308 heads on a 340 block??

[Suregrip391]

Hi Guys, Im looking to run some 308 heads (89-91 360 heads) on my 340 when I rebuild it in a couple weeks. Will I need to swap out anything to make these babies work? I currently have the "J" heads with the 915 casting.(pushrods, rockers, shafts, need changed over??) Also, how do you guys feel about running the 1.88" intakes with the 1.6" exhausts in those 308's??? I was told the 340 would be better off if I opened them up to the 2.02" on the intakes??.....Whadda ya think?? Thanks.
:thumbup:

 

[perfacar]

hi, the 308 exhaust port does flow somewhat better over early heads, however, from zero lift to .200 it outflows the intake, this causes the headers to turn red hot, and loss of cylinder fill. as for the intake, the 915 and 587, have a way fatter flow curve over the 308,this is where the valve spends 90 % of it's time. and is very important for cylinder fill. plus the 308 has a taller and narrower intake port, compared to early heads. a mismatch! the 1.88 valve will flow more than enough air to a 340. especially if you use the mopar sst. intake valves. I have flow tested these heads. the valve gear all interchanges. the 1.88 /1.60 is a good combination,

 

[moper]

"from zero lift to .200 it outflows the intake, this causes the headers to turn red hot, and loss of cylinder fill."
What? Please explain your thinking...

" as for the intake, the 915 and 587, have a way fatter flow curve over the 308,this is where the valve spends 90 % of it's time. and is very important for cylinder fill."
Again, what? The X and Js are fairly close, the 587s and 308s are also fairly close, but less than the X and J. I think (opinion) the 308 is a better street head in any case.


plus the 308 has a taller and narrower intake port, compared to early heads. a mismatch!

Not that I've ever seen...They all have the same rough opening. Magnums are a little narrower from what I've found.

 

[340mopar]

The 308 head is a raised port head and is supposed to be a good head for modifying for performance use. I have not tryed this, this is what I read in the Mopar LA Engine manual.

Chuck

 

[D-mailman]

What Moper said....I strongly agree! Terry.

 

[moper]

The 308 head is a raised port head and is supposed to be a good head for modifying for performance use. I have not tryed this, this is what I read in the Mopar LA Engine manual.

Chuck

I have heard that too. I'll let you know , My 418 "F.A.R.T." Swinger will have them on it when I get around to finishing it.

 

[perfacar]

hi, when the valves are in the overlap stroke is where you do not want exh outflowing the intake. the exh will pull some of the intake charge out,and burn it ,therefore the headers turn red. plus, the cylinder does not get a full fill. a motor is an air pump, more air in= more power. the 587 head will outflow a 308. the 894 which is the x head flows good. the 915 doesn't flow as good as the 894 and 587. i have ran the 915 and 587 heads, 587's are faster. plus, all these heads have been flow tested on a super flow 600 w/flow com.
the 587 head has a higher port velocity. that relates to better fill, and throttle response. mopar pushed the 308 and the 576 head, they will work, with lots of port work done to them. flow curve is more important then the big peak number.

 

[340mopar]

With the exhaust pulling intake charge into the cylinder at overlap and being the exhaust valve closes while the piston is still traveling down the cylinder why would the cylinder not get a full fill when the intake is full of a fuel/air mixture.

I can see how some unburnt fuel/air could get in the exhaust from what you described. What you described, and I know you know this, is scavageing, which is a good thing to help move air. To much scavaging would be a waste if you are sending unburnt fuel into the exhaust without using it, I get that part also.

It just seems to me that with what you describe that the cylinder would fill better or at least with a less deluted charge of fuel/air and spent exhaust gases being too much fuel/air is being scavanged.


I am just trying to understand,

Chuck

 

[BJR Racing]

Maybe what he's saying is too much reversion, this is a major part of head flowing and most camshaft choosing, this is why alot of engines don't respond like they should. You have both intake and exhaust pulses that make differences, one is as detrimental as the other. Just my opinion.

 

[perfacar]

hi, the 576/308 heads were installed on a 340, motor is a 9.1 cr, on VP C 12 race gas. the first pass, motor had detonation in 3rd and 4 th gear like mad.
rejetted carb up a lot, next pass detonation in 4th gear only, car was slower.
next pass, took timing out of motor, no detonation, but car was slower.
the header pipes ran red , could see them. came back home, removed heads, went back to old 587 heads, set motor back to original setup. ran faster, no detonation. few years later, had a cam made to help this problem, it worked well, but, the car still wasn't as fast with the 576/308 head as the 587 was.
discussed this with a S/S mopar racer, he stated, that the 576/308 head requires a ton of portwork to make them work. over scavanging does pull part of intake charge out, this is what turns pipes red color. the fine line is to scavange the cylinder, without pulling any intake charge out. this is where flow and cam durations , overlap are critical.

 

[moper]

"hi, when the valves are in the overlap stroke is where you do not want exh outflowing the intake. the exh will pull some of the intake charge out,and burn it ,therefore the headers turn red."


I dont think that's really true. In fact, if that were the case, batch fired V-8 fuel injected engines would routinely melt things, because on every firing cycle one cylinder's valves are open when the injector fires. What turns headers hot is high exh gas temps and a beneficial lack of thermal transfer. More heat in the exh means more power out of the cylinder, because less heat energy is being absorbed by the piston, exh valve and port walls. The other reason would be lean mixtures that will cause temps in the chamber way up and the result is is more heat into the exh system.

" plus, the cylinder does not get a full fill."
Cylinder filling is a result of a combination of things. Fluid dynamics, pressure wave tuning and inertia, thermodynamics, port and cylinder physical shape and material, cam timing, stroke and rod length, fuel type...What you describe above is scavenging effects of inertia. That is nothing but a plus or bonus in an engine of any type.

" flow curve is more important then the big peak number."

I agree whole heartedly to that one.

 

[340mopar]

How can you pull intake charge out when the exhaust valve closed before the intake closes at overlap. You may pull intake charge out at overlap while the exhaust valve is open but once the exhaust valve closes the pulling out stops and the cylinder continues to fill because the intake valve is still open.

Reading your track test, it sounds like the 308's were more efficient by the heat generated. It sound like you guys gave up to early and could have made them run well if you spend more time on it.

I am only going by what you have written, because obviously I wasn't there while you tested.

Chuck

 

[perfacar]

on the overlap stroke, the exh valve is closing, the intake is opening, not closing. this is the start of cylinder fill. the 308 is not more efficent , if the header pipes are glowing red then its burning fuel in the tube, as for giving up early?? super stock guys flog parts every way they can. you have no idea what is involved in building class motors. stock and super stock racers flog every part, and try to make the part work better than it was made to do.
on the 340 that we tested on,it was a stocker, no grinding, porting is allowed
only a very precise 3 angle valve grind is allowed. the fact remains, they don't work very good, most stocker guys have went back to the older 587, heads. by flogging parts and conbinations is why stockers and super stockers go fast!!

 

[lenweiler]

My last car was a '69 Dart 340 4 gear. I built a mild 9.5 engine with a 750 DP and dual plane. I never had a lean or detonation problem with the 308 head. The bowl configuration was a breeze to work with. The car ran 12.7's with serious traction problems in all four gears. Max speed was 112mph with a 3.91. FYI.