Increase in power against cost

[Nigel curry]

Hi all
i have a question that has interested me for a while,I have a l48 1976 corvette coupe which has a 350 auto box.any modifications made were made by previous owner but they appear to be a Holley street avenger carb on a Edelbrock inlet with headman headers which may make me sound as though I know what I’m on about.Believe me people I am just learning,it’s just that my son in law says that the exhaust system which to my knowledge is standard could need to be upgraded, so my question is would this make an appreciable difference against what the cost may be.I might add I am definitely not a speed enthusiast I just love corvettes
any thoughts greatly appreciated nige

 

[johng]

If you've still got the original exhaust which had both pipes feeding into 1 catalytic converter, then it probably would make a noticeable difference. Have you got long tube or shorty headers? Also, do you know how your ignition timing is set? If that is still stock you can make a noticeable difference by re-curving your distributor.

 

[Roscobbc]

If the only modifications to what was originally a low compression smog era engine were carb, aftermarket inlet and headers.........then significant changes to the exhaust system will be unlikely to make a lot of difference. Had you said you also had aftermarket heads, performance cam, pop-up hi-comp pistons etc.......then yes the exhaust system would certainly be a essential requirement.

 

[Roscobbc]

The 'real trick' is to buy a Vette that a previous owner has already replaced a few 'smog era' items with aftermarket parts. Costs of new cylinder heads, cam, pistons along will be perhaps £3K or so - few buyers during tight economic times seem willing to pay you extra for the tuning bits you've added.
I worked out that it perhaps cost me £50 for each additional horsepower I gained when putting the new engine in my '68 some 10 years or so ago,
When factoring-in the other items other than engine components you'll have probably needed to replace (so clutch/flywheel or modded auto box/torque converter, suspension/brake upgrades for the addtional power) I would say the figure rises to £65 per horsepower.
As the septics over the pond say "going fast, and faster than all the others is purely down to the depth of your pockets".............

 

[teamzr1]

Hi all
i have a question that has interested me for a while,I have a l48 1976 corvette coupe which has a 350 auto box.any modifications made were made by previous owner but they appear to be a Holley street avenger carb on a Edelbrock inlet with headman headers which may make me sound as though I know what I’m on about.Believe me people I am just learning,it’s just that my son in law says that the exhaust system which to my knowledge is standard could need to be upgraded, so my question is would this make an appreciable difference against what the cost may be.I might add I am definitely not a speed enthusiast I just love corvettes
any thoughts greatly appreciated nige

Hi Nigel
The bigger the balls you have, the more horsepower and torque you need

All below is from GM engineering for the L-48 engine in the 1976 C3 has

Exhaust, does not flow well at all since it has exhaust pipe for each head but than is shoved into 1 pipe to 1 CAT and then limited flow to mufflers
Here you see what the pipe size is
What type of headers you have, are they long in length, mid or short lengths ?
Best is, throw the CAT away, measure the diameter of the end of header where exhaust bolts to
If that collector is 2.5 inches than go same diameter for each side and run duals to 2 mufflers which have also 2.5" inlets
Your stock ones are limiting exhaust flow where the duels with headers will flow better and that helps make more HP and Tq


Work on the heads with cheaper mod costs
1. you see the rocker ratio for L-48 is only 1.50:1
Replace rockers with like 1.7 or 1.8 ratio has move valve travel helps performance



L-48 as you see has no balls and these numbers are at the flywheel
15% loss through drivetrain so by the rear wheels a VW can beat ya

With that auto tranny rear end ratio is only a 3.08 :-(
At least change that gear with the 3.55 the L82 came with will move the mass of the car better off a stoplight



L-48 has only compression ratio of 8.5:1 (L82 had 9.0:1)
Could either replace stock head gaskets with thinner ones, will raise that a bit
or have someone mill the bottom of heads to raise compression up more



Add some timing, change jets in carb and then see ya at the drag strip

 

[Nigel curry]

As usual you lads do the business thanks for the info/ advice that was REALLY helpful
thanks again lads nige

 

[Nassau65]

My own memories of using a 1976 corvette being used in/around London, various rides to the home countries were very pleasant. More than enough power for the London traffic, keeping up with everyone on the M25, M1, M11, A12, A13 etc. Standard car without any modifications. OK, it wasn’t a “brute” but it was always fun.

 

[Fat Jogger]

I have a camshaft and associated components up in the "parts for sale" area here. It's nothing excessive, but would pep things up a bit, and is not an unusual upgrade on a mid 70's L48 auto. Although I suspect you may not be able to do the work yourself, so getting the work done by a third party would probably make it an expensive option.

Happy to chat though - via DM.

Regards,
FJ

 

[Forrest Gump]

Another good benefit of changing from a stock exhaust to an aftermarket system is you may be able to make it sound better (and louder if that’s what you want). You’ll sound like you’re going faster, even if you‘ve not gain much actual power!

If you do have a standard exhaust at the moment, an upgraded system will help unlock the “flow”potential of the carb, intake and headers you already have fitted. The next restricting point will be those heads, which in the mid seventies were about the worst flowing of all the heads used unfortunately (I used to have them too). Upgrades to cam and lifters will give a bit but be limited by the heads and compression ultimately.

As John said in post #2 the timing is definately worth checking out. Make sure all the timing (36deg) is coming in by 2800 revs, if not fit lighter bob-weight springs to get it - A £10 upgrade that will make a difference!

 

[Chuffer]

Nigel : As Forrest Gump said , making a car breath more freely at both ends ie induction and exhaust is the easiest and cheapest way to release a few more bhp. Ditch the cat if still fitted and get a more free flowing exhaust system and you can enjoy the noise and thrill your neighbours too !! I run a straight through system de catted with Supertrapp tail mufflers and timing 34 degrees `all in` with a Hypertech Street / Strip dizzie on my 1980 . The engine however is a 9.5 : 1 compression Camaro IROC -Z 305 rather than the standard Corvette 8.2 :1 compression 350 lump and has Edelbrock Torker 2 inlet and Edelbrock `Performer ` 600 cfm carb .

 

[Nigel curry]

Yet more valuable replies and info,much food for thought thanks guys
nige

 

[Roscobbc]

Normally the two items that will make the most significant improvement to an engine are the cylinder heads.
Smog era American V8's from the mid 70's through to early 80's perhaps (certainly before MPI injection) have compression ratios perhaps 2 or 3 points below the 10/10.5 to 1 of pre '72 non-emission engines. In an ideal world you'd simply just fit a set of hi-comp pistons......but not a straightforward as that is it?.
Many USA cam manufacturers used to provide RV type cams with opening/closing events that advertised improved performance from stock low compression engines, giving stronger lower rpm 'grunt' and better driveability from relatively stock engine components with na simple (?) cam swap.
Aftermarket performance cams can help make-up for the 'breathing' inadequacies of most OEM cylinder heads. Most cylinder heads (even aftermarket) have restrictive exhaust flow. The very best aftermarket heads will contour and route the exhaust ports differently to stock- often creating installation difficuties with headers and engine compartments clearance. Irrespectively and pro-rata its going to be expensive...........