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Rebel Machine Intake vs Stock 1970 390 4bbl Intake |
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Ram Air Rick
Supporter of TheAMCForum Joined: Jul/04/2007 Location: United States Status: Offline Points: 2667 |
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Posted: Jul/28/2009 at 8:43pm |
A while back I had posted some pictures of a 1970 Rebel Machine Intake manifold (casting number 3199762C) and compared it to a Stock 1970 390 Intake manifold ( casting number 3195532C).
The reason for the comparision was that I wanted to know why the Machine's 390 made 15 more horsepower over the stock 1970 390 (340 hp vs 325 hp ) respectively, and more specifically what role the intake played and how.
I know that the increase was because of the intake manifold and exhaust manifolds. Both exhaust manifold types Machine and Non-Machine ) are of the log design .I believe the diffference is that the Machine log has a 2.25 " exit,vs. the non-Machines standard 2.0" exit .
Below are picture comparisions of the Machine and stock 1970 4bbl intakes ,with captions.
While the pictures show the apparrent differences,there may be differences that are not visible from the outside(although I doubt it). Someday I may run a borescope down the runners,to see if there is a difference,and I might try to "CC" the runners although intitially this doesn't seem like a practical way to compare these,as since the plenum design is so different?
I hope this helps.
Here are the pictures with captions:
Machine intake (3199762C) (left) Stock 1970 390 4bbl Intake (3195532C) (right) (No real height difference as shown.)
Machine intakes casting number
Machine intakes right side manifold carb flange,to manifold floor measurement.(same as stock) Also shows plenum design.
Machine intakes left side carb. flange to manifold floor measurement.(Same as stock)
The Machine intakes real difference as I can tell so far. Oval left and right plenum openings,and radiused edges cast into ovals as they enter runner area.
Stock 1970 390 4bbl intake (casting number 3195532C)
Stock 1970 390 right side manifold carb flange,to manifold floor measurement.(same as Machine). Also shows plenum design.
Stock 1970 390's intakes left side carb. flange to manifold floor measurement.(Same as Machine)
Stock 1970 390 intake .Note the four round hole configuration,and the sharp edges as it enters the runners.
Edited by Ram Air Rick - Jul/28/2009 at 9:02pm |
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amx39068
Supporter of TheAMCForum Joined: Feb/21/2008 Location: Arizona Status: Offline Points: 11576 |
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The machine intake's greater plenum depth provides a much better flow when combined with the radiused edges. It is my understanding (as I have not done this myself) that the Machine intake can be signficantly improved upon so that it is good for close to 7K RPM when opening up the runners which is pretty darn good for a cast intake.
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Dan Curtis-Owner and CEO AZ AMC Restorations; Facebook: https://www.facebook.com/amcmusclecars/ & Curtis Real Estate Development
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Ram Air Rick
Supporter of TheAMCForum Joined: Jul/04/2007 Location: United States Status: Offline Points: 2667 |
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That's just it.Besides the radiused edges and the oval plenum design by sides ( more open) vs the four individual intake plenum holes ,I didn't measure any difference in the plenum depth.They were the same between the two intakes.
It is possible that the only real differences are the ones we see it the pictures ie oval plenum openings per side,and radiused edges. In fact,the more I looked at the two intakes,the more I was convinced that the Machine intake was made by modifying the stock intakes mold in the plenum area? They look that similar to me otherwise.
And boy,if in modified form they were good to 7K ,I'd say that's a killer cast iron manifold !!!
Rich C.
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Peter Marano
Supporter of TheAMCForum Joined: Jul/01/2007 Location: Kenosha WI Status: Offline Points: 1650 |
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I researched this while I was in the employ of American Motors, the drawings confirm the findings of Rich C.
There was also a camshaft change durring the 1970 model year, perhaps the camshaft change along with the improved manifolds netted the 15 horsepower increase. At any rate grinding the sharp corners into radii is probably something anyone with performance in mind should do. |
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Class Guy
AMC Addicted Joined: Jul/02/2007 Location: Arkansas Status: Offline Points: 969 |
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Probably the 15 hp number is an advertising value and not necessarily real. Does anybody have access to actual factory dyno information that shows that the 15 hp was the result of actual testing? Probably not, and probably not now nor in 1970. Advertised HP numbers in the automotive industry are historically for marketing purposes and not reflective of actual peak or average hp. That's the reason you have numbers like "300 hp", or "155 hp" and not "303 hp" or "152 hp". I'm not saying that the numbers are totally bogus, just that the number are only "kinda" indicators of the performance of the engine. A good frind of mine worked in Chevrolet engine development for 30 years + and he said the marketing folks spent lots of energy in discussion about the numbers they would decide to advertise for new engines.
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Peter Marano
Supporter of TheAMCForum Joined: Jul/01/2007 Location: Kenosha WI Status: Offline Points: 1650 |
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I agree, but the best lies are based on some truth. It does follow that the changes were made to increase horsepower, a ~5% increase is reasonable.
Perhaps someone with dyno time could comment. |
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Class Guy
AMC Addicted Joined: Jul/02/2007 Location: Arkansas Status: Offline Points: 969 |
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Jeff Lee may have some dyno information that would be relevant.
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Steve_P
AMC Addicted Charter Member Joined: Jun/28/2007 Status: Offline Points: 3759 |
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There is an SAE paper on the rebel machine and it goes into good detail on the engine. It does have dyno curves showing 390 from 69/70/70 machine hp/tq curves on one graph. And the machine is at 340. How real this is, who knows? AMC said 315 HP for 68-9 390 but the Edelbrock (?) dyno (in the '69 Car Craft AMX buildup) said something like 260 (I have mentioned this before).
The paper states a 7% improvement on exh manifold flow vs the '70 2" stock manifold.
It states the intake manifold revisions were "simple changes that involved casting modifications to increase the radii at the btm of the risers, and to open the ports bet the primary and seconday bores..."
They also state that the new cam, intake, and exhaust give the gains to 340 hp.
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Alliups
AMC Addicted Joined: Mar/25/2008 Location: S.B. county Ca. Status: Offline Points: 1409 |
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The crux is the difference between a 390, Ram Air, Javelin or AMX in '70 and the Machine. Unless the cam is different the only separators are the 1/4" larger logs and the intake discussed in the S.A.E. papers. The exhaust being slightly larger and the intake flowing better means the engine is becoming a slightly better air pump. Since the most power gains are generally through the heads- de-restricting flow through them nets the most gain. A 5% gain is highly reasonable. So instead of say, 260 net hp, it's now 270 in the Machine. Then "super tune" it as has been done time and again for Factory Stock racing. In fact there's the one article with a '68 or '69 AMX 390 that delivers every bit of the gross hp rating from back in the day net.
Steve |
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Steve_P
AMC Addicted Charter Member Joined: Jun/28/2007 Status: Offline Points: 3759 |
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WHat are you saying in your first sentence??????? I'm sorry but your posts rarely make sense and this is no exception
Ram air is irrelevant on a flywheel dyno- the engine is run in a room. The cam IS different on the machine 390 vs the "early" 390 at 325 hp. This is stated in my last sentence. Of course the same cam was from then on also used in the non machine engines but they were not re-rated. Edited by Steve_P - Jul/29/2009 at 12:07pm |
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