kartkevin39
Member
Just got a new GX390 and was wondering how much I might be looking at to make it an OPEN engine and who could help me do so?
Gx390
- Thread starter kartkevin39
- Start date
jsstump#70
Member
"Open" is a very subjective term but lets take a look. Piston and rings, $150, Rod $150, Flywheel $200+, Cam $135, Pushrods, $20, Valves, springs, retainers $100+, rockers $200, machine work(porting, 3 angle grind, head shaving, unshrouding valves, honing, clearancing) $200 to $500 depending. Some added stuff may be bracing, welding, billet side cover, tall valve cover to clear rockers, all this can be $200 to $500. Now you need a carb. That's super important and is a huge part of making power on this and most other engines. $250 to $400. Might need a special adapter for it to work. $30 to $50. Header, $50+
Obviously, if you have the parts and the machines and the knowhow, these prices will be less. Middle of the road on the prices I listed is $1950. This is ON TOP of the cost of the engine. Its expensive. That is why so may are using MX style engines when rules allow. They can be had for lees or equal to this and will turn 10,000+ rpm all year. That 390 you build WILL let go. It might just be a beast and may last awhile but it will let go in spectacular fashion one day. Its a 3600 rpm engine being asked to do a lot. It will run but its not bullet proof. More "open" you make it, the shorter the life span. Good luck.
Obviously, if you have the parts and the machines and the knowhow, these prices will be less. Middle of the road on the prices I listed is $1950. This is ON TOP of the cost of the engine. Its expensive. That is why so may are using MX style engines when rules allow. They can be had for lees or equal to this and will turn 10,000+ rpm all year. That 390 you build WILL let go. It might just be a beast and may last awhile but it will let go in spectacular fashion one day. Its a 3600 rpm engine being asked to do a lot. It will run but its not bullet proof. More "open" you make it, the shorter the life span. Good luck.
kartkevin39
Member
Lol Spectacular fashion! Alright, thanks for the info. I was thinking the Yamaha would be better for open either way but got this engine for free and its brand new. Got it from work. I'll find something to do with it or sell/trade it. Thanks again.
When you say open how much power and RPM are you talking? I use a gx390 because I need about 30-35hp for my application and it's a reliable (but slightly heavy) way to do it compared to a screaming GX200. The motor isn't too stressed at that level. If I was needing 40-50 or more HP it wouldnt be a reliable or cost effective option. It's not even cost effective now compared to a MX motor to be honest.
kartkevin39
Member
I see. I was just asking being that I have one and just seeing what it would take.
CarlsonMotorsports
Site Supporter
Very good advice from both of these guys.
I can build a very reliable 32-33 CHP GX390 engine that turns 7200 rpm all day long. No, it will not hang with a MX450 engine on a big track, but it sure beats twisting a stroked animal to the moon for about the same cost ($2800.)
If you're needing cheap and reliable -- it's hard to beat a MXer that's designed and built to run 100,000 miles on pump gas.
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Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
28 years of service to the karting industry
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
I can build a very reliable 32-33 CHP GX390 engine that turns 7200 rpm all day long. No, it will not hang with a MX450 engine on a big track, but it sure beats twisting a stroked animal to the moon for about the same cost ($2800.)
If you're needing cheap and reliable -- it's hard to beat a MXer that's designed and built to run 100,000 miles on pump gas.
-----
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
28 years of service to the karting industry
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
kartkevin39
Member
Thanks for the advice. May try to sell/or trade
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So what kind of HP can a GX390 make at Max 5500rpm running 93 Octane with the best Parts for Endurance?
CarlsonMotorsports
Site Supporter
That's a very open ended question.
First, you'd have to take into consideration the parts used in your build.
Stroke crank, compression piston, valve sizes, carb size, cam lift & duration, etc etc.
Running gas is not a huge deficit to alcohol (although I prefer alcohol as a race engine fuel) when it comes to making power.
That will not be your limiting factor. Detonation at higher compression ratios, less heat control, etc is more of an issue with highly modified race engines on gasoline.
Then consider that everyone's dynos will read differently....some builders offer extremely inflated numbers as part of their sales pitch or simply to dismiss the fixation racers have on "numbers."
Throwing a 50 CHP number out there may sound real impressive, but even if it's accurate...If it's in a build that's not reliable or at a usable rpm, then what's the point? My point is, it's pretty easy to build a reliable 30 CHP that's somewhat affordable. It's quite another thing to build 50 CHP. (ie it's not easy OR cheap!)
