.21 Engine Exhaust Timing

[Norm Doerr]

You gonna run A-hydro with us Tom???

Hell yeah !!

Let the fun begin. Last count I have with just the locals is around six 20s. Andy sold his 20 too soon LOL NOw back to the subject here!

 

[ek kustomz]

You gonna run A-hydro with us Tom???

Hell yeah !!

Let the fun begin. Last count I have with just the locals is around six 20s. Andy sold his 20 too soon LOL

Thats sweet Norm.

Damn Tom, who's gonna pit for me???

 

[Andy Greene]

Marty- great thread,....... I will stay out of the way and TRY not to ask too many questions - LOL.
Andy

IMPOSSIBLE...... !!! :lol:

Marty Davis

TOO TRUE - LOL . so is that a yes or no !!!!

 

[Marty Davis]

There is another great reason to use low timing.

It makes the engine larger....

Imagine the point where the exhaust port is closed by the piston. On the low timing engine, the exhaust port closes earlier. That means MORE TRAPPED CHARGE. That is the reason that the engine is effectively larger. There is more trapped burnable product by closing the port earlier.

How much larger, I will leave that to the mathmatics experts. It would represent the difference between a cylinder of .640" in diameter and a height difference of about .015". The stroke is .670".

What does that make the difference in percentage?

Marty Davis

 

[Marty Davis]

You gonna run A-hydro with us Tom???

Hell yeah !!

Tom:

That is good news.....

Marty Davis

 

[aaron alberico]

Marty and guy's

i like high exhaust timimgs and other timings,

with the mods i have been doing to my engine's i have awesome rpm's and still have

good torque,

my cmb 67 with high timings and ops 67 nitro pipe at around 9-3/4 to 10in will rev harder then alot of 45's out there and can still pull decent props[h48's] my heat race boat,

novrossi 35plus 21 stock timings 191 exhaust, 131 transfers and and boost and ops 3280 pipe at 7-1/4in had to problem pulling h7 46mm

with 3.75cup, pulled it like it was'nt even on the boat, and this was the engine's first outing, still not run in

the old saying there is more then 1 way to skin a cat

regards Aaron

 

[AndyBrown]

MartyI have a greenie and a Mac both with the same numbers right to the tee and the greenie is a lot stonger than the mac.I don't no if the opening in the case is different or what.Don't get me wrong the mac and cmb or both great engines.

Dave Roach

I would be curious about the Crankcase volume of each. Why don't you measure it with the piston at the bottom on both engines and let us know what you find.

Marty Davis

There are three different case port volumes in the MAC/bluehead and the MAC/Greenie.

The fisrt batch of MACs had small port volume. Not what I designed , but what CMB choose to put there.

Those first MACs were very low on torque.

The following MACs had larger ports (and more torque), but still not what I designed.

The MAC greeny had the biggest ports. They are about the size that I would hog out our personal engines.

Exhaust timing and pipe length have less to do with rpm than several other factors.

I've seen 200 degrees and short pipes that would not turn up.

 

[Chris Wood]

.

Exhaust timing and pipe length have less to do with rpm than several other factors.

I've seen 200 degrees and short pipes that would not turn up.

I'll bite....

what is it?? props.. hull drag.. areo? squish band??

chris

 

[moparbarn]

this isn't nitro theory, but, on cars the current thought is that the ex. needs to flow at least 70% of intake. balance is the key.

both ends of the flow tract have to work together and compliment each other. ex. timing mods and pipes won't work well without similar work on the crank and intake. i do know that i have tried to adj. squish clearance by shimming the sleeve (on a k&b .21 ob), and the motor was slower. only moved it .002". went back down, shimmed the head, ran MUCH better. this was on a mod motor-turbo crank, turbo head, "ported case", skirt cut on intake side, etc...

 

[aaron alberico]

volume, flow and intake timing have alot to do with rpm's



regards Aaron

 

[Chris Wood]

volume, flow and intake timing have alot to do with rpm's

regards Aaron

aaron,

I get that.. i guess i am interested in pictures.. always curious to see what people are doing.. unless it is a secret.....

chris

 

[Tom Foley]

Adding to the fray here. Perhaps instead of looking at just the exhaust timing, whether high or low, what about it's relationship to the intake timing. The blowdown timing (when both intake and exhaust ports are open) would have a huge effect on the pipes ability or lack thereof, to do it's work. Too high and you can have a peaky somewhat torqueless motor, too low and you can have an easily driveable motor that won't have very high peak power. Dave Marles for one, stresses the importance of this when looking to modify a motors timing. After all, the entire system has to work together. Just a thought.

I believe , Blowdown is the period after TDC before the exhaust port opens . I seem to remember this from Gordon Jennings book .

 

[Preston_Hall]

If you are swinging the wrong wheel and trying to push a brick or crooked popsicle stick around the lake you will never get any rpm. A stock engine can do wonders if it is set in a good boat setup. Perfect example is Don Ferrtte's twin hydro record with stock engines. So ask yourself, "If I have two stock MACs, why can't I go 107mph"? SETUP.

