What is the Honda gx390 piston dish volume?

KartFab

Member
I've calculated compression ratio before, but i have one unknown right now: stock honda gx390 dished piston volume. I wasn't planning on modifying the compression ratio, but changed my mind after i put it in the block and don't want to pull it to CC the dish.

Heres my stats
Bore 88mm
Stroke 64 mm
Piston .005" in the hole at TDC
Head gasket .040-.043"
cylinder head volume stock ~ 41cc
piston dish volume ????

I am trying to figure out how much to mill off of my head to get a 10.2-11.0 MAX compression ratio (i want to keep pump fuel and keep it somewhat reliable).

I cant find anywhere where I can get the dished piston volume.

With that said, i looked at the vegas karts site, and they had a little guide, but i think its for the flat top pistons... (copied and pasted below)


Stock = 41cc (8.75:1 CR)
Milled 0.025" = 37cc (9.5:1 CR)
Milled 0.050" = 33cc (10.2:1 CR) *Max for 87 Octane
Milled 0.075" = 30cc (10.8:1 CR) *Max for 91 Octane
Milled 0.100" = 27cc (11.5:1 CR)
Milled 0.125" = 24cc (12.3:1 CR)
Milled 0.150" = 21cc (13.3:1 CR) *Maximum Recommended

So i'm thinking between .050" and .075" shaved should be good provided that the valves clear the piston. But id rather just do the math with the dish volume to be 100% sure. I'm going to measure the piston to valve clearance tonight before I get the head milled tomorrow.
 
Okay guys thanks! I did some backwards math with the summit racing static compression ratio calculator for hondas stock stats; results show that the dish should have a volume of around 6cc (again that's from my fuzzy math).

I had the head milled around 72 thousandths, then sanded it down on a flat fake granite coutner top to get rid of the mill marks. I CC'd the head and it came out to 31.3CC so, i'm satisfied with the results and it should hold me just at or a bit under 10:1 compression ratio so i can still use premium pump gas.

I looked around for some modeling clay, but could only find some spongy play dough. Gotta get the clay and will check piston to valve clearance before buttoning it up. Also, since I cut out some from the head, ill need to use those cut to length push rods. im thinking ill just take off 72-82 thousandths from the stock length ones. not sure about head gasket compressed thickness, its either .050 or .040.
 
I had a fully tuned gx390 about 4 years ago, i bought it ready done and developed it much further myself until it blew up in a big way.....that was a good lesson not to use a standard cast piston with 25-30hp

Anyway, the skimmed and ported 390 head was just too big in volume to get it anywhere close to 12:1 or more.....mine was 10.5:1....im told the trick was to start with a gx270 head which has a much smaller chamber volume.
 
Agree on the 270 head......Seek out a" double hump " clone head...You can get to 25-27 CC chamber volume easily with these. Definately opt for the MLS layered head gasket also.
 
I had a fully tuned gx390 about 4 years ago, i bought it ready done and developed it much further myself until it blew up in a big way.....that was a good lesson not to use a standard cast piston with 25-30hp

Anyway, the skimmed and ported 390 head was just too big in volume to get it anywhere close to 12:1 or more.....mine was 10.5:1....im told the trick was to start with a gx270 head which has a much smaller chamber volume.
What was the source of the failure if you don't mind me asking? My max revs are 5-5.5k per the cam. Cam specifies 11:1 ratio or lower, so Im hoping i could be in the clear there (my estimates put me at 9.8-10.3 without actually measuring everything i have in there). Oil clearance is good, and im only at 26 DBTDC, will be running 93 octane only. Im thinking this should keep pre-detonation risk at a minimum. Not sure how often i should check the connecting rod bearings though.

Agree on the 270 head......Seek out a" double hump " clone head...You can get to 25-27 CC chamber volume easily with these. Definately opt for the MLS layered head gasket also.

Its a little late for that, already got the head milled, and a MLS head gasket will put me over the edge for a compression ratio. Im not trying to go all out here on rpm's, just a mild build that gives me lots of torque. But i do like squeezing in those added efficiency modifications just because.
 
It was hard to conclude but i suspect the piston just failed due to fatigue, it literally disintegrated it to hundreds of tiny pieces, i also suspect the piston crown had been machined down so it was thinner than normal, a real shame as it had loads of billet parts like the rod and rockers, but a forged piston was not fitted, it would rev to about 7200rpm. Compression was about 10.5:1 which i was shocked at why it was so low when i measured it.

Also i seem to remember the builder telling me the block had been machined down too. So probably why i never saw a dished piston

Really i was always fighting it and when it did blow up i was quite relieved!
 
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