502 Intake Question.
08-30-2011, 11:47 AM
#11
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Here's a stock 502MAG bottom end with fresh rings and bearings, AFR 265 heads, Imco Powerflows, RMbuilder cam, Vic Jr. single plane intake, running all its accessories on 89 octane. How much more power/torque do you think it would make with a dual plane intake?
08-30-2011, 12:08 PM
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Thank you.
Anxiously awaiting the reply.
08-30-2011, 12:16 PM
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If you do testing down to 2800 (if you can get the dyno to hold) you may see a dip or two in the torque graph down there. These are from intake vs cam vs exhaust tuning frequencies. The dual plane will smooth those out. But does it matter ? Nope. A small 7-15lb dip Under 3k is pretty much meaningless.
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To othrs for this threads discussion:
Another thing with the AirGap rpm's is that they are not heated like the non-air gaps. Heated intakes will make for slightly better idle and of course faster warm up times. Heat not great for performance.
So, the low speed drivability/idle is that much closer between the airgap style and single plane.
With proper initial timing and carb tuning, single plane motors will start right up and idle fine. Drivability is often described as 'like fuel injection.'
I don't use chokes either unless a bone stocker. How dare I. LOL.
08-30-2011, 01:23 PM
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As far as the rest of my build. 502mpi mags got rid of the injection due to vapor locking issue.
RHS Pro Topline 320 cc heads, custom ground cam just alittle smaller than crane 731. holley 850's sitting on RPM airgaps, timming locked at 38, as far as jetting 84 in the front and 90 in the rear. running cyclone shortty headers to replace my Gil's , ( better accsess to plugs and drains). Labbed 28's @ 5600 81mph, stock 28's should put me back to around 5300 were I want to be. will know this weekend on numbers with them.
RHS Pro Topline 320 cc heads, custom ground cam just alittle smaller than crane 731. holley 850's sitting on RPM airgaps, timming locked at 38, as far as jetting 84 in the front and 90 in the rear. running cyclone shortty headers to replace my Gil's , ( better accsess to plugs and drains). Labbed 28's @ 5600 81mph, stock 28's should put me back to around 5300 were I want to be. will know this weekend on numbers with them.
08-30-2011, 02:15 PM
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Of course the difference is in the cylinder heads.
The small port Air Flow Research cylinder heads you installed have an intake port volume of only 265 cc's. These have high velocity intake ports that are MUCH smaller than the stock Chevy/Mercruiser rectangle port heads and will boost low end torque very well. It would seem a dual plane manifold would flow more than enough for these small port heads and increase idle to 5000 rpm performance. Never the less, the small intake ports could help the high speed single plane manifold pull up the torque figures to a credible amount at low speed.
I suggest doing a test with the same Victor Jr. single plane intake manifold and the huge 320 cc (stock) Chevy/Mercruiser rectangle intake port head.
The test would show a considerable difference especially below 3000 rpm. Low speed torque and fuel economy would suffer from the loss of intake charge velocity (speed). This comes from exposing both sides of the carburetor to all eight of the single plane manifold runners and all eight of the large rectangle intake ports.
I wonder what the reasoning is for going smaller on the intake ports (than stock) AND installing a high rpm style single plane intake manifold?
Dennis Moore
The small port Air Flow Research cylinder heads you installed have an intake port volume of only 265 cc's. These have high velocity intake ports that are MUCH smaller than the stock Chevy/Mercruiser rectangle port heads and will boost low end torque very well. It would seem a dual plane manifold would flow more than enough for these small port heads and increase idle to 5000 rpm performance. Never the less, the small intake ports could help the high speed single plane manifold pull up the torque figures to a credible amount at low speed.
I suggest doing a test with the same Victor Jr. single plane intake manifold and the huge 320 cc (stock) Chevy/Mercruiser rectangle intake port head.
The test would show a considerable difference especially below 3000 rpm. Low speed torque and fuel economy would suffer from the loss of intake charge velocity (speed). This comes from exposing both sides of the carburetor to all eight of the single plane manifold runners and all eight of the large rectangle intake ports.
