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Old 08-19-2009, 09:08 PM   #1
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'88 Winnebago - GM 454 power/MPG issue

I would think this would be a pretty common subject, but a search didn't seem to surface anything relevant, so here goes:

Just bought a clean 27 foot '88 Winnebago Super Chief with 58K miles on the clock. All in all in great shape both cosmetically and mechanically, but the power seems low for the size of the engine vs the MH, and the MPG really sucks at a bit over 5 MPG with careful driving at 55 MHH or so - lots of mountain grades here in eastern Oregon, John day area, and these grades require range 2 with the TH-400, and in high, not enough power on any but the lesser and shorter grades - we're talking 3000-5000 ft. elevation.

This engine has dual exhausts, a 4 barrel carb, and the smog pump setup - and I guess my basic question is, is this the best this rig will do in regards to power and MPG, or is there likely something practical that can be done to improve things?
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Old 08-20-2009, 02:59 PM   #2
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Quote:
Originally Posted by Gary - K7GLD View Post
I would think this would be a pretty common subject, but a search didn't seem to surface anything relevant, so here goes:

Just bought a clean 27 foot '88 Winnebago Super Chief with 58K miles on the clock. All in all in great shape both cosmetically and mechanically, but the power seems low for the size of the engine vs the MH, and the MPG really sucks at a bit over 5 MPG with careful driving at 55 MHH or so - lots of mountain grades here in eastern Oregon, John day area, and these grades require range 2 with the TH-400, and in high, not enough power on any but the lesser and shorter grades - we're talking 3000-5000 ft. elevation.

This engine has dual exhausts, a 4 barrel carb, and the smog pump setup - and I guess my basic question is, is this the best this rig will do in regards to power and MPG, or is there likely something practical that can be done to improve things?
Nope....When I picked up the 83 Itasca (33) it was sick and delivered 4.5 MPG and both ends leaving the Coachell valley (Palm Springs) were 2nd gear only.....
Ran hot.."Ignition timing retarded" so in go the proper ignition center and weights (375 center and 41 weights) which now handed out a 22 mechanical advance and bumped up the base advance from 4 to 10. You will hafta cut the vacuum advance back to 10 or so...
Next step was the "Thorley headers" with 2 1/2 pipes along with some flow masters 70 series mufflers....
Dang near 8 MPG but that was not e'nuf so into the carb for the jet change (73) and the power rods spring which from the factory is setup at 11 inches of manifold vacuum and they pull up and dump a ton of fuel down the primary barrells...
Swapped out the 11 spring for a 6 inch spring and now I'm a dab over 9 MPG while towing a Saturn SC2...
Once I quit being lazy I'll install a (LM1) fuel sniffer and go from that point..Enjoy.

Jim
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Old 08-20-2009, 03:53 PM   #3
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Jim has you on the right path.

If it was me I think I would delve into the carb first instead of later. If it is the original QuadraJet carb, they are notorious for vacuum leaks and dumping excess fuel. They can be fixed and tuned as Jim describes or you can get a new replacement if yours is badly worn, but you'll still have to do the fine tuning on it.
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Old 08-20-2009, 04:31 PM   #4
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Jim has you on the right path.

If it was me I think I would delve into the carb first instead of later. If it is the original QuadraJet carb, they are notorious for vacuum leaks and dumping excess fuel. They can be fixed and tuned as Jim describes or you can get a new replacement if yours is badly worn, but you'll still have to do the fine tuning on it.
Nick, The reason to check the carb last is you will not know how rich (doubtfull) or lean condition the headers will create...
With the ignition fixed 100% and locked down exhaust should be next..On my 87 Itasca the jets were changed from 76 primary to 73s along with a 1 step colder spark plug.
Now with the 83 the " Thorleys" combined with the (X-Pipe) leaned it out e'nuf during the summer running and when the colder weather drops in I might go 1 jet lower....Depends on what the (LM1) prints out...

Jim
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Old 08-20-2009, 04:56 PM   #5
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Hmmm, right you are Jim. I was thinking a carb rebuild and tuning would give the quickest "bang for the buck" but then, you are correct, those exhaust changes would likely require further carb tuning.

