520 Contential updraft vs down draft.

[rowdy1]

I have a true aircraft 520 w/ the double down intake and updraft carb. Has anyone ever really done the testing to see which carb setup works best? I realize the updraft is how they run on aircraft, but is there any advantage to a manifold with a Holley or other carb set up.

[twister69z]

Continental never made a 0-520 ,only conversions done became 0-520's the original carburetor on the 0-470 is converted for the extra cubic inches. Larger Venturi modified nozzle and changed economizer settings make up the changes. The only advantage of down draft I can see is cost of the carburetor and ease of adjustment. Downfall is when you shut it off you have no mixture control to use the idle cutoff.
Twister69z

[rjonesboy]

I think what twister is saying is that there were no true aircraft carbureted 520s. Only injected. IO-520, GTSIO-520, TSIO-520, etc. Somebody put a carb on it at some point.

[helicsher]

My gitso 520 is all aircraft with 10 to 1 pistons 285 hp
I have a holly 750 on it, and it is bad a$$

[fletchster76]

Granted I've got a 520 conversion but cylinders are the same. I have had both down draft and up draft on mine. I felt like the downdraft (had a holley 500, then a edelbrock 600) rapped up quicker, but the down draft ran smoother and burnt less fuel. No difference in HP I can tell.
helicsher, if your engine has 10 to 1's in it, pretty sure it's got more that 285hp. A 520 with 7 to 1's is usually 285hp and with 8.5 to 1's it's 300hp. Those 10 to 1's are custom made the way I understand and very pricey.

[rowdy1]

I don't know about all that, I just want to know if there is an advantage or dis-advantage to running the Holley mounted in front of the engine as opposed to the up draft carb mounted in the front of the engine below the oil pan.

[GMAC 76]

I would think the down draft automotive carb would allow a bit more of fine tuning ie: bigger jet selection, different accelerator pumps and cams and more cfm choices to really tune the carb to the specific engine....with the updraft it kind of is what it is with not a lot of modification choices available...I have worked on many 4 and 6cyl engines lyc and cont with the down draft carbs and they seem to work well when tuned right plus a rebuild kit for an automotive carb doesn't cost $4-$600..

[John Fenner]

I would think the down draft automotive carb would allow a bit more of fine tuning ie: bigger jet selection, different accelerator pumps and cams and more cfm choices to really tune the carb to the specific engine....with the updraft it kind of is what it is with not a lot of modification choices available...I have worked on many 4 and 6cyl engines lyc and cont with the down draft carbs and they seem to work well when tuned right plus a rebuild kit for an automotive carb doesn't cost $4-$600..

What he said plus better fuel economy.

[rowdy1]

Better fuel economy on the up draft or the down? I have never known a Holley to get better fuel economy in any application!

[John Fenner]

When properly tuned, a Holley can get great economy. And if you were to go with a 4v, I'd recommend a 550 cfm with the rear metering plate kit to properly jet it.

[rowdy1]

I did a "cfm calculator" on 520 CID @ 3100 rpm, yesterday. The charts show a "stock" engine only needs 490cfm. Even a "performance" engine was less than 550cfm.
What it did not account for was how much fuel was needed to cool an air cooled engine. I had the brilliant idea to mount 1 very small 2bbl carb on each side of the motor. Something like those 300 cfm rodchester carbs that came on older sbc. I would leave the cross over in the back to equalize the sides. I don't know if it would work, but my theory was to shorten the intake runner length, for better velocity and less fuel fall out. I can fabricate and tig weld aluminum so it is not a difficult project.

Again I don't know if it would work, but it sure would look cool!

[trailerdon]

When I ran the cfm calculator on the jegs site; 520ci @ 3000 rpm it came up with 650 cfm carb. That is what's been running on my gitsu for two years. very strong, and fairly good on fuel. "about the same as a friends injected 0540". I would think a two bbl would leave a lot on the table.

[John Fenner]

Yes, a 650 will work, likely without tuning, however, a 450 properly tuned will give a harder snap and if the secondaries are tuned right it will pull hard on top end while using less fuel than the marvel ma4-5 or 6