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Old 02-10-2020, 07:32 PM
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Default GX390 Build

In 2013 I purchased all the parts to build a GX390, and they've been sitting in a box ever since. Im working on four other projects at the moment, but I really would like to get this together so I'm trying to fit it in.

Here is most of the parts for this build.



Got the rod, piston and camshaft from Mike Clements. His parts are real nice. The connecting rod is longer than stock which is something I always try to achieve because it reduces stress.
I've had it explained to me how it reduces stress a couple times, but it sounded real technical. something about rod angles, anyway I just use the rule the longer the rod the better, and I stick to that.



To reduce drag on the internals I filed on the dipper to the shape of streemline tubing.
Crankshafts do not need shovels full of oil dumped on them to work. This only makes drag which slows the engine down considerably. Just a mist of oil is perfectly fine.
Here is the dipper before and after filing.





Here is the shape of the streamline tubing I copied. This tubing has 90% less wind drag than round tubing.
In past expirences the more I've filed off dippers the faster engines run until the dippers get so thin you would think they might break off.

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Old 02-10-2020, 07:35 PM
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how much did all of this cost.
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Old 02-10-2020, 09:15 PM
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how much did all of this cost.
In the neighborhood of 5k. I know thats too much for most people to care, but for me I really enjoy building things so please don't hold it against me.

I only tell you for your info in case you would like to do something similar.

---------- Post added at 07:23 PM ---------- Previous post was at 07:18 PM ----------

Did some of the same work to the crank and camshaft as I did to the 420 built previously.
Ground off the helical cut gears that cause major drag and replaced them with straight cut gears, then made the valve timing adjustable. I lightened the cam gear even more than before.




Wont be running a counter balance shaft so I balanced the crank with a proper weight ground from .250 steel plate.




Ground the flywheel flush.




Then welded the weight on. This actually solves several problems. It gets rid of the counterbalance drag and balances the crank shaft to the rod and piston. It also makes the weight of the flywheels about even. When the flywheels arnt even it causes vibration problems.



---------- Post added at 07:38 PM ---------- Previous post was at 07:23 PM ----------

I never paint cases because it holds in heat. Heat in most cases is not a good thing. Metal expands when it gets hot causing parts to get out of spec which causes drag. Enough drag to slow the engine down 1000 rpm or more.

In effort to combat heat I use several tricks. One is to coat the parts in heat dissipating ceramic, and another is to coat the exhaust port in a ceramic thermal barrier.
Here is the stuff used.



This is what it looks like



This is the thermal barrier on the exhaust port.



---------- Post added at 08:15 PM ---------- Previous post was at 07:38 PM ----------

I ported the head by making everything properly sized.
Valves are 38mm and 33mm. Ports and valve seats are 80% the size of the valves.
The intake valve should be 31% 32% the size of the bore
Welded in the low side of the exhaust port to straighten the port out.
Here is a comparison to stock.
Looks funny to have a lip like that in the port, but the benefits out weigh the negatives by a substantial margin.



Ground the valves and valve seats.



Ground the 90* edge off the exhaust valve so the escaping gas will pass by easier. This one move gains 2 to 3 percent. Here is a pic of the rounded edge.



Welded up the squish band which made it a 30cc head. The squish should be about 30% to the bore.



Added a compression release right next to the spark plug to avoid what happened with the high compression Predator 420cc.



Built a steel flange for the exhaust port.



Built a proper length intake tube for a 40mm carburetor. This is something I've mentioned before. Building a proper length intake can yield a 13% increase in power.

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Old 02-10-2020, 09:23 PM
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Looking forward to seeing this develop. Nice looking motor!
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Old 02-10-2020, 09:42 PM
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The piston was 15 thousands shy from the top of the cylinder so I milled .015 off.



Now the piston comes right to the top. Thats one of the main rules. It has to come to the top. If it doesn't it will cause detonation between the piston and the squish band.