Good call!
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Old 08-20-2009, 05:40 PM   #6
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Nick, Another bang for the buck is cold intake air.......
Measured mine in the heat (115) in July 07 with a digital temp gauge from Walmart and stock with the 8-9 inch air cleaner snout just hanging in the hot engine bay air the gauge read about 165.....Not good. And the probe was in the air cleaner next to the carb inlet
With a 4 inch air tube off a 84 chebby blazer that would extend a few inches beyond the radiator the air temps droped by 32, Not good e'nuf for me so I insulated with double sided aluminum bubble wrap everything on the air cleaner and the extended tube..
Another -8 gained for a total of 40, Point is for every 1 cooler air you will capture 1 more horse....So in essence 40 more horses were recaptured and that's something you can feel while climbing a stout hill.....
There is a formula for that by the way but "Tony Pedragon" (funny car driver) said it the best...

Jim
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Old 08-20-2009, 11:44 PM   #7
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You da man, Jim.

Its funny - I was thinking some more about this message thread as I was driving home from work and thinking that a good cold air intake would be the icing on the cake. Then when I get home I see your comments on it. Great minds, huh!
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Old 08-21-2009, 10:39 AM   #8
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With the (OP) Gary he stated the altitudes are 3,000 to 5,000 feet while driving around, Take the 5,000 foot.....That eliminates 15% of available HP right off the top.
I've yet to test any RV engine that runs lean unless they have a severe vacuum leak, The Rochester carb right off the shelf has a serious disavantage on a heavy vehicle with the power rods set to open up at 11 inches of manifold vacuum, Hard to find a RV that isn't cruising around the 10-12 inches of vacuum and plunging downward at the slightist headwind or hill....
Now just with the ignition mods many can cruise down the flat slab right around 12-15 inches of vacuum.....Mine with the other ~stuff~ on it runs 14-17 on the flat road and it's rare to run below 6 inches on a climb....Even outta the Coachella valley eastbound on the I-10...
The vacuum gauge is your friend and I install them (2 1/8 gauge) permanetly in the dash, With the ignition set and LOCKED DOWN and the exhaust (with X-PIPE) completed the carb is sniffed out and setup for the jets and depending on the age/mileage on that carb I will spend the money at the "Carb shop" for a stage 1/2 setup which includes the power "Spring" and maybe a small jet change depending on the "Snapon" sniffer....
Gotta get off my lazy rear end and install the (LM1) fuel analyzer (ain't cheap) just for testing cuz some better mileage might be rung out of it....

Jim
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Old 08-21-2009, 01:54 PM   #9
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Originally Posted by JimElliott View Post
Nope....When I picked up the 83 Itasca (33) it was sick and delivered 4.5 MPG and both ends leaving the Coachell valley (Palm Springs) were 2nd gear only.....
Ran hot.."Ignition timing retarded" so in go the proper ignition center and weights (375 center and 41 weights) which now handed out a 22 mechanical advance and bumped up the base advance from 4 to 10. You will hafta cut the vacuum advance back to 10 or so...
Next step was the "Thorley headers" with 2 1/2 pipes along with some flow masters 70 series mufflers....
Dang near 8 MPG but that was not e'nuf so into the carb for the jet change (73) and the power rods spring which from the factory is setup at 11 inches of manifold vacuum and they pull up and dump a ton of fuel down the primary barrells...
Swapped out the 11 spring for a 6 inch spring and now I'm a dab over 9 MPG while towing a Saturn SC2...
Once I quit being lazy I'll install a (LM1) fuel sniffer and go from that point..Enjoy.

Jim
AH - thanks for all the GREAT info - my MH seems to already have a pretty decent dual exhaust setup - 2 1/2 inch pipes or better - but mufflers, while good sized as well, are NOT at all a straight-thru design - the headers are also apparently better than what I would consider "stock" or OEM - more of a performance-type design, but maybe that was the OEM style common to this year and class Winnebago/GM big block setup?

The 4-barrel carb and distributor, however, seems OEM - engine was fully tuned up and carb rebuilt prior to our purchase, but I doubt either has seen any actual modifications or tuning towards power or economy.

SO, question is, is there any central vendor that can provide the materials or assemblies for what was describe in Jim's post? I live in the John Day Oregon area of eastern Oregon - pretty small population and isolated for many products/services - so where might be a good online vendor source to start for distributor and Carburetor mods - or ready built replacements?

