K&B 3.5 Compression Ratio

[chunk t]

All you Engine guys... Compression questions.

I measured my 3.5 K&B and come up with a compression ratio of 12.4:1.

First... did I figure the ratio correctly.

Second, what compression ratios can I get away with on a Piped 3.5 K&B o/b?

I know that changing compression, different plug, different percentage nitro will affect detination. How do I factor these in, to find optimal compression so detination doesn't occur too early?

Bore - .65

Stroke - .635

= swept volume of .2107

Head clearance - .02

Head shim - n/a

= deck volume of .0066

dome diameter - .425

dome depth - .125

= head volume of .0118

BDC volume/TDC volume = 12.4:1

thanks,

chunk t

 

[LH7018]

Seems your calculations are correct, but you calculated the geometric compression ratio. What needs to be looked at is the trapped compression ratio. This is the CR after the exhaust port closes, so you will need to know your exhaust timing to do this. This is why altering the exhaust timing also changes your trapped CR (raising exhaust timing lowers TCR requiring removal of head shims or, barring that, head button machining to maintain your desired TCR).

 

[chunk t]

Uhhhh... yeah I knew that. :huh:

Doh!

Thanks, I completely missed that one. I was so intent on getting the formula right I began with the wrong numbers for Swept Volume.

thanks, and good catch!

Back to question #2, how high can I run compression before detination begins too early?

chunk t

 

[Jerry Wyss]

Uhhhh... yeah I knew that. :huh:

Doh!

Thanks, I completely missed that one. I was so intent on getting the formula right I began with the wrong numbers for Swept Volume.

thanks, and good catch!

Back to question #2, how high can I run compression before detination begins too early?

chunk t

Well,gee,,I think you can get most engines to detonate by running them too lean,,I know I've ran at worst

over 12:1 CR and kept detonation at-bay by keeping things rich enough :huh:

 

[chunk t]

LH,

Got it right this time. My TCR is (8.4 : 1).

I'm looking to raising the the compression ratio up to 10.0-10.5:1. I have two ways to achieve this. I can either...

1. reduce the deck volume by taking the head clearance down to .004"

or

2. reduce the volume of the dome/bubble of the head which would allow me to maintain more deck clearance.

...either way I have to chuck the head and turn it. Anyone have any info as to which way is preferable for increasing the compression ratio and horse power? Or is it a "push"?

thanks,

chunk t

 

[Jerry Wyss]

I know What I would do but probably wouldn't be what someone else would do. I would take about .012

minimum off the face of the squishband and then take whatever it takes to get me .006" headspace off the bottom of the button surface that sit's on the top of the sleeve flange.

A shim could be added under the button to take it up to .008 or even more if you need it.

You can also take the route of puttin "sleeve shims" in under the sleeve flange to raise all the ports, .006

wouldn't hurt. But this means that you will now have .012" plus .006" plus the (.004" that you say you can have) Thats .022" overall,,,.020" removed should give you the .006" you want. That should give you the

option of putting in more sleeve shim or more head shim to vary the headspace.

All this work can be done on the "flat" surfaces of the button which is "coolaid" for a small lathe.

 

[LH7018]

I'm no expert, but I'd have to agree with Jerry on this, as this is what I do also and it works for me. If you have the facilities to access a lathe, even a small one, or someone that knows how to use one, it is really a simple task to turn the head button. To summarize what Jerry and I are talking about is this; if you want to raise the compression, machine what you have to on the head button to keep a tight deck clearance (and room to add/remove a shim if necessary for fine tuning) and adjust the volume of the combustion chamber to get your desired compression ratio while keeping the tight deck clearance. Hope this helps.

 

[chunk t]

Jerry,

Yeah, I was thinking that same way. I can shave the face of the head and reduce the head volume from .0113 to .0098 ci and hold the head spacing to .010" and still fall within my desired 10-10.5:1 ratio. Taking a little extra off of the head where it rests on the sleeve and using different thickness shims to adjust the CR.

I guess I've got it in the back of my mind that .004" - .006" deck spacing is a bit tighter than I'm comfortable with.

My transfer ports are already at 128-degrees in stock configuration, so I can't shim the sleeve without screwing up their timing.

I've got two types of heads, ones with a "stepped" hemisphear, and ones with out a "step" (just a constant radius) one better than the other?

chunk t

 

[Jerry Wyss]

The answer to your last question would probably be better answered by someone like "Marty Davis"

I know he has done work in the varied bubble shape,, I have not. I know that getting the fuel/air

mix to flash at the optimum time is what it's all about.

 

[chunk t]

Jerry,

The heads are all K&Bs, the ones with a constant radius bubble are from the early '80s, the stepped bubble heads are more recent from the late '90s. I'll probably turn a few of each to different dimensions and roll the dice.

chunk t

 

[LH7018]

Also, if you do take some material off the face (squish band) of the button, be sure when you're finished to take the sharp edge off of the point where the squish band meets the combustion chamber. You don't have to remove much ( I use a piece of 600 grit paper), just enough to break the sharp edge. That edge if left sharp is a source for detonation.

 

[Jerry Wyss]

Also, if you do take some material off the face (squish band) of the button, be sure when you're finished to take the sharp edge off of the point where the squish band meets the combustion chamber. You don't have to remove much ( I use a piece of 600 grit paper), just enough to break the sharp edge. That edge if left sharp is a source for detonation.

That's a Great point! That sharp edge has been the "culprit" of Many blown plugs over the years

 

[chunk t]

Jerry,

There was an earlier post you and Gareghty were talking head volume (.19, .18, etc) for your Thunder Tigers. Were those volumes cubic centimeters? Just so I know what measurement system you used as referance.

chunk

 

[Jerry Wyss]

Jerry,

There was an earlier post you and Gareghty were talking head volume (.19, .18, etc) for your Thunder Tigers. Were those volumes cubic centimeters? Just so I know what measurement system you used as referance.

chunk

Yes, "cc", I had a couple set's of .17 , .18 and .19 cc headbuttons made last Spring ,I'm having another machinist do some more, mostly for the guy's in Oz.