Wake-Up time for that mid/late 70's 'cookin' C3

[Vetman]

We have a large number of C3 owners in the club that date from the USA's emission era (so '74 thru '80) - these are typically 'soft-tune' engines with low compression and mild cams. They'll perhaps have 'the show' but not always the 'go' (certainly not enough 'go' for some members!)
There is plenty of information out there on how to 'warm' things up a little but very little information in terms of actual facts and figures relating to the horsepower and torque gains......i.e 'bang for yer buck', so to speak. This thread from the USA's Corvette Forum addresses that issue.
Scott has a '77 L48 which sounds much like many of our CCCUK members Vettes. He is also lucky enough to have a friend with a chassis dyno. Read the changes he has made to his car with dyno results recorded at each step........quite interesting.......and still on basically what is a stock engine (so no cylinder head or camchaft changes........yet)

1977 L48 Basic Mods with Dyno Results - CorvetteForum - Chevrolet Corvette Forum Discussion

C3 Tech/Performance - 1977 L48 Basic Mods with Dyno Results - My 1977 L48 was a 100% stock, 4-speed car, 96,000 miles, purchased from the son of the second owner, came with a pile of records and receipts showing it was well cared for, and it ran fine. Slow, but fine. So, this past year I...

www.corvetteforum.com

What a lot of people forget when comparing the smog year power output with the glory years is the change from gross to nett rating. The attached page indicates how from 71 to 72 the 350 LTI lost 75 bhp just due to the reporting change. Hope you can expand the photo enough to see the engine data. Note the two examples are geared differently.

 

[Nassau65]

Those were the days I’m afraid. It’s a pity that young drivers nowadays will never experience that.
Probably all big cities have the same issues now.

 

[teamzr1]

What a lot of people forget when comparing the smog year power output with the glory years is the change from gross to nett rating. The attached page indicates how from 71 to 72 the 350 LTI lost 75 bhp just due to the reporting change. Hope you can expand the photo enough to see the engine data. Note the two examples are geared differently.
View attachment 22520

Marketing game back then due to all the smog crap taking a hit on engine output
So they switched to gross brake HP which is testing output as real world of functions connected to the crank like A/C, Alt, steering pump, etc
and with exhaust on

Before that it was output of engine standalone but of course that was bloated to what output is of a full functional vehicle
so the feds here mandated engine testing as a fully functional vehicle

But even that is not real world as the drivetrains cost like 15/20 % loss to the rear wheels
and then correcting for elevation, weather, etc as you see with this OBD-II scanner data of my modified 99 C5
what the brake HP and rear wheel numbers and corrected D/A is a big difference

 

[Roscobbc]

A better comparison perhaps if using the LT1 option would have been to use the '70 LT1 (instead of '71) - with its higher compression ratio (so 11:1 vs 9:1) irrespective of nett or gross HP rating the '70 should have made more power presuming that that suitable fuel was used?

 

[Vetman]

A better comparison perhaps if using the LT1 option would have been to use the '70 LT1 (instead of '71) - with its higher compression ratio (so 11:1 vs 9:1) irrespective of nett or gross HP rating the '70 should have made more power presuming that that suitable fuel was used?

I chose 71 to 72 because the engines are the same in my examples so the difference between gross and nett bhp can be seen readily.

 

[Chuffer]

We have those bloody speed cameras everywhere in North London. A friend got caught a few weeks ago, 24 mph in Holloway Road ( 20 mph zone) 3 points and a £150 fine.
The other day I tested myself on the way into central London to not exceed 20mph. It was very very difficult to do, you really have to look at your speedo all the time. What a stressful journey that was.

That`s the problem in this country these days , you spend more time watching your speedo than watching the road ahead which is a damned sight more dangerous . It`s not speed that kills , it`s inappropriate speed that causes accidents . I did a speed awareness course abot three years ago having been caught by a lurking camera van in small village . I was in the Vette doing 33 mph in a 30 limit . Absolute b*ll**ks if you ask me . I took great delight in provoking the speed course lecturer into an arguement about the pros ans cons of road safety and being too paranoid watching the speedo instead of total awareness of your surroundings and other road users . Needless to say he would not engage in the discussion as he was pre programmed to spout government speil !!