Made a head gasket out of .040 copper which will add 7cc. Forty thousands will make a safe distance from the piston to the head while using an aluminum rod.
This will set my compression to 11.5:1



Using only ceramic bearings on this.





This is about all I can think of right now. I'm still sorting through pictures.
I have an issue with a piston ring I will try to sort out tomorrow.

Really trying to set time aside to get this built. Its been sitting too long.
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Old 02-11-2020, 07:06 AM
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Nice nice nice nice NICE!

Looks like a killer build, and it looks like you really know what you're doing!
What kind of experience do you have?

Really? No stroker crank? ARC stroker cranks are already aero-edged, billet aluminum, plus the extra cc's.
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Old 02-11-2020, 08:08 AM
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Anything I know about engines comes from a friend of mine thats in his mid 70s. He is one of those people that has an unbelievable amount of knowledge stored in his head. When it comes to engines, racing or history it's truly impressive.
Ive been hanging out at his shop for the last 20 years bugging him with any questions I can think of. His answers can go for hours.

With strokers Ive been taught a longer stroke is almost never the way to go to get HP. I know this is the exact opposite of what most people do.
When you make a longer stroke engine you may increase the cc's and gain some hp. but it puts more stress on the crank, rod and piston which is not what you need on a performance engine.

I always try to achieve a shorter stroke. This reduces stress and allows the engine to rev higher which means you can lower the gear ratio to increase the torque to the wheels while still achieving the same top speed.

Here is the the combo I strive to get. 2.5:1 bore to stroke ratio with a connecting rod thats 2.5 times the length of the stroke.
I never actually get to those numbers do to limits in the engine, but thats the direction I push.

Making HP comes from an engine that can breathe. This is where the large bore and short stroke comes into play. The larger bore allows for larger valves and ports, and the shorter stroke allows the engine to fire more times per second.

This is the basis that formula one uses for there engines which are the most efficient combustion engines in the world.
Here are there numbers. 98mm bore with a 40mm stroke and a connecting rod length of 2.56 times longer than the stroke.

I will mention it again because its been ingrained in me over the years. Real horse power comes from an engine that can breathe.
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Old 02-11-2020, 08:20 AM
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Very interesting! Glad you can absorb that information from your friend! I know the type, I go to him whenever I have a performance question.

One thing you didn't mention that I read in a David Vizard book, is that a longer stroke also puts more stress on the rings and bore. Some of those strokers out there can make some earth moving torque, though!
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Old 02-11-2020, 06:44 PM
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I’ve done 64mm stroke and 70mm stroke and it doesn’t affect the engines ability to rev higher. Bigger stroke just makes more torque, especially down low. Seems like the cam and springs determine max RPM more than anything else. I’m curious about the flow numbers on the head. I don’t know if it was you or someone else but this is the second time I’ve seen the intake filled in like that.

Also on breathing, the longer stroke makes the piston dwell more on TDC and BDC, which allows an extra amount of time to get a bit more exhaust out and a bit more intake in. Also makes more vacuum, so you get a better fuel signal at the carb.
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Old 02-11-2020, 09:05 PM
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Quote:
Originally Posted by Mammoth View Post
[/COLOR]

In the neighborhood of 5k. I know thats too much for most people to care, but for me I really enjoy building things so please don't hold it against me.

I only tell you for your info in case you would like to do something similar.

---------- Post added at 07:23 PM ---------- Previous post was at 07:18 PM ----------

Did some of the same work to the crank and camshaft as I did to the 420 built previously.
Ground off the helical cut gears that cause major drag and replaced them with straight cut gears, then made the valve timing adjustable. I lightened the cam gear even more than before.




Wont be running a counter balance shaft so I balanced the crank with a proper weight ground from .250 steel plate.




Ground the flywheel flush.




Then welded the weight on. This actually solves several problems. It gets rid of the counterbalance drag and balances the crank shaft to the rod and piston. It also makes the weight of the flywheels about even. When the flywheels arnt even it causes vibration problems.



