exhaust manifold question

[mattyrich]

1981 mercruiser 140 serial number (6018808)

Manifold from 1991 mercruiser 140 3.0 serial number of motor it came from
(OD619732).

Swapped ma am planning on buying a new manifold and exhaust elbow. I need a manifold that uses a Mercarb and is compatible with my head, from the 81' 140, and fits the 45 degree angle exhaust elbow so it will match up with my alpha one outdrive. I do have the head from the 91' motor as well but im really not sure of the condition of it whereas the 81' is running good right now.

I have a total of three manifolds, one form the 91' that is one piece & i want to use because i know it will line up perfectly with my carb and throttle linkage on the valve cover. ( i swapped to the 81')

two with endcaps that both came from the 81' one is set up for a rochester carb, and the other is set up for a Mercarb. I cant use the one thats set up for the rochester carb because there is no arm on the throttle body, to hook up to the cable in the boat. also, i dont think that i can mount a 45 degree angle elbow to it?


The one with the bolt patter for the mercarb might work but not sure. problem is the exhaust elbow lining up, and/or the manifold itself is more narrow than the later style one piece and im not sure if the throttle cable has enough play to work with the throttle cable.

Sorry for the long post, but i would like to get the most accurate response as possible

Are the bolt patterns the same on these manifolds(to mount to the block) also, all of these manifolds are in used condition, and if im gonna put anymore time into this motor, i think i should just buy new. I want to make sure i get the right parts.

Please help

 

[mattyrich]

Re: exhaust manifold question

Sorry for the double posts. It was a mistake...

Any help?

I just need to know if a manifold from 1991 fit the head on a 1981? both are mercruiser 140's.

Thnks, Matt

 

[jerryjerry05]

Re: exhaust manifold question

The manifolds should fit.The risers won't match up with the exhaust tube.
You shouldn't use an old manifold/riser.
Contact Barr or OSCO manifolds.They will help with what you need.J

 

[mariner1900]

Re: exhaust manifold question

Having recently fitted a 91 engine to replace a 74 model engine I found the following difference between the manifolds.

The 91 manifold is bolted on with only five bolts.
The 81 manifold is bolted on with a combination of 6 bolts & studs.

The 91 manifold has smaller and different shape ports. Ports are approx 2/3 the size of the ports on the 81.

So whilst the manifold may bolt to the head the you will have 1 less bolt and the ports will be partially blocked due to port misalignment, size and shape. This will probably lead to bad flow and a loss of power.

The 45 degree riser from the 91 manifold will bolt straight to the 81 manifold. The gasket is the same. The only difference is that the 45 is held on with just 2 bolts instead of 4 studs. I am currently running the straight, old style, riser on a 91 manifold with no problems. So see no problem the other way.

 

[mattyrich]

Re: exhaust manifold question

Thanks Mariner, I have been looking at yours, and fishrdan's posts for a while now because i notice your into the same mess i am. lol... I'm not sure which direction to go at this point. I dont want to put the money into a new exhaust manifold and elbow if it is not going to perform. SO basically you were doing the exact opposite swap that i was. I have the straight back elbow and probably some other good parts you may want as back ups. Any chance that we could speak on the phone? PM me if your up to chat about this swap. Thanks, Matt

 

[mattyrich]

Re: exhaust manifold question

Exhaust manifold for a 1981 mercuiser 181, 140 hp. It is goin into a boat with alpah one gen II outdrive. the boat had a 91 mercruiser 3.0 140 hp. I have been told that the manifold from the 91' ( 608687A 8) should fit the head from the 81'.(81? motor serial number 6018808). I know that the head is attached with six bolts and the new manifold is five bolts, and the ports are different sizes. I konw it will line up, but are the differences enough to cause performance issues. I have been getting mixed answers on this subject and want a solid answer berfore i purchase. I need to have a manifold that will fit the 45' exhaust riser, so that it fits up to the exhaust on the alpha one drive that is in the boat. It also must fit the Merc-Carburator that lines up with the throttle control cable that is in the boat ad attaches to the valve cover. I was considering the manifold that was made in the 80's that has two end caps and fits the mercarb. Will this model fit the 45'd exhaust riser? I need to know if the width difference on the manifold can be compensated with the cable i n the boat, as the 91 manifold is a wider and the intake sits about 4-5 inches further away from the head? Thanks for any advice, Matt

 

[elkhunter338]

Re: exhaust manifold question

I put a manifold for a 1971 140 on a new 2007 engine. The reason was the manifold on the 1971 was almost new and I had to replace the engine. Michiganmotrz sold me a special manifold gasket, metal composit one. My local mechanic said with that type of gasket the larger port manifold will be fine on the smaller port head. All other bolts matched and bolted right up.
130 hrs later everything is running fine.