 

[teamzr1]

On this subject
As tuning the engine and tranny of GM vehicles I often find that sometime in the life of a vehicle, someone changed something
and either did not know or care that now the speedo is reporting incorrectly
This could be wheels, tire sizes, different rear end gear size or even gears in auto tranny or even different type of tranny

Now at the least the tranny is no longer doing its up/down shifts at the correct MPH, but worse get stopped by a cop
for a speeding ticket where the vehicle owner believes they were doing close to the road's speed limit and cop claiming they were going faster

Before the C5 (or other vehicles about that time) if making changes as above, then there are different inner or outer gears to sway in speedo cable to make the gearing adjust so speedo is now correct

Even 3rd party vendors make an electronic unit that is put between the speedo cable and tranny to make speedo gauge report correctly

Starting with the C5 this all changed by electronics control and if changes are to be made by any of the above functions I can correct that in the engine and tranny controls

As you see, there are a good deal of math variables that are needed, so correct MPH is reported and also to assure the auto tranny is shifting at the correct MPH

At the least to know where your vehicle is at on this is having a GPS or an APP for cellphone and when driving as example when speedo
says 40 MPH, what do your tools say the true speed is

This way, you'd know what math you need to do in your head for not exceeding the true MPH

Or have someone tune the controllers or get the proper speedo/tranny gears to make the correction

As example this is a 2008 C6 as to the PCM (left) and TCM (right) where the value changes are done

 

[phild]

I have a buddy who runs a luxury airport consierge service.......speaking with him recently and he said that if he gets one more 20 mph speeding ticket he's 'out of it'. Too many distractions for even experienced drivers what with bus lanes, yellow box junctions, newly introduced one way streets, traffic light cameras......and of course not forgetting the 'hazards' of dodgy other drivers in major cities.......the Uber type drivers whose first priority is to look only at their phone/satnav.......the zero hours delivery drivers who have daily workloads of perhaps a hundred or more parcels.......the older black cab drivers who think that the roads are only for them and cruise along a perhaps 5/10 mph below the speed limit in the middle of the road.......the worlds nationalities of drivers who perhaps didn't get their license in the UK or had someone else take it for them.
I used to love driving in London.....both for work and for leisure. Would take young family on a Saturday or Sunday 'up West', wander around the large department stores, prestiege car dealers, have lunch, no parking fees after 1:30pm Saturday/all day Sunday..........them's were the days!

I moved out of London in the '90's and would never live there again, especially now. I occasionally need to go by train to London on business and all I see is people looking miserable, unless they're p****d up after being in the pub. People are so rude and self entitled. I was walking down the platform at London Bridge station recently keeping pace with the inevitable crowd. Someone barged out of a carriage and collided with me causing them to drop their phone. Apparently it was my fault, a sign of the times I suppose.

 

[teamzr1]

No wonder you cannot drive fast in the UK, they have almost no roads

the map reveals that more than 14 percent of the roads can be found in the US, while 1.24 percent are in Britain.
Where does all the public tax dollars or fees taken for roads go ?

Countries with the most roads

Rank	Country	% of world's roads
1	US	14.34%
2	China	7.92%
3	India	4.87%
4	Mexico	3.43%
5	Argentina	3.25%
6	Brazil	3.15%
7	Russia	2.88%
8	Australia	2.87%
9	Germany	2.61%
10	France	2.42%

 

[Forrest Gump]

We have a large number of C3 owners in the club that date from the USA's emission era (so '74 thru '80) - these are typically 'soft-tune' engines with low compression and mild cams. They'll perhaps have 'the show' but not always the 'go' (certainly not enough 'go' for some members!)
There is plenty of information out there on how to 'warm' things up a little but very little information in terms of actual facts and figures relating to the horsepower and torque gains......i.e 'bang for yer buck', so to speak. This thread from the USA's Corvette Forum addresses that issue.
Scott has a '77 L48 which sounds much like many of our CCCUK members Vettes. He is also lucky enough to have a friend with a chassis dyno. Read the changes he has made to his car with dyno results recorded at each step........quite interesting.......and still on basically what is a stock engine (so no cylinder head or camchaft changes........yet)

1977 L48 Basic Mods with Dyno Results - CorvetteForum - Chevrolet Corvette Forum Discussion

C3 Tech/Performance - 1977 L48 Basic Mods with Dyno Results - My 1977 L48 was a 100% stock, 4-speed car, 96,000 miles, purchased from the son of the second owner, came with a pile of records and receipts showing it was well cared for, and it ran fine. Slow, but fine. So, this past year I...

