L-48's

[Barry52]

I own a very nice 1975 "driver" with the L-48 motor and a four-speed. The vehicle has 89K original miles and all the numbers are matching. My question, to anyone that can help, is: how much more performance can be obtained if all the emmissions junk is removed and a true dual exhaust is installed? The car is now 28 years old and does not need to pass emmissions testing during annual inspection. Just curious.

 

[Tom73]

If you only remove the emission equipment from the engine you may see a 5-10 hp improvement. What is really needed is a performance tune. Recalibrate the carb for performance, re-curve the distributor for performance. They are factory set for emission control. With only that change you should see a huge difference in performance (with or without the engine emission equipment). Now, '75 was the first year for cat converters and they were VERY restrictive. Replacing that system with true duals would make the biggest improvement of any change you could make except a total performance rebuild. HP #'s, no idea, sorry.

One thing to keep in mind about emissions. Your state may no longer require emission testing to get the license plate but the car is still subject to the emission laws. Those are Federal and the state does not have any authority to exempt you from them. But the state does determine the enforcement Do they use spot checks or roadside sniffers to catch gross polluters? If so they could still nail you. As said you are only exempt from the testing, not from the requirements.

tom...

 

[dmrodco]

Tom makes a good point. At one time here in Utah they had vans that would set around differant spots with sniffers they would pull over and ticket folks spewing too much noxious fumes. I haven't heard of it in the last 6 months or so however you know big brother never really goes away he just slips into the shadows until you forget about it then bam !

 

[Dave L.]

As Tom said the true dual exhaust would give you about 30 more horses. Add an Edelbrock 2101 intake and you'll get another 15.
2 easy mods without having to do internal engine work. I've done both of these to my stock 81(along with removing all emission items) and it made a big difference.

Dave

 

[80vette_ca]

;stupid and when you do the exhaust put some headers on as well, just to let the car breath a bit better and give a bit more a rumble.

 

[Kritter]

Yea, and if you go with headers think about some header wrap. I recently wrapped the headers on a 73 with some heat wrap and it makes all the difference in the world with eng compartment temps. They also say it boosts HP.

 

[78SilvAnniv]

I would be concerned...

The vehicle has 89K original miles and all the numbers are matching.
The car is now 28 years old...

Am I being over-cautious?
I would be concerned with trying to get more power out of an aged engine. I know that when I had dual chambered exhaust, rebuilt carburator and a tune up done on the 78, I now drive her much harder than I used to. I like the sound and fresh power.

You may be better off pulling the numbers matching engine and installing a crate engine for plenty of hp. My 78 had a CA L48 (about 165 hp w/ CA emissions) and the GM crate engine I had installed is a 4 bolt main with 250hp that is now 5 years old. The exhaust, carb and tune did wonders for it!

So, am I being overly concerned?
Heidi

 

[Ganey]

See Why Exhaust is problem for HP on 75-81
what many refer to as my Exhaust topic.
http://www.corvettefaq.com/c3/ganeyexhaust.htm

Needs more cam as well.

:cool

 

[Glensgages]

If you are NOT going to be drag-racing this car (missing a shift with the stick COULD be a worry here), I don't see why you couldn't build an 'honest' 300-325 HP motor from your short-block, as long as you'll drive it sanely.
The motor in my '79 Z28 was assembled 14 YEARS ago, and has literally THOUSANDS of 1/4-mile runs on it, yet it shows no signs of 'giving-up' anytime soon, and it was built with GM/Chevy parts (4-bolt-block, steel crank, 'pink' rods, etc.).
I've shifted it at 6500 RPM, and as low as the current shift-point, 5800 (it goes no faster at 6500; why abuse it?), which produces 12.80s @ 105 MPH ETs. I'd think my useage would rival the 89k miles on your L-48, so, modify away!
If the "numbers-matching" thing concerns you (no offense, but I don't think 'numbers-matching' L-48s will ever be a big-ticket item, the 4-gear not-withstanding), get a new crate-motor.
If you go THIS route, why not step-up, and get either the H.O. 454, or H.O. 502? Now, THAT would be fun!!!!!

 

[matchframe]

When I got my '78 L-48, the first thing I did was got rid of the CAT, true dueled it out. I then added a mild hydraulic street cam, and an Edelbrock Perfomer intake. Cleaned up the quadrajet carb and added a performance kit to it. I have no idea what the HP was, but it was a Whole Lot Better! I could hold my own with most cars out there.

Like Heidi pointed out, the easier long run path would be to drop a nice crate motor between your fenders, and you are set for a long while.

Personally, I am in the process of changing out to a Big Block. It will be a while before I get it back on the street however, I now have the body off of the frame for a complete update to today's technology.

Keep us up to date to what you do, and we love pictures!

Bill

 

[Jack]

I own a very nice 1975 "driver" with the L-48 motor and a four-speed. The vehicle has 89K original miles and all the numbers are matching. My question, to anyone that can help, is: how much more performance can be obtained if all the emmissions junk is removed and a true dual exhaust is installed? The car is now 28 years old and does not need to pass emmissions testing during annual inspection. Just curious.

If you decide mods are OK in your area ... Your exhaust is a major constraint ... your intake & carb are not ... your intake & carb will support much more cam. Read Ganey's exhaust link ... and his cam recommendations are proven. True duals, step up the cam and recurve the ignition is all you'll probably need for a easy-to-drive, reliable 75-100 hp increase.
JACK:gap

 

[Buckskin7T7V]

The best bang for the buck is changing the rear end gearing. Go to lower gear...4:11 etc.

Second would be the ignition system. Have the distributor re-curved to 40 or 45 degrees advanced at 3500 rpm.

Next would be a mild cam change 286 Comp cam.
Then headers.
Headers designed to fit on a stock body frames are top end designed. You only see the performance increase at the higher rpms.
A true LT-1 dual exhaust system would help everyday driving.
Swap the head with older 441's (1969) would help it breath better. The tops of the L-48 pistons are flat and if you get into the motor, change them to the L-82 pistons. They have a slight dome to them, but you can still run on today’s gasoline.

 

[KANE]

My car was originally an L48. However, I changed over to the L98 fuel injection in 1997 with the crate motor I replaced the original with.

The best modifications to my L48 were these-
Upgrade to dual exhaust
Full length headers
Free-flow cats and mufflers
Accel HEI distributor
HOLLEY spread bore carb (4150??? This was 10 years ago!)
Set the timing to the engine based upon the mods

Big difference.