-----
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
29 years of service to the karting industry
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
First, you'd have to take into consideration the parts used in your build.
Stroke crank, compression piston, valve sizes, carb size, cam lift & duration, etc etc.
Running gas is not a huge deficit to alcohol (although I prefer alcohol as a race engine fuel) when it comes to making power.
That will not be your limiting factor. Detonation at higher compression ratios, less heat control, etc is more of an issue with highly modified race engines on gasoline.
Then consider that everyone's dynos will read differently....some builders offer extremely inflated numbers as part of their sales pitch or simply to dismiss the fixation racers have on "numbers."
Throwing a 50 CHP number out there may sound real impressive, but even if it's accurate...If it's in a build that's not reliable or at a usable rpm, then what's the point? My point is, it's pretty easy to build a reliable 30 CHP that's somewhat affordable. It's quite another thing to build 50 CHP. (ie it's not easy OR cheap!)
-----
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
Carlson Motorsports on Facebook
29 years of service to the karting industry
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
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Not really really open ended. I said, "So what kind of HP can a GX390 make at Max 5500rpm running 93 Octane with the best Parts for Endurance?"
So you have a Base GX390 a Set CC to use (88mm x 64mm) 389cc to work with. NO Big Bore or Stroker, just 389cc!
I didn't say for Racing, I said for Endurance!
I said using 93 Octane Gas, so that set's the limit on what Max CR you can use, guessing 11.5cr, probably Max CR I would want to run anyway.
You have a Set Max 5500rpm to use to pick the CAM.
So You Pick the CAM based on this Max Rpm, Valve Sizes, Carb Size, and other HD Internal/External Parts needed for Endurance!
Think of it being used at Continious 4800-5000rpm on a Mini Air Boat or Snow Glider, Hence Endurance is the key Factor. Yes, I know there are Bigger, maybe even better Engines out there, I want to keep it as light as Possible and the GX390 being the Smallest I want to use.
So Carb, CAM, Valve Sizes, CR, and HD Parts needed for Endurance, Roller Rockers, Push Rods, FlyWheel, Rod, Bolts/Studs.
I did forget to mention I want Electric Start. Will be using one of them light Weight Lithium Batterys. A 240CCA 2.4lb one turns my Yamaha Vmax 600(95hp) Snowmobile Engine better than the Old Style Lead Acid Battery ever did.
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A Stock GX390 is 11.7hp@3600rpm with 8.2cr is 75% Volumetric Efficiency! If just turned 5500rpm at 75% Volumetric Efficiency = 19hp. So you have from 75% to 100+% Volumetric Efficiency to work with and still be durable!
I believe the Stock CAM is like .260 Lift. So a Higher Lift CAM = +HP
So in using 93 Octane, with the Base Stock 8.2cr you have to maybe 11.5cr to use for +HP.
We have the Stock Muffler, to using a Tuned Header Exhaust for +HP!
We have the Stock Air Cleaner, to an Improved Air Filter like a K&N to use, for +HP!
I don't know what Size MM the Stock GX390 Carb is, but since it was Designed for 3600rpm, probably is to Small for 5500rpm, for +HP!
Then you have Stock In/Ex Valves ?? mm vs Over Size Valves to use for +HP!
All using the best HD Parts to make it Durable at Max 5500rpm. So just 10% more than Stock, I don't think so with the Mods you can do.
On my 2 Strokes used at 6500rpm using 11.5cr with a Tuned Pipe, I figure it takes 7cc to make 1hp. A +1.0cr = 2hp, so a +.5cr = 1hp!
So a 2 Stroke used at that 6500Rpm, with 11.5cr and a Good Tuned Pipe can make, 389cc/7cc = 55.5hp. That comes in at 92% Volumetric Efficiency. My Stock Skidoo 670 using 11.5cr, Dynoed 115.7hp at 7750rpm, comes in at 93% Volumetric Efficiency. So I would guess a GX390 4 Stroke 389cc to be about Half that 55.5hp maybe 28-35hp@5500rpm.
A Skidoo/Rotax 380HO(368.4cc) Twin using 11.2cr, 30mm Carbs, Stock was 48hp@7000rpm. With just a better Tuned Pipe it Dynoed 57.26@7000rpm. Made around 52-53hp at 6500rpm. 368cc/7cc = 52.5hp@6500rpm.
