svejkovat

I would like to streamline this GX35 for mounting it with as narrow a profile as possible. Ibraze and tig weld, so the possiblities are not limited.
I anticipate routing the exhaust and carburator toward the back (output shaft faces aft for this application). Prop vacuum/pressure is not an issue since this is not for flight but for a ground vehicle.

Do Ireally need a formula? Or will Ibe pretty much OK simply maintaining proper ID and fabbing an intake that wraps around the cylinder rearward while trying to keep the length as short as practical?

gkamysz

My Feedback: (19)

Yes just hold the diameter and avoid tight corners. Think bent tubing rather than 90° mitered elbows. If welded elbows are the choice, maybe a three piece to avoid restrictions. These things are fairly low on power so they won't be that critical.

spaceworm

Maybe take a look at Toni Clark's website. He has adapters, both cast and fabricated for 2 strokes using Walbro carbs. Perhaps not to buy from him, but to get an dea of what is available and what works. Again these are for two strokes, but I think four strokes are less sensitive to intake and exhaust header length. I could be wrong, though. I have several Ryobi 4 strokes, I am going to convert for plane use and I am looking for a Stihl 4-Mix to convert. Good luck.

http://www.toni-clark.com/ftp/cat_11..._1_2010_en.pdf pages 84 and 85

Sincerely, Richard

av8tor1977

My Feedback: (6)

Just yesterday I found this site and page that might highly interest you....

http://www.scooterparts4less.com/ALL...e_Manifold.htm

AV8TOR

svejkovat

Thanks, great ideas there.

Here's an elbow from Clark's. Gives me the right idea.



And here's one from my pile'o'parts bin.



And here's a really very ideal one from that scooterparts link. It appears as if it might actually tuck the carb up behind the jug. And only twenty bucks shipped.





Unfortunately, the last two are 20 to 22mm ID. I suspect (I don't have the engine yet) that the GX35 is 10 to 12mm. Would that be a problem?

And the flange holes are typically 90 degrees "wrong" on these for use with the GX35. They're also pretty unlikely to be spaced appropriately. So I'd be paying for the cast 's' shaped tube section and refabbing appropriate flanges and alum tigging them on.

But these are giving me excellent ideas.

How do Iupload images from my desktop here? I"click here to upload images and files"and browse then upload them and get an OKmessage finally.... but Ican't seem to navigate to them afterward. Where are they located now? On other forums I've generally seen a "place file inline" option to put the uploaded image in the post. Ok, nevermind that... they didn't show in the post "preview" but are now below. How would Imove them from there to the main body of the post?

spaceworm

The scooter parts are a lot less expensive. I wonder if they have details on the dimensions?

As you have seen the pics do not show up until after you have posted. Sometimes it's a crapshoot as to what I end up with

Good luck.

Sincertely, Richard

Nosedragger

Going from the small carb bore into the larger intake is going to cause problems. I wouldn't move carb unless you don't have a choice at all.

spaceworm

Perhaps borrow on Toni Clark's fabricated adapters. It should not be to hard to cut and drill a couple of brass plates to match your engine and carb inlet/outlet manifold shapes. Then select some brass or copper plumbing connectors the size of the carb (or engine) throats and hard solder or braze the shape you need. It may be that 1/2 inch copper pipe fittings would be close to the 10-12 mm size mentioned. Once you find that it will work for you, you could refine the design as needed. Should be an interesting exercise.

Sincerely, Richard

av8tor1977

My Feedback: (6)

Whether you fabricate, or find something close that will work you should be fine. I would try to keep the i.d. close to original, and the added length down as much as possible, but it should all work with no problems. Don't forget to isolate the carb in some manner frorm heat however. Even if you just use a plastic, nylon, delrin, or a resin/phenolic spacer. I once made an isolator out of 3/8" plywood coated with epoxy. If you don't, the heat soaked carb will cause the engine to be difficult to start once it is warm.

AV8TOR

spaceworm

Had that problem with my '60 Corvair. Also had the carb ice up in the winter, until I found it had carb heat like an airplane via a shutter on the cooling air duct[8D]

Sincerely, Richard

pe reivers

Often some intake duct length (like 8 - 10" ) will add a substantial amount of power in the 9000 rpm band.

If the engine rev's lower, the duct must be longer. (duct is total length, including air filter fittings)

Nosedragger

no matter what the standard formulas recommend going beyond 5" from valve to atmosphere is a waste of time

pe reivers

how do you know?
Here is a successful 50cc Honda effort. Remove the long duct, and the bike ran 10mph less top speed. Tests on a 125 twin also showed reduced power on shorter intake headers.

Nosedragger

Apples and oranges. 50 cc with a different bore and stroke is not even close to the GX35.Different cam timing,different intake valve size, different intake runner size,different carburetor size etc.

