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Square vs Dog Leg Heads

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Javelin_GT View Drop Down
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    Posted: Nov/11/2009 at 1:32pm
This has probably been answered/debated but I cant find it.

I know everyone days dog legs flow 50% over the square ports.

The HP only increased by 10 from 69 to 70 on the 390 and I thought part of that was the ram air.

Are dog legs really that much better?    My main reason for asking is this.....

I have a nice pair of early heads with good valves and some port work (bolt on ready) and a pair of good but stock/still dirty 502's.

For doing a build on a 360 that will go 6500rpm is it really worth dumping the extra money do get the 502's up to where the square ports are already at?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote stonedblue Quote  Post ReplyReply Direct Link To This Post Posted: Nov/11/2009 at 1:54pm
Just a layman's opinion, but, I think the gain you get with the tighter cc square heads will give more hp than the higher flow 58 cc DL heads. That said, I was gonna use the 390 SP heads on my 360 till a set of 6090 DLs dropped in my lap ready to bolt on. The SP heads needed a complete rebuild, so, 'Rambler Mentality' kicked in. Cutting the 6090s for a bit more comp. was way cheaper! JMHO.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ram Air Rick Quote  Post ReplyReply Direct Link To This Post Posted: Nov/11/2009 at 4:51pm
I'm sure that it's been done here somewhere, but we flowed the exhaust side 
of a set of 291 Dogleg heads vs 558 squiare ports ,and found a 20% increase,not a 50% increase. Nothing super scientific. My friend just asked a machine shop that does a lot of work for and with him, to put the heads on their flow bench,and check them out on the exhaust side. I do not have the flow numbers handy,but at the end of the day it was 20%.
 
Rich C. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote turbo Quote  Post ReplyReply Direct Link To This Post Posted: Nov/11/2009 at 5:16pm
having driven cars with both types of heads that have been worked on in a similar fashion I can say that I really didn't notice any difference in my butt dyno.  nothing about the doglegs really jumped out at me.  you would think that it would be significant difference just looking at them but not as much as a lot of people would have you believe.  
they call me Capt Fun.......!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Red Devil Quote  Post ReplyReply Direct Link To This Post Posted: Nov/11/2009 at 6:20pm

Someone posted the SAE paper on this a while ago and it was the 20 - 25%, like Rick measured, not the 50% urban-legend often quoted .... but ported numbers may be a different story.  If you've already got a nice set of ported early heads, I'd say use 'em. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote bigbad69 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/11/2009 at 6:35pm
Originally posted by Red Devil Red Devil wrote:

...not the 50% urban-legend often quoted ...
It's technically not an urban legend. AMC printed it in "Performance American Style". Whether the "typo" was on purpose or a mistake is unclear. What is clear is the doglegs don't outflow the square ports by 50%.

On a mild street engine, there is probably no benefit from the doglegs alone. They do offer more intake and exhaust choices though.



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amx39068 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amx39068 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/11/2009 at 6:35pm
The engine has to have enough inbound flow to take advantage of any improved outbound flow. In other words if your intake side is equal to or less than the capacity of the rectangular port heads then doglegs will make little if any improvement in performance. And unless you use 291C heads lower compression may offset any potential gains from the heads.

Intake, carb, ignition, exhaust and even gearing are all part of the equation.The engine's flow is only as good as its weakest link and if that weakest link is not the heads then different heads will not make it flow any better.

Spend your money on a good intake, carb and exhaust that are well suited and matched to your engine's build and you will see much better results than swapping out your heads to doglegs. On the other hand if you are buying one or the other then by all means go with dogleg if all other things are the same such as port work, chamber size, etc. IIRC dogleg heads have 1.68 vs 1.625 exhaust valves although I donKt remeber if 70 and 71 doglegs have the big valves as well.
Dan Curtis, AZ AMC Collector Quality Restorations & Parts - amcmusclecars.com
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_P Quote  Post ReplyReply Direct Link To This Post Posted: Nov/12/2009 at 8:29am
as said, the high lift gains are 19-20%.  the low lift gains are less.  50% is a PAS fantasy for AMC to sell heads or maybe a typo LOL.  There are flow data curves in the SAE papers.
 
Ram air/cold air hood scoop is irrelevant to flywheel dyno readings (advertised HP) and does not enter into the equation- the engine is on a dyno and in a room.  However, in the real world it was usually a ~1-2% gain in MPH on a stock car in the quarter mile with cool air enabled from a scoop. 


Edited by Steve_P - Nov/12/2009 at 8:30am
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