---------- Post added at 07:38 PM ---------- Previous post was at 07:23 PM ----------

I never paint cases because it holds in heat. Heat in most cases is not a good thing. Metal expands when it gets hot causing parts to get out of spec which causes drag. Enough drag to slow the engine down 1000 rpm or more.

In effort to combat heat I use several tricks. One is to coat the parts in heat dissipating ceramic, and another is to coat the exhaust port in a ceramic thermal barrier.
Here is the stuff used.



This is what it looks like



This is the thermal barrier on the exhaust port.



---------- Post added at 08:15 PM ---------- Previous post was at 07:38 PM ----------

I ported the head by making everything properly sized.
Valves are 38mm and 33mm. Ports and valve seats are 80% the size of the valves.
The intake valve should be 31% 32% the size of the bore
Welded in the low side of the exhaust port to straighten the port out.
Here is a comparison to stock.
Looks funny to have a lip like that in the port, but the benefits out weigh the negatives by a substantial margin.



Ground the valves and valve seats.



Ground the 90* edge off the exhaust valve so the escaping gas will pass by easier. This one move gains 2 to 3 percent. Here is a pic of the rounded edge.



Welded up the squish band which made it a 30cc head. The squish should be about 30% to the bore.



Added a compression release right next to the spark plug to avoid what happened with the high compression Predator 420cc.



Built a steel flange for the exhaust port.



Built a proper length intake tube for a 40mm carburetor. This is something I've mentioned before. Building a proper length intake can yield a 13% increase in power.

How do you determine the proper intake size? Iím doing a 14.5 ci small block with a 34mm carb, and Iím going to have to make my own intake.
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Old 02-11-2020, 10:13 PM
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My understanding its not the longer stroke that makes more torque its the displacement being larger that makes more torque.

It would be the same if you took a short stroke engine and bored it out. The end result would be more torque.

Shorter strokes have been proven to yield higher rpm's.
My guess why you didn't see a difference between those two different stroke cranks would be due to this oiling systems design and how bad it is.

The dipper system design is terrible due to the huge amount of drag it produces.
It will rev to a certain height and then it hits a brick wall. The oil is flying around in there ricocheting off everything and then the crank has to cut through it.

Thats how they came up with the idea of a dry sump system. They realized the oil crashing around inside stopped the engine from revving any higher. Once they dry sumpped it they broke through that barrier reaching rpm's never achieved before.

Ive done quite a few things to combat the drag in this oiling system like streamlining the dipper or getting rid of the counter balance.
I ground the low oil sensor mounts inside the cases so the oil doesn't ricochet off and hit the crank and cam. Here is a pic before and after.





One of the reasons I chose to get a billet side cover is it doesn't have fins on the inside for oil to bounce off like the stock cover.





Progress was made today after getting the rings sorted out. I was able to install the rod/piston for the last time hopefully.



Got the rockers installed.





Determining the length of the intake tube is quite simple. Take 86000 and divide by the rpm you want the engine to run at.
Sample, 86000 divided by 6500 equals 13.2. That means the intake should be 13.2 inches from the valve to the back of the carb.

This works by timing the back up of the incoming air in the intake port.

The intake valve opens and the piston starts to move gases through the intake port.
Then the valve closes and the moving air jams up and compresses. This is where the length of the tube comes into play. The length of the tube determines the timing when the gases compress, and you want it to be fully compressed right when the valve opens again so it shoots into the cylinder kind of like a mini supercharging effect.

The shorter the intake tube the faster the timing. The longer the tube the slower the timing.

Here is the flow chart numbers comparing a stock head to this head.

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Old 02-12-2020, 05:22 AM
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Quote:
Originally Posted by Mammoth View Post
My understanding is its not the longer stroke that makes more torque its the displacement being larger that makes more torque.

It would be the same if you took a short stroke engine and bored it out. The end result would be more torque.

