Modded 394 lean tuning issue WTF...

[Chainsaw Jim]

Good point about backing the H needle out, that has never occurred to me to do.

I popped the nozzle out on the wj to check the feed holes, they're huge.

I didn't drill the main bypass jet because the useable tuning range on the H was less than a couple turns.
Is this a correct assumption or am I missing something...?

That sounds about right. As a last resort I drilled one because it wouldn't richen up until the screw was backed about halfway out.

 

[Jimmy in NC]

A bump in compression does generate more heat and higher fuel demand. You can have some compression increase due to carbon in the chamber and on the piston crown as well. I think that compression gaining whatever the cause may be the best theory.

Playing a little more into that, you may be in the territory where 87 is on the borderline of not being enough octane. That will generate alot of excess heat too which leans the combustion mix because some of the fuel mix is burning off early.

Really not much carbon on the piston crown or chamber.

As for 87 octane, I and many others run it in saws with more comp than this one and I've yet to see or experience any signs of detonation or other issues with it.

MAHLE motor sports division has done extensive testing on dynos for the argument of high octane fuel. What was found is that with the "fixed" timing on small engines, as you increase RPM, you hit a point where lower octane actually runs better.

Wrap your head around this.. at 14,000 RPM that's 233.3 revolutions per second. It's moving so fast the fuel cannot burn fully. Lowering octane causes a quicker burn and more of the fuel is burned before the exhaust is open generating more expansion and thus more power.

They also said in these engines they had a hard time generating detonation. Yes it can be done with water or other contaminants in the fuel but with clean fuel it's a mute point as the piston speed is just too fast.

The test mule was a single cylinder quad. I forget the model but the principles are the same. To get the most out of the fuel with high octane, you need more ignition timing as the RPM comes up. With saws that's not a reality that most of us can afford.

 

[wcorey]

I seem to recall something where these smaller motors can get away with much higher comp, maybe due to the relatively narrow squish band/small combustion chamber.

I know tighter squish clearance also combats detonation but not sure where the actual threshold is or if it's relative to bore size.
I've heard .040-.060" batted around in relation to larger bore bike motors.

 

[srcarr52]

I seem to recall something where these smaller motors can get away with much higher comp, maybe due to the relatively narrow squish band/small combustion chamber.

I know tighter squish clearance also combats detonation but not sure where the actual threshold is or if it's relative to bore size.
I've heard .040-.060" batted around in relation to larger bore bike motors.

I shoot for 0.050-0.060" on 800cc twin cylinder snowmobile engines.

 

[RIDE-RED 350r]

MAHLE motor sports division has done extensive testing on dynos for the argument of high octane fuel. What was found is that with the "fixed" timing on small engines, as you increase RPM, you hit a point where lower octane actually runs better.

Wrap your head around this.. at 14,000 RPM that's 233.3 revolutions per second. It's moving so fast the fuel cannot burn fully. Lowering octane causes a quicker burn and more of the fuel is burned before the exhaust is open generating more expansion and thus more power.

They also said in these engines they had a hard time generating detonation. Yes it can be done with water or other contaminants in the fuel but with clean fuel it's a mute point as the piston speed is just too fast.

The test mule was a single cylinder quad. I forget the model but the principles are the same. To get the most out of the fuel with high octane, you need more ignition timing as the RPM comes up. With saws that's not a reality that most of us can afford.

Very interesting, I hadnt thought of fixed timing and how it would play in this area. Thanks for posting.

Ive kinda had this question in the back of my mind why my Eddie Sanders big bore 250r with race fuel dome blowing 200psi requires 108 octane minimum but we seem to get away with more compression on these saws and can still run pump fuel. I know raw compression isn't the only factor, we can get into corrected and uncorrected compression ratios and on down the line. Seems like just about the time you have a pretty good grasp on things, another facet changes it all up on ya! Lol

 

[MustangMike]

But I have advanced the timing on all my saws, plus the Stihl manual says that higher octane fuel burns cooler, so I will continue to use it.

