gx270open build


New member
open to suggestions on how to build a gx270 to make decent hp without breaking the bank. I would like to stay around $500.
Advance the timing, Shave the head .100 or so, mild porting, get a better carb, billet rod. Dont know whats available for the 270 but a cam would be nice , around 300 lift or so. If a flattop exists, try that. Compression first then flow. Good luck.


New member
These are pretty good motors. I've ran them.

13hp GX390 carb bored to 24mm and GX390 insulator

GX390 cams fit these motors. So any cam made for GX390 will fit. Mod-1 cam from Dyno cams on a cast core should be a good choice for you.

Put stainless 36mm intake and 31mm exhaust. That's 13hp size valves. Get them at vegascarts.com. Also get the dual springs and keepers and retainers from them as we'll.

ARC racing has billet rod and flywheel. Advance the timing.

(Optional) 1mm ring and light weight Honda 270 piston from NR-Racing

Get the .010" steel head gasket and have the piston .025" in the hole for .035" total clearance.

(Optional) You might want to mill the piston flat and deck the block. And have it .025" in the hole.

Get a good port job on the head and I would run some reinforced 1.1 ratio rocker arms from NR-Racing.

Put it together and see how much room you have to mill the head by using modeling clay to measure the valve clearance.

This should put you around about $600. But the flywheel and rod really cut your budget in half.

You are located in Kentucky. I am too. I've been messing with the GX270 and GX390 for a while now. 3 years now. I'm working on a GX270 right now that I've already put $1000 in it just so I can really stomp everybody at my local garden tractor pulling club in their 8hp altered class.
Are you still running the GX270's if so you still agree with the budget build in this post are do you have some better choices now


New member
shaving the head .100" is that not a lot? If i'm not wrong, thats about 2,5mm? We shaved ours 1,2mm and used the thin head gasket which puts you at about 2mm, do you think running 2,5mm shaved and thin head gasket is viable for endurance racing? Thats a lot! running .280" cam and stock rockers!

alvin l nunley

New member
shaving the head .100" is that not a lot? If i'm not wrong, thats about 2,5mm? We shaved ours 1,2mm and used the thin head gasket which puts you at about 2mm, do you think running 2,5mm shaved and thin head gasket is viable for endurance racing? Thats a lot! running .280" cam and stock rockers!
just willy-nilly cutting the head can get you into trouble. Two things, valve clearance and compression ratio. An increasing in compression is no good if the fuel you run does not have the octane to match. Knowing the final compression ratio is important when choosing the correct fuel. Air density will tell you what jets to run.

PD Power

New member
I believe that you are in over your head in regard to making power in the 4 cycle engine.

Compression ratio, as a result of displacement and combustion chamber size, is a nice thing to know.
However the compression that counts, is that which is in the engine when it is running.
It is called cylinder pressure. Cylinder pressure is altered by a number of variables in the 4 cycle which are unrelated
to mechanical compression ratio.
You constantly say that compression is the "holy grail" for making power. This is an erroneous statement.
In the field of compression, dynamic (running) compression is what matters.
There are many factors which much operate in harmony to make for a powerful 4 cycle engine.

Give serious thought to the fact that "the whole is greater than the sum of it's parts."

PD Power

New member
jsstump said:
Compression first then flow. Good luck.
This is very bad advise. Flow is always the determiner of power potential. Compression ought never to interfere w/flow.
Further.....compression is radically different depending upon the cam profile.

You really need to study this stuff, from a book like "The Racing Engine Builders Handbook". By Tom Monroe. HP Books, is the publisher.
This is by far the best I've seen on engine building, and how stuff like head ports, cams work, and why.
Good luck.

71 chevy

New member
I'm in the compression first then flow camp too.
My opinion is with higher compression you can run a bigger cam and still have the same pressure. Bigger cam means more airflow, So that can negate done of the effects of head flow.

Obviously both work hand in hand but imo compression comes first.
This is very bad advise.
Over a year to get this sage introspective. No actual advise for the original poster as to his question however. In this particular style engine that the poster asked about. the compression ratio (static and dynamic) in stock form is very anemic. Not from poor flow but from a huge combustion chamber and very dished piston. As was asked by the poster "how to make decent HP on a budget", first raising the compression will give him the quickest and cheapest gains. Its very simple and cheap to raise the static compression on these. It already will flow relatively well. On a budget with a big block GX engine, raising the compression first will give much larger gains than increasing the flow will. On a big block GX, you wont see the gains a big cam, porting and a bigger carb have to give until you get the factory 8 to 1 compression ratio up some.
Interesting you mention Tom Monroe's book. I met him while working at one of the companies he mentions and frequented in his "acknowledgement" section of the very book you mention. I will quote from it: "Within limitations, increasing compression is the single most effective easy way to increase the power output of a naturally aspirated engine." (page 4) While I don't discount the benefits of camshaft grind/timing, airflow, chamber efficiency, or port flow and understand (and as you recommend, study) these things, for the question at hand, I would use the $500 budget to raise compression before flow. Good luck.


New member
Giving this a bump because I am wondering if there has been any developments in parts since 2015.
A flat top piston and head with a smaller chamber are my interest.