I wonder what the reasoning is for going smaller on the intake ports (than stock) AND installing a high rpm style single plane intake manifold?
Dennis Moore
08-30-2011, 02:47 PM
#16
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Very seldom does a single plane work good for a relatively low rpm marine engine (an engine that operates below 5900 rpm). Single planes are best for car racing where only one carburetor is allowed and engine rpm's are at 6000 to 7500 RPM.
The single plane intake manifold was the best manifold in the early 1970's Super Stock drag race cars. These cars where limited to stock carburetors but could use any intake manifold. These flow limited engines required a huge plenum chamber to make power at high speeds. Low speed torque was not required at all.
A modern dual plane manifold (with a large carburetor) will make as much horsepower as a single plane manifold but have more low speed torque (when operating in the rpm range of most boat engines).
Dennis Moore
The single plane intake manifold was the best manifold in the early 1970's Super Stock drag race cars. These cars where limited to stock carburetors but could use any intake manifold. These flow limited engines required a huge plenum chamber to make power at high speeds. Low speed torque was not required at all.
A modern dual plane manifold (with a large carburetor) will make as much horsepower as a single plane manifold but have more low speed torque (when operating in the rpm range of most boat engines).
Dennis Moore
This thread, and thus my answer is for a 502cid BBC.
Depends on cam and heads.
If running a modern cyl head and a modern HR cam (even low duration) a single plane will win. I spent many hrs on a dyno with wet exhaust testing different intakes, spacers, etc,etc with a short duration, relatively high lift cam.
Also proved it on the water with that engine and others.
500+ cids, good heads, and good cam = old rules change.
454, OEM Rectangulars - 95% of time I'd side with the AirGap RPM.
This is nothing new, but some still want to make or believe a marine low compression 502 look like a tow truck engine. It's not.
Depends on cam and heads.
If running a modern cyl head and a modern HR cam (even low duration) a single plane will win. I spent many hrs on a dyno with wet exhaust testing different intakes, spacers, etc,etc with a short duration, relatively high lift cam.
Also proved it on the water with that engine and others.
500+ cids, good heads, and good cam = old rules change.
454, OEM Rectangulars - 95% of time I'd side with the AirGap RPM.
This is nothing new, but some still want to make or believe a marine low compression 502 look like a tow truck engine. It's not.
Would be like saying a Dominator (1050) has no place on a marine engine. In one's head you may have been thinking a small cam 454, but not thinking about 540+cid engines with powerful heads, cams, and exhaust.
As I mention many times:
A 350 is not a 383 which is not a 396 is not a 406 which is not a 427 which is not a 454 which is not a 502 which is not a 540 which is not a 556 which is not a 572 which is not a 632................ then add all the different wants and goals and thus possibilites of the many different engine builds.
Of course the difference is in the cylinder heads.
The small port Air Flow Research cylinder heads you installed have an intake port volume of only 265 cc's. These have high velocity intake ports that are MUCH smaller than the stock Chevy/Mercruiser rectangle port heads and will boost low end torque very well. It would seem a dual plane manifold would flow more than enough for these small port heads and increase idle to 5000 rpm performance. Never the less, the small intake ports could help the high speed single plane manifold pull up the torque figures to a credible amount at low speed.
I suggest doing a test with the same Victor Jr. single plane intake manifold and the huge 320 cc (stock) Chevy/Mercruiser rectangle intake port head.
The test would show a considerable difference especially below 3000 rpm. Low speed torque and fuel economy would suffer from the loss of intake charge velocity (speed). This comes from exposing both sides of the carburetor to all eight of the single plane manifold runners and all eight of the large rectangle intake ports.
I wonder what the reasoning is for going smaller on the intake ports (than stock) AND installing a high rpm style single plane intake manifold?
Dennis Moore
The small port Air Flow Research cylinder heads you installed have an intake port volume of only 265 cc's. These have high velocity intake ports that are MUCH smaller than the stock Chevy/Mercruiser rectangle port heads and will boost low end torque very well. It would seem a dual plane manifold would flow more than enough for these small port heads and increase idle to 5000 rpm performance. Never the less, the small intake ports could help the high speed single plane manifold pull up the torque figures to a credible amount at low speed.
