Increasing Intake manifold volume on a 59ci mercury

[smitherik33]

I apologize if this is being posted twice. I don't think my initial post uploaded.

Does anyone know if increasing intake manifold volume is beneficial? I recently designed and installed a 3/16" intake manifold spacer on my 59ci mercury. I don't have a vbox or a dyno so I don't really have any way of measuring its results. I also had some prop work done and some other modifications done when I put the spacer on. I've heard of guys running 3 gaskets between the manifold and reed plate to increase performance, that is what actually sparked my idea of making a spacer. I'm thinking that this would especially be beneficial on high rpm powerhead with some aggressive port work. What's everyone's thoughts on my idea? Do y'all think this could be a beneficial bolt on part? I also have a few of them available if anyone is interested.

I have uploaded a photo of one installed on my motor. Notice the bare aluminum plate under the manifold.

[Powerabout]

generally more volume in there makes its lazy at the bottom end when you run carbs?
I can see one area to fix, your carb entry's a squared off, that's not conducive to flow

[Mike Schmidt]

My vote would be NO. Over the years we have found that the closer you can get the carburetor to the motor the better off you are. Two data points. Years ago I did a lot of Yamaha KT 100 kart engines to help fund my boat racing. We tried a lot of stuff on the dyno (that would not pass tech) then stretched the rules in that direction on the race motors. Milled the carburetor mounting surface on the cylinder to the minimum dimension, milled the adapter plate to the minimum dimension, shortened the carburetor to the minimum dimension (all off the mounting flange side) and then assembled it without any gaskets. Was worth 1/2hp on a 15hp motor.

I built several Kawasaki KX 125 based 250 opposed twin 250 motors. Spent all winter building a new crankcase, with the reed cages kicked up toward the sparkplugs for better air flow. With this layout the two carburetors crashed into one another, so I welded up a set of carb mounts that looked like they came off a Pro Stock race car. Filled the corners in with epoxy so everything flowed real nice. Looked trick, but put the carburetors 3 inches or so further from the reed cages. The motor was a dog at best. Tried all kinds of stuff, but was still a dog. Ripped it all apart, welded up the crankcase, put the reeds and carburetors back in the "normal" location. Won the 250 Hydro National Championship at Depue with it.....

Michael

[John Schubert T*A*R*T]

My vote would be NO. Over the years we have found that the closer you can get the carburetor to the motor the better off you are. Two data points. Years ago I did a lot of Yamaha KT 100 kart engines to help fund my boat racing. We tried a lot of stuff on the dyno (that would not pass tech) then stretched the rules in that direction on the race motors. Milled the carburetor mounting surface on the cylinder to the minimum dimension, milled the adapter plate to the minimum dimension, shortened the carburetor to the minimum dimension (all off the mounting flange side) and then assembled it without any gaskets. Was worth 1/2hp on a 15hp motor.

I built several Kawasaki KX 125 based 250 opposed twin 250 motors. Spent all winter building a new crankcase, with the reed cages kicked up toward the sparkplugs for better air flow. With this layout the two carburetors crashed into one another, so I welded up a set of carb mounts that looked like they came off a Pro Stock race car. Filled the corners in with epoxy so everything flowed real nice. Looked trick, but put the carburetors 3 inches or so further from the reed cages. The motor was a dog at best. Tried all kinds of stuff, but was still a dog. Ripped it all apart, welded up the crankcase, put the reeds and carburetors back in the "normal" location. Won the 250 Hydro National Championship at Depue with it.....

Michael

That's exactly what I did on the SST45 & 60 motors. Milled the back side of the reed mounting block & the front of the CC. No tech spec. Found out years ago the the MK25 CC on the popper accelerated better then the 20H CC so tried it on the 45 & 60 & beach starts were phenomenal.

[hightide]

Tarver put out pretty much the same thing a while back, they help. I think it is more about allowing the airflow to straighten out rather than increasing volume. The reed plate and intake are very tight over the reeds and I think possibly limits flow. Jeff Dunn makes a 1" spacer that goes behind the carbs that people swear by as well

[fs5]

I agree with what hightide said . Also porting the reed block helps a little to.
If you do a search I think I had on old thread on it

[smitherik33]

Guys I think a few of you are going a little on the extreme side. 3 inches is obviously way too much of a volume increase. That just sounds like a bad idea, thats why I only went 3/16. I have recently spoken to a few guys that have done what I have done and they all said they increased top speed, without noticing any drop off in bottom end. My opinion is that this modification is better suited for a high rpm race engine that spins somewhere in the 7-8k range. Also, fs5 you actually sparked my idea when you recommend that I should run 3 gaskets between the manifold and reed plate. I think this 3/16 spacer would basically be the equivalent of running 6 gaskets, but with a lower possibility of an air leak.

[smitherik33]

Hightide, I agree with your train of thought. I also think people get intake volume and crank case volume confused. On a separate post a guy tried telling me that I was decreasing crankcase pressure. He failed to notice that the spacer was between the manifold and reed plate and not between the reed plate and crank case. Its all about finding a happy medium, im sure this would be counter productive if it were spacing the carbs out several inches.

[rumleyfips]

Try " Name that outboard " page 25 on this site. On a 22ci OMC, a 1/4" spacer between the reed plate and the intake manifold was good for 1 hp.

[smitherik33]

Try " Name that outboard " page 25 on this site. On a 22ci OMC, a 1/4" spacer between the reed plate and the intake manifold was good for 1 hp.

Having trouble finding it. If you have a link handy I'll take a look at it. Sounds logical though. There's probably some motors that benefit from it and some that don't.