258 Intake Manifold Gasket

[JeeperChris]

Interesting but watch the Offy. I believe the design intent is that the primaries sit inboard and give advantage to low speed. The volume of the seperate planes are different and move air differently.
If your carb doesn't need that (sounds like the carb has atomization and low throttle beat in its design) I might consider a single plane intake manifold.
The Offy is not split between cylinders like you would expect on say a V8 application.
They are split on each cylinder for low and high throttle.
Not sure there is any advantage with an Offy on that carb. Not much anyway.
Different story when dealing with an old dogleg carb. There is a marked improvement in that application.

Not sure, as I said before, I am not familiar first hand with your carb.

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I've been reading the Offy page about the DP manifold and i see what you were saying. I'll make sure it goes on correctly, thanks.

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[JeeperChris]

Do you have a sloped riser for the intake to mount the new carb on? That 1" riser makes a difference.

This is another part I'm having a hard time finding. Is this the one they call a 'wedge'? I see a lot of 2" angled spacers but not many 1". Where did you get yours?

 

[mitchelllogic]

This is another part I'm having a hard time finding. Is this the one they call a 'wedge'? I see a lot of 2" angled spacers but not many 1". Where did you get yours?

No. A wedge will be a significant angle. Those are meant for pretty dramatic engine tilt. I would have to know how much engine tilt your dealing with, or what you have for a lift, dimensions of your driveline and angles, etc. I'm guessing if your valve cover is a 1/2" from your firewall and your motor mounts are still close to stock location your likely similar to mine at about 7degrees.

I think you will need an "angled 1 inch spacer. Reduces to about .9" if I recall. Maybe a bit more.

The idea being you want the carb to sit parallel to the frame regardless of engine tilt.

It's not critical but it does help with fuel delivery on road.

A 1 inch riser by itself seemed to help mine run better but putting the angled one in was even better.

Carbs do get finicky up here though.

MoPac (Canada) J.Bs Power Centre has them as well (saw on last weekend). Just about any speed shop should have them. There are ones for Holly and Eidelbrock. Not sure if those are a fit for yours but the Holly should be really close.

Speedway motors has some in red anyway Part # 3912502. But check all dimensions because I'm not familiar with that one. Seems pricey though.

In the photo below you can barely notice the angle of the spacer (the photo angle doesn't help) but you can see the tilt on the valve cover. That's all your compensating for.

Oh and your air filter will sit properly instead of on an angle tipping back.

Most would leave it and just bolt and go. I'm just fussy that way.

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[mitchelllogic]

Let me know what your thinking about hooking or not hooking up the water ports on the intake manifold to your coolant system.

For the record I firmly believe the coolant ports (2 of the 3) need to be connected to the coolant system and have benefit up to about 100 degrees F ambient temps.

The third port will need a plug. (Points at the engine midspan). Don't overtighten fittings or plugs. Use a high temp sealant and let it full cure before adding coolant.

If you route the return line over the valve cover to the heater core (5/8" lines) you create a bit of a high spot so it's a good place to install an auto vent to purge air.

I haven't done that yet but it's on my list.


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[JeeperChris]

Just looked up that part number on Speedway Motors and that's not the right one (for anyone also looking at it), the bottom of that spacer is not flat and will disrupt the separation between primaries and secondaries on the Offy Dual Port manifold. I think the contours on the underside of that one may be why it costs so much. My Demon is just a fine tuned Holley and shares 99% of the parts so I'll find the right spacer now that I know exactly what I'm looking for.

My Jeep is still stock- no lift, so my engine still has that stock angle to it. When I do lift it next year it will get that 1" skid plate drop though.

I will absolutely be hooking up the water jackets on the Offy manifold. I'm in the process of doing it to the current OEM manifold I put on the Jeep so it will be an easy transition when I put the Offy on there. I even have the vent line inserts to purge the air.

I also purchased one of those Lokar throttle cables and a bracket for my current Weber carb which I've been trying to sort out for a few months now. I'll pick up the 4-bbl bracket when I get the manifold towards Spring time.

 

[mitchelllogic]

Just looked up that part number on Speedway Motors and that's not the right one (for anyone also looking at it), the bottom of that spacer is not flat and will disrupt the separation between primaries and secondaries on the Offy Dual Port manifold. I think the contours on the underside of that one may be why it costs so much. My Demon is just a fine tuned Holley and shares 99% of the parts so I'll find the right spacer now that I know exactly what I'm looking for.



My Jeep is still stock- no lift, so my engine still has that stock angle to it. When I do lift it next year it will get that 1" skid plate drop though.



I will absolutely be hooking up the water jackets on the Offy manifold. I'm in the process of doing it to the current OEM manifold I put on the Jeep so it will be an easy transition when I put the Offy on there. I even have the vent line inserts to purge the air.



I also purchased one of those Lokar throttle cables and a bracket for my current Weber carb which I've been trying to sort out for a few months now. I'll pick up the 4-bbl bracket when I get the manifold towards Spring time.

Good Luck!
Post your progress. I'm not the only one interested to watch the build I'm sure.



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