Look at NASCAR. Everbody has about the same horepower but they obviously they do not have the same setup.

 

[dave roach]

Preston

Don engines are stock.I don't think so.They have Brads piston in them.Stock is when it come from the factory.LOL LOL Just messing with you.

DR.

 

[aaron alberico]

Preston,

i'm with you to,

in 2002 at the SAW trials with my oval 67sg boat and a bone stock cmb 67

i went 103mph, the hole package got to work!

regards Aaron

 

[Preston_Hall]

Thanks for agreeing. Now go tell those other guys why a plane won't fly off a conveyor.

 

[Chris Wood]

i am lost.. with all the crooked 2 x 4's and busted bricks

one person says high intakes works and exhaust.. then says low numbers are working?? which is it?? i now there is a thousand ways to do it. i now what works for me past and present..

I guess i am just trying to compair numbers from what others use vs mine..

but when i see people saying one works and then they say the other works i am lost.

oh well

 

[aaron alberico]

Chris

it's a funny game..lol

i have been told by someone, HOW THE HELL DO YOUR ENGINES WORK WITH THEM TIMING NUMBERS,

its the hole package in song with each other :lol:

regards Aaron

 

[DaveMarles]

i am lost.. with all the crooked 2 x 4's and busted bricks
one person says high intakes works and exhaust.. then says low numbers are working?? which is it?? i now there is a thousand ways to do it. i now what works for me past and present..

I guess i am just trying to compair numbers from what others use vs mine..

but when i see people saying one works and then they say the other works i am lost.

oh well

Chris, This is a complicated subject but some of it can be simplified to a great extent. When you are comparing numbers its no good listening to people quoting exhaust timings. The exhaust timing and transfer/boost timings are tied together. For example, an engine with 180 exhaust and 120 transfers will have similar characteristics to an engine with 190ex and 130 transfers because the blowdown at 30 degrees is the same (blowdown is the period from exhaust opening to transfer opening ). By similar, I mean that if one will pull away from the shore and not sag in the corners and rev out OK then the other will too. There will be a difference in performance but possibly not very much. The pipe length has to change because that is tied more to the exhaust timing than anything else. If the pipe suits the lower timed motor better than it does the higher timed motor then the lower timed motor will have a better overall performance including peak rpm than the higher timed motor.

Now if you took the motor with 120 transfers and upped the exhaust to 190 then its never going to work because the blowdown has increased to 35 degrees and from experience 35 degrees just won't cut it. This would be the same if you had transfers of 110 and exhaust of 180, it just won't work. This difference between exhaust and transfer timing is absolutely critical for good performance. The blowdown requirement of all of our racing nitro motors will be slightly different foir engine size and application but not by much. For example I don't run riggers but in my 21 geared Valvola running 35000 rpm, the blowdown is 31. If I go to 32 degrees then it will not accelerate reliably if I come of the throttle for more than a fraction of a second. My geared .45RS running 29,500 rpm has the same blowdown, my 91RS 30 degrees.

A motor with a lower blowdown will have more torque/better throttling but will have less overrev potential past peak hp, but that may suit certain types of boats. Conversely a motor with a high blowdown will be more peaky and will rev past peak hp more easily which is good for straightaway but maybe not good if your heatracing boat gets out of shape when it gets loose on some choppy water.

This is a simplified answer to the question of exhaust timings and transfer timings because when altering the blowdown this is not just altering the timing duration its also altering the blowdown time area. In general almost no one is measuring time areas in model boat engines because its not easy on a small motor and lifes too short. We are altering time areas by trial and error when we alter timings and port widths and experienced engine modders will most likely have already found the optimum for their particular application.

The question of optimum exhaust timing is a separate issue. I've run transfers up to 140 degrees and exhaust up to 205 degrees with good results but now I like transfers much lower. Performancewise on my 3.5, it runs fastest with transfers of 130 to 131 but its not reliable enough like that and doesnt pull hard enough in the turns and so I actually run 127/128.

With higher transfers the exhaust timing has to be higher for the above reasons but then as the ex timing gets higher the powerstroke becomes shorter and then, no matter what, there will be a loss of torque. In some types of boats, thats acceptable but for me its not.

I have generalised here because as I said its a complicated subject and there are more things to consider than this but for me this is the part of engine modding that needs the most careful attention.

I agree with the experienced racers here though that boat/engine SETUP is the most critical factor, but bear in mind that if you have a good setup and a correctly modded motor and you can drive well, then you have the best chance of success at the races. (plus a bit of luck).

Dave

 

[Chris Wood]

Finally, thanks dave.. I run numbers similar to you but have heard some people use boost and transters in the 140 to 150 and i cannot for the life of me figure out how the motor evens runs..

I guess for the average guy that doesn't have the machines to do this stuff.. it can be tuff to change things.

thanks again.

chris