance. I would aim to 1.0-1.5mm range. On the other hand over 12:1 is quite high compression ratio for an air cooled street engine. It may work or it may cause detonation. I don't have much experience on high compression N/A engines so I can't say for sure. So ideal solution would be to adjust piston to head clearance to 1.0-1.5mm and then machine piston tops or combustion chambers to get suitable compression ratio, maybe something like 11.5:1.

Got it I was going for 2mm but 1.5 is possible.

I have pushed these GS's to their limit on the street and a couple of my bikes are at 10.7:1 comp ratio which I think is right at the ragged edge for these air-cooled engines.
They don't knock or ping unless stuck behind a Hardly-Ableson parade when it is over 90 degrees out.
All future creations will be limited to 10.5:1.

So you have no problem finding 100 octane gas there?
How much do you have to pay for it?

Eric

Hardly-Ableson parade

Good to know
I work at an airport 100LL av gas It would be possible at 2.50 a liter but I'd rather not go that way.Max at the pump here is 92.

they ran over 12:1 in racing with out problems,it will just need tuned properly, new bikes are 14:1,timing and jetting ,need to be altered ,check into old school racing there should be a lot of useful info to work with
but high compression means more strain on cranks,and rods ect... so good luck with crappy designed metals,a thick head gasket,or some head chamber relief work would bump it down to a smoother 11:75:1 maybe i dont think 12:1 will be bad if the crank is good

Never herd of "old school racing" But I'm searching for it now.
The bottom end is good and well within plastigauge tolerances.
I could retard to timing to make it a little easier on it.

If you need to drop compression one way would be to machine a recess in the piston crown. You'd leave about a 6mm wide ledge around the edge to make the squish work but gain chamber volume with the recess.
It's not uncommon to use a recess like this and looking at the depth of your valve pockets it should be quite safe.
The alternative of adding volume to the head space is actually quite difficult with the 4v heads - they're a very good shape to start with.
Remember what I said early on in this build...these heads have a rep for cracking...so don't overdo the compression.

Following you through this is good mental exercise for me as i'm doing race motors most of the time - but currently mainly 2V and 2 strokes....

Good to know its racking more than one brain

According to the engine calculator

78mm bore
56.6mm stroke
17cc ( head + valve recesses)
1mm squish

makes it 13.4:1

I need another 6cc's to get down to 10.5:1

1.5mm squish
makes it 12.2:1

I need another 4cc's to get to 10.5:1

So cutting a recess in the piston you say? hmm,, That's possible and would be better over all.

I measured the amount of piston top I have to play with and it is 4.32mm in the center at the thinnest spot and gets thicker as it spreads out.

I could dish it out 67mm.

1.7mm dish depth would make 6cc's and that's what I need with a 1.5mm squish.

Again with the engine calculator

1mm squish calculations
78mm
56.6mm
15cc head only
4cc piston dish 1.1mm dish depth.
comp ratio is 12.4:1

Need more dish

6cc piston dish 1.7mm dish depth.
comp ratio is 11.5:1

1.5mm squish calcs

78mm
56.6mm
15cc head
4cc piston dish 1.1mm dish depth.
comp ratio is 11.1:1

more dish. Need 5-6cc's

6cc dish 1.7mm depth
comp ratio 10.5:1

5cc dish 1.4mm depth
comp ratio 10.7:1

[7981GS]

You can mix one gallon of Avgas to five gallons of 91 octane to get 95 octane real gas.
Of course that would be illegal as there is no "Road use tax" on it.
We have a VP Racing gas joint here that sells 100 octane unleaded for road use that we
had to put into the tank after we left the H-D parade. Of course it is over $8.50 per gallon.
I think that we saved the engine that day.

Eric

[gearhead13]

I am running 11-1 rated pistons, cold dry compression of 180+ psi. Best gas at the pump here is 94 octane, the timing is set to stock advance. Havent had any pinging problems. Pistons had zero deck height head gasket .043" Running it on the rich side.
The metric hurts my brain.

[7981GS]

I am running 11-1 rated pistons, cold dry compression of 180+ psi. Best gas at the pump here is 94 octane, the timing is set to stock advance. Havent had any pinging problems. Pistons had zero deck height head gasket .043" Running it on the rich side.
The metric hurts my brain.

We can only get 91 octane P*ss-water at our pumps and 10.7:1 just barely makes do with that.
It won't be long before they make us run that 90 octane crap that is the highest octane available in California.
Then, I will HAVE to mix gas for two of my bikes.
I also have an 11.5:1 car that must have the 100/91 mix or it will do nothing but knock!

Eric

[Mekanix]

Called cometic today for gasket prices

The base gasket will have to be 1.60mm (0.063)

putting the piston in the hole by 0.25mm (0.010)

That was the closest option to the 1.45mm (0.057) I could get in coper.

The head gasket now needs to be 0.25mm (0.010) smaller to get the 1.5mm squish.

That makes it a 1.27mm (0.050)

I called JE pistons to see if its ok to dish out the center of the piston the 1.7mm.(0.067)

Didn't get a call back.

[TeamDar]

Should have bought turbo pistons. They come already dished.

[Mekanix]

Didn't have much of a choice for a 78mm piston. There were two on the shelf and I didn't have to buy a whole kit so I saved some money there. Everything else was much higher compression than these.

[GregT]

If you do need to gain more chamber volume, the side walls could be laid back to match the bore size or the diameter of the piston dish.
Don't touch the front or rear squish areas.

[7981GS]

You can also enlarge/make deeper the valve reliefs to drop a cc or two.
Especially if you are installing over-sized valves anyway.
I have had them cut .040 deeper with no problems. You can probably go deeper safely.

Eric

[Mekanix]

This how much area the piston will take up. There is room on the sides to loose a few cc's

Its 15cc's stock
I need to gain at least 6cc's

There is already 2cc's in the valve reliefs.
I need another 4cc's at least,

Do you think I could get that from the sides ?

With the deck height back to 1.25mm(0.050) I need to get at least 21cc's to get below 11:1

You can also enlarge/make deeper the valve reliefs to drop a cc or two.
Especially if you are installing over-sized valves anyway.
I have had them cut .040 deeper with no problems. You can probably go deeper safely.

Eric

Never thought of that How much meat is in the valve seats ?
Since these heads have a habit of cracking what effect would recessing the valves in the seats have?

[TeamDar]

You could connect the valve reliefs in the pistons. Make one long pocket for the intake and one long pocket for the exhaust.

[7981GS]

In your case, you could un-shroud those valve pockets quite a bit also and increase flow.

Eric

[Mekanix]

In your case, you could un-shroud those valve pockets quite a bit also and increase flow.

Eric

Do you think there is that much room ?

You could connect the valve reliefs in the pistons. Make one long pocket for the intake and one long pocket for the exhaust.

I was thinking of just making a 67mm cylinder in the middle of the piston 1mm deep.
But now looking at the piston and valve reliefs and squish bands I would like to keep the triangles between the valve reliefs and just take out the material in the middle.

[7981GS]

Yes there is room. Look at where your valves are at the flat of your combustion chambers.

These, cannot be opened up any more as there is simply no room with these custom-made over-sized valves and pistons...








Those are for a 13.5:1 compression engine.

No, it will not run on 87 octane pump gas. LOL!!!


Eric