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Upgraded to headers... New Intake Manifold next?

Thore826

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Mar 18, 2008
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41
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New York
Corvette
1975 White, 1984 Black, 1996 Green
I've got a '75 L-48, engine's all stock but I just replaced the exhaust manifolds with Hedman Hedders ending in glasspacks, and no cats so its pretty free-flowing. I've heard that when you do headers you should do them alongside a larger intake manifold, I guess to "compliment" the increased exhaust flow (more out, so more in). If I'm using the stock Q-Jet carb, should I bother getting a new intake manifold, or I'd need a better carburetor in order to get a better flow? I'm not looking for any big gains, just to get the most out of bolt-on for now.
Thanks
 
An engine is merely a big air pump , if you derestrict one section , it will give you very few tangible benefits unless you derestrict it all...
An intake manifold at this point , is probably the WORST bang for the buck ...you might get a handful more HP but you will be paying for the bling more than anything else. If you HAVE to add anything... a new Holley Street avenger or a MSD recurvable distributor would be my choice..
If you have an Auto then a higher than stock stall will give you the most seat of the pants performance increase.
 
Adding headers and an aftermarket intake to an otherwise stock L48 is a poor investment, very little bang for the buck. Low compression, mild cam, 'smog heads' and conservative ignition advance curve are all working against you.

A Qjet is all the carb any SBC will ever need, no need to waste money in that department.
 
You've read various views here...even a suggestion to drop in a ZZ4 (which if funds are not limited is a good way to go), but I suspect, your project budget and the complexity of the task are key issues in this instance.

The L48 is a typical base engine with heads of limited flow, restrictive exhaust, mild cam that limits power at high rpm and a restrictive intake.

First, this disclaimer...my suggestions are given with the assumption that the engine is relatively low mileage (less than 50,000) and is in good condition, ie: no excessive oil use, no high cylinder leakage, no worn valve springs and etc.

If your goal is to do the best you can given the OE heads and camshaft, then adding headers, taking off the cat and going to true dual exhaust was a wise idea.

I'd also suggest an aftermarket intake because, even with the stock heads and camshaft, a manifold using current technology (40 years newer) makes sense because it removes the majority of any restriction upstream of the cyilnder heads. I'd suggest an Edelbrock Performer.

I'd have your stock Quadrajet properly jetted for use with lower restriction exhaust and intake. Finally, it's wise to recurve the distributor but....it needs to be done in a manner that limits centrifugal advance to work with an increase in initial. Often this means changing the distributor cam and ones with short slots are hard to find. I'd say, as long as you use 91-oct fuel, you need to start with 34-38° total adv. with 10-12 initial advance. Start with it all in by 3500 rpm then work down to get best performance without detonation. Once you have the centrifugal set, then add an adjustable vac. advance mechanism and limit vac. adv. to 8-10°

Have fun and good luck.
 
Ballpark on the HP gain:

You've read various views here...even a suggestion to drop in a ZZ4 (which if funds are not limited is a good way to go), but I suspect, your project budget and the complexity of the task are key issues in this instance.

The L48 is a typical base engine with heads of limited flow, restrictive exhaust, mild cam that limits power at high rpm and a restrictive intake.

First, this disclaimer...my suggestions are given with the assumption that the engine is relatively low mileage (less than 50,000) and is in good condition, ie: no excessive oil use, no high cylinder leakage, no worn valve springs and etc.

If your goal is to do the best you can given the OE heads and camshaft, then adding headers, taking off the cat and going to true dual exhaust was a wise idea.

I'd also suggest an aftermarket intake because, even with the stock heads and camshaft, a manifold using current technology (40 years newer) makes sense because it removes the majority of any restriction upstream of the cyilnder heads. I'd suggest an Edelbrock Performer.

I'd have your stock Quadrajet properly jetted for use with lower restriction exhaust and intake. Finally, it's wise to recurve the distributor but....it needs to be done in a manner that limits centrifugal advance to work with an increase in initial. Often this means changing the distributor cam and ones with short slots are hard to find. I'd say, as long as you use 91-oct fuel, you need to start with 34-38° total adv. with 10-12 initial advance. Start with it all in by 3500 rpm then work down to get best performance without detonation. Once you have the centrifugal set, then add an adjustable vac. advance mechanism and limit vac. adv. to 8-10°

Have fun and good luck.

I'm in the process of doing the same thing. I have the L82 and I went with true duals with no Cats, and long length headers. I just had my R Q-jet rebuilt and had different metering rods installed. Also did the curve kit. I'm not wanting to get into new heads and a cam now.
My mechanic helping with all of this is now suggesting a new intake manifold to give me more HP.
What do you think the ballpark on HP gain is on what I have done so far, and what additonal HP would I maybe gain with a new intake? By the way, the engine has 46,700 miles on it.
Thanks, :w
 
I recon you will be lucky to get an additional 5-7hp...if that...
Your best seat of the pants and TRUE performance increase without increasing HP would be a rear gear and, if using an auto box , a shift kit and stall change
 
5-7 hp

I recon you will be lucky to get an additional 5-7hp...if that...
Your best seat of the pants and TRUE performance increase without increasing HP would be a rear gear and, if using an auto box , a shift kit and stall change

Is that 5-7 hp after eveything is done including the intake m. , or 5-7 hp more after the intake man. is installed.

I would think that doing just the exhaust part including the headers would give me around 20 hp extra.
 
A high rise will be a disaster , you will need huge shooters to cover the bog as the fuel drops out of suspension and at lower RPM the car will be a dog , you need radical cams and really good heads + 10:1 cr for a high rise to work and should preferrably be racing the vette.
You have to analyse your driving habits and proceed from there - you need to see at what rpm you are spending most of your time and what you really want outa the car...robot to robot - top end daemon..highway waffler ?
I would build for most torque in the RPM range you drive in , torque is grunt and HP is mostly to max the car out at high speed - its where wind resistance = hp that you stop accelerating.
It's not quite as simple as just adding HP...
 

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