In addition, since I mix at 40:1 (not 50:1), that further lowers the octane rating, so I use 93 in my mix.

 

[Deets066]

But I have advanced the timing on all my saws, plus the Stihl manual says that higher octane fuel burns cooler, so I will continue to use it.

In addition, since I mix at 40:1 (not 50:1), that further lowers the octane rating, so I use 93 in my mix.

To each their own

 

[Deets066]

If I bump comp up to 160 psi in my 700 triple, there is NO way it will live without 110 octane.

Fact

 

[srcarr52]

If I bump comp up to 160 psi in my 700 triple, there is NO way it will live without 110 octane.

Fact

I had a Polaris CFI4 800 that was at 175psi, it ran on 91... at 10k elevation. Now I just leave the engine stock and add a turbo.

 

[p61 western]

I had a Polaris CFI4 800 that was at 175psi, it ran on 91... at 10k elevation. Now I just leave the engine stock and add a turbo.

How much compression do you think you lose as you go up in elavation?
Edit not being a azz just curious. I know it goes down, just wondering how much.

 

[Deets066]

How much compression do you think you lose as you go up in elavation?
Edit not being a azz just cutious. I know it goes down, just wondering how much.

A lot, jetting is way different too

 

[srcarr52]

How much compression do you think you lose as you go up in elavation?
Edit not being a azz just curious. I know it goes down, just wondering how much.

At 10k I think it's around 20%.

 

[Brewz]

yeah that sounds about right
I live at sea level and went to work at 3000 meters for a couple months and the first week almost killed me.
Looked it up and the air is about 20 to 25% thinner

 

[smokey7]

I mainly build vintage stand up jet ski motors specifically kawasaki 440 and 550 engines. They start out at about 125 psi on a fairly healthy one and seen a few at 150. They run huge squish upwards of .120 and domes 36- 40cc when im done squish is .050-.060 domes i target for are 27.5 cc on a 550 and 24 on a 440 porting changes a whole lot and my god do they run different. When im done they usually blow 185 with minimum of 100 octane and usually set up to run bouys. If it was a freestyle motor the porting changes some but they get way more timing and definatly cant hold them wfo for more then 5- to maybe 10 seconds before you seize them right up. I could go on for hours about these engines and i practally have. But it seems stroke, load on the engine, compression, rpm, jetting, fuel, application and engine opperating temp all play into the game of keeping a high performance engine alive.

 

[RIDE-RED 350r]

I agree, seems there is a world of difference between high performance power sports two-strokes and saws. I have aspirations of porting my own 250r cylinder in the not too distant future.. Going to be a welcome change working with a cylinder that's 72mm bore and has a removeable head! Lol!

 

[srcarr52]

I agree, seems there is a world of difference between high performance power sports two-strokes and saws. I have aspirations of porting my own 250r cylinder in the not too distant future.. Going to be a welcome change working with a cylinder that's 72mm bore and has a removeable head! Lol!

Durations go up with larger cylinders... and pipes. So, they are nothing like saws.

 

[RIDE-RED 350r]

Hence the need for reed or rotary valves

 

[Lee H]

Durations go up with larger cylinders... and pipes. So, they are nothing like saws.

I always thought that too. Just like back in the day or even today with muscle cars.
Putting a lot of duration in a big block would be ok but put the same in a small block
and it's world of difference.

 

[MustangMike]

A High duration cam can truly transform an engine. My fastest Mustang was my 1968 390 GT. I ordered the Ford Solid Lifter cam with 308 duration, but got the one with 320.

It did not have enough vacuum to open the secondaries on a 735 Holley, but with an 800 Double Pumper with mechanical secondaries it was faster than either my 428 CJ or my 427 Ford motor (the 427 had the 308 duration cam). It was an animal, it would pull away from a 440 6 pack Super Bee (w/headers) like nothing!

 

[Brewz]

wow we have gone from a 394 Husky to big block Ford's