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Discussion Starter #1
Hey all,

First time posting here.

My question is about hipo exhaust manifolds for my mostly stock 1966 289 auto, no ac, no ps. I was looking to upgrade to the hipos as I do not want to deal with leaks, heat, etc. with headers. I was hoping to get a little extra exhaust noise and a little hp gain.

I read several topics on here about them. I've seen many people mention port matching. I'm a bit new, so I watched some videos/did some research on this. However, if removing the heads/engine is currently not an option for me, would port matching the exhaust manifolds against a gasket be beneficial on its own? And not port matching the head? Is it possible to port match the head without removal? Any other suggestion?

Thanks in advance,

Dave
 

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The problem with the SBF is the extremely restrictive exhaust ports. If you don't open them up it won't make any noticeable improvement to add HiPo manifolds or headers. That would be like connecting a 2" fire hose to your 5/8" garden hose. You're only going to get a dribble out of the end of the fire hose.
Due to the close fit of the exhaust ports to the shock towers it would take a contortionist to open up the ports with the heads on the engine in the car.
 

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You can gasket port match and blend toward the valve a little bit with the heads on the car. I've done it.
It is a PITA. It's way better practice (and much more effective) to do the job right with the heads off the car.
The hipo manifolds are just a prettier manifold. You won't get much out of that swap. We used to extrude hone
the ports for cheater use. (street racing) Even then they aren't as good as a tubular shorty header.....
 

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Discussion Starter #4
Thanks for the quick responses. So would there be a benefit to gasket matching the exhaust manifolds but not the heads?

Also, is there even any benefit at all for hipos other than looks? Not even a little bit louder?
 

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Welcome to the forum.


As stated above, the restrictions are in the heads and you have to port the heads to see some HP increase. I don't think you will feel anything by opening the exhaust manifold. I personally would pull the heads. Sounds more scary than it is. You will have much better access to do the job right. Keep in mind it is not a job which can be done in 5 minutes. Porting takes time if you want to do it right.



I heard about dyno tests with stock and HiPo exhaust manifolds without showing a significant increase in HP. Going from stock exhaust manifold to heads is a different story, but has other drawbacks.



Keep in mind, an engine is a complex system. Changing one piece doesn't necessary mean you will gain anything. At the end everything must fit together (carb, intake, heads, exhaust, cam,...). If you want to get some experience get the desktop dyno simulation program. I don't think it is very accurate, but it will show you the effect of different mods.
 

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Fellow VMF member Josep from Ibiza bought a '66 Ragtop in Grand Rapids, MI. I flew up there and drove it to my house in Texas. It has a C code 289 that has been rebuilt once with .030" over pistons but I don't know if a custom cam was installed or not. It had a throttle body EFI on a Performer manifold and dual exhaust added.
When I got it home I pulled the heads and found the huge Thermactor bumps in the exhaust ports. I ground them out and opened up the ports per 22GT's diagram. I installed repop HiPo exhaust manifolds. The modified heads with the HiPo manifolds made a noticeable improvement in the engine's performance.
 

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When I did my last engine rebuild I gasket-matched my heads and exhaust. I'm not sure it did much but I'm a fan of every little bit helps. I'm not sure I would pull the heads off to do this though, as stated before, you're not going to gain much.
 

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If you don't plan on doing any port work to the heads, here is what I'd do.

a. Take some scrap boxboard (cereal box type) and using the STUD HOLES in a new set of cheap paper gaskets punch holes in the piece of boxboard, place over the studs up against the heads. Secure with washers and nuts.

b. Using a nice, sharp X-Acto knife, cut out the port openings in the boxboard.

c. Transfer the boxboard templates to the HiPo manifolds, securing in place with nuts and bolts. Mark any place where you can see metal through the holes with an ink pen or magic marker.

d. Remove the boxboard templates, get out your Dremel and either sanding drums, burrs, etc., and remove the area you marked plus an additional 1/16" at the same place. No need to remove any more since the holes are now bigger than the ports in the heads.

Why the extra 1/16 inch, you ask? Better to have a slight ridge protruding "on the way back" to lessen reversion pulses and also allows for some variance in making your holes.

If you don't want to be "daring" to try installing the HiPo's like the factory did, without gaskets (I'd only do this if I had the manifolds surfaced first) then take the cheap paper gaskets, lay them out in the bathtub soaking under water overnight, and install them wet. When you have everything hooked up, fire up the engine to dry them out.
 

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Personally I don't think the hipo manifolds are much if any improvement over stock. For one, they still have a 2" opening like the stock. One of the Mustang on line magazines a few years back did a chassis dyno test on a 289 67 Mustang. In one day on the same chassis dyno they did back to back to back tests between the stock, K code and try y's. The stock and K code made exactly the same power! The try y's made something like 18 hp more to the rear wheels.

A few years back when I put my GT40P in my 66 I was going to use stock exhaust manifolds temporarily. I took a exhaust gasket as a template and slightly opened each port along with smoothing out the transition. I also opened up the outlet of the manifolds quite a bit. Unfortunately the Z bar adapter bracket interfered with the inner exhaust stud so that put a end to that. I would have loved to see how they worked.
 

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One of the Mustang on line magazines a few years back did a chassis dyno test on a 289 67 Mustang. In one day on the same chassis dyno they did back to back to back tests between the stock, K code and try y's. The stock and K code made exactly the same power! The try y's made something like 18 hp more to the rear wheels.
I take a lot of these write up's with a grain of salt, especially when the article ends on the left facing page and the right facing page is a big advertisement for the product "winning" the test! :pirate:>:)
 

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It sounds like different mufflers are what you really need. I haven't noticed a change in sound on my cars even when going from stock manifolds to full length headers. I think mufflers are the only thing that affect sound.

My favorite first mod is mandrel bent exhaust pipes sized for performance with summit turbo mufflers. There is always a good increase in sound and power. The summit turbos are at the top in flow tests for turbo mufflers. They have a great sound but definitely too much of it for your girlfriend or wife. At first you will be probably worried about getting arrested but I never have been. Straight through mufflers like glass packs, magna flows or borlas are another step up. One of my cars definitely has magnaflows in its future.

For a 250hp engine the charts show that dual 2.25in exhaust or single 2.5in would be the right sizes.
 

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The problem with the SBF is the extremely restrictive exhaust ports. If you don't open them up it won't make any noticeable improvement to add HiPo manifolds or headers. That would be like connecting a 2" fire hose to your 5/8" garden hose. You're only going to get a dribble out of the end of the fire hose.
Due to the close fit of the exhaust ports to the shock towers it would take a contortionist to open up the ports with the heads on the engine in the car.
And I know you are a very good contorsionist as you have already done this job before.!


Josep
 
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