Example: A Simonini Victor 1, 2 Stroke Singles comes in using just 9.5cr and 6500rpm:
362cc = 44hp@6500rpm
384cc = 48hp@6500rpm
400cc = 54hp@6500rpm
CarlsonMotorsports
Site Supporter
How long do you wish it to endure at increased HP?
I consider this an "open ended" question as to how best to build your engine.
You mentioned nothing in your post about application, intended use, or how long you wish it to last.
Performance is almost always a trade-off with durability.
I'm sorry I wasn't able to help you.
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An Engine doesn't know what it's on, only People Do!
Engine Life(Hrs) has more to do with the Person who Builds it, the Person using it, and Person Maintaining it, using Good Fresh Octane Gas with Good Oils, and doing Good General Maintenace, Plugs, Timing, Oil Changes, etc!
Old Stock Cont. 084ci, 4 Stroke Motors, off Military 10kw Generators used on Kitplanes at 3600rpm made over 1000hrs, 20+ years ago. Today, you have better Oils, Metals, Technolgy, to use in these Motors. Simonini 2 Strokes have a 600hr TBO, Hirth 2 Strokes a 1000hr TBO, Rotax 2 strokes 300hr TBO, all using Higher Rpms. Rotax 912(80hp) 4 Stroke 1500+hr TBO used at 5500rpm continious.
Yes, I did say, "Continuous 4800-5000rpm on a Mini Air Boat or Snow Glider, Hence Endurance is the key Factor.
You have Set variables Max 5500rpm, a Max Safe CR for 93 Octane, Max 389cc, everything else is up for whats best to use!
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Cavitation is for Boat Props used in Water, not other Type Props. Tip Speed is when they go Super Sonic is.
Whether used on an Air Boat, Hover Craft, Snow Glider, Ultralight, etc. you would use a Prop Belt or Gear Reduction Drive. Like for these Honda's and Briggs they use on Avg a 1.8 Ratio. So 5500rpm/1.8 = 3,055.5rpm for Prop Speed. Every Size Prop has a different Max Tip Speed where they go Super Sonic. A Longer Prop turned at lower rpm is more efficient than a Shorter Prop turned at High Speeds. Lenght of Prop used dictates Thrust made at a certain say engine is Designed for say Max HP@5500rpm you would Pitch it for 5400rpm/1.8 = 3000rpm for Prop. Example: a 60" Prop Piched at 14, Pitched for 5400rpm = Static thrust = 318.07 pounds. Required engine power = 33.282hp. As Prop Speed goes up, so does HP needed. So you can Play with Prop Speed using different Ratio's to Tuned it. Your Engine is Designed for say Max 5500rpm. The Highest I have ever seen any of the Briggs Big V Twins 993 is 6200rpm. Then after takeoff they reduce rpms to usually 75% Power. Rpm used depends on if you want Speed or best GPH use.
Prop Calc.
http://godolloairport.hu/calc/strc_eng/index.htm
Here is a Briggs 623 Big V Twin with an ACE Belt Reduction Drive. The Highest Stock 993 HP I have seen is 40hp@3600rpm and they have turned them up to 6200rpm. That 2 Place Snow Gilder with a Rotax 447(40hp@6500rpm) is 80+mph. So for my 61 Year Old Bones, 28-35hp, 50mph is plenty.
I think these Honda GX390's Improved for more HP would be good Engines to repower some of these Old Snowmobiles that originally used 12-35hp 2 Strokes. I just bought a 1972 Skidoo 292 TNT Single Cylinder Sled, Stock 22hp@6500rpm for some Vintage Racing.
Snow Glider with 100hp BMW Motor Cycle Engine.
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Yes, very True, but with every Engine, there is a Point on How Big a Carb you can put on it, a Limit on Max In/Ex Valve Sizes you can use, and using 93 Octane there is a Max Safe CR you can use, probably 11.5cr. Most Skidoo's use 11.5cr, some 11.2cr or 11.8cr for 91 Octane, and 93 Octane is the Highest I have available at the Pump around me. So there is a Point on all Engines where they can become maybe over Carbed, or over Cammed.
GX390 389cc at 5500rpm at 130% Volumetric Efficiency makes 49cfm, which a 28mm Mikuni is 52.1cfm.
Here is a 1971 Skidoo/Rotax 246 Blizzard 29hp used a 38mm Carb. 292 Blizzard 31hp, 340 Blizzard 36hp, used on 300lb Sled.