Shorter strokes have been proven to yield higher rpm's.
same displacement longer stroke (smaller bore)
makes a tad more torque though than a shorter stroke (bigger bore)
reason is simply the lever on the crank being bigger
and by ~ the same amount gained in the combustion
the lever's working against you during compression
that's why longer stroke enignes tend to slow down more than short stroke
during compression.. hence short stroke yields higher rpms

the sweet spot is dependend on compression ratio AFAIR..
too long ago to remember correctly
but I have something like "1:1 bore/stroke ratio for a 9:1 compression is about in the right realms" in mind *shrugs*.

Anyways... this is a very interesting project
I still can't help but wonder how the equivalent of an additional matchstick dragged through some oil could cause a monster like that to run any slower
the massive inertia should deal with that on it's own (at least in my head )
Ah well you definitely look like you perfectly know what you are doing,
so I just get back to my bucket of popcorn and watch you
getting closer and closer towards the 45horses that are hidden
somewhere in a gx390

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Old 02-12-2020, 08:03 AM
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I was in such awe with the pictures and what not read only a little of this and it sounds bad ***.

What are you hooking this thing up to? A wood spliter?
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Old 02-12-2020, 08:41 AM
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More like a boulder splitter!

This thing is gonna be bonkers when it's done!
But yeah, what kart is it going on?
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Old 02-12-2020, 05:16 PM
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Love it, remember building 327's because being square bore/stroke you could rev to the moon and beat the 350s and big blocks...great build
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Old 02-12-2020, 08:27 PM
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This will be going on a cement mixer for sure.

Actually was thinking of installing it in a mini jet boat. Anybody could use one of those in their life.
If you haven't seen one, youtube mini jet boat.

The longer stroke topic is quite interesting to ponder.

You may get a little more torque if the displacement stays the same. I can understand more leverage on the power stroke and that same leverage works against the net torque on the compression stroke. What it ends up to be may be a positive torque gain.

All the info I've put together says the winning combo is: short stroke, large bore, with big valves and large ports.

The larger bore allows more gas to be compressed and also allows for larger valves which goes along with larger ports, and the short stroke allows it to fire more times per minute with less stress.
I like the sound of that.

Im going to use this plug with the gap at 25. This is a hotter plug that usually gives a 1 to 2 percent gain.



Decided where the height of the oil is going to be. Right under the cam gear, but not touching. This will put the dipper in the oil .450 thousands.



Added a dip stick to the oil plug.







Started on the valve timing, but I ran out of time.



Here is a article from Hot Rod Magazine that talks a little about the loss due to the fins on the inside of the side cover.




Anybody can study the effect of oil on the internal parts with simple easy test.
Fill the oil to the top of the dipstick and then do a top speed run with your speedo app on your phone.
Then drain the oil to the very bottom of the dipstick and do another top speed run.

Comparing the two top speeds should give an interesting result.
Note, this test only works if you have the governor removed.

Long ago I took a Kart out for a midnight rip, and I remember thinking the whole time that this kart seems way faster. I ripped it for several hours before taking it home and checking the oil. It had oil in it, but it was so low there was none on the dipstick.
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Old 02-13-2020, 07:49 AM
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Interesting, never knew that oil windage could make such a difference!
From what you've said, I'm liking big bores even more!

Personally I like Autolite plugs, and Paul Dempsey also agrees they're good for karting.
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Old 02-13-2020, 07:12 PM
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Got the valves timed which makes it a good day.

I set the intake to 102.9 and the exhaust ended up at 113.25.

The intake lobe duration was 259 and and the exhaust was 256.

The camshaft has .350 thousands lift and the 1.2 ratio roller rockers lifted the valves .420 thousands.
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Old 02-13-2020, 08:11 PM
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How did you set the valve timing?
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Old 02-13-2020, 08:38 PM
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Quote:
Originally Posted by Whitetrashrocker View Post
How did you set the valve timing?
When I converted the cam to a straight cut gear I slotted the bolt holes which makes the gear adjustable.

Under the button head bolts in this pic the holes are slotted which allows the gear to be advanced or retarded.

When you get a non adjustable cam from the factory your stuck with whatever timing the cam was manufactured with, which can very.


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