Carburetors, intakes, and Spacers. Oh my!

Stepping back in time a little. 

The engine in question is a Ford 302. It will be going in a boat. The RPM range will be idle to 5000. It will spend a lot of time in the 3000-3500 range. The carb in question is a Holly 600 cfm. 

Starting the assembly of the SBF 302 going in the boat. The cast iron intake is a nasty mess from saltwater being pumped through it. I'm switching to a closed cooling system which will allow me to run an aluminum intake. My business partner gave me an Edelbrock Performer 289 aluminum intake. It is dual-plane but has a slightly larger plenum area than the original. 

I also need a phenolic spacer under the carb to insulate it from the intake. They come with 4 ports to match the carb, 2 ports to match the intake, and one big opening. They also come in a wide assortment of thicknesses. 

Is the larger plenum going to cost me much in the way of low-end torque? 

As for the spacer, thicker or thinner? I assume a thick spacer makes the plenum larger. Is that also the case if the spacer is ported to match the carburetor or only if the spacer is ported to match the intake? Which is better for my situation. 

Does any of this even matter enough to be noticeable or am I overthinking it? 

Plenum will be fine. Seems to affect throttle response more than torque. 

A thicker four hole spacer would probably be what is neded. The four hole help the carb signal from what i understand. 

In reply to Dusterbd13-michael (Forum Supporter) :

Thanks. I wondered if it was as big of a deal as the advertizers make it out to be. 

I've used 4 hole and open spacers and not noticed any significant difference. On a stockish motor in a boat I doubt they'd make any noticeable difference.

For a small block in a boat and your intended RPM range I wouldn't worry about it. 
I used a really nice looking tapered bore four port spacer on my 351C a few years back.  After a few days of driving with it I was struggling with the throttle sticking, a problem I'd never had before.  The beautifully tapered bore holes left the webs so thin that the webs distorted and interfered with the throttle plates.  Summit gave me a full refund on that one.  

Car Craft of Hot Rod did a monster test on this kind of thing back in the day.  I think they had a few intakes, all the possible carb spacer configurations, and a couple different carbs.  Their test motor had a lot of chest hair, IIRC... 406 with 18 degree heads and 11:1 with a circle track cam.

Results didn't follow logic.  In the case of a 4-hole spacer where you might expect HP to not change much and Torque to increase a bit, sometimes the opposite happened.

The dual plane you have (performer 289) is a true dual plane, so my guess is that a 4-hole spacer will have zero effect, a two-hole spacer that matches the intake will have zero effect, and the open spacer will have a very tiny effect of possibly giving you 3 more HP at the expense of possibly losing a bit of midrange torque.  There will be a point in the harmonics where the "sharing" will favor one side and then the other.

I would personally go with the 2-hole that matches the plenum.  You could also do both the 2-hole and the open and try them both.  I have a feeling you'll never notice a difference.

As far as thickness, don't stress.  First, the heat that the carb sees isn't the end of the world even on a car, so in a boat situation where you have an endless supply of cold sea water you won't have the soaking effects of air-water cooling like a car.  Then, think about the overall distance from the throttle plates to the valves.  They're already vastly different from one port to the next, so it's not like the thickness of the spacer will make the magic harmonics happen.  Even if it did, it would make the magic happen for one or two runners, and only for one or maybe two points in the RPM range.

TL;DR... I think you're overthinking it.  Pick a spacer and put it on.  If this were a race engine where the last 5hp was important, I'd take the time to check it out.  On a boat engine which is engineered specifically for low end torque, I don't think you'll ever notice anything.

Another thing to consider, in a boat you're riding on water with exponentially greater resistance than in a car in the air.  In a car, you might be able to tell the difference with 15 more hp.  In most traditional boat hull designs, it takes about 20 more hp for a gain of 1 additional mph.  As long as the spacer you choose doesn't cause bogs, mixture issues, or a vacuum leak, it won't change your top speed.  The rule in my boat was that if the captain needed to go faster, all passengers agree upon boarding that they may be thrown overboard at any time without warning.

In reply to Curtis73 (Forum Supporter) :

This engine is getting a closed cooling system and a 190-degree thermostat instead of the usual 150-degree thermostat used in raw water cooled engines. I'm concerned it will have heat issues a car won't due to limited airflow. It will be getting a couple of large bilge blowers to try and keep the engine compartment temps under control.  

Creating a bog or a dead spot in the middle of the RPM range was not something I wanted to figure out. 

Motor Trend TVs Engine Masters just did an episode on carb spacers. It was episode 61 titled "Carb Spacers, Gimmick or Gold. If your a Motor Trend subscriber you can watch it online if not do a search to your cable provider to see if it is being replayed. They did testing on both single and dual plane manifolds with four hole and open spacers. I really don't remember all the details but seemed the open spacer had a slight advantage.

Toyman01 + Sized and said:

In reply to Curtis73 (Forum Supporter) :

This engine is getting a closed cooling system and a 190-degree thermostat instead of the usual 150-degree thermostat used in raw water cooled engines. I'm concerned it will have heat issues a car won't due to limited airflow. It will be getting a couple of large bilge blowers to try and keep the engine compartment temps under control.  

Creating a bog or a dead spot in the middle of the RPM range was not something I wanted to figure out. 

