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#79152 02/18/14 11:33 PM
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I'm new to the forum and thought I would post my plan for my 1978 250cid in my Camaro. I found a 45k mile garage stored non-running '78 base model Camaro in bright red with a 250cid L6 and 3spd. My plan was to just swap out a 350cid and 4spd into it and store the L6 for rebuilding and putting back in someday to keep it original. But I noticed the rubber fuel lines were rotted, negative battery cable was frayed, spark plug wires were on the wrong cylinders and a few other things which got it running again. So after a hard last year and a half financially, I've decided to cut my costs and just keep the L6 in there and make some budget upgrades to make a nice street level daily driver that goes hard from stoplight to stoplight and can cruise at highway speeds. Plus, I hate to take the L6 out and loose the attention that it gets online and around town when people notice something other than a small block in a second gen Camaro.

I've been reading all the chevy L6 websites and got Leo Santucci's book researching what upgrades can be made. I obviously want to keep it as low budget as possible and match the components to each other and the intended use. I can also do the build in stages so I can afford it as I go along. Here's the overall evolving plan for the end product:

Carb: Holly #8006 390cfm with electric choke and vacuum secondaries
Air filter: 9" with 2" filter (until a ducted fresh air intact can be fabricated)
Intake manifold: Offenhauser 5416-LK

Cylinder head: pre-1975 open chamber, screw in studs, hardened exhaust valve seats added, 4 angle + valve job, back cut valves, clean up casting flash, blend throat to .274" less than valve size, polish chamber and exhaust port (may upgrade to 1.84"/1.6" valves, remove head bolt bosses, install intake lumps and port match if funds allow)

Gaskets: Fel-Pro 8006 with 0.40" thick with 3.875" bore and 9786 intake/exhaust seem like the only choice

Camshaft: hydraulic with between .450" and .500" lift, price-wise Comp Cams cam & lifter kit, springs, and keepers/locks seem like the best buy

Pistons: stock for now, the short block is low mileage and in good shape, so I'd rather not rebuild it yet (later on probably 307cid flat tops for proper quench and tailoring the dynamic compression with cam, head chamber and deck height)

exhaust: 292cid heavy tuck exhaust manifold with 2.5" outlet, 2.5" single exhaust and muffler

Ignition: stock HEI with some an Accel Super Coil, replacement cap and rotor, spring kit and 8mm wires


That's parts and work list, but as I said it's evolving with new knowledge. I'm trying to get the most bang for the fewest bucks.

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That sounds like a good plan for a fun, reliable, peppy, daily driver. Some here can not let go of their inner racer. They will be here soon to complicate things. Please don't mention a 194 head. \:D


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 Originally Posted By: Beater of the Pack
That sounds like a good plan for a fun, reliable, peppy, daily driver. Some here can not let go of their inner racer. They will be here soon to complicate things. Please don't mention a 194 head. \:D


That's why I said open chamber for the cylinder head. That's the recommendation I keep seeing to avoid valve shrouding, better to start open and mil the head.

I forgot to put my pictures up, here's my before when I bought it:


And after I degreased and did some painting:

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I forgot to mention ARP cylinder head studs on my list, both because they are recommended for cylinder head durability and because I will probably take it off at least once during the upgrades and the studs will pay for themselves at that point over using bolts.

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looking good! Standing by for more detail and progress!

Last edited by moregrip; 02/19/14 03:48 AM.
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Thats what under the hood of my Nova looks like! What kind of gas mileage do you get ? When you pull the head will you measure and post how far down the hole the piston is at tdc please? Jay 6155

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 Originally Posted By: intergrated j 78
Thats what under the hood of my Nova looks like! What kind of gas mileage do you get ? When you pull the head will you measure and post how far down the hole the piston is at tdc please? Jay 6155


Yes, the Camaro and Nova, kissing cousins. My first car was a 1976 Camaro and the owners manual was a combination for both the Nova and Camaro.

I figured out my mileage and posted it at nastyz28.com awhile back. I'll have to go reread my thread to find out what I got. But it's a bit low. The previous owner had decided to sawzall the catalytic converter off and replace it with a turbo muffler, so the lack of exhaust back pressure makes it impossible to fully tune it. I've got a shop who is going to replace the entire exhaust pipes with 2.5" single system and performance muffler. Then I should be able to get it properly tuned and find out its real baseline. I discovered the half exhaust manifold on the integrated head has the exit in the same place as the HD truck 292 manifold and has a three stud connector, so I can put the exhaust pipes on it now that I'm going to be running with the eventual manifold swap.

