Honda GX390 Stroker Build

[bob58o]

Google Compression ratio calculator.
Need to know CC'c of chamber, but can figure it out using the 8.2.

Or give me 10 minutes

---------- Post added at 04:57 PM ---------- Previous post was at 04:46 PM ----------

I'm guessing 48cc chamber.
90mm x 0.045 gasket is 3cc
90mm x 0.010 gasket is 0.75cc

Can loose 2.25cc via gasket.

Turn 51.2cc into 49cc
And you get 8.5 instead of 8.2.

---------- Post added at 05:00 PM ---------- Previous post was at 04:57 PM ----------

Wait I'm late. But you need to mill!!

 

[KartFab]

hmmm. I'll do a little research and get back to you on this. i find it hard to believe that the compression ratio has not changed by increasing the bore and stroke.

Edit: stock head is 42cc

 

[bob58o]

https://www.rbracing-rsr.com/compstaticcalc.html
3.54" piston
2.75" stroke
0.01" gasket
0.01" deck height
0 cc piston top volume?
48cc chamber volume?

9.65 : 1 CR

For 10.2 : 1
You'd need a 45cc chamber and 0.010 gasket.
Or a 42.5 cc chamber and the stock gasket.

How much to mill to get the required volume in the chamber...
Well that is a Third Class in College level Calculus.
They have charts for the 196/212cc. Haven't found them for the 390/420.

I could be way off here. In a hurry before work!!!
So double check my stuff and call Dynocams and see how much they say to mill.

---------- Post added at 05:52 PM ---------- Previous post was at 05:43 PM ----------

I blindly say mill 0.075" Lol

Only one eye was closed. Lol

There is always octane booster

 

[Joe-405]

hmmm. I'll do a little research and get back to you on this. i find it hard to believe that the compression ratio has not changed by increasing the bore and stroke.

Edit: stock head is 42cc

The problem with more bore is that you just added to the combustion area instead of shrinking the area. And stroke is just how far it moves. Piston will stay in the same spot with more stroke.

 

[Poboy kartman]

The problem with more bore is that you just added to the combustion area instead of shrinking the area. And stroke is just how far it moves. Piston will stay in the same spot with more stroke.

Yeah....nothing in this post makes any sense. Stroke means the piston moves further up and down in the bore...how can it stay in the same place? And bore increases the volume of the charge which is now going into a SMALLER combustion chamber.

Let's just take my 351w for example....with a 3.85 stroker crank...stock rods and stock pistons....the piston would push the head off....however...with a shorter pin height and same bore...will put the piston in approximately the same location as the stock 351w non-stroker. Now boring....a .030 over will get you in the 10:1 neighborhood with aftermarket heads. (bear with me...I'm going off memory so numbers are not exact.)

Now bore .060 over and the CR goes way past pump gas with the same heads.

 

[bob58o]

https://gokartmods.com/2014/05/08/gx390-cylinder-head-milling-and-porting-the-cheap-way/

---------- Post added at 01:03 AM ---------- Previous post was at 01:02 AM ----------

I found it.

---------- Post added at 01:05 AM ---------- Previous post was at 01:03 AM ----------

42cc head??
1cc per 0.007" milled?

---------- Post added at 01:14 AM ---------- Previous post was at 01:05 AM ----------



http://karting.4cycle.com/showthread.php?31312-GX390-help
Here is a chart for head milling from a company call Vegas carts here is there link :http://www.vegascarts
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

 

[Flyinhillbilly]

Any time you increase the swept volume of the cylinder by stroke or bore you will raise the compression ratio. Poboy is right on the money.

 

[KartFab]

Valves, Retainers, and Electric Start

Got electric starter in the mail as well as stainless valves and better retainers. Took the engine to the engine shop and they will be boring it out this week.

 

[Jfive]

My 383 sbc was gonna be 10.2:1 but it ended up being bored. 060 and ended up a 10.4:1 388.

 

[KartFab]

Math Problem

Ok, I am in need of some help calculating the difference at TDC given the following:

Known:
1) aftermarket piston height at TDC +.093" above the deck
2) stock length rod
3) stroker crank of 69mm

Unknown: all variables the same, except stock crank. What is the differece at TDC between crankshafts 69mm vs 64 mm. Im thinking i just take the difference between the two (69mm-64mm=5mm) to get the excess stroke, and since stroke is total distance traveled, we are looking at piston top position +2.5 at the top and -2.5 from stock position. So all else the same, if i had put a stock crank in there, we would take off 2.5 mm(0.0984252 inches) from +.093 to get (.093-0.0984252= -.005" in the hole at TDC)

am i way off here or is this way of thinking correct?

 

[bob58o]

Makes sense to me. Half of the difference in stroke on each side.

But no stroker then
Could you cut the piston top that much?
I know people cut 0.060" off some Pistons.

