BJR Racing

Racing Engines & Heads


Small Block Cylinder Head Technology

I know a lot of people try different things and do some trial and error on different things, and I'm going to try and help you folks out by letting you know what I've found in past experiences. I will cover these heads in groups of 273/318, and 340/360 and 383/400 and 440's.
OK. First the 273/318 heads:
I'll mainly be talking about the 318 as they are more common and more available. But the same will apply to both engines. For most people, they are looking for good pickup for the street and in this case you would need to stay with the 318 casting heads as the 340/360 heads have too much port volume in them for the bore size of the 318 - even at .030 or .060 overbore.
I know that some are going to say that they never had any problems with the 340/360 heads on their 318, and you more than likely didn't. But you left some HP and TQ on the table. The smaller ports in the 318 heads from 67-90 will have more port velocity than the larger heads and they will use less fuel, and nowadays who couldn't use more fuel mileage?
So, my suggestion would be to use the smaller intake valve that comes in the heads (1.78) and use the larger exhaust valve out of the 340/360 heads (1.60). What this will do is give you more HP from the high velocity intake, and more TQ from the larger exhaust size valve. Most 318 engines won't ever need more than 136 cc's of port volume. Most stock castings are 120/122 in port cc's and the 302's are 118/120 from the factory. Now by opening up the bowl and gasket matching the heads they will gain about 6-8 cc's, depending on how much work you are willing to do. Heads in this state should make about 210/215 cfm's with a good valve job. This is enough to get over 450HP out of the engine. With the right components cam, intake, and carb. and exhaust, of course..
Another thing that most people have done is to add the 1.88 intake valve to the 318 heads and this only hurts the flow at low lifts and port velocity. Basically the port becomes less efficient, and we all know that the more efficient a engine is the more HP and TQ it will produce. The reason that I don't suggest the larger intake valve is the shrouding effect of the head and cylinder wall. Let alone, the drop of flow and low speed operation of the engine. Basically the throttle response won't be there.
Anytime a port becomes larger, it will require more fuel to stay efficient. Another thing that has a direct effect on the port size is the rod length, what this does is determine the piston speed. The faster the piston speed, the larger the port will need to be, as there will be less suction time for filling the cylinder. This is the reason the big block Chevys need a port with 290 cc's as their piston speed is very fast due to the short length of the rod (6.135). And a big block Mopar needs a smaller port in the 175/180 cc range due to it's rod being 6.768.
With heads in the 125 cc range and flow in the 215 cfm range, this will satisfy most all 273/318 engines for street/strip use. With heads of this type they will be able to turn 6,000+ RPM's on the small engine with ease. The only reasons that they won't will be due to under carburetion, under intake'd, under cam'd or not enough exhaust flow. Obviously springs will have a direct effect also but I included this with the cam as most will change the springs when they change the cam. This pretty much gives a general dos and don'ts for the small engines.
Now for the 340/360 engines:
The 340 and 360 will be covered together just as the 273/318 engines were. Once again the same will apply for the 340 engine just as the 318 mods applied for the 273 engine. A good 340/360 engine shouldn't need more than 160/168 cc's of intake port volume and 250 cfm's of air flow. More airflow is a plus but not at the cost of a larger port runner.
Even though the strokes are different in the 340 and the 360, the rod length is the same. The piston speeds are different but the port can remain the same and this is why. On the 340 engine you should use the 1.88 intake valve to keep the port velocity up in the larger runner. The same modifications that were done to the 318 heads will apply here also. A good 340 head with the 1.88 valve should flow in the 235/240 range @ .500 lift and 160+ @ .200 lift if the port is properly done. The same air flow will apply to the 360 engine. The specs stated above would be a very good street/strip head and would satisfy most 340/360 engines.
There is no need for a 2.02 valve until you're racing at high RPM's above 6500. The valve size is just too large for any low RPM engine, and bottom end TQ and throttle response will be suffer.
With the air flow stated above, the 340/360 should be capable of making over 550 HP. The need for more than this will result in you spending big $$$$ and won't be very streetable. Anything more than this would be considered race only.
Just for reference for stroker engines, a 318/349 engine would use the 160 cc 1.88/1.60 valve 340/360 head. And the 360/408 engine would need a 180/185 cc head and air flow in the 280/290 range or more with a 2.02 intake valve and a 1.625 exhaust valve.
In any case, the intake to exhaust port flow ratio should be kept to 70% for best results. Superflow states that anything above 60% won't really be noticed in a street or drag engine. Ratios above 80 and 90% have been seen and only help high RPM engines like NASCAR where high efficient exhaust flows are needed to make HP at 9,000+ RPM's. Obviously heads like this aren't cheap or useable for street use.

Engine Size Usage Port CC Head Flow CFM Valve Size Intake Exhaust
273/318 Street 118-124 190-205 1.78/1.50 M-1 Dual Plane
or Edelbrock RPM
38-40" length
273/318 Strip 124-134 210-230 1.78/1.60 M-1 Single Plane
or Victor/Super Victor
1-3/4" - 1-7/8"
34-36" length
3" outlet
340/360 Street 150-155 200-215 1.88/1.60 M-1 Dual Plane
or Edelbrock RPM
38-40" length
340/360 Strip 160-168 230-250 1.88/1.625 M-1 Single Plane
or Victor/Super Victor
 1-7/8" - 2"
30-34" length
3" outlet
Hopefully this will help you street enthusiasts and weekend racers better understand how to get more out of your engines with better efficiency.