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11/16/15 5:35pm - Original Message: 'headers and timing advance/tuning?'
jim in oregon
Former Member
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Milwaukie, OR - USA

Vette(s):
'78 Silver Anniversary T Top coupe,58k orig miles. THM350 auto, 350 L48, JBM headers,Oyster White interior,.as of Nov '15 in process of painting.

Joined: 10/19/2015
Posts: 71
I have a new (to me)  78 Corvette with the L-48 350 cid engine.

The previous owner had JMB  performance headers put on the car and dual stainless exhaust that each have a cat converter and a muffler.

The car is running very well since I had the Rochester Quadrajet rebuilt with an upgraded brass float by a competent old school mechanic and put on new stock plugs( gap .045) and new wires and dist cap etc.

Timing is currently set at 8 degrees advance which is stock setting.

I have read that the performance and fuel efficiency can be improved by proper headers and perhaps a bit more advance to the timing..

Does anyone have any experience in this area?..and IF so what other issues need tweaking for the new tune up?Removal of vacuum advance?.. 

The car AS IS runs like a top  and accelerates as fast as I need it to and will go much faster than the law allows and do all of that quite smoothly..
BUT, I am curious..

I'd like to take full advantage of 2K in header-exhaust system if that can be done simply and intelligently..

I appreciate the responses ..Jim


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11/16/15 6:00pm - Reply: 'Re: headers and timing advance/tuning?'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

Joined: 11/8/2003
Posts: 19457
You can prolly get a couple more dyno HP by advancing the timing some. Would you actually be able to feel the improvement? Possibly. Depends on how far off the timing is to begin with, and how much slop is in the timing chain set.
BEST way to set the timing is by using a vacuum gauge....NOT a timing light. Timing lights are for checking to see where it is AFTER actually setting it, for future reference. Tongue

Hook a vacuum gauge up to a manifold vacuum port, then adjust the timing, AND carb settings to achieve the highest vac reading you can get. You'll prolly have to go back/forth between the carb and dist a few times to get the best reading, but once you have it, lock everything down....your timing is set where the engine wants it...not the EPA. Wink
 If you want, now you can put a light on it to see where it shows to be, and use that as the correct setting in the future, assuming the outer ring on the balancer is not moving around.


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Joel Adams
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11/16/15 8:12pm - Reply: 'Re: headers and timing advance/tuning?'
jim in oregon
Former Member
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Milwaukie, OR - USA

Vette(s):
'78 Silver Anniversary T Top coupe,58k orig miles. THM350 auto, 350 L48, JBM headers,Oyster White interior,.as of Nov '15 in process of painting.

Joined: 10/19/2015
Posts: 71
Thank you for the thoughtful reply..I'll digest all that and see what if anything I do..
Perhaps you may know what the chief difference beside the aircleaner air input between the 350 L48 & the L82..Timing is a bit different, the rest of the carb & plug items seem to be the same ..Has a different exhaust setup to reduce back pressure a bit..Jim


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11/20/15 2:36pm - Reply: 'Re: headers and timing advance/tuning?'
nosal1 Gold Member
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Dunedin, FL - USA

Vette(s):
1980 Corvette Torch Red with Gold metallic 370HP/350 CU.IN.Turbo 350 w/shift kit.Originally CA.Car with 305 CU.IN.

Joined: 3/21/2006
Posts: 238

AS usual Joel is spot on setting timing by vacuum not by the light!!!

Sal C           nosal1


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