NR has $20 flat top that maybe you could cut?
Have Joe-405 call Russel on the Bat Phone. Lol

---------- Post added at 09:12 PM ---------- Previous post was at 08:34 PM ----------

I know in my 420cc
With 90mm piston and 66mm stroke.
I should be able to get a 69mm (Edit ) 0.120" crank.
A 4.41" Rod
Aftermarket flattop cut 0.060" for zero deck height


What piston do you have?
What is compression height can stock?

 

[KartFab]

Well, the problem I'm running into is that the bore is set to the piston to get proper piston to bore clearance. The other problem I'm running into is that the stroker crank makes this particular piston pop out of the bore by .093" at TDC. A friend of a friend that used to make lots of Hp (yes I know sounds sketchy) said that the top should be cut .080" so the piston top doesn't heat up too much.

This flies in the face of arc racing that said this particular piston was a stroker piston. It's complete crap. I specifically asked them and they said "this stroker piston should fit .010" in the hole at TDC with the stroker crank. Since then, they re-titled the piston to not say -.050" on it and put a blurb in there saying it may need to be milled .050" to get the proper compression height....still calling it a stroker piston. Last time I checked, a stroker piston had a shorter distance between wrist pin and piston top. This one just seems like it has a dhorter skirt.

I'm just trying to make it work and I am worried that it already sticks out too far, which will probably heat up my rings too much as well. The tolerances are pretty tight anyway for the ring end gaps.

 

[bob58o]

Howard's outdoor power equipment cuts Pistons for $30.

 

[KartFab]

Howard's outdoor power equipment cuts Pistons for $30.

Dennis cuts them for free WOOT WOOT (we pay eachother in favors ok!)

Alright. Just got off the phone with ARC Racing. Kind of a depressing conversation. I need to sort through the haze. Here are the facts:

their 88 vs 90 mm stroker pistons are only different by .050 from wrist pin to top of piston. Confirmed by them measuring it. Their 88 stroker pistons will be near TDC as is, the 90 mm needs .050 off the top to sit at the top of the deck at TDC.

1) Option 1: They confirmed they sent me the right rod (I need to verify it in the box though). If wrong rod was sent, then it would account for the pop up. If not, see option 2

2) Option 2: They in fact sent me the 'wrong' piston (highly unlikely). I will measure distance between wrist pin and top of piston to compare against the stock piston that the engine came with to see if in fact the piston is the problem. If not, option 3

3) option three is to cry, and mourn the loss of the stroker crank, because the actual engine block was shorter than the rest of his GX390 buddies. If in fact option 1 and 2 check out, this leaves me with no other recourse (short of buying a real block ok maybe thats a possible option #4... ok now there is an option 4 (for now) But i hate that option. So yeah.... option 3 is what I really deep down inside think is the issue. but i really really want the issue to be option #2 or #1.

 

[KartFab]

They sold a stock compression height piston. They are making me a custom sized rod because they would rather do that than let me return their stroker crank yay!!!

Also, the rod ratio is about 1.6 so we are still above the danger zone of <1.5 rod ratio

 

[bob58o]

So will this rod put you flush?

What clearance are you talking about?

 

[KartFab]

This rod will put me out .043" at TDC. After I CC the head, I'll figure out how much gasket and milling I will need to get a static compression ratio of 10.5:1. I am leaning towards milking more off piston and having a thicker head gasket, so if I ever decided to switch cams it would be easy to adjust ratio by swapping out to a thinner gasket.

I don't know what clearance I was talking about, but I had to grind out part of the block (windage tray?) so the connecting rod and rod bolts wouldn't hit it. Also checked the rod to crank clearance. Seemed good, was well over .030", but didn't measure exactly.

 

[Poboy kartman]

This rod will put me out .043" at TDC. After I CC the head, I'll figure out how much gasket and milling I will need to get a static compression ratio of 10.5:1. I am leaning towards milking more off piston and having a thicker head gasket, so if I ever decided to switch cams it would be easy to adjust ratio by swapping out to a thinner gasket.

I don't know what clearance I was talking about, but I had to grind out part of the block (windage tray?) so the connecting rod and rod bolts wouldn't hit it. Also checked the rod to crank clearance. Seemed good, was well over .030", but didn't measure exactly.

Didn't measure exactly??? Say what???? Please explain...I mean....that's really cheap and easy!!!!

 

[bob58o]

Windage tray is what I call my seat at the Taco Buffet.

 

[KartFab]

Didn't measure exactly??? Say what???? Please explain...I mean....that's really cheap and easy!!!!

feeler gauge.

Windage tray is what I call my seat at the Taco Buffet.

That's what I call anything i sit on lol. I'm also an avid crop duster.

So here is the piston with the old rod. Ill be putting the new one in today, measuring pop up, then calculating what i need to get the right CR with a gasket and piston milling. Makes me glad i can keep this a stroker and rod ratio still around 1.6.

Also as an FYI, arc fixed the problem and im glad they went above and beyond to fix it as it was their bad. They have since fixed the description of the piston on their website, its just a standard 90mm. So max that can be milled is .060. I'm thinking ill get somewhere between .043 and .053" off when it is all said and done.