my 250

[Anonymous]

Hey I started pulling down my 250 this arvo in plans for a rebuild and to find what was done to it.
found that the head is ported(dont know how far) and has been fitted with double springs.
Pulled the head off to find an ACL race series gasket
I checked the valve sizes they are IN 46mm and EX 41mm
Had a look at the pistons to see if they were O/S but had nothing stamped on them so I checked the bore and it was 96mm
I dont know the sizes of the std gear so I dont know if they are O/S or not.

 

[addo]

That sounds like 10 over. What are the head numbers (casting marks), which chamber, how many CC's? Just curious. It's got 1.81 and 1.61 valves, BTW.

Take some pics as you go.

Adam.

 

[alloydave]

Do you want to check the bore size again ...96mm ?
standard bore is 93.472 mm .I havent seen any 2.5mm oversize pistons before

cheers Dave

 

[Anonymous]

I checked the bore again this time with verniers and it looks to be 95.5mm
So thats like 80thou?
Addo
the head is cast iron and has 17 M6 cast next to No1 intake spring
it is an open chamber and I will cc it after i give it a bath and will also check the tension of the springs.
Im going to pull the bottom end down tonight to check the specs of the crank,rods,bolts and cam.
The cam is fairly wild but I will change it for some thing more suited for nitrous use.

 

[xctasy]

Same as the head from my XC Falcon!

The bore is 60 thou over.

 

[Anonymous]

60# ???
Isn't 40# equal to 1mm?

 

[Anonymous]

Pulled the sump off 2 nite all fairly standard stuff
Crank,rods and bolts are std but have been balanced,crank has 20/20 grind
Pistons are ACL duralite
It has a JP hi vol oil pump,the sump has been modified to clear the pump
Still need to pull the cam

How much will the duralite pistons take before they will fail?
GASSED... :nox:

 

[xctasy]

3.76" = 95.5 mm = 80 thou douh!!!!!

 

[Anonymous]

Is it posible to go further with the bore if I need to?
Or would that leave me with a wall thickness like a piece of 1 ply dunny paper

 

[addo]

Let's work through this...

Standard bore 3.680 inches. 3.680×25.4=93.472mm

Given bore 95.5mm. 95.5÷25.4=3.76 inches (approx)

3.76-3.68=0.080 inches. This is diametral.
Divide by two: 0.040 inches.

You can go another 20 thou on the bore if you need to.

Now we can all get some sleep!

 

[Anonymous]

smart ass chippies

 

[xctasy]

Hey addo, we need some oversize 80 thou over pistons made of Hardwood. We will be leaning on this engine with Nitrous, a turbo, and the forged ones are too expesnive. So how 'bout it!

 

[Anonymous]

So the duralite's are that bad
Ill get the father in law to spin up a forged set for me then
I got a 250hp kit so I might as well use it

 

[addo]

I would consider the ACL's. See what the engine builders you trust think. There are a lot of pluses in a motor running hypers, that you lose with forgies. Bore flex is more of a concern. Sure you can't get away with the existing?

 

[Anonymous]

OK I got a new cam and Iv got some pics but i dont know how to post them
Computers arn't a strong point

 

[Anonymous]

I found this when pulling the cam
The last owner had changed the timing chain but fitted the wrong washer on the cam bolt causing it to bottom out
I could turn the gear an 1/8 of a turn before the cam moved

 

[Anonymous]

I finally cleaned my head up and it has 47cc chambers its an open chamber iron X flow I have no idea on what the STD cc is but im sure that someone here would.
Would someone know what the comp ratio would be it has flat tops and std 250 rods.
Cheers GASSED250

 

[xctasy]

53 cc was what it was as a stocker. It's been shaved about 30 thou.

 

[Anonymous]

Would that increase comp by much?

 

[xctasy]

The old XC ran an 8.8:1 compression. The details on piston trough and gasket thickenss I don't have. I also don't know how the low compression regular fuel version used for taxis was made. It was most likey putting deap trough pistons from the 250 into the 200 sixes.

Anyway, 6 cc is like going from 8.8:1 to 9.4:1. Each 1 cc taken off is 0.1 points up, in simple terms.

I use the http://falconperformance.sundog.net/compcalculator.asp calculator. A very good piece to use.

The figures I put in were

Input Values:
Number Of Cylinders 6
Bore Diameter [inches] 3.68"
Stroke Length [inches] 3.91"
Combustion Chamber Volume [cubic centimeters] 47cc
Head Gasket Compressed Thickness [inches] 0.41
Head Gasket Bore Diameter [inches] 3.81"
Piston To Deck Clearance [inches] 0.025"
Select Piston Type Flat Top Dished Domed
Dish/Valve Relief/Dome Volume [positive cubic centimeters] 22cc
Volumetric Efficiency: 60% 65% 70% 75% 80% 85% 90% 95% 100%
RPM: 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000


Results:
Displacement, [cubic inches] >250
Displacement, [liters] >4.1
Static compression ratio >9.4:1
Cubic Feet per Minute required @ 3500 rpm, [cfm] >202
Estimated Horsepower >@ 3500 rpm
Assumes altitude of sealevel, barometric pressure of 14.696 and 60° air supply to carburator >122 bhp