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carb choke size advice for 427cc cylinder

Hi everyone,

The posters here are fantastic and I thought I would tap into the collective wisdom here for advice on this one. I'm about to fire this engine but am finding myself at a stalemate in terms of where to start with a choke size for these 40DCNF carbs. Stock was 32mm and the displacement and several other things are very different now, mainly displacement which is up from 2.926 to 3.419L. Should be a fantastic motor, but the Ferrari community very rarely ever does anything like this so there's not a lot of experience out there in baselining this arrangement. Matter of fact, I'm the first person to ever do this setup which is essentially a Ferrari 360 crank in a 308 block bored to 83mm.

Here's what we have: 427cc cylinders, 44mm (stock is 42mm) nailhead style intake valves (2v head), ported intakes (flows well), feeding a 79mm stroke engine with a 83mm bore flat plane Ferrari V8. The intake cams are .427 lift with 249 duration @ .050. 10.5:1 compression. I've set the cams at 108 exhaust LC and 104 intake LC. The pistons are pretty much flat top and I'm seeing 185psi cranking compression before running the engine. Hemi plug placement on these heads.

The engine is a street engine, but of course a 'hot' street engine. It's going to operate within stock rev range which is up to 8k which should obviously be plenty. I'm thinking of running 36mm chokes but I'd like to lose as little as possible at the bottom end as I know with these cams vacuum signal at idle and below 3k is sort of an unknown. I'm not sure what the Porsche crowd does with a cylinder setup like this and your knowledge and input would be much appreciated.



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Last edited by lvporschepilot; 03-13-2012 at 09:22 AM..
Old 03-13-2012, 06:01 AM
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The first thing that comes to mind for you is to look to the motorcycle industry.
You could look up 500cc engines both street and race, and find out what they use.
Carb size and jetting from them will get you in the ball park.
Bob
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Old 03-13-2012, 09:52 AM
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It all boils dwn to CFM
Old 03-13-2012, 09:54 AM
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Originally Posted by HawgRyder View Post
The first thing that comes to mind for you is to look to the motorcycle industry.
You could look up 500cc engines both street and race, and find out what they use.
Carb size and jetting from them will get you in the ball park.
Bob

I'm checking with a Ducati forum as well. Not many people know this stuff...... I think I may end up on a dyno for a day or so working this out.

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It all boils dwn to CFM

Yes, but it's not just CFM. Port velocity and carburetor signal with these cams make this somewhat of a dance. I know some of the smaller 911 hot street/race motors must have displacements about this size so I'm wondering what works for them.
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Old 03-13-2012, 10:59 AM
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Venturi Size=20√(V/1000 x N/1000)

V = Cylinder capacity in cm^3

N = rpm for peak horsepower

Main jet = 4 x Venturi

Air Corrector = Main Jet + 50

Not perfect but it will run and give a point from which to start.

Assumes one carb choke per cylinder

Last edited by chris_seven; 03-13-2012 at 12:07 PM..
Old 03-13-2012, 12:05 PM
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The other basic rule for engines: a smaller throat size will give better low end torque.
The larger the throat size (up to a point) will give better high RPM and HP.
Bob
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The other basic rule for engines: a smaller throat size will give better low end torque.
The larger the throat size (up to a point) will give better high RPM and HP.
Bob
Bob, I agree I like the venturi to be a maximum of (carb bore size 1.25)

Some people say 1.1 but this seems to close to a tube to have much depression at the main jet until gas speeds are high.
Old 03-14-2012, 10:46 AM
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Thanks all. Sticking with 32mm then. I might play with 34 and 36 but from what I'm hearing it's not the best choice for this application.
Old 03-14-2012, 10:58 AM
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I drove this engine over the weekend. Really stunning power with excellent torque. Feels more like a 360 than a 308 now. I kept rpm below 5k rpm as I still have Brad Penn break in straight 30w in there but I'm fairly certain the rings are about seated now.

Regarding fuel delivery, this engine has tons more vacuum. I noticed the 32mm chokes were running very rich as the larger vacuum signal is just sucking more fuel from the carb. I could go leaner mains and idles, but rather than that I'm going to go a bit high and try 36mm chokes and see how much that decreases the signal. That should lean out the jets that are currently installed a good bit also raising CFM which will probably yield a bit more power and torque at peak airflow which should be around 5600-5700rpm. Fabulous power though.
Old 03-19-2012, 04:50 AM
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