www.corvetteforum.com

I’m not sure there is much appetite among many C3 owners these days for making them more powerful, Ross. They’re becoming quite old and precious cars now.
My ‘75 has followed a very similar path to Scotts thread. It already had a non-cat, 2 1/2“ exhaust on it when I got it but still the stock cast iron exhaust manifold, and rest of the engine stock. The carb and intake were still stock. The car made a nice noise and tried hard but was pretty slow really. Rather than mess about rebuilding the carb I just stuck on a new Holley double pumper - the classic “performance“ carb, and changed the intake for a Weiand Street Warrior dual plane. Those two items instantly made it a pretty quick car, not blindingly fast but certainly good fun now, and for not too much money. Presumably the horsepower and torque numbers were similar to Scott’s, but I would say the real life performance exceeded what you might imagine those [low] numbers would feel like. I think its the V8 flat torque curve that just makes these cars pull continuously. My car is a manual which always has some advantage over auto given the same flywheel output. Later I changed the exhaust manifold to the same Flowtech headers that Scott fitted. This didn’t actually make much noticeable difference, except the looked better!
A few years on and I took the engine out and went the whole hog, 383 stroker crank, new pistons, AFR performance heads, performance cam…….Now that taken performance to a different level altogether, but not cheap to do.

 

[Vetman]

I’m not sure there is much appetite among many C3 owners these days for making them more powerful, Ross. They’re becoming quite old and precious cars now.

My ‘75 has followed a very similar path to Scotts thread. It already had a non-cat, 2 1/2“ exhaust on it when I got it but still the stock cast iron exhaust manifold, and rest of the engine stock. The carb and intake were still stock. The car made a nice noise and tried hard but was pretty slow really. Rather than mess about rebuilding the carb I just stuck on a new Holley double pumper - the classic “performance“ carb, and changed the intake for a Weiand Street Warrior dual plane. Those two items instantly made it a pretty quick car, not blindingly fast but certainly good fun now, and for not too much money. Presumably the horsepower and torque numbers were similar to Scott’s, but I would say the real life performance exceeded what you might imagine those [low] numbers would feel like. I think its the V8 flat torque curve that just makes these cars pull continuously. My car is a manual which always has some advantage over auto given the same flywheel output. Later I changed the exhaust manifold to the same Flowtech headers that Scott fitted. This didn’t actually make much noticeable difference, except the looked better!
A few years on and I took the engine out and went the whole hog, 383 stroker crank, new pistons, AFR performance heads, performance cam…….Now that taken performance to a different level altogether, but not cheap to do.

I’m not sure there is much appetite among many C3 owners these days for making them more powerful, Ross. They’re becoming quite old and precious cars now.
My ‘75 has followed a very similar path to Scotts thread. It already had a non-cat, 2 1/2“ exhaust on it when I got it but still the stock cast iron exhaust manifold, and rest of the engine stock. The carb and intake were still stock. The car made a nice noise and tried hard but was pretty slow really. Rather than mess about rebuilding the carb I just stuck on a new Holley double pumper - the classic “performance“ carb, and changed the intake for a Weiand Street Warrior dual plane. Those two items instantly made it a pretty quick car, not blindingly fast but certainly good fun now, and for not too much money. Presumably the horsepower and torque numbers were similar to Scott’s, but I would say the real life performance exceeded what you might imagine those [low] numbers would feel like. I think its the V8 flat torque curve that just makes these cars pull continuously. My car is a manual which always has some advantage over auto given the same flywheel output. Later I changed the exhaust manifold to the same Flowtech headers that Scott fitted. This didn’t actually make much noticeable difference, except the looked better!
A few years on and I took the engine out and went the whole hog, 383 stroker crank, new pistons, AFR performance heads, performance cam…….Now that taken performance to a different level altogether, but not cheap to do.

I have upgraded the L48 engine in my 75 C3. This was done in stages. Initially, dual exhaust, headers, Performer dual plane manifold, and Carter 625cfm carb. Later work included Performer Plus cam, Cloyes timing chain, and Holley 600cfm carb. More recent work included 64cc alloy E- Street heads, Performer airgap manifold, electronic ignition, roller rockers, and oil cooler. Latest work included cowl and forward cold air induction improvements, and AFC monitoring to allow better carb tuning.
I believe these mods have resulted in about 315 gross bhp and about 381 lbs.ft gross torque. These figures are based on Edelbrock curves for their mid-performance top end package

 

[Forrest Gump]

I’m not sure there is much appetite among many C3 owners these days for making them more powerful, Ross. They’re becoming quite old and precious cars now.


