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'90 HF Exhaust Conversion (??)

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'90 HF Exhaust Conversion (??)

Postby festivus31 » Thu Feb 09, 2017 7:07 pm

When I rebuilt my HF's motor, I noticed some obvious melting of the catalyst in the cat converter. I didn't have the money at the time to deal with it, but I do now!

I haven't been able to find a direct bolt-on replacement converter, as the HF's converter is bolted directly to the manifold and appears to have a non-standard design. It's not even shown in my Haynes manual.

To avoid any other headaches involved with the HF's "unique" exhaust layout, I was wondering if there would be any obvious problems with sticking another crx or civic's exhaust on my HF, so I could use the standard under-car catalytic converter layout.

Is there a specific sub-model's exhaust that would be the easiest fit/find for the HF? I know I would have to replace the manifold, any D-series manifold will fit the head, right?

Any info is appreciated.
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Re: '90 HF Exhaust Conversion (??)

Postby Baker » Fri Feb 10, 2017 6:54 am

Any 88-91 Crx Si or "Dx" model exhaust will bolt up, but you will need the entire exhaust, from exhaust manifold back.
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Re: '90 HF Exhaust Conversion (??)

Postby 4drSpankR » Fri Feb 10, 2017 11:33 am

You may get poor performance or less MPGs' with the cat located underneath the car on an HF. The cat is specifically designed to be in that location due to the fact that it needs to heat up very quickly (and maintain that heat) because of the low displacement of the engine. Although I'm sure your options are already pretty limited with that engine as is.

msc can provide you with a bit more info.
1991 HF - previously '90 B18A1 now '97 B18C1
1989 Si - running parts car/gone
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Re: '90 HF Exhaust Conversion (??)

Postby msc » Fri Feb 10, 2017 11:55 am

There are no conversions or upgrades for a stock HF exhaust system. All the details that are special to the HF exhaust were done for a reason. If you want your HF to run like an HF you need to keep the exhaust as stock as possible.

The O2 sensor is placed very close to the head where the exhaust from all four cylinders is merged together. This is to keep the O2 sensor up to operating temperature. Move the sensor further downstream like in all other manifolds and the O2 sensor will not function properly under all conditions. Expect issues in cold weather, cursing under light loads, or city driving where you do a lot of idling.

The cat is placed as close as possible to the head to get the exhaust into the cat while it's hot enough to bring the cat up to operating temperature. Move it down stream and you may have trouble meeting emissions if you have to test. If you don't have to test but maintain the cat because it's the right thing to do for social responsibility you will not be getting the full benefit of having a cat.

Increase the pipe diameter from the stock 1.5" and you tune the exhaust to a higher RPM thus loosing some of the low end torque needed to push the tall gears.

The good news is there is still a proper cat available. The Magnaflow 50210 looks like a correct fit to bolt to the HF manifold and incorporates the forward pipe section (A-pipe) with the cat. The MagnaFlow 24446 looks like a correct fit for the cat alone. The DEC Converters HON1611 also looks to be a direct fit and might even be C.A.R.B. accepted.

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Re: '90 HF Exhaust Conversion (??)

Postby festivus31 » Fri Feb 10, 2017 4:44 pm

Thanks for the info msc...that was very helpful. I'm going to go ahead and get the Magnaflow 24446. If the picture is anything to go by, everything looks right except the bolt posts sticking off either side. Shouldn't be a big deal though. Otherwise, every site I've been to states this converter is compatible with the HF specifically. I'll report back with the results when it arrives.

Thanks again!
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