Box Stock

Choosing a carburetor for an open class without limits?

flattop1 said:
Not sure where animal came in . My understanding was a clone type engine .
still high compression and advanced timing is hard on rod bearings .
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The OP's stock compression ratio (8.89:1) should not damage the bearings. There are thousands of stock engines run on gas everyday and, as mentioned in another forum here (L206), some last 9 seasons on the stock components. I agree that the timing is fairly high. The prolong over rev beat the piston to death and I am sure the rod felt every pounding.
The Animal I mention is just for reference for a OHV motor running on gasoline.
 
laser3kw said:
I read in another post that you are using gasoline / petrol for your fuel?

In the other thread post, you state that your ignition timing is 30 btdc. I think that is too much. On the Briggs Animal, on gasoline, we found best timing was around 26 or 27 btdc with 87 pump octane.

add that to the engine specs and your actual (calculated) C/R is 8.89
bore 70mm, sweep volume 208 cc, head 18cc, head gasket(.010 thk) 1cc, piston dish 1cc and piston clearance (down the bore)3.4cc
From what I read between the two posts is:
you do not have "too much" compression - a 72mm flat top piston & .010 head gasket would be a nice 9.71:1. with a .020 head gasket, 9.28
you have too much ignition timing
you are running at too high of sustained RPM. The rod might take it but the piston won't (see your picture with the detached piston top ring set)
you may need slightly thicker oil -10w40
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Thanks for the advice, I'm trying to find a 72mm piston. I also have 2 heads with different combustion chambers. I have to collect all the parts together and then I will understand what compression ratio is obtained. Now the motors are disassembled. We use gasoline with an octane rating of 100, which is 93-94 by US standards.
 
good afternoon dear forum users. I ordered a 72mm piston, while it was on delivery I decided to build a second spare motor for a 70mm piston. I measured the clearance of the piston and the top of the cylinder, it is 0.0276 inch, the standard piston is 1cc, the thin gasket is 0.10. crankshaft 55mm
 
Displacement + combustion chamber CC's divided by combustion chamber CC's = compression ratio.

Combustion chamber CC's = head CC's + gasket hole CC's + piston in the hole CC's.

Convert cubic inches to cubic centimeters, (CC's) multiply cubic inches by 16.387064

Calculate cubic inches; (3.1415 X (radius divided by 2)) X stroke, (or thickness) in inches.
 
This is
 

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70mm x 55mm stroke (2.165") (212cc sweep volume) / .010" head gasket / piston clearance -.027" / 16cc head / dish piston -1cc = 11.77:1 C/R
good combo - set ignition to 27° for gas / petro - do not rev it past 7000 rpm (with billet rod) for prolong time (piston is too heavy)
 
laser3kw said:
70mm x 55mm stroke (2.165") (212cc sweep volume) / .010" head gasket / piston clearance -.027" / 16cc head / dish piston -1cc = 11.77:1 C/R
good combo - set ignition to 27° for gas / petro - do not rev it past 7000 rpm (with billet rod) for prolong time (piston is too heavy)
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not quite right, the correct number is 10.7, I was wrong, today I carefully measured the piston, and found out that the cavity volume is about 2.3cc. measurements showed that the diameter is 54mm, and the depth is 1mm. I think it's about 2.0-2.3cc.

I also measured the head gasket with a micrometer, it turned out .014.
 
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not quite right, the correct number is 10.7, I was wrong, today I carefully measured the piston, and found out that the cavity volume is about 2.3cc. measurements showed that the diameter is 54mm, and the depth is 1mm. I think it's about 2.0-2.3cc.

I also measured the head gasket with a micrometer, it turned out .014.
Click to expand...

Yes! when I plug in those numbers I came up with 10.76:1 (using piston dish volume 2.3cc)
I recommended 27° ignition for gasoline / petro based on experience dyno tuning with similar engine. It's a good start. I would also find the lightest weight piston for that type of set up. Makes life on the rod (billet with insert bearing) a lot easier.
My primary experiences with the longer sprint / road course tracks tell me that you can take away some ignition timing on an engine that will run at prolong higher rpm - like a long sprint track / enduro road course. Of course you have to jet it accordingly for best power.
 
laser3kw said:
Yes! when I plug in those numbers I came up with 10.76:1 (using piston dish volume 2.3cc)
I recommended 27° ignition for gasoline / petro based on experience dyno tuning with similar engine. It's a good start. I would also find the lightest weight piston for that type of set up. Makes life on the rod (billet with insert bearing) a lot easier.
My primary experiences with the longer sprint / road course tracks tell me that you can take away some ignition timing on an engine that will run at prolong higher rpm - like a long sprint track / enduro road course. Of course you have to jet it accordingly for best power.
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Thanks for your advice, now I'm waiting for the delivery of the 72mm piston. But for this motor, I still have an 18cc head, according to my approximate calculations, it will be the same 10.5-10.7. But I'll know for sure when I get all the parts.

I also have 2 carburetors, this is a PZ27 with an accelerator pump, and PWK30.
I decided that I would try different options on these motors.
 
good afternoon, dear forum members. I received a delivery of a 72mm piston. And after the calculation, I found out that with it the compression ratio will be 11.26:1, do you think this is normal? Not too much for our long distances?

Piston flat top 72mm
Crankshaft 54mm
Piston clearance to the top 0.039.
Gasket 1.35 cc
Head 16cc
 
What was the previous compression?
Do you think you've got the rod issue figured out .
My calculation comes in at 14.74 . likely i am wrong .
 
oneman said:
Piston flat top 72mm
Crankshaft 54mm
Piston clearance to the top 0.039.
Gasket 1.35 cc
Head 16cc
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what I use for calculations:
bore 72 (2.8346)
stroke 54 (2.126)
sweep volume 219.86cc
head volume 16cc
piston clearance (in the hole or pop up)(.039 in the hole) 4.03cc
piston dish volume (zero if flat top, stock dish 2cc) 2cc
head gasket volume (bore and thickness) (2.8346 x .013) 1.34cc
using those numbers with any decimal accuracy : 11.37 C/R

Myself, for racing where I will be at a prolong higher rpm and load, hot day (temperature) I might might consider a thicker head gasket.
.025 (.64mm) = 10.68
.020 (.51mm) = 10.96
your choice, jet and ignition timing accordingly.
After seeing the beating the piston took from prolong 8500 rpm (plus ign timing), I think the rod was collateral damage. The damage was from the piston melt down
 
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