K&B 3.5 Compression

[chunk t]

I'm looking for suggestions as to what should be the optimal compression ratio for an engine I'm setting up. I think I'm settling on 9.75:1, any reason(s) I should lower or raise the ratio?

here's what I've got going on..

-K&B 3.5

-Irwin stamped pipe

-128 trans port timing

-125 boost port timing

-181 exhaust port timing

-50-60% nitro

-.14cc single bubble head w/squish band area of 59.5%

-deck clearance of .018

I can make the compression adjustments with a head shim.

chunk t.

 

[Jerry Wyss]

I don't think there would be any harm taking the c/r to 10.7,,,and the key to making HP

is making Heat. You can have the most perfect numbers in the World, and not make

much HP by running it too cold :huh:

Rich and Hot with them numbers should show a much better result.

 

[Bob Kensill]

Get rid of all that head clearance! Try .003" head clearance. You may be a little high on the exhaust timing to pull much prop,but it should rev good.

 

[Jerry Wyss]

Get rid of all that head clearance! Try .003" head clearance. You may be a little high on the exhaust timing to pull much prop,but it should rev good.

Can anybody compute them numbers above to find out the c/r with .003 H/S,,

My Glasses are all steamed up :blink:

 

[Grimracer]

Chunk,

Big differance between 50 and 60% fuel.. might want to test both.. The squish are seems rather large to me.. Im not sure and would need to run some numbers on the heads i have now.

.018 is a bunch to much for a 21.. You are better off at say....008 deck height and some place around 10 to1 comp.. Hight comp on any OB is not all that needed but deck hight is very important.

Grim out

 

[chunk t]

I can manipulate deck height (take it down to a negative value if desired) and change head volume & squish band %, by swapping out different head buttons and head shims. I guess the better question might be what CR is good for the port timing I have. I even have head buttons with tapered squish bands that I could use.

Jerry, I have my own calc. formula on EXCEL that I can plug numbers into, so I can manipulate any measurement on paper to see what it yields, and bounce numbers against eachother until I get the right mix of clearances and timings to hit a desired CR.

chunk t.

 

[Jerry Wyss]

Hi Mr. Chunk T,, I guess I'm in Marty Davis's corner on the 3.5 OB's port timings.

He say's he runs the EX port timing consistantly over 180deg. My Tigers won't turn on

until It Is up there, 182,184,,course those engines have "gear reduction" lower's

,1.04/1 gear ratio, yours might not like it,,But,

I run smaller diamiter props,,, Yeah,,so what,, where does it say you have to run

full 40mm props anyway,, I'll tell ya, reduce the average X series for the 3.5 and you'll

be shocked,,I was.

I took the X640 and reduced it to 37.5mm,, and I been wonderin' why I didn't do that

years ago.

My Tigers like the greater pitch ratio of the "X6"series,, I think the K&B likes the lesser

"X4" series. Take a M440, take no more than 0.035" off the outside edge,, thin, balance

and give'r a shot, you can probably run that EX port timing after all,, and you can pitch

the trailing edge a little if you need more.

The .14cc bubble makes me a little nervous,, that's kinda small in my way of thinking.

 

[chunk t]

I've got head buttons from single bubble .12cc all the way out to .19cc double bubble. Like I said earlier I can change out head buttons. I'm just looking for a concensus on the "sweet spot" to set my engine at with the timing my sleeve has.

I'd posted a thread a while ago about head volume, squish band area & flat vs. tapered squish band hoping Marty would chime in... Never did.

Rod... you out there??? What's the effect of different deck heights? What's key to consider?

chunk t.

 

[Jerry Wyss]

I've got head buttons from single bubble .12cc all the way out to .19cc double bubble. Like I said earlier I can change out head buttons. I'm just looking for a concensus on the "sweet spot" to set my engine at with the timing my sleeve has.

I'd posted a thread a while ago about head volume, squish band area & flat vs. tapered squish band hoping Marty would chime in... Never did.

