Is 7 inches of manifold tubing too much?

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thingswithwelding

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I have a 79cc predator 4 stroke build Im working on. I have bent the tube and made one of the flanges. I cut it to where I want it and its about 7 inches total. I could cut it more but is this ok? Will it run good? Ive seen the 212cc with some pretty long manifolds. Im asking here so I can know feom peoples experiences. My plan is to use a 19 mm Delorto or a PZ 19 or something like that. Cheers Thingswithwelding.
 
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Agreen

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It'll have some low end grunt for sure, but with every bit of experience (and "theoretical knowledge") that I have, the top end will most likely suffer. That's a seriously long intake runner for anything. Even my TPI Chevy engine has problems with high rpm power because of the runner length.

Most small engine intakes are 1-2" long at best. Results may vary.
 

thingswithwelding

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It'll have some low end grunt for sure, but with every bit of experience (and "theoretical knowledge") that I have, the top end will most likely suffer. That's a seriously long intake runner for anything. Even my TPI Chevy engine has problems with high rpm power because of the runner length.

Most small engine intakes are 1-2" long at best. Results may vary.
Here is a photo. I finally got my tablet app downloaded to resize these pictures. I want to have it as long as possible. I seen a predator online with long ones. the engines are probably built. I dont know if that makes a difference. Do you think cutting it at that last bend will be ok? Thanks
 

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thingswithwelding

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I seen them long and I think it will look cool. If It cant work of course I will have cut it down. Maybe I will take 3 inches off it. My carb will be here in a few days.
 

curtisfox

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Just roll with it and see how you like it. If it limits you too much, cut her down a bit. I agree unless you are going for high performance ( racing ) Don't think it will matter that much................Curt
 

FFV8

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We run them that way all the time. Actually pretty close to proper length for 5k rpm tune...





Both of those are 79cc Lifan engines.
 

curtisfox

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Nice bikes, nice long tail pipes. Do you have mufflers on them or just straight? Can't really tell in the pictures..................Curt
 

FFV8

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Straight pipes. Not loud at all.

38 inches from the valve head is on tuning for 4000 rpm. Intake length is a little short for 4k, actually right for 5500 - that flattens out the curve a little.

.
 

curtisfox

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Thanks! KOOOOOOOOOOL Learn a little more each day................Curt

PS. Same size as the port?
 
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Venice Motor Bikes

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I prefer short manifolds... They work very well & have great performance.
This is on a 99cc Felt that I did several years ago.

image-2603644506.jpg image-1955499435.jpg
 
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Gbrebes

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Straight pipes. Not loud at all.

38 inches from the valve head is on tuning for 4000 rpm. Intake length is a little short for 4k, actually right for 5500 - that flattens out the curve a little.

.
Hey FFV8, I would like to get some technical knowledge on the length of intake manifolds in relation to four stroke single cylinder engines.

You have always seemed to me as knowledgeable guy when it comes to internal combustion and such.

When you talk about "tuned to 5,500 rpm", I don't understand the theory of what happens in between the carburetor and the engine head. I have always tried to make my intakes as short as possible, but I don't even know why theoretically. It seems like you are saying that a longer intakes helps in the higher rpm range? So how does a short intake (say 3" or so) affect a four stroke engine? I would also like to know what "flattens out the curve" means. I am guessing your talking about horse power or torque range over different rpm's?

I assuming a two stroke engine scenario is completely different. I am running a 5hp Briggs engine at the races and I am always interested in squeezing out a little more speed in the straights. I feel like I can hold my own in the turns, but sometimes I'm overpowered in the straights, i.e., high rpm situations.

Thank you in advance for any light you can shed on this.

Gilbert
 

FFV8

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Gilbert:

The 4 stroke engine operates on a 720 degree cycle. The valves are in motion for about 120 degrees of that cycle - depending on cam grind. Flow stops & starts.

There are two radically different flow systems. The Intake, which is at a little less than atmospheric pressure & cold. The Exhaust which is hot, and pumped out well above atmospheric pressure.

The exhaust gas tuning is the inverse of a two stroke. The plan is to have most of the exhaust charge traveling down a tube at high velocity when the exhaust valve starts to close. This creates a slight suction on the cylinder, removing as much exhaust gas as possible.

The intake tuning is built around maintaining a velocity & containing a standing wave. As the intake valve opens, a flow through the carburetor is established. Fuel is mixed. The air / fuel mixture is cold & dense. When the intake valve closes, this charge stops suddenly. If you have enough velocity, the charge packs in against the closing valve, continuing to flow as the valve closes. Improving the cylinder fill as much as possible.

The standing wave. When that intake charge stops against the valve, the energy has to go someplace. It reverses direction. This is why some engines need a long velocity stack. When we consider the intake tuning length we include the length of the carburetor and velocity stack (if one exists)
It is common to see a fuel fog standing above a carburetor on the dyno. VW engines with 48IDA Webers were famous for this. The fuel charge was pushed back through the carb & up the stack. Tuning had to take in to account the fuel that went past the venturi twice. If you ran the engine in a vehicle without an air cleaner wrapped around the velocity stacks, it would lean out as the fuel fog was blown away in the wind.

If you have the cam timing events for your engine and the displacement the calculations can be done. You need to know how many degrees BEFORE Bottom dead Center the exhaust valve begins to open, and how many degrees AFTER TDC the exhaust valve closes. For the intake you need opening Before TDC, and closing After TDC.

.
 

MEASURE TWICE

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You mention in the first sentence of the 2nd to last paragraph where intake charge after intake valve is fully closed, there is a reversal of the vaporized charge. If this has too short a stack, does this mean it could dampen (even wet) a paper air filter as a result?

I had what I believe to be a different reason for my Briggs 4 stroke engine to cause paper air filter wetting with fuel. The valve timing for opening the intake was off due to wearing with age. This was since both worn valve and seat changed gap.

While this was equivalent to reducing the tappet/valve gap to zero, the intake valve opened too soon while the piston was still traveling upward toward the cylinder head. I've heard something called cross over that allows for it opening a little soon, but not where 1/2 the travel of the compression stroke caused a venturi effect of fuel spray out the wrong way at the air filter.

Since I adjusted it, now it is a bit better, but I may want to eliminate this cross over all together. Sometimes still the engine won't start until either the paper air filter dries out, or you keep swapping in new ones like an idiot.

An oil bath air filter I used on the engine at one time, even though it was not probably what it originally had, never had a problem starting as fuel spray the wrong way at the air filter just meant a little thinning of the oil bath with fuel. This was not a problem for what happens in wet paper filter and loss of air induction due to paper no longer being exactly porous.
 

Venice Motor Bikes

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This is my final 'two cents' on the long manifold debate...

One of the best bits of advice I ever got about engines & racing was this... "If you want to know what parts really work best?, look at what the fast guys are using!"

So I leave you all with this simple fact... There aren't any serious motorcycles (or even go-karts) that run intake manifolds that long... (because shorter manifolds work better). ;)


Also... Longer intakes 'frost up' & cause problems with the atomization of the fuel. :(
 
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Gbrebes

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Thank you very much FFV8 for your detailed response, it was highly informational. I think I might still have the specs card for my ARC 94ss cam. I am using a Briggs raptor block, so I think that is 206cc? So if I can get my timing info for the cam, I would love to try some calculations for the intake.

On a side note regarding exhaust, I did notice when I tried an open pipe on the exhaust on the track I got less power than with my little lawnmower muffler attached. Would you possibly know the explanation for that?

Thanks again for the help, Willow Springs race is only 10 days away,

Gilbert
 
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