I have upgraded the L48 engine in my 75 C3. This was done in stages. Initially, dual exhaust, headers, Performer dual plane manifold, and Carter 625cfm carb. Later work included Performer Plus cam, Cloyes timing chain, and Holley 600cfm carb. More recent work included 64cc alloy E- Street heads, Performer airgap manifold, electronic ignition, roller rockers, and oil cooler. Latest work included cowl and forward cold air induction improvements, and AFC monitoring to allow better carb tuning.
I believe these mods have resulted in about 315 gross bhp and about 381 lbs.ft gross torque. These figures are based on Edelbrock curves for their mid-performance top end package

That must go pretty well now! How do you think the Air Gap intake has affected your torque / hp?

 

[Vetman]

Sorry for mistakes in my previous post. I am using a small iPhone with fat fingers.

75 C3 cars had low factory output due mostly to panic smog mods that stopped it breathing. Many owners have reacted to this with engine mods. The factory output figures were nett and therefore look worse than they are when compared to earlier C3 cars with gross output figures.

 

[Vetman]

That must go pretty well now! How do you think the Air Gap intake has affected your torque / hp?

The figures I quoted from Edelbrock used the EPS manifold but probably similar to airgap to within a few bhp.

Using the air-gap manifold was part of the bigger exercise to get more cold air into the engine. I think a bigger improvement would come from reducing the temperature of the petrol in the carbs. I believe drag racers used to put ice around the carb to achieve this for a short burst.
Overall I am pleased with the performance achieved and the car is still well behaved. In truth the satisfaction comes from the engineering and spanner work.
I haven’t checked my fuel economy but thought I got about 12 miles per gallon with or without mods. I know for certain that any stab on the accelerator pedal resulted in a noticeable downward movement of the petrol gauge needle.

 

[teamzr1]

For those interested in a good estimate
I gave several ways to calculate HP & Tq
and many other formulas on my website at

Team ZR-1 all Corvette years Racing Automotive Pit Stop

Team ZR-1 Motorsports is a Club related to Corvette, racing, LS1, Chevrolet, Automotive, and Free Membership

teamzr1.com

 

[Roscobbc]

The figures I quoted from Edelbrock used the EPS manifold but probably similar to airgap to within a few bhp.

Using the air-gap manifold was part of the bigger exercise to get more cold air into the engine. I think a bigger improvement would come from reducing the temperature of the petrol in the carbs. I believe drag racers used to put ice around the carb to achieve this for a short burst.
Overall I am pleased with the performance achieved and the car is still well behaved. In truth the satisfaction comes from the engineering and spanner work.
I haven’t checked my fuel economy but thought I got about 12 miles per gallon with or without mods. I know for certain that any stab on the accelerator pedal resulted in a noticeable downward movement of the petrol gauge needle.

Edelbrock inlet manifolds are arguably ahead of the curve with many of their inlet manifolds (perhaps less so on earlier inlet manifolds and other items like heads and cams) - can only speak for big block Chevy engines but I very specifically chose the dual plane Performer Air Gap over a single plane Victor Junior as the overall power figures were broadly very similar.......the taller Victor flows a little better between 6000 and 7000 rpm.....but not significantly so (and I wouldn't be revving my engine much over 6000 rpm anyway).
Interesting that you are looking for a cooler petrol flow. Does your fuel system have a 'return' pipe? (that helps) mine tends to run 'fluffy' in town driving in hot weather.....I know that is fuel percolation.......I have 'wrapped' some of the fuel piping in the engine bay with insulation (not sure if it does anything though).
I'm surprised with your 12 mpg though.

 

[CaptainK]

I'm always looking to improve the power of my small block. Not by huge amounts though, just a little bit more pep. Swapping to an Edelbrock 600 carb and Performer manifold and having it setup on a rolling road did make it more responsive. I would like to have alloy heads and the Edelbrock Pro-flow 4 EFi at some point, but time and money like usual. At the moment it drives fine enough, so I just enjoy what it is (but still dream of tinkering for a bit more power).

 

[Chuffer]

