Manifold Mods (Padgett) - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1987-1991 Parley with regards to your 1987 to 1991 Bonneville, Olds 88 or Buick Le Sabre Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 09-07-2004, 08:53 PM   #1
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Default Manifold Mods (Padgett)

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Originally Posted by padgett
T'were me, the first thing I would do is to open up that horrible restriction where the oulet pipe is attached to the manifold.

Top picture - as from the factory
Bottom - about 30% more flow area


If that were not enough, the front manifold is not that bad but for the rear I'd look into separating the crossover and the rear manifold and bringing a new pipe down to a collector about a foot or two below the current location. Of course this would also require a new head pipe.
Padgett, I took this from a post in another topic and just wanted to start one because I'm curious as to your ideas.

First, is the above mod one you have done to yours? Or one you heard about?
If you've done it, do you notice any differences?
Also, can you explain a bit more about the last paragraph, and about the new head pipe.

Thanks
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Old 09-07-2004, 11:40 PM   #2
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Have modded mine but the exhaust is not the real restriction. First get a much better air filter (have been looking for an airbox for the later AC 1281). Feels faster and definately changes the exhaust sound (deeper).
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Old 09-08-2004, 12:13 AM   #3
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This will vary from car to car. Padgett'* pics are similar to Don'*. I've seen some that are fine, though. From the factory. The only way to know is to pull it and look. Or hire a really small rodent to go in there through a valve and inspect.

Good mod, especially if it looks like that when you open it up.
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Old 09-08-2004, 12:46 AM   #4
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That'* basically limited to the pre 92 cars. If you have an L27 you're probably fine, but the older one has issues.
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Old 09-08-2004, 01:40 AM   #5
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Wrong Jason.

Quote:
I guess there is no L27 on earth thast has a restriction problem at all at the rear manifold.... The manifolds for the L27 in the H and C bodies are slightly larger than the 88-91 Vin C bonnevilles, but I'll bet there are some that have the issues as well... The tubular manifold was used from 87-95 in the bonneville, there were manifold changes with the series II engines...
That comes from experience far beyond you and I combined. This is a function of the manufacturing process. Some cars deal with this problem, some don't. The same style manifold was used all the way through the 95 L67.
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Old 09-08-2004, 03:27 AM   #6
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the new pipe he'* talking about is something I've mentioned I'm doing too... basically get rid of the crossover pipe between the 2 manifolds. cut off and cap the cross over flange on the rear manifold and run a Y pipe to join the 2 manifolds down @ the cat (or where the cat was if you've/going to remove it)

on a side note... Jr3800 mentioned before that the actual buick cars with the tubular manifold, aren't supposed to have the restriction at the rear manifold... but the major "problem" beyond the restriction on the rear mani is the fact that the cross over from the front mani goes right into the rear mani. hense the rear mani is trying to push twice the amount of exhaust through the same size flange as the front

I seriously think on these cars, a good intake, and full exhaust including the manifold mods would be worth 15hp atleast... maybe more... I think it'* very possible and likely to get this particular engine to 200 hp ( or damn close N/A ) with intake and head work (porting/matching and polishing, shave), the exhuast mods, intake, chip, 180 Tstat and type 2 ignition to keep it firing nice and freindly

I'm talking about the Vin3 engine though... and personally I'm not sure which of the 2 you guys are talking about
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Old 09-08-2004, 10:04 AM   #7
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My experience is with the code "C" 3800s.

Have not studied the tube length closly enough to calculate the pulses but for a street "even fire" engine, you will have smoother flow through one exhaust pipe with six cyl than for one cyl.

Six cyl with sane cams do not overlap the exhaust pulses and a properly designed exhaust system can get a scavenging effect from a single pipe. The stock 3800 cams are quite dead so this is not an issue.

The right hand bend and terrible opening from the rear manifold to the head pipe is and it looks like the crossover pipe was selected for fit rather than tuning. Further I suspect theshort pipe was to ease installation of the powertrain module. It would not surprise me if collisions were occuring (ever hear the drone of a well tuned E-Jag or a Corvair on trombones - that is what a six should sound like. Of course both have split manifolds but then they also have lots more cam.

My experience with opening up that ragged hole pictured (was the stock manifold from a '90 Bonneville) both smoothed and deepened he exhaust note. Whether it resulted in any more power is questionable, maybe 5 hp peak.

Remember that for everything before the "L" engine, the limitation is as much in the intake as the exhaust and if you haven't got the gozinta, the gozouta won't matter.
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Old 09-09-2004, 03:46 AM   #8
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it isn't the fact that it'* going into 1 pipe... it'* before the pipe where the front Xover pipe tries to shove its exhaust into the rear manifold ... in with the exhaust the rear manifold is already trying to get the hell down into the pipe... basically what I was saying was get rid of the mani to mani X over and make a Y pipe.

and as I added in above post... without the intake, full exhaust, etc to back it up .. the gains wouldn't be that great.. but all together they should compound and add up nicely

then again I'm talking about the Vin 3.. and not that up on the Vin C
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Old 09-09-2004, 10:01 AM   #9
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About what I was thinking: For "low price", clean up the rear manifold , remove the crossover pipe and seal the opening. The Fab a new crossover pipe that extends past the current exhaust and connects to a new head pipe that bolts to the existing rear manifold.

For even better flow keep the front manifold but fab a new rear to include the new collector as above. Use the same redesigned crossover.

Issue is that it would have to be a labour of love since it would probably be cheaper/generate more power to just drop in a L-67.
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Old 09-09-2004, 06:35 PM   #10
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maybe for you ... L67 for me is 4K and up .... then add on down time for a complete rewire of the dash forward on the car... and atleast in my case... a total ring/seal/gasket freshen up on said motor...

so yeah... for that cost and time ... I can do... my nitrous motor, my turbo motor (if ofcourse assumptions of mine and DrJays are accurate) and a performance rebuild to the 4t60 to handle 300+hp and prolly still have enough left over to buy some paint and make it all look pretty


that'* just me though... what really kills me on cost is shipping ... all the ones I've found thus far worth buying were 2800 US ... add shipping to Van island BC, convert to canadian funds... then add taxes and duties...it gets pretty steep in price real fast
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