Mirada 318cid - Heads and exhaust manifold?

[Buick_T]

First off, the car specs:
Dodge Mirada 1983
318cid (stock) / 904 / stock 8.25 diff.

I have been rebuilding the engine for street and leisure cruising (the whole car actually) and so far I have done/bought these:

-Cylinders bored 0.02"
-New pistons 0.02" oversize
-Crankshaft polished (stock size)
-Bearing surfaces polished
-New cam/crank bearings

The heads/cam:
-Bought: Comp Cams xe256h (20-221-3)
-New lifters, push rods, timing kit etc.
-The heads (other one at least) had this number stamped: 4027183? (looks like 4027188...)
-Took the heads to shop

Original Carter Thermoquad
Original 4-barrel intake

For the heads the mechanic/owner (who has built many street and racing engines) recommended milling, new valves (stock 1.75" / 1.5"), new valve guides and grinding the exhaust side ports larger for better performance.

Here the problems begun:
The shop has been making up excuses for why the heads are not done (other projects, health issues...), it's been 10 months and now I called and told I'll fetch them back and go elsewhere.

The questions which have risen:
-Is the recommended XE256H a good cam for stock engine?
-Does it need new valve springs?

-Are the stock heads good enough? The small exhaust ports are my biggest concern. Should I look for other heads? Try to get someone else do the grinding as the previous shop promised? Or are they good as they are? Of course new valves, guides and milling the surface will be done.

-I read the (Grand Cherokee) Magnum exhaust manifolds should flow a lot better. And if they would be out of a 93-95 G.C. they don't interfere with the firewall unlike the 96-98 and for example Pick-up ones.
-Is there anything to gain with better exhaust (manifold) with stock small exhaust port heads?

I had really readied for refurbing the heads, not buy new ones.

Sorry for long post, but I am lost with the subject...

 

[Oldiron440]

Here’s what I think in general,
New cam - new springs especially when everything is fifty years old.
I like using new valves because once they are ground they sit lower in the head and they can alter head flow.
As far as porting or head work any machine shop can take a few thousand's off your heads but I’m on the fence about cleaning up the ports, I usually unshroud the valve in the head, but I’m not sure with a mild cam it would benefit you. But it is possible to do with a die grinder either electric or air powered and a few flapper wheels that you can acquirer locally. You can look into it on YouTube and get an idea if it’s something you want to try it’s worth maybe ten hp on a stockish engine. I would recommend looking into steel head gaskets to help with compression.
It doesn’t sound like the place your heads were at was doing you any favors good luck moving forward.

 

[Mikes5thAve]

You should have switched to the later 80s swirl port heads but hopefully the machine shop did some nice work to what you have.

There are some old posts about the magnum exhaust manifolds incuding some flow numbers for different models.

 

[Duke5A]

I'm guessing with you being in Finland means parts are hard to come by. So telling you to find better heads is not going to work. Those heads are probably 4027163 castings; which are open chamber, 1.78/1.50 valves and tiny runners. Ideally you'd want the later 1985+ 302 castings that are closed chamber. They don't flow any better, but you can use a piston with a quench pad and run a higher compression ratio. Your factory heads will work, but not much more than 250HP or about. I wouldn't spend the money to have 1.88/1.60 valves installed as the runners are too much of a bottleneck.

Don't worry about Magnum manifolds. I forget which side, but one of them is going to hit the firewall. Besides, factory 318 manifolds will flow enough to support 300HP and you're not going to be anywhere near that.

It shouldn't take a machine shop more than a week or two to turn around a set of cylinder heads.