I am sure that my 1980 C3 `s performance could be improved on with time and money but it suites me just fine and will no doubt see me out . During it`s previous ownership or ownerships it has had an engine swap to from the 350 sb to a 1985 Camaro IROC -Z 305 which has a 9.5 : 1 compression ratio instead of the standard 8.2 : 1 CR . I always run on Esso 99 RON unless desperate for fuel . The IROC -Z `s had Tuned Port Injection but mine came with a Torker 2 single plane inlet and a 750 cfm Holley . With a B&M Mega Shifter on the TH 350 auto and a Hypertech Street / Strip high voltage distributor it all rather showed the intensions or maybe pretensions of the previous owner !! I have since replaced the old Holley with a new Edelbrock 600cfm Performer carb, Edelbrock fuel rail with in line filter and an Easy Flow high cfm intake and filter. Once properly set up with timing at 34 degrees all in the car was transformed . I accept that a dual plane inlet would be better suited to general road use but I didn`t fancy throwing any more money at it . I compensate the low speed drivability by dropping into 2nd to the revs up which means I can punch away when back on the gas when needed . It certainly keeps pulling well enough as the revs rise . I am running with a 3.07 : 1 diff that I had fitted new in 2017 and I removed the cat at the same time from the 2 & 1/2 inch exhuast with Supertrapp tail mufflers and standard cast iron manifolds . Power output is anybodies guess but then the engine is getting a bit tired now at its age . But aren`t we all ?

 

[Roscobbc]

I am sure that my 1980 C3 `s performance could be improved on with time and money but it suites me just fine and will no doubt see me out . During it`s previous ownership or ownerships it has had an engine swap to from the 350 sb to a 1985 Camaro IROC -Z 305 which has a 9.5 : 1 compression ratio instead of the standard 8.2 : 1 CR . I always run on Esso 99 RON unless desperate for fuel . The IROC -Z `s had Tuned Port Injection but mine came with a Torker 2 single plane inlet and a 750 cfm Holley . With a B&M Mega Shifter on the TH 350 auto and a Hypertech Street / Strip high voltage distributor it all rather showed the intensions or maybe pretensions of the previous owner !! I have since replaced the old Holley with a new Edelbrock 600cfm Performer carb, Edelbrock fuel rail with in line filter and an Easy Flow high cfm intake and filter. Once properly set up with timing at 34 degrees all in the car was transformed . I accept that a dual plane inlet would be better suited to general road use but I didn`t fancy throwing any more money at it . I compensate the low speed drivability by dropping into 2nd to the revs up which means I can punch away when back on the gas when needed . It certainly keeps pulling well enough as the revs rise . I am running with a 3.07 : 1 diff that I had fitted new in 2017 and I removed the cat at the same time from the 2 & 1/2 inch exhuast with Supertrapp tail mufflers and standard cast iron manifolds . Power output is anybodies guess but then the engine is getting a bit tired now at its age . But aren`t we all ?

Can't say for sure re. the Torker inlet on a small block on a big block its certainly regarded as a non-performance item. The name Torker itself is a major misnomer as it doesn't add torque to the engine, except perhaps at higher rpm (where its not required) - I used a Torker 11, along with a Holley 750 double pumper as the first upgrade on my original 427. It certainly didn't produce any addition power over 4000 rpm, but did make the engine more responsive between 2000 and 2500 rpm.
You've already indicated that the 3.07 rear end is probably too much of a ratio for off the line performance.........a dual plane inlet would theoretically improve low end 'out of the hole' performance.......but would possibly be higher than the Torker, creating clearance issues.

 

[teamzr1]

I am sure that my 1980 C3 `s performance could be improved on with time and money but it suites me just fine and will no doubt see me out . During it`s previous ownership or ownerships it has had an engine swap to from the 350 sb to a 1985 Camaro IROC -Z 305 which has a 9.5 : 1 compression ratio instead of the standard 8.2 : 1 CR . I always run on Esso 99 RON unless desperate for fuel . The IROC -Z `s had Tuned Port Injection but mine came with a Torker 2 single plane inlet and a 750 cfm Holley . With a B&M Mega Shifter on the TH 350 auto and a Hypertech Street / Strip high voltage distributor it all rather showed the intensions or maybe pretensions of the previous owner !! I have since replaced the old Holley with a new Edelbrock 600cfm Performer carb, Edelbrock fuel rail with in line filter and an Easy Flow high cfm intake and filter. Once properly set up with timing at 34 degrees all in the car was transformed . I accept that a dual plane inlet would be better suited to general road use but I didn`t fancy throwing any more money at it . I compensate the low speed drivability by dropping into 2nd to the revs up which means I can punch away when back on the gas when needed . It certainly keeps pulling well enough as the revs rise . I am running with a 3.07 : 1 diff that I had fitted new in 2017 and I removed the cat at the same time from the 2 & 1/2 inch exhuast with Supertrapp tail mufflers and standard cast iron manifolds . Power output is anybodies guess but then the engine is getting a bit tired now at its age . But aren`t we all ?

Car would react much better at low to mid RPMs with like a 3.73 or a 4.11 rear end gear

Wow, how we forget what we had in the 1970s

Setup for my C5, I went with 11.3:1 engine compression