I get my info from Honda mini four stroke guru who has been doing this longer than anyone and has a dyno.Too bad hes only doing private work or I'd have him come over and educate you.

pe reivers

hear-hear. So you have no personal experience, nor any knowledge to boast, yet you say that intake duct length does not matter.
Here is a link to a tuning calculator if you want to do some research yourself.
http://www.mez.co.uk/mezporting/exhaust_length.html

Nosedragger

never said it doesn't matter,I said the above information was incorrect for this engine and that my information comes from someone who knows what they are doing. I've owned 3 engines from this guy I have plenty of first hand experience with them and with my engine builder, I posed the question and posted the reply here. If you had any experience or working knowledge of this engine you would understand where you are wrong.

captinjohn

PE, It would seem to me your calculator is best used for making a tuned exhaust for a 4 cycle engine. Also the motorcycle shown is kinda a downdraft carb setup with probably a exhaust tuned to it.In other words not much help in a lot of engine conversions. Capt,n

gkamysz

My Feedback: (19)

Any engine can have a tuned intake and/or tuned exhaust. Mild timing and mild valve sizes will only lead to mild response to the tuned intake. Just because we don't see many on model airplanes doesn't mean it doesn't work. Virtually, every production automobile engine since the 90's has some form of tuned intake, although we in the US were a little slow in the game.

I've caught a few posts here and there about a guy who builds Honda GX engines. What do you use them for? Does he have a website?

You can only teach somebody what they are willing to learn.

Greg

pe reivers

John, the engine in the picture is s downdraught, the carb however is a standard Mikuni updraught slide carb. However that does not change the way the intake power tuning works. I also worked on 125cc twin Hondas in a go-cart with open exhaust pipes of tuned length. Being OHC engines the carb tube is high! In this engine as well, using shorter intake tubes caused quite severe power loss. One has to consider here the 2nd, 3rd and 4th order resonance. First order is very long, and very peaky. IIRC, the 3rd order length was 75000/rpm (inches) rule of thumb. The higher the order, the less gain, and the less peaky.
BTW, The calculator also has intake duct length as output, and responds to engine cam timing well.

Edit:
some corrections: see http://www.irday.com/html/Engine%20f...0412/8777.html
I was wrong about the harmonic order. Here are the correct data
"
Probably the single most important induction parameter is cross sectional flow area, versus cylinder displacement.

There will be an optimum flow velocity that produces a torque peak somewhere in the Rpm range. Below peak torque velocity cylinder filling poor. Above the peak, pressure drop will reduce cylinder filling.

This peak can be moved around somewhat by also tuning the runner length, but the peak cannot be moved very far doing this. If the valves and ports are far too small, the engine is not going to make massive top end power, no matter how short the runner length is made!!
Pressure waves in the intake runner can be used to shift the torque curve around, and the intake length can resonate at more than one Rpm. It can be tunes to a different harmonic.

Second harmonic tuning is the strongest where length in inches = 108,000 divided by Rpm.
Third harmonic tuning = 97,000 divided by Rpm
Fourth harmonic tuning = 74,000 divided by Rpm
Fifth harmonic tuning = 54,000 divided by Rpm.

While second harmonic tuning is the strongest, you can only get one peak, and the runner length can be inconveniently long. It works best on very peaky high Rpm engines.
On a sports car something much shorter that uses fourth and fifth harmonic tuning is better because it will give a better spread of torque and be less peaky. For instance a ten inch runner will peak at 5,400 7,400, 9,700, and 10,800 Rpm. The peaks get higher at higher Rpm.
Having a double hump at both 5,400 and 7,400 might work rather well on a road car. It might also work well on a motorcycle engine peaking at 10,800 Rpm.
But tuning the runner length is only going to be effective if the induction flow area is sufficient to support enough flow at that Rpm.
"

Nosedragger

No website and he doesn't offer services to the general public anymore. I modified them to mount on aircraft after he modified them for more power.
Being a former customer I still have his email address.So I asked.
The application(aircraft,boat,land,powertool) has nothing to do with the reason a 8-10" intake won't work as expected, if it worked the stock engine on the trimmer stick would have a 8-10 long intake on it.
It has to do with stock cam timing being so short.
And such a small diameter intake, frictional loses in such a long pipe have a negative effect.

captinjohn

I have seen many Honda cycles & other brands over the years...seems like no long intakes on them, and the RPM they reach is pretty high.Cross section and cam duration works ok with the right exhaust system. Capt,n

Nosedragger

My email paid off and I'm meeting up with the Honda guru this weekend.

I don't see how anyone could make a recommendation on intake length without knowing the exact cam timing.
Most of the formulas out there are outdated.Hes using wave tuning software and computational fluid dynamics to arrive at the starting point for intakes.
The only thing I can share with you guys is to make a test intake manifold that you can shorten 1/4" at a time and test.

gkamysz

My Feedback: (19)

What is the application too put so much effort into a relatively low performance engine?

Nosedragger

He said ground vehicle, could be buggy,truck,crawler etc In a crawler application a two stroke can not match the performance of a four stroke.They don't have the bottom torque.

Its only low perf when its stock, a full mod is another story.

pe reivers

If max power is not the objective, There will be no use in intake duct tuning, because low end torque is very much more depending on valve timing and other construction differences. Engine size has nothing to do with the policy behind the rationale.
A garden appliance engine BTW is not the best engine for powering vehicles (air or land), whichever way you look at it. They are intended for mostly stationary rpm use, and do a more than decent job of that. Best torque, or best power implies some form of competition. That would need, beside the fancy software for duct tuning, machining possibilities to "pocket" the valves for better gas flow pulses and many other  techniques. The standard cams also would probably have to go. From this point of view, it is hardly appropriate to talk about a single object like duct length. The whole package must be considered, all directed to the goals that are envisaged.