And would removal of the entire smog pump setup be worthwhile, or only marginal in benefit?
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Old 08-21-2009, 02:45 PM   #10
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SO, question is, is there any central vendor that can provide the materials or assemblies for what was describe in Jim's post? I live in the John Day Oregon area of eastern Oregon - pretty small population and isolated for many products/services - so where might be a good online vendor source to start for distributor and Carburetor mods - or ready built replacements?

And would removal of the entire smog pump setup be worthwhile, or only marginal in benefit?
The smog pump is OK, I would leave it alone.
Ignition mods are "tuff" to do since one needs the distributer machine to set the curves or has the time to play with one..Maybe on some of my other car sites I can find someone to properly set one up...
There is a good carb setup person in Hunting Beach Kalifornia that is a true wizard once he knows youe engine setup, I'll grab that info for you...
For a ornery thought when the ignition/exhaust is setup to/for what you want you can send a spark plug along with the carb so can lean/richen it up to what ever the plug readings are....

Jim
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Old 08-21-2009, 05:41 PM   #11
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http://www.smicarburetor.com/products/sfID1/28/sfID2/9

The above site is the carb "Guru" when you get the other items ironed out.....
Sean Murphy is theeee best and used to work at the "Carb shop" in Ontario Kalifornia..
Still digging for a good ignition person.

Jim
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Old 08-25-2009, 01:42 PM   #12
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JIM - earlier you said:

Quote:
.."Ignition timing retarded" so in go the proper ignition center and weights (375 center and 41 weights) which now handed out a 22 mechanical advance and bumped up the base advance from 4 to 10. You will hafta cut the vacuum advance back to 10 or so...
Places like Moroso sell what they call "distributor re-curving kits" - but those might not be specifically what you are talking about above - where is a good source for those particular parts - and I assume you limit the maximum advance with shims at the weight stops?

The goal seems to be more base timing and a corresponding limit of maximum timing at full advance - but what are we looking for at midrange, FASTER advance with heavier weights and/or springs - or SLOWER advance with lighter weights/springs?
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Old 08-25-2009, 02:36 PM   #13
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JIM - earlier you said:



Places like Moroso sell what they call "distributor re-curving kits" - but those might not be specifically what you are talking about above - where is a good source for those particular parts - and I assume you limit the maximum advance with shims at the weight stops?

The goal seems to be more base timing and a corresponding limit of maximum timing at full advance - but what are we looking for at midrange, FASTER advance with heavier weights and/or springs - or SLOWER advance with lighter weights/springs?
All of those "Over the counter" centers & weights are junk...Stock 375 centers and 41 weights are what you are looking for cuz they provide a 22 mechanical advance. Might find some at the salvage yard, Just look at some older points distributers cuz they will fit the HEI.
The 375 & 41 are self stopping around 22 mechanical....Lighter springs are for faster advance, The present springs are good for garage doors..
Springs are difficult setting up and thats why I say find someone with the ignition curve machine for proper setup, With the 3 speed trannie I would have the mechanical advance ALL IN by 3,000 RPM.
When you get that mess setup then the vacuum advance must be cut back cuz the norm is 25 vacuum advance which will be way toooo much and thats what GM sets the stock settings (smog).
(Stock) 4 base, 16 mechanical along with VA of 25 = 45 runing down the slab with good vacuum (above 6-7")...My way is
10 base, 22 mechanical and vacuum can set at 10 for a total of 42, Remember with hill climbs with little to NO vacuum the stock setup is limiting you to 20 max...New setup with the properly curve dist with NO VACUUM is 32 up the hill which is a huge difference and allows your engine to run much cooler..

Jim
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Old 08-26-2009, 05:25 PM   #14
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Gary, Have been looking for a "workaround" for your distributor....You may have a "tuff" time finding the ignition guru in the area soooooooo.
Summit racing has a dist for slightly over $90.00 which comes close to some decent settings (18-19) mechanical advance OR your local "Chebby" stealor can obtain a ZZ4 ignition that is right on the money....
Depending on how many miles your engine has on it I would examine the ignition gear presently on yours and IF it's in great shape I would reuse it on the new ones....
You will still need to dial in the curve (springs) which can be time consuming plus it takes two people and can turn into a 4 beer job....

Jim
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