Calculating compression ratio - which method?

[David Ashcroft]

Polishing of the rod bottom can take off just enough material for Clearence of the internal case bottom for rod stretch due to rod bearing wear.

 

[Terry Keeley]

Terry, I have one of the last VAC's sold here in Aus. The head has an angled squish and a very small flat section on the outer edge. Flat piston.

It's interesting that the detonation on your motor occurs across the entire piston. Mine only seems to occur mostly on the exhaust port side. I have also noticed that a majority of the pitting occurs on the head, not the piston.

Interesting they've gone back to an angled squish band, I've never seen a completely flat squish before in a stock head. My understanding is it increases the MSV (mean squish velocity) allowing for a higher C/R before detonation, but don't quote me on this, it's been a while since I've read about it. I know Andy used to run flat buttons as well Marty has played in this area quite a bit, maybe they'll chime in.

If you look closely the majority of the deto is on the exhaust side, the little gouges are from a plug wire that got loose.

Here's the button and piston from my 90 VAC after about the same number of runs, you can see how the piston was hitting the head at 0.012" clearance, I've since increased to 0.015".

Now here's a guy that runs LEAN!

 

[Ray Sametz]

Few things I was told to do with 45 vac, from a good source.

1 Lap the carb barrel

2 Open button 24 thou with 1/2" ball mill. Don't change the depth. Gave me .29cc head vol.

3 On open side of drum cut .010, not the housing.

4 Polish bottom rod journal with 1000 grit.

5 Check oil slit bottom of rod.

6 Check run out on flywheel.

Thanks Ray

Can you explain 4 & 5?

What does the polishing on the rod journal do?

What do you mean by "check" the old slit?

Hi Tim oil slots on the rods should always be checked on a new engine for burrs which can cause blockages ..Get a sharp razor blade and scrape the slit clean for good oil flow,,and could balance flywheel if keen..

Polishing of the rod bottom can take off just enough material for Clearence of the internal case bottom for rod stretch due to rod bearing wear.

Tim, David covered it.

 

[Marty Davis]

Terry:

I used to run flat squish bands and fought detonation all the time. I always try to run my engines just at the point where they start to 2 cycle and never lean. After we developed the Engine Analysis Software and added the Squish Velocity plugin, I found out why I was detonating so much. The squish velocity was off the chart.

I also found that if I didn't take the angle on the squish band completely off the edge that area would detonate and degrade. I now run 2.5 degrees of squish angle on all my engines and make sure that the angle runs all the way out. MUCH less detonation now and I can run engines much longer before replacing the piston and had button.

 

[dwilfong]

On the .45 eng I do not run a flat band on the squash either.

Marty's eng soft where is the bomb for calculating what is what.

MSV is very important in the calculations. This is what every thing is designed around when I make a button.

2.5 to 3 deg of angle on the squash and 70% area has worked the best.

Also 9 / 9.2 calculated trapped CR.

MSV at max RPM 60MPS.

I adjust the chamber CC's around this with a .012 to .013 clearance.

You have to jockey the numbers around depending on the ex timing you have.

Also Brass head button and special cooling cap that only sits on top of the button with a small channel for the water to flow in.

Don't have to cut back the water at all if done like this.

OD99 have worked the best with 55% nitro.

Marty showed me how to make my first brass button with flat chamber top and sides and use his program.

Many thanks to Marty for his insight as it has served me well.

 

[Terry Keeley]

Thanks for your experience guys. Here's a couple tech articles I found on Marty's site:

http://www.1nitrorc.com/sv.html

http://1nitrorc.com/table_water_restrictor.htm

http://1nitrorc.com/water.htm

http://1nitrorc.com/heads.htm

http://1nitrorc.com/chambers.htm

 

[Tim van Schyndel]

On the .45 eng I do not run a flat band on the squash either.

Marty's eng soft where is the bomb for calculating what is what.

MSV is very important in the calculations. This is what every thing is designed around when I make a button.

2.5 to 3 deg of angle on the squash and 70% area has worked the best.

Also 9 / 9.2 calculated trapped CR.

MSV at max RPM 60MPS.

I adjust the chamber CC's around this with a .012 to .013 clearance.

You have to jockey the numbers around depending on the ex timing you have.

Also Brass head button and special cooling cap that only sits on top of the button with a small channel for the water to flow in.

Don't have to cut back the water at all if done like this.

OD99 have worked the best with 55% nitro.

Marty showed me how to make my first brass button with flat chamber top and sides and use his program.

Many thanks to Marty for his insight as it has served me well.

Thanks David

Would you be able to provide a picture of the cooling cap design you use?

 

[dwilfong]

Tim

Here is what I do for a cooling cap.

I got the idea from a pic posted by one of the racers down under.

It is just a aluminum disk that sits on top of the head button.

Nothing fancy about it.

Not the do all end all but I like it.............................

Keeps the chamber nice and hot.

 

[Tim van Schyndel]

Tim

Here is what I do for a cooling cap.

I got the idea from a pic posted by one of the racers down under.

It is just a aluminum disk that sits on top of the head button.

Nothing fancy about it.

Not the do all end all but I like it.............................

Keeps the chamber nice and hot.

Beautiful stuff David.

Thank you!

 

[NeilLickfold]

Interesting stuff. I calculate the height taking into consideration the float of the rod. It is my view that the piston on the higher revving engines is retained by the rod.

My engines all have alloy rods, so I think that the tdc of the piston in the engine cold and hot are about the same. In a steel rod engine, the tdc will be slightly lower at working temp, so maybe the solder test for measuring the tdc, compensates for this. Certainly on the smaller 2.5 ss engines, the solder test will make it over compressed for the same numbers compared to how I currently do the tdc position.

Seeing the higher nitro results on the pistons and head buttons, what are you using to see when it runs best ?

If it is run slightly richer than the settings that showed the pitting on the head and piston, do you see the engine /boat run slower ?

Thanks,

Neil