increasing compression on Saito 4S..!?
 

Howdy, in order to minimise/eliminate nitro I have milled the crankcase on two of my Saito engines to increase compression.
Saito.72 was milled 0.5mm which increased compression from 10.1(measured) to 12.7 (calculated) and indeed the engine was less dependent on nitro after that but still needs 5% or more to run really well.
Saito.56 was milled 1.0mm which increased compression from 8.1 to 12.2 and it now runs beautifully on 5% nitro but still not well enough on 0%. Clearance between piston and valves is still about 1mm measured with a clay dimple on top of piston. Further, I had to shorten the pushrod and intake tubes a bit. The engine has no bad habits like kicking off props or signs of detonation.....in fact it runs much better after these modifications.
So now I intend to stretch things a bit more and plan to mill two more Saito.56 cases - one 1.2mm which will take compression up to about 13.8 and another one 1.4mm giving about 15.6 compression ratio.
Before I start I will consider any relevant viewpoints on what the limitations will be provided piston and valves do not hit each other.....Thanks..!..and Cheers/Harald

RE: increasing compression on Saito 4S..!?
 

Wow, now that is taking the bull by the horns and doing something about a perceived problem. Good for you.

You said, IIRC, that you milled the case. That means the actual crankcase that holds the crank (duh!)? I take it that that piece is less expensive to replace in case of an error? Haven't looked. Am I correct in which piece you milled? I just woke up, so pardon me if this is a stupid question.


Ed Cregger

RE: increasing compression on Saito 4S..!?
 

canard, I have two early Saito with high comnprerssion, an .80 and a 1.50, one has 15.6 and the other has 14.4. I belive the .80 is the one with 15.6 to 1 as it will start up unassisted on Davis Diesel Fuel but after a brief warmup has too much compression and cannot be safely throttled beyond 5,500 rpm at which point it starts making black and gold grenade sounds. Some guys that have these have complained that they launch their props into space too easily but only when you get them too lean and any fourstroke will do that. I like them that way.

RE: increasing compression on Saito 4S..!?
 

One of the guys in my club has been milling his Saitos for quite a few years now to run them on zero nitro and he's found that 13.5:1 compression was ideal. Coincidentally, that's exactly the same compression I've found best on 2 strokes as well. I'm not certain of his total oil % (I think it's 20% though) but its a blend of 90% Castrol M castor and 10% Mobil Jet Oil II. The jet oil is simply to keep the engine clean from any carbon.

RE: increasing compression on Saito 4S..!?
 

thanks guys for encouraging input so far, here some further remarks:

Ed: yes indeed it is the crankcase I mill down as shown in the attached set-up but with the help of a friend who has got really professional milling equipment to do it.

Hobbsy: so you would not advise against going all the way up to 15.6..?!?!..well I could make and add a paper gasket if needed.

downunder:my standard homebrew fuel contains 12% ML70 and 6% castor which I use for both 2S and 4S and if I could get away with 0% nitro I would be happy. Mobil Jet Oil - is that a type of ATF oil..??..never tried it.

All: The critical thing with the Saito 4S engines is that they become extremely touchy on the high speed needle at no/low nitro fuel. With the HS needle set on the ground the engine starts to blobber and run rich when unwinding in the air. If set a bit on the lean side on the ground it usually quits when below half tank in the air... [&o]...Cheers/Harald

RE: increasing compression on Saito 4S..!?
 

I believe the ML70 is a very good oil but apparently not available outside of Europe, much like Castrol MSSR. Mobil Jet Oil II is a synthetic gas turbine oil and is the only synthetic I've ever dared try in any of my engines. BP2380 is also suitable (another gas turbine oil). Something else that could be worthwhile trying in zero nitro fuel is 3-5% acetone but if you're using Shell A racing fuel then it's already got 3% acetone in it.

With the high speed needle, I'd guess the taper on it isn't fine enough to control the much lower fuel flow you have with zero nitro so possibly there's another finer tapered needle that might fit or do some very careful reshaping of the needle.

RE: increasing compression on Saito 4S..!?
 

downunder, thanks again for good info. Second thoughts make me inclined to back off a bit and mill 1.2mm and 1.3mm not to overdo it.
Will look for Mobil Jet Oil II and BP2380 since my can of ML70 is running dry soon.
Must say that I have tried acetone as additive but not seen much effect but maybe compression has to be closer to optimal before evaluating it with certainty. Good idea about needle taper - will put a needle in my micro lathe and give it a try later on....Cheers/Harald
PS have you got the forest fires under control yet..?..nothing on TV up here lately..DS

RE: increasing compression on Saito 4S..!?
 

Alright, so I want to do this to my Saito .82. Does anyone have any idea how to calculate how much I'd need to increase the compression to maximize power with 15%-25% nitro and 16%-18% oil?

RE: increasing compression on Saito 4S..!?
 

You will have to start by determining the combustion chamber volume at top dead center. Then you can calculate the rest by using the bore and stroke to calculate the swept volume which should be 13.8cc or .82 cubic inch. Or if you know the existing compression ratio, you can calculate the rest.

RE: increasing compression on Saito 4S..!?
 

How do I determine how much nitro is equivalent to how much compression? I would need to know this in order to set the compression correctly.

RE: increasing compression on Saito 4S..!?
 