Rod... you out there??? What's the effect of different deck heights? What's key to consider?

chunk t.

I'd give Rod a call if you have his #

Most are running .008" headspace,, the "sweet spot" has to be found for every boat, motor &

pipe config, then prop to suit.

I took me months to find mine and there's always room for improvement.

Flat squish is mostly favored,, .18 to .19cc volume is mostly what I use.

 

[chunk t]

I'll dig at home, I've got Rod's number somewhere.

I did some quick number crunching, if I lower the deck to .008" and swap the .14cc head with a .18cc head, my C/R is going to jump to 13:1. I'm no expert, but I'm of the mind to think that's too high. A simple change of a few .000" makes a stark change in the C/R. That's why I'm now becoming curious about the true importnce of deck height. Changing the C/R & bubble shape & squishband area should have a greater effect on temperature and detination than a simple matter of clearance.? :huh: right...

chunk t.

 

[james irwin]

I'm looking for suggestions as to what should be the optimal compression ratio for an engine I'm setting up. I think I'm settling on 9.75:1, any reason(s) I should lower or raise the ratio?

here's what I've got going on..

-K&B 3.5

-Irwin stamped pipe

-128 trans port timing

-125 boost port timing

-181 exhaust port timing

-50-60% nitro

-.14cc single bubble head w/squish band area of 59.5%

let me tell you a little story about myself ?? as a lot of you know , i purchased a few of the rossi piranas ( red caes motors ) 3.5 i could not make them turn the 44.000 as they were advertized. i have a friend here in muncie that used to fly rc combatt. he ran my fuel exclusivly ,he called and need some , when he arrived we found i was out , so i showed hims how to Mix it , now comesthe fun part , he called the next day , he was frantic , he said jim, this oil is seperating ??? it cant it is only 30 % when he arrived , we took all the cans he had used , gues what . i had givin him two can,s of nitro , he he what he had was 100 % nitro .so we broke it down and remixed it . i know , you are wondering what the hell has this got to do with boats / remember the pirana that started this articall . went to the celina race , fIrst hEAT , MAN THAT ENGINE TOOK OFF LIKE A BEAR , FOR TWO DAYS NO ONE COULD EVEN COME CLOSE . GAVE THAT ONE A LOT OF THOUGHT , YOU WILL NEVER GUESSE WHAT I DID TO MAKE THAT THING REALY COME ALIVE , TOOK THE HEAD CLEARANCE . FROM .007 TO .012 I WAS RUNNING THE 30 % I HAD DANNY MIX. THEN WENT BACK UP TO 60 % GOD BESS

 

[chunk t]

let me tell you a little story about myself ?? as a lot of you know , i purchased a few of the rossi piranas ( red caes motors ) 3.5 i could not make them turn the 44.000 as they were advertized. i have a friend here in muncie that used to fly rc combatt. he ran my fuel exclusivly ,he called and need some , when he arrived we found i was out , so i showed hims how to Mix it , now comesthe fun part , he called the next day , he was frantic , he said jim, this oil is seperating ??? it cant it is only 30 % when he arrived , we took all the cans he had used , gues what . i had givin him two can,s of nitro , he he what he had was 100 % nitro .so we broke it down and remixed it . i know , you are wondering what the hell has this got to do with boats / remember the pirana that started this articall . went to the celina race , fIrst hEAT , MAN THAT ENGINE TOOK OFF LIKE A BEAR , FOR TWO DAYS NO ONE COULD EVEN COME CLOSE . GAVE THAT ONE A LOT OF THOUGHT , YOU WILL NEVER GUESSE WHAT I DID TO MAKE THAT THING REALY COME ALIVE , TOOK THE HEAD CLEARANCE . FROM .007 TO .012 I WAS RUNNING THE 30 % I HAD DANNY MIX. THEN WENT BACK UP TO 60 % GOD BESS

:blink: Jim... So you're saying an engine can run like hell with the higher head clearance which I'm setting up with. Right...? Back to my original, original question. is 9.75:1 a good C/R to target? Or should I go into the 10.5-11.0:1 range because I am setting a greater head clearance?

chunk t.

