Every 318 (LA) budget rebuild I read about, in preparation for mine, usually involves increasing the 1.78" to 1.88" intake, but leaving the exhaust alone? So why not increase the stock 1.50" to 1.60"? It makes little sense to breathe better in only one direction, right?

Any thoughts on this matter, or on rebuild directions?

I have a 72 cuda, and have decided on my path (see below). Feel free to comment and suggest or advise, as long as you stay away from the token comments about going to a 340 or 360, which is dumb considering I already have a 318.

Anyway,
This is a cruiser that I want a bit more out of.
I love the idea of a stock looking 318 that is torquey as hell.
349ci stroker kit is down around $1300.
That will take my CR to 9.5:1
Cyl hd pors worked a bit with 1.88" / 1.60" valves at about $400
Holley 650 is new and laying around
Edelbrock performance
Headers
904 trans with shiftkit
8.75 rear with 3:23

I am also wondering what exhaust system is recommended.


Thanks all

 

Sounds just right, I would always upsize both valves. I always liked the look of the 370c.i. 3.79" stroke that's available.

I should also suggest Larry Shepard's book How to Hot Rod Small Block Mopar Engines.

 

The reason that the intake is much MUCH more critical than the exhaust is PRESSURE.

The only thing forcing air in through the intake on a non- super/ turbo engine is atmospheric pressure, and on a carb engine, THAT is reduced by the restrictions in the carb, air filter, manifold, AND the fuel itself

The exhaust on the other hand has a nice high pressure, high temp FORCE behind it.

 

Thank you for the suggestions.
I asked about the valve size because I recall reading about someone experience with upsizing just the exhaust valve for better torque, instead of losing critical velocity with larger intakes alone. Thus I was confused why many would consider one, not the other, yet the two together seem to make more sense. You move more air into the same space, you then need to move more out.
Or am I missing something very fundamental here?
Thanks.

 

cuda -

You're sorts on the right track.

Think about it this way, when the A/F mix ignites, it pushes the piston down. That's where the torque comes from. The exhaust stroke just gets rid of burnt gases through the exhaust valve and in itself doesn't do anything for power production - UNLESS it produces a resistance to flow, robbing power. That's why you want a free flowing exhaust system down stream.

Sure making an exhaust valve bigger provides less air resistance, but too big a valve will actually slow down exhaust velocity. The engineers who design this stuff are supposed to figure out the optimal size for a given application.


So, the question becomes: when is the exhaust valve big enough to allow minimal resistance to exhaust flow, while still allowing enough exhaust velocity? When it is, going bigger really doesn't help.

Usually the safest bet is to pick up a pair of HP 340 heads (I did that on my 318 years ago).

At least that's my understanding.

If you're not planning a race only rig, swapping out the 904 for a 727 might be something to think about too.

Archer

 

Don't use the 2.02" valves on a 318 as they get close to the small bore cylinder wall and limit flow. Look at the Hughes main girdle to strenghten the two bolts, not alot of money and it works like a crank scraper too.

 

There sure seems to be a lot of opinions regarding performance updates, and I appreciate them all, but have to sort through them.

Couple things,
I see no benefit to a 727 trans, which is heavier and slower and far more money...considering I already have a great 904.
Also, everyone that runs the big valves in the 318, seems to suffer from low end. Not good for a cruiser.

I was talking with a machine shop today and he suggests just working those 318 heads, with performance grinds, cleaning up the bowl areas, possibly bigger intake valves, and a cam and intake and carb. Also port match the intake to head, and tune it all good and she should run strong. I also want to push up compression with the right valves, thin gasket and decking. I think I can hit 9:1.

I hope that is all good for about 100-120hp.

 

cuda -

The 904 is lighter than the 727 and that makes it a good choice for 1/4 mile runs, if you beef up the small block, you might start having problems with the 904 as a street car. Yes, you can always use it, and not worry about upgrading until something happens. If it doesn't - nothing lost.

Can't tell you about after market valves / sizes, but I never had a low end problem with the 340 heads on a 318, a mild cam and headers. Valve size and flow is related to cam size. The trick is get everything to match. Yes, that might include the torque converter stall.

If you deck the heads too much + a "thinner" gasket, you might have to work the intake manifold too. Not a big deal, just don't be surprised if it happens.

Archer

 

Excellent point, for every .010" milled from the head .0095" should be removed from the intake face of the head.

Did you happen to see this discussion. https://moparforums.com/forums/f81/b...or-help-10440/