Somewhere in here it has been mentioned what the compression ratio of most of the Saito's is?

To make things simple, if the .82 engine has a 10:1 compression ratio, the combustion chamber volume is .082 cubic inches. (there is an error here for the maximum volume you will need to use the swept volume plus the combustion chamber volume when considering the compression ratio.)

If you want a compression ratio of 13.5:1 then you will need a combustion chamber volume of .0607 cubic inches.

To change a 82 to 13.5:1 in this case, you would need to reduce the volume by .0212593 cubic inches.

With a bore of 29mm (or 1.1417323 inches) The top of the piston has an area of .3258881 square inches This is wrong. (Actually .325881 times pi = 1.02 "^2)

You would have to take 65 thousands of an inch off the deck of the block This is wrong. Read on down the line to other posts)

I'm standing by for someone to check on me and make sure I'm correct?

Jim

NOTICE There are several errors in this post so read down the line for corrections

RE: increasing compression on Saito 4S..!?
 

That is done by trial and error. Due to minor differences in combustion chamber shape each engine will have require a slightly different compression ratio for optimum ignition and combustion. But, I don't think you need to increase compression ratio for 15-25% nitro as Saito recommends 10-15% for their engines. If anything a small reduction in CR would allow a leaner needle setting without detonation. The only way to find out is to try.

Serious competitors running glow adjust compression ratio to suit current atmospheric conditions. The engines are run so close to the limit that they must do this to get peak power without blowing up engines.

RE: increasing compression on Saito 4S..!?
 

You should consider the volume of the combustion chamber in the total uncompressed volume. So your .82ci with .082ci combustion chamber would really be 11:1.

RE: increasing compression on Saito 4S..!?
 

A new block costs $27 in the USA The push rods are $4.25 and the tubes are $9

You could build it back up a little with a gasket under the jug if you have to?

RE: increasing compression on Saito 4S..!?
 

Greg, -- Thanks for correcting my lack of observation

RE: increasing compression on Saito 4S..!?
 

Piston area would be 1.02in^2 would it not? So if the engine started at 11:1, it would need only .016" removed to get to 13.5:1. That's not much so some aluminum or brass shim stock would fix it if you weren't happy. You could probably take out that much in the valve adjusters to try before shortening the rods. This all of course depends on the real CR.

RE: increasing compression on Saito 4S..!?
 

That is correct, I forgot to multiply by pi

RE: increasing compression on Saito 4S..!?
 

Well, I've got my valves adjusted down to whatever the thickness of a cigarette pack wrapper is.

I may give it a try if I can get a hold of some good machining equipment. If I do, I'll post the results.

RE: increasing compression on Saito 4S..!?
 

Hi guys, my Saitos have the following comp.ratios measuring the "head space" with ATF oil in a pipette and calculating as follows:
(head space+swept volume)/head space
Saito.56 has(had)only 8.1:1
Saito.40 Special has(had) 7.8:1
Saito.72 has(had) 10.1:1
All of these engines have now been further milled in the crankcase to take them up closer to 13.5:1 compression.
Also put the head of an old Enya.60 in my lathe to shave off some 1.5mm to increase its meager 8:1 original compression-see pics below.
Cannot wait to put them all in the test stand as soon as the snow melts away...Cheers/Harald

RE: increasing compression on Saito 4S..!?
 

RVM,
If you combine very high compression with high nitro you will loose plugs frequently. This is why we have to reduce compression on the eastern european engines that are setup for no nitro.

Dave

RE: increasing compression on Saito 4S..!?
 

Well, I know that my .82 can run up to about 40% nitro in rather warm conditions. This indicates to me that there is still a bit of compression left to play with. I wanted to maximize power with 15% - 25% nitro fuel, because nitro still nets a higher gain in power than raising the compression. Basically, I want 40% nitro power out of my engine with 15% - 25% nitro. :D

RE: increasing compression on Saito 4S..!?
 

(RVM wrote)....Basically, I want 40% nitro power out of my engine with 15% - 25% nitro...
Good point RVM and I stretch it a bit further by wanting X% nitro performance with NO-NO nitro fuel.... Therefore I try to "swiss cheese" my engines as far as I dare go which up till now has worked fine with no parts failures yet. Attach a few pics as example....Cheers/Harald

RE: increasing compression on Saito 4S..!?
 

Some of those modifications are extreme. How much runtime do you have on that piston after modification? I'm curious as to me, such a small amount of piston side area is sure to overload the lubricant and lead to scuffing. I think the lifter mod will lead to worn bores over time. To save weight there I would make new ones which were hollow. Enya uses aluminum tube pushords with hardened steel tips, and are very light.

Cool engine mods. Very similar to what I've done with my four stroke diesel conversions. No nitro, no methanol, no glowplug. ;)

RE: increasing compression on Saito 4S..!?
 

Canardlover, that is the most interesting thing I have seen on RCU yet. I have read so many glow plug this or needle valve that on here that boredom was setting in. You are way out there and I hope you will tell us more, like how many more rpms or longevity. How much fuel have you burned in these swiss cheese engines? You are the man. Please send more info.

RE: increasing compression on Saito 4S..!?
 

Very impressive. I wish I had the money and knowledge to do that.

How is the engine running now? How is vibration? I'd imagine that the balance would be all over the place with the lightening you've done. What kind of power increases are you seeing? What about longevity?