Air Filter Housing

[frober262]

Does anyone know if the air filter housing for the Rochester 4 BBL and the TBI are the same? I am looking at switching my stock Rochester on the Motorhome for a TBI set up from an 90 c3500 454 and we are trying to eliminate as many problems as we can before we get our hands dirty.

[EagleMark]

Yup, some are not high enough for TBI, but circumfrence is the same.

[gregs78cam]

There is a spacer between the TB and the Air filter housing that can be used to get it back to the right height. Actually there are two different heights. I think they are something like 1" and 2". It also has the filtered air port for the PCV system.

[frober262]

Thanks - actually the lower I can get it the better! The MH has a Banks Air Filter on it with a KN filter and it just clears the dog house now. So I'm thinking that I may have to eliminate the OEM heated adapter and go with an after-market non heater adapter, except that I have heard that if I drive it in cold weather that I risk icing up the TBI.

[gregs78cam]

If you are talking about replacing a Quadrajet, I don't know if you know this but the throttle body is actually shorter than the carb. TB is 2", Q-jet is 3.25".

[EagleMark]

and then the monstrosity of a TBI adapter GM used (because same engine went into carb trucks as well) is about the missing 1.25 inches. So same height. Aluminum adapter would save about 3/4 inch.

The big block TBI had a tendency to ice under load like pulling a trailer up a mountain 80 to 100 MAP. So much fuel and air mixing and evaporating it iced but only lower ambient temps and I don't know numbers. For idle they all have an issue if ambient temp is close to 32 with high humidity, that is the reason for the hot air tube of the exhaust. The heat riser valve is in the small block base, no vacuum lines and works great in all cold weather, closes when hot. The BB ones I've seen don't have one? (more info?) The SB 5.7L air filter fits the BB 7.4L TBI with no spacer if it needs a spacer use a Power Bowl, can also add another snorkel on other side and use a BB lid for bigger air filter. Guy on FullsizeChevy did a cool one.

[frober262]

Sorry, I didn't explain myself properly. You got the picture. I'm swapping the Q-Jet for a TBI in the hopes of getting better gas mileage and making it easier to pass Ontario's bi-annual emission test. This year it took 3 tries @ $90 a pop, before the tech took pity on me and leaned the idle out to pass and then reset it by ear so that we could drive it. Gas mileage went in the sh** tank. Now he wants to sell me a rebuilt for $425 installed. With gas @$4.50 - $5.00 per US gal, you can see why we are headed into TBI country.

[EagleMark]

Emmissions will be better as well. Don't expect miricles in MPG in a motorhome with a BB, but it will improve. There's no drawback other then installing it.

[907 Chevy]

Sorry, I didn't explain myself properly. You got the picture. I'm swapping the Q-Jet for a TBI in the hopes of getting better gas mileage and making it easier to pass Ontario's bi-annual emission test. This year it took 3 tries @ $90 a pop, before the tech took pity on me and leaned the idle out to pass and then reset it by ear so that we could drive it. Gas mileage went in the sh** tank. Now he wants to sell me a rebuilt for $425 installed. With gas @$4.50 - $5.00 per US gal, you can see why we are headed into TBI country.

Holy chit, they still charge you if you don't pass?

I agree with mark, big block and motor home equals bad gas mileage no matter what. My 28' 88 Jayco with a 460 and C6 transmission gets 7 mpg. That's a fuel injected big block as well....

[gregs78cam]

Qjet=3.25", Heated BB TBI adapter plus throttle body = 3.25"

First post, top three pics....
http://www.thirdgen.org/techboard/tb...ter-plate.html

[frober262]

Thanks Greg that link hit the mark and your height measurements along with Marks comment about the diameter closed the deal for me. As a side note, I am told that the vanes on the adapter are to promote better distribution of the A/F mixture to cylinders, apparently some cylinders tended to run lean on the BB. I agree with Mark & 907 trying to push a 6 ton brick down the road at 60 MPH isn't going to give you great gas mileage; but, 6 MPG versus 5 MPG would have made a difference of 80 US Gal on our trip in 2011 .

Yes, 907 they charge you every time they conduct the test. On a car it is only $35 - 1st time and $17.50 - 2nd try then they cap the cost at $450 to repair. If the estimate for the repair exceeds the limit they give you a conditional pass in most cases . However for MH's and similar heavy vehicles there is no "get out of jail free" card and you must pass regardless of the cost.

[907 Chevy]

Does your motor home have an overdrive transmission?

Most likely a turbo 400 behind that 454, I can't imagine it being anything else. Its lack of overdrive thats killing your fuel economy. I'd swap in a 4L80e since you're already planning on converting it to fuel injection.
Swapping to TBI might gain a little more mileage, but that big block is still gonna be turning high revs on the highway and is still gonna be using a lot of fuel as a result.

[frober262]

I have heard about the 4L80 , one of our local shops offered to rebuild one for me with heavier bands & plates but he can't install it. That will be the next upgrade .

[EagleMark]

Then may want to rethink TBI upgrade? The ECM you have should be a 1227747 and will not run the eTrans. You could repin new connectors and a few wires for a 16187427 when the trans comes around.

[JeepsAndGuns]

I agree with mark. If you are thinking about swapping in a 4L80E, then might as well set up the TBI system to be ready for it now. Like mark said, just re pin the harness for the 7427, swap it in and tune it. Even if you never swap in the 4L80E, you will still have a better tbi system with the 7427.