Exhaust crossover in intake

[tayto]

Looking for recommendations on what to do with the exhaust crossover in the intake manifold. I daily drive my truck and do not live in warm climate (west coast of Canada). My Qjet has an electric choke, I am running stock "log" exhaust manifolds with NO heat riser valve and currently my intake has no provisions for an EGR valve. While most say to just block them off completely, I do not think this is the way to go. An intelligent source said to block one side complete and restrict the other side with a 1/4" hole. I think the intake gaskets I got sent 2 plates with a 1/2" hole in them.

Any suggestions here?

[Kenth]

You need to understand the principles of the internal combustion engine to make the right decision for the use of the car.
Heat is a factor of three that converts liquid fuel into a combustible gas.
The others are the amount of fuel and the air velocity through the venturi.
If you remove or reduce one of the factors, you must increase the others accordingly.
This does not worry race guys as they start at 3000 rpm or higher and drive only 1/4 mile at a time. For them, driveability and fuel consumption are not a priority.
The crossover channel is there for a reason.

FWIW

[Cliff Ruggles]

For what you are doing leave the exhaust crossovers open in the intake.  It's a wet-flow system and heat is your friend with these things.  Heat improves thermal efficiency and allows for faster warm-up of the engine and eliminates problems associated with fuel globbing up on the cold intake runners until the engine fully heat soaks.

You will so a lot of conflicting information on the subject, and a high percentage of it is highly inaccurate.  There is NOTHING waiting for you but issues, especially in cool/cold weather if you block off the crossovers.  I'd like to have a nickel for every person who has build a 350 SBC with the later "Vortec" heads and calls up here complaining about poor performance in the Fall and Winter months bit don't have too many issues in the summer........Cliff

[tayto]

I had no intention of blocking it, so don't worry about that! I am just curious, I guess, of why they used restrictors, if at all. Two 1/2" diameter restrictors were sent with my intake gaskets, so that's what I used. I recently acquired a 14096242 "bowtie" intake that i want to try with my setup, so would like to "correct" anything while i'm in there.

[old cars]

Here's what Brzezinski Racing has to say:

In stock form it will produce more power between 4500 and 7000 rpm than any other stock Chevy manifold. Commonly referred to as a “Marine” intake, this manifold is a cast-iron copy of a Z-28 high rise aluminum intake. The marine intake is equipped with both carburetor bolt patterns, is 1.250" taller than regular intakes and was not available – in cast iron form – on any production cars.

In stock form this intake will out perform stock Q-Jet and 2 barrel manifolds in most applications where the engine is running between 4500 and 7000 rpm. Because of its huge internal volume, the Marine Intake lacks throttle response below 4500 rpm compared to other stock intakes.It is equipped with Holley and Q-Jet carburetor bolt patterns.  It will not bolt to Vortec or Vortec Bow-Tie heads.

[Cliff Ruggles]

To make power and shine in the 4500-7000rpm range would require a LOT of camshaft plus some good flowing heads on a typical SBC 350 build.  Any cam making power in that range isn't going to be user friendly for "normal" driving anyhow and would be more of a "race" application.

What a lot of folks don't realize is that a bigger intake manifold does not always make more power as we've been led to believe over the years.

I 2004 I dyno tested 3 different intakes on a very well prepared Pontiac 428 (434cid) engine.  The engine was topped with aftermarket aluminum heads flowing 260cfm, 10.6 to 1 compression, and custom ground hydraulic roller cam with 284/296 @ .006 and 236/242 @ .050" with close to .600" lift.

First I tested a factory cast iron intake, stock other than opening up the four holes to two large openings and giving it both bolt patterns.  That intake made 497hp.

Next up was an Edelbrock Performer RPM, which is over 1" taller and larger runners than the factory iron intake.  The very next pull with no other changes was 491hp.

Last I tested a Pontiac 455 aluminum HO intake modified to two large openings just like the cast iron and Edelbrock RPM.  The runners were cleaned up some but stock sizes.  It made 487hp.  The 455 HO intakes are actually a little shallower than a cast iron intake so slightly smaller plenum volume.

So even at 434cid with aftermarket aluminum heads and pretty hefty HR cam we LOST power topping that engine with an Edelbrock Performer RPM intake.

Look at how many folks buy RPM and RPM Air-Gap intakes for smaller 350 SBC engine builds not making anywhere near 500hp or moving  nearly the air or the much larger 428 engine......some food for thought........Cliff

The pic below shows the cast iron intake used in that testing (right).  The intake sitting beside it is an early attempt by Edelbrock to make a spread bore aluminum Pontiac intake, P4B-QJ.

[old cars]

Cliff. Not seeing how your tests as you stated are relevant. Max horsepower ( at what rpm ) is only one part of the equation. What carburetor? where are the torque specs.

Basically you can figure that on a 350 chevy making , say 325 hp on the stock intake , you could be looking at 355 hp when the intake is swapped for a current high performance air gap style intake. On a 383 the power increase will be greater.

[old cars]

If you live in  northern climates and have to use your street rod for daily commuting , then an exhaust-heated or better yet a water-heated intake are the way to go . The question here is do we need manifold heat? If you live in the southern part it can probably be done away with. If yo have an annular discharge carb you can run a cool manifold, because it's worth a sizeable amount of ft-lbs. Typically 10 ft-lbs. If a cooler intake is used , about 3/4 of a compression ratio increase above the previously limit is possible, and that's worth a like amount.

[Cliff Ruggles]

Just putting out accurate numbers from DIRECT testing.

Edelbrock tells us that bolting on a Performer or RPM intake to a Chevy 350 engine makes more power.  OK, I bought into that BS for decades so decided to do some back to back testing. 