I suggest doing a test with the same Victor Jr. single plane intake manifold and the huge 320 cc (stock) Chevy/Mercruiser rectangle intake port head.
The test would show a considerable difference especially below 3000 rpm. Low speed torque and fuel economy would suffer from the loss of intake charge velocity (speed). This comes from exposing both sides of the carburetor to all eight of the single plane manifold runners and all eight of the large rectangle intake ports.
I wonder what the reasoning is for going smaller on the intake ports (than stock) AND installing a high rpm style single plane intake manifold?
Dennis Moore
Last edited by SB; 08-30-2011 at 02:55 PM.
08-30-2011, 04:43 PM
#17
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Air gap vs Vic jr.
I have an old Gm muscle car with 548" Chevy in it.Canfield 305 cc heads and an old school Crane 741 hydraulic roller cam.10.2 compression , street headers and 3" exh system, .Originally assembled with an Air Gap.Motor pulls like crazy to 5500 rpm shift point.Tried a Vic Jr out of curiosity.Lost a liitle torque below 4500, picked up a little above 4500.Overall the Air gap felt better and I put it back on.Fits under the hood better too.I doubt the Victor would do much for you on a milder 502,maybe 1 or 2 mph, but the only way to be sure is to swap manifolds and run the boat on GPS.
09-01-2011, 07:37 AM
#19
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Just an interesting side note, I have the DynoSim dyno simulation software. I put in the specs for my motor which has been run on a dyno and the results in the software matched the real world almost exactly. I have since rebuilt the motor and put in the new head flow numbers and compression, etc into the software. I ran simulations with a dual plane intake and a single plane intake. This is where it gets interesting, I get better torque and hp averages with the single plane then the dual plane. Now I have a 524 CI BBC with a fairly stout cam .623/.611 & 242/252 @ .050", 10:1 compression, ported AFR 305 heads, and Lightning headers. My engine shop has also dyno'd many similar motors and has noted that the Brodix HV2000 series single plane intake produces more power from the mid range on up than an Air Gap. As was stated previously, everything is different, and everything matters.
09-01-2011, 08:37 AM
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Of course the difference is in the cylinder heads.
The small port Air Flow Research cylinder heads you installed have an intake port volume of only 265 cc's. These have high velocity intake ports that are MUCH smaller than the stock Chevy/Mercruiser rectangle port heads and will boost low end torque very well. It would seem a dual plane manifold would flow more than enough for these small port heads and increase idle to 5000 rpm performance. Never the less, the small intake ports could help the high speed single plane manifold pull up the torque figures to a credible amount at low speed.
I suggest doing a test with the same Victor Jr. single plane intake manifold and the huge 320 cc (stock) Chevy/Mercruiser rectangle intake port head.
The test would show a considerable difference especially below 3000 rpm. Low speed torque and fuel economy would suffer from the loss of intake charge velocity (speed). This comes from exposing both sides of the carburetor to all eight of the single plane manifold runners and all eight of the large rectangle intake ports.
I wonder what the reasoning is for going smaller on the intake ports (than stock) AND installing a high rpm style single plane intake manifold?
Dennis Moore
The small port Air Flow Research cylinder heads you installed have an intake port volume of only 265 cc's. These have high velocity intake ports that are MUCH smaller than the stock Chevy/Mercruiser rectangle port heads and will boost low end torque very well. It would seem a dual plane manifold would flow more than enough for these small port heads and increase idle to 5000 rpm performance. Never the less, the small intake ports could help the high speed single plane manifold pull up the torque figures to a credible amount at low speed.
I suggest doing a test with the same Victor Jr. single plane intake manifold and the huge 320 cc (stock) Chevy/Mercruiser rectangle intake port head.
The test would show a considerable difference especially below 3000 rpm. Low speed torque and fuel economy would suffer from the loss of intake charge velocity (speed). This comes from exposing both sides of the carburetor to all eight of the single plane manifold runners and all eight of the large rectangle intake ports.
I wonder what the reasoning is for going smaller on the intake ports (than stock) AND installing a high rpm style single plane intake manifold?
Dennis Moore
