340-2 to 340-4 conversion(?)

HarryLeSabre

HarryLeSabre

Member
Hi,

first of all, I'm pretty satisfied with my 340-2 - so far its the perfect cruising engine for my taste (and car).

But you know ... more hp is never a bad idea, and if its easily obtained it gets even better ;)

So what would I need to convert my 2 barrel engine to have 4 instead?

- 4 barrel carb (like this one https://www.ebay.com/itm/402675496824 ?)
- matching intake manifold - are those easy to find or rather rare?
-...

Is this all it takes or does the 340-4 have more compression and/or another camshaft as the 340-2? Is the ST300 gearbox able to handle this increase in hp or do you have to "beef up" that one as well?

And in general, is this conversion a good idea at all? Is there a big difference between 340-2/340-4 power-wise and is it still as suitable for daily use as the 340-2?

Cheers,
Harry
 
Harry the 2 barrel 340s had an advertised compression of 9:1 while the 4 barrel 340 had a 10.25:1 advertised compression ratio. The difference was in the pistons. The 10.25:1 had a smaller dish in the piston.

The 4 barrel 340 manifold is pretty common and you should be able to pick one up fairly reasonably. There is not much demand for them.

You will have your choice of carbs. The rebuilt OE will be the easiest to set up, but It will not be that difficult to use an aftermarket Edelbrock in the 600-650 CFM range. These will require you to modify the linkage to work with your transmission switch that is attached to the linkage. There is a Edelbrock part to accomplish this that has been discussed here before, but I cant seem to find it anywhere. Their AVS carbs are reasonably priced and easy to tune.

There is no difference in camshafts between the 340s and the ignition timing remains the same. The ST300 trans is plenty strong enough for this upgrade and will handle all of the 340 engine modifications you can throw at it.

I don't know if this will give you a truly detectable increase in performance, but the 4 barrel will sound like it is making more power when open. Your mileage should be similar unless you open the secondary's a lot. The larger CFM of the aftermarket carbs might provide the most gain and you will be ready for more modification down the road.
 
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