Increasing runner size, height of the intake and plenum volume LOST power on a much larger engine with excellent flowing heads.  How can as bigger intake help a tiny little 350 SBC making over 100-150 LESS HP?

Tell me how bolting an RPM Airgap intake to a 350 engine in a 1978 Chevy truck, or even a 1979 Corvette L48 or L82 makes "more power" than the stock intake that was removed?  I woln't even get into the fact that the taller intake in most vehicles requires a drop-base air cleaner for hood clearance, which moves the lid closer to the carb killing off quite a bit of power with some set-ups (tested that deal too).

Let's even go one step further and say that they just "rebuilt" the engine and added an "RV" cam like 95 percent of the "population" does with these things.  Tell me how a bigger/taller intake magically improves power (torque), average power, and makes the vehicle faster?  Even your post above says that the Marine intake makes more power in the 4500-7000rpm range.  Most 350 SBC builds woln't make power much past 5000rpms and even with good heads and most of the cams chosen these days are DONE before 6000.

The engine in my Ventura is also set-up to test intakes, I remove the water crossover from the intake so I can swap them in about 5 minutes at the track.  I've tested as many as 5 intakes back to back at private track rentals on several different occasions.  The only intake that ran close to the iron intake was the Tomahawk intake and it took a 1" spacer to do it.  The "modified" iron intake actually ran quicker in short times and ET, the big single plane intake ran nearly 2 mph faster on top end but still got outran by .03 seconds in ET.  For reference the intakes I've tested were the Performer (complete TURD), Performer RPM (very good intake but the drop base air cleaner KILLED power so I had to make a custom filter for the Shaker opening and ditch the lid), Torker 1, Torker II, Holley Street Dominator, P4B and Tomahawk (aftermarket single plane designed by Dave at SD Performance).

What the big Tomahawk intake and spacer did was "shift" power and pull harder in the upper mid-range and top end, but lost average power/power in the loaded RPM range.

None of this matters much in the big scheme of things, Edelbrock has the entire population brainwashed into thinking you can spend $200 on one of their intakes and go right out and pull bigger wheelies, longer burnouts and run quicker times at the track.  About the only true statement they make in their advertising is that the new shiny part removes some weight from the front of the vehicle.......FWIW......Cliff

[Cliff Ruggles]

The question here is do we need manifold heat?

Yes, in ALL wet flow systems a cold intake is never a good thing.  Heat is actually your friend with these things and allows us to run a leaner mixture and more efficient than if the intake was cold.  The engineers really did know what they were doing with these things.  They even went too far in the mid to late 1960's and routed some additional exhaust gasses directly under the carburetor and used a steel plate to transfer heat directly to the carburetor to make things more efficient.  That move didn't work out so well so by 1970 all of those intakes had disappeared, but they would have survived if the additional heat didn't have negative effects like boiling the fuel in the bowl of the carburetor in some scenarios.

As far as blocking off the crossovers, about all that does is make the engine not run well until the intake heats up, and it's going to eventually heat-soak anyhow.  Even with the crossovers fully blocked off try touching one after you've driven 20 minutes to work and shut the engine down, your fingers will nearly melt to it!.....

[Kenth]

Any engine not reached the normal operational temperature is cold.
Thats why engineers uses the crossover channel to heat up the manifold as quick as possible.
And, thats why engineers uses the choke on carburator to add fuel until the engine reaches the normal operating temperature.
All parts of the principles of the internal combustion engines, same today as 120+ years ago.
This has NOTHING to do with ambient temperatures, winter or summer.

[tayto]

just for the record, i had no intention of blocking the exhaust cross over completely off. i was more curious as to why the factory would use restrictors, or no restrictors or a combination of both so i could make a decision on what to use.

as for the intake, I found very in little info on the marine intake. it apparently is a copy of the old Z28/LT-1 intake where I didn't find a lot of negative "reviews". However, I like to try things for myself as most information found on the internet isn't always correct.

Saying that, Cliff you haven't steered me wrong. if you think it's a waste of time then i wont bother.

[68rs/ss]

I see that you modified your intake at the divider and eliminating the four holes into two sides with a small opening between dividers. I would like to try this on my BBC street engine. Are there other modifications that can be done with a die grinder to made the 68 stock Q-jet cast manifold perform better? Some of the corners going into the runners are sharp. Guessing smoothing these would benefit? The stock cast intake weighs a ton.
Phil

[Cliff Ruggles]

My intake underwent a series of "modifications" about 20 years ago, followed by dyno, street and track testing.

Right on the dyno it outran the larger Performer RPM by a solid 6HP with no other changes.  A company the CNC ports cylinder heads actually has a CNC scan of it to modify intakes in similar fashion.

Removing the exhaust crossover shaved a lot of weight off of it as well.  I would actually still be using the crossover but the aftermarket aluminum heads on my engine have no provisions for exhaust gas flow to the intake.

It's worth the time but be aware that those old cast iron intakes are difficult to grind and machine.  I have about 8 hours alone just opening up from four holes to two larger openings.......Cliff

[Too Fast]

Cliff I see a 1968-69 iron intake for sale for $145.  First place I looked has one. From your testing you say this would be a great intake if I modded it like you did the iron one?  I have time this winter to do that. I have a .030 over Pontiac 400/Kauffman 74cc heads/4 tube 2.5" mandrel bent exhaust with X-pipe from Pypes.  Currently I have an Offenhauser 360 that you previously said to get rid of.  Seems to run fine and easily pulls to 5800 RPM but I am always looking for improvements.