 

I did see that you were using a closed cooling system.  Doesn't matter.  Your cooling system is water-water with water-cooled exhaust manifolds.  You won't have even one tenth the engine room heat issues of an inefficient air-water cooling system in a car.  You could run that thing for two hours at full throttle and be able to lay your hand on the exhaust manifold and not lose any fingerprints.  Heat soak in the carburetor is going to be a complete non-issue.

Edit to add:  I also wouldn't worry about extra blowers unless you have done something to change the design of the boat or it is a displacement-hull application.  Every boat manufacturer has to comply with US Coast Guard safety testing, which - for inboards and I/Os - includes a CFM of passive vented airflow through the engine room rated at a certain speed.  That airflow must be greater than X cfm (which I can't recall right now, but it's a lot more than a blower will produce).  Even prior to USCG certification, it still had to comply with BIA but I don't know what those rules were like.  I think if you put a temp sensor in the engine room, you'd see 100F tops compared to 250 or more in some car engine compartments.  In a car you have all the heat from the radiator, 1000 degree or more exhaust manifolds, and the only thing you have to keep it cool is the incredibly weak density of air.  You're taking the heat of the engine, transferring it to air, then blowing that air into the engine compartment.  In a boat, none of the engine components ever get hotter than about 200 degrees and instead of hot air trying to cool things, you have the incredible efficiency of water to directly dump it back into the sea.  That heat never makes it to the engine room.

Regarding the spacers and plenum design:  An old-school trick to get a few more ponies was to take a dual-plane intake like yours and cut a chunk out of the plenum divider.  Now, instead of two barrels feeding four runners, every runner is open to all four barrels.  It effectively increases plenum volume without changing the length or velocity of the runners.  What it can sometimes do is reduce low and mid-range torque as increasing the volume in the plenum reduces it's velocity inside the plenum.  This can cause mixture/puddling in the low and mid range.  Until you get to a high enough harmonic frequency, it can reduce torque.

That plus the fact that it takes a buttload of HP increase to effect just 1 mph gain is why I suggested just matching the spacer/gasket to the plenum and calling it a day.

My Baja came with a 305 making 170 hp.  It was good for 45 mph.  I replaced it with a 383 with vortec heads making 325 hp and I was able to squeak out 60 mph.  Nearly double the power got me 15 mph.   Hence why I suggest not chasing power with spacers as it won't translate to knots.

If you want to experiment with spacers I encourage it.  You won't be making any more bogs or mixture issues than you will have to tune anyway.  Even a brand new carb needs to be tweaked when you bolt it on, so playing with spacers might mean a wee tweak to an accelerator pump or an idle screw, but nothing complex.

In reply to Curtis73 (Forum Supporter) :

The manifolds are the last thing cooled in the cooling system before the coolant heads to the heat exchanger. They will be above engine temperature and will probably be pretty warm. The coolant will be 190 or so when it gets to them. I'll have to measure that when I get it up and running. The manifolds on my Mercruiser with full closed cooling were way too hot to touch at anything above an idle. I'll grant that they won't be anywhere near 1000 degrees. I hope. 

My I/O boats all had gunnel vents for the engine compartment. This boat, being an inboard with the engine in the middle of the boat and nowhere near the gunnels or transom, has no provisions for passive ventilation. Once the engine cover and deck plates are down, the bilge and engine are pretty well sealed. It's getting two large blowers and I'll have to duct them back to the transom. The engine cover is also the driver's seat so I'd like to keep it as cool as possible. Keeping it from exploding is a nice bonus as well. 

The carburetor spacer is strictly for a thermal break between the engine and carburetor but I also don't want to install one that hurts performance out of ignorance. Having dealt with hot start problems before caused by boiling fuel in the carburetor, I'm trying to error on the side of keeping the carburetor cool. The way my luck runs, I'll probably end up with icing problems now. 

In reply to Toyman01 + Sized and :

Well, they'll boil at about 230, so let's hope they don't get that hot.

I'm interested in the cooling system.  I've never encountered a system where the exhaust manifolds were last.  Most of them are pumped equally from a crossover/stat neck.  I think I'd be a little concerned with a 190 stat and the manifolds trying to prevent boiling if they start with 190 degree coolant.

But, admittedly, I rarely encounter closed systems.

The engine room has to be vented somehow.  Probably under the floor.  I'm sure it's not a sealed compartment under the floor.  If it was a commercially manufactured boat, it had to pass some kind of test for engine venting.

Can't hurt to put an extra blower under there.

I think you'll be pleasantly surprised.  I think you'll go for a day of skiing and be amazed at how cool things stay.

In reply to Curtis73 (Forum Supporter) :

I sure hope so. 

This is a Half Kit. It is probably the most common setup and cheaper because the heat exchanger is a good bit smaller. The block is cooled by the coolant run through a heat exchanger. The manifolds and risers are cooled by raw water. They work very well in freshwater but not so well in saltwater because you have to replace the manifolds and risers every 3-5 years due to corrosion. 

What I'm using is a Full Kit. The manifolds are cooled by the engine coolant after it leaves the thermostat housing. The risers are still raw water cooled and will need replacing every 3-5 years in saltwater. 

The only difference between these line drawings and my setup is the I don't have a outdrive pickup or pump. My raw water pump will be drawing water through a seacock under the engine. 

If it's like the other kits I've seen, the stat housing mixes hot and cold.  I don't think they are suggesting that the manifold only gets water after the stat opens.  You would boil coolant within 30 seconds of startup.

That's pretty much what I have seen of other closed cooling systems.  I think it will do well.