 

[Grimracer]

I have has good luck with the follwoing

.18 to 19cc V

.008 to .011 HS

10to1 CR

1/2 sqish (inner) cut at .5deg..

lots of nitro (Odonnell 65)

Med hot plug (Odonnell Red Dot)

That will run great with the timing numbers you have set up on the engine. I do not run my EX that high but thats more a heat racing thing then a full bore speed thing.. for me the engine/boat HAS TO FINNISH HEATS...

What are your goals for the engine?

Grim

 

[Ray Sametz]

Chunk T, From what you posted if you drop head cl. to .010. You should be at 9.1 Comp Ratio. Very reliable with 60% for heat racing. You can drop Head Cl. to .008 to test but not to .003. My 2 cents only. Ray

 

[Jerry Wyss]

I'll dig at home, I've got Rod's number somewhere.

I did some quick number crunching, if I lower the deck to .008" and swap the .14cc head with a .18cc head, my C/R is going to jump to 13:1. I'm no expert, but I'm of the mind to think that's too high. A simple change of a few .000" makes a stark change in the C/R. That's why I'm now becoming curious about the true importnce of deck height. Changing the C/R & bubble shape & squishband area should have a greater effect on temperature and detination than a simple matter of clearance.? :huh: right...

chunk t.

I think (and have seen) the general reason for the .008" headspace,,it provides the cooling

layer for the piston crown (not too large so it catches on Fire) and just large enough to keep

your blown plug elements from turning your headbutton into a tiny shrimp fossil sample.

Could be wrong but I doubt it

 

[chunk t]

:lol: Thanks Jerry.

My rational behind this - with the combination of 50% nitro, near 10:1 C/R (which think is on the high side for 50%) and reduced bubble, I want to avoid early detination... So I'm using the deck height as the control for when the fuel is igniting (and controling temperature).

Is my logic on course or off-track?

chunk t.

 

[Jerry Wyss]

:lol: Thanks Jerry.

My rational behind this - with the combination of 50% nitro, near 10:1 C/R (which think is on the high side for 50%) and reduced bubble, I want to avoid early detination... So I'm using the deck height as the control for when the fuel is igniting (and controling temperature).

Is my logic on course or off-track?

chunk t.

As far as the "deck height" goes, 8 to .010 with the higher EX timing should be ok,

lower EX timing would require more head shim.

Mine has been at .008 and works well

with the .18 and .19 button with 184 EX timing and .013 sleeve shim with the GadjetCraft carbon pipe @ 8 1/8th inches and X640 @ 37.5mm dia. Thats my TT's "sweet spot".

I was surprised to find out this arrangement gave me 9.87:1 c/r

 

[chunk t]

All you engine guys. Direct question here.

Why do I see a patern of beliefe that higher %nitro requires less deck height. Other than personal experience. I am not questioning your answers given in earlier replys; I am just looking for a point of referance to those answers given. Anyone can answer this question.

thanks,

chunk t

 

[Jerry Wyss]

All you engine guys. Direct question here.

Why do I see a patern of beliefe that higher %nitro requires less deck height. Other than personal experience. I am not questioning your answers given in earlier replys; I am just looking for a point of referance to those answers given. Anyone can answer this question.

thanks,

chunk t

I haven't seen that pattern but if it's ok with you chunk,

I'd like to invite RodG on here,, he could give ya the straight poop. I only know what works for

me concerning the 3.5 I run.

(I think He way too busy)

 

[Jerry Wyss]

I hate to see this topic die,, this is a very important area for the performance of these

motors.

The lack of participation by those that "do know" allot about this subject is "no help"

dispelling some of the confusion.

I was "schooled" by a young man that definetly knew about this stuff, because I run

motors that are a "working testament" to that knowlege.

Now, Come On and let fly,, thats what this forum is all about,, inquiring minds need to

know :huh: