318 to stock 340 power level questions.

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1968FormulaS340

1968FormulaS340

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So……

After having my car sit for 2 years because I haven't wanted to de-tune my original 340 (12.1 compression, solid roll cam, Holley 800DP, Victor 340) I have reached a decision.

Well, the decision was made for me. I just got a 60,000 mile, 1968, 318 that is complete (carb to pan with all brackets and manifolds) for free.


So here is my question.

This 318 build will be using (I have all the stuff below and want a stock looking engine compartment)……….

Stock 68, 340 intake manifold

Stock 68, 340 exhaust manifolds

Stock 68, 340 AVS carb

4 Speed

3.91 gearing

Stock tires

I will be cracking open this 318 and checking everything.

Using the 318 stock bottom end and heads what mods would get me to a stock 1968 340 power level (or close to it given what I will be working with).


I will be setting my original engine on a stand and want this engine for nice reliable street cruising.
 
cam. carb, maybe some headwork, intake. i think would bring you close.
 
So, you want to bump your '68 318 up from its stock ~230 hp to 275 hp (stock for '68 340)?

Other than the displacement, the 340 runs a 4v manifold and carb, hotter cam, and more compression.

Assuming you want to run the stock pistons and not rebuild the entire engine, I'd have the heads milled to up your compression ratio a little and up the size of the valves to 1.88/1.60. Go with a 4v manifold and carb. Add a hotter cam with a more modern grind and you should be close.
 
Have the heads rebuilt with performance in mind. Meaning 1.88/1.60 valves and a 5 angle valve job. Have them milled .030 on the decks and intake surfaces. Cut the spring seats for dual springs, and then stick a modern cam like the 268 series Lunati or XE Comp and you will be at the 300hp level thru the manifolds. Peice of cake. If you find int needs pistons, stick in some hyper flat tops with valve reliefs and you'rea cam change away from more power later.
 
The ports on the 318 heads are smaller than those on the 340. A 360 head will have bigger ports but it also has a larger combustion chamber. This will reduce the compression ratio. The 68 318 already uses the tallest piston in the "stock" piston inventory. If you want to recover compression, I'd suggest an aftermarket piston rather than milling the head and manifolds.
 
moper said:
Have the heads rebuilt with performance in mind. Meaning 1.88/1.60 valves and a 5 angle valve job. Have them milled .030 on the decks and intake surfaces. Cut the spring seats for dual springs, and then stick a modern cam like the 268 series Lunati or XE Comp and you will be at the 300hp level thru the manifolds. Peice of cake. If you find int needs pistons, stick in some hyper flat tops with valve reliefs and you'rea cam change away from more power later.
Click to expand...

THIS is how you do it! And you don't need alot of cam. The engine will produce.
 
2 Darts said:
The ports on the 318 heads are smaller than those on the 340. A 360 head will have bigger ports but it also has a larger combustion chamber. This will reduce the compression ratio. The 68 318 already uses the tallest piston in the "stock" piston inventory. If you want to recover compression, I'd suggest an aftermarket piston rather than milling the head and manifolds.
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Larger ports aren't necessary for what the OP is looking for. In fact, larger ports aren't really needed until you're up past the 350 hp mark. The 302 heads that everyone seems to like are small port heads. With some porting, these heads are capable of 400 hp.

And the larger 360 chambers would require a severe milling, or new pistons in order to maintain compression (I don't think the OP was looking to do a rebuild.)
 
Did the make a production 360 quench head for pre magnum motors that would keep his compression, already have 1.88/1.60 exhaust and large ports to match up to his intake and exhaust manifolds??

If so, could that save in his machining cost and be cost effective?
 
a large port 340 intake manifold on a set of small port 318 heads is going to cause a bottleneck regarding air flow
 
autoxcuda said:
Did the make a production 360 quench head for pre magnum motors that would keep his compression, allready have 1.88/1.60 exhaust and large ports to match up to his intake and exhaust manifolds??

If so, could that save in his maching cost and be cost effective?
Click to expand...

I think those would be the 308 casting heads
 
No, there chamber (308) is still open, but is a good flowing head.
 
autoxcuda said:
Did the make a production 360 quench head for pre magnum motors that would keep his compression, already have 1.88/1.60 exhaust and large ports to match up to his intake and exhaust manifolds??

If so, could that save in his machining cost and be cost effective?
Click to expand...


Factory didnt, but the Engine Quest Magnums are about as close as you can get. Thier bigger brother would be the RHS magnums.
 
moper said:
Factory didnt, but the Engine Quest Magnums are about as close as you can get. Thier bigger brother would be the RHS magnums.
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That was my next question. Do those head have pre-magnum intake bolt provisions?

Those would allready have the valve size and spring pocket machined, correct?

Are those cost effective and would you end up with a better head rather than sinking all that cash/effort into a 318 head?
 
IDk about the EQ heads, but the Indy RHS heads come in both sytle. "LA" and Magnum.
Unless your doing the work, I think the Indy head is a better deal and way to go about it.
 
I think my 318 has about 330 hp based on my 13.34 time slip. I used the KB 845 piston(domed) and opened up the"302" head intake port to 360 gasket size. I did some bowl work, but used the stock 1.78 intake valves. i used a comp XE-274S solid cam with 273 rockers. I also used a air gap manifold & a 750 carb. So if you use a zero deck flat top, "302" heads, a comp 268 hydraulic cam and a 650 carb I don't see any reason you couldn't make 275 hp, especially at sea level. My 330 hp guess is up here at 5800'. Maybe just a little work on the intake side of the heads would be helpful.
 
to autox,
better head for a bigger motor, yes.
but I don't believe so at this level of 318.

I have a 318 in the shop that has .020 stock pistons, 318 closed chambered '234' truck heads, 340 crane blue print cam w/4* advance, and headers, eddy performer/600 eddy carb.
It runs hard to 6100rpm then turns off like a light switch, but it make great power and breaks tires shifting with a 4 spd, it's my back up motor.

the heads were un touched too.

just keep it simple bro and you'll be good.
 
1wild&crazyguy said:
to autox,
better head for a bigger motor, yes.
but I don't believe so at this level of 318.

I have a 318 in the shop that has .020 stock pistons, 318 closed chambered '234' truck heads, 340 crane blue print cam w/4* advance, and headers, eddy performer/600 eddy carb.
It runs hard to 6100rpm then turns off like a light switch, but it make great power and breaks tires shifting with a 4 spd, it's my back up motor.

the heads were un touched too.

just keep it simple bro and you'll be good.
Click to expand...

So you keep the stock 318 valve sizes on those? If you are not messing with changing the valve sizes, I totally understand just working with the 318 head.

There was talk about changing the 318 heads' valves to 360 size and cutting the valve spring pockets. At that point, I thought there might be other viable options.
 
autoxcuda said:
So you keep the stock 318 valve sizes on those? If you are not messing with changing the valve sizes, I totally understand just working with the 318 head.

There was talk about changing the 318 heads' valves to 360 size and cutting the valve spring pockets. At that point, I thought there might be other viable options.
Click to expand...

yeah stock valves, they had a valve job at some point but not by me.

on the valve change out idea, it would only be a set of valves and some home bowl blending after the VJ, but yeah I can see it snow balling from there into guides,milling,etc..
 
1968FormulaS340 said:
I realize I will have a port mismatch with the cast iron 340 intake and the 318 heads but it is something I will have to live with.

So, for a came you are recommending one of these?

http://www.summitracing.com/parts/LUN-60403LK/

http://www.summitracing.com/parts/LUN-60402LK/
Click to expand...

Both of those are great cams. I agree with the other comments, the 60403 would probably be at the upper limit for an otherwise stock 318, a 60402 would be a piece of cake.

The 60403 goes to .513" of lift on the exhaust, make sure your springs can handle that. Seems like .500" is the cut off for quite a few spring options.

The factory didn't make any production heads with 360 sized ports and a closed chamber, unless you count magnum heads. 308's are open chamber, usually from 65-72cc's. I was fortunate enough to find a set at 65cc's, but from what I've heard most have larger chambers than that.

Magnums are a good choice if you're going to run different heads, they're pretty cheap, flow better than 318 heads, and are usually around 60-62cc's. There's been a couple of magazine builds that have pulled 400 hp out of magnum headed 318's. They do require oil through lifters and push rods, a new rocker set up and a new intake manifold though.

The stock heads are still more than enough to get to 300 hp though. Bigger valves would help, but aren't completely necessary. If the heads are going to be off and getting work anyway, I'd open them up to the 1.88/1.60 size and mill them .030 as suggested, the compression will be better and the larger valves will help the top end.

As far as the port mismatch, Edelbrock's performer manifold is pretty close to having 318 port sizes. The ports are a little larger, but not as big as 360 ports. It would be pretty simple to port match a set of 318 heads to a Performer intake. This is the performer intake, not the Performer RPM or Airgap, both of those have 360 ports. You could also look for an edelbrock streetmaster intake, or a holley street dominator. Both of those older intakes have true 318 sized ports.
 
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