Yes, my 43k mile 250 is untouched since it was first assembled, so it's like an archeological dig when I start to take the top end off. I plan to CC the heads and piston tops to see what the factory specs were, take a micrometer to what I can recover from the head gasket, and measure the deck height. I've been playing around with a dynamic compression calculator trying to guess what the factory combo was to get the 8.1:1 static compression ratio, so I can start imputing changes to the cylinder head and cam in an attempt to get it as close to 8.5:1 DCR as I can without having to pull the short block to deck it and replace pistons yet. The Fel-Pro 8006 0.040" thick head gasket is really making it tough to keep the CR up, but I don't know of any thinner ones that are available. When the temperature warms up again, I'd like to get out in the garage with TDC gauge and try to estimate the ABDC for the stock cam too.

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 Originally Posted By: Lifeguard
[quote=intergrated j 78]..... The previous owner had decided to sawzall the catalytic converter off and replace it with a turbo muffler, so the lack of exhaust back pressure makes it impossible to fully tune it.....


Just a general comment on this concept of "lack of exhaust back pressure" causing a problem. This statement is not uncommon on the internet, though not technically correct. Lower pressure at the exhaust port when the exhaust valve is opening and closing is desirable, with some qualifications. Exhaust pressure is affect by 1) quantity of flow, 2) pressure pulses communicating from other exhaust ports (a good example is on oem cast iron exhaust manifolds), and 3) pressure pulses reflected back from the end of transitions (exhaust runner terminations at collectors or mufflers or catalytic converters). Removing the catalytic converter may have negatively impacted the tuning of the exhaust system, probably altering the timing of the positive and negative pressure pulses at the exhaust ports, but not because it reduced general back pressure.

Last edited by Winter; 02/20/14 11:50 AM.
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One part of my build that I'm questioning is the carb and intake manifold. The 390cfm Holley is the smallest 4 barrel to fit on the Offenhauser, but when you run the math on it the max RPM that would justify that size is closer to 6,000 rpm. So I'm wondering if even the 390cfm is over carbed for a street build? I know I see a lot of guys throw 750cfm carbs on a small block Chevy for the street, which makes more peak power but at the expense of power for everything below that. I'd rather have as much power off idle on up and a lower peak power, because for a street driven car that is just more fun. Am I better served with a 2 barrel? If you use a 2 barrel, should the stock truck intake manifold just be used?

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 Originally Posted By: Winter
 Originally Posted By: Lifeguard
[quote=intergrated j 78]..... The previous owner had decided to sawzall the catalytic converter off and replace it with a turbo muffler, so the lack of exhaust back pressure makes it impossible to fully tune it.....


Just a general comment on this concept of "lack of exhaust back pressure" causing a problem. This statement is not uncommon on the internet, though not technically correct. Lower pressure at the exhaust port when the exhaust valve is opening and closing is desirable, with some qualifications. Exhaust pressure is affect by 1) quantity of flow, 2) pressure pulses communicating from other exhaust ports (a good example is on oem cast iron exhaust manifolds), and 3) pressure pulses reflected back from the end of transitions (exhaust runner terminations at collectors or mufflers or catalytic converters). Removing the catalytic converter may have negatively impacted the tuning of the exhaust system, probably altering the timing of the positive and negative pressure pulses at the exhaust ports, but not because it reduced general back pressure.


I'm not sure if I was clear on the state of my exhaust, everything from the catalytic converter on back was removed, and then the header pipe just has a muffler on it, so there's not much exhaust left which I'm sure affects the tune of the exhaust pretty negatively. I can put the idler screw all the way in with the HEI vacuum plugged, and I can't get it to go below 1k or 1.1k rpm to get to the recommended 800 rpm idle on the first step on the engine tuning decal. It just races at idle. My assumption is that the lack of exhaust is the problem, so I'm going to get that fixed for that reason and just to make it sound like a grown-up's car. If I can tune it after that, I found the problem, if not I guess I'll keep digging.

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 Originally Posted By: Lifeguard
One part of my build that I'm questioning is the carb and intake manifold. The 390cfm Holley is the smallest 4 barrel to fit on the Offenhauser, but when you run the math on it the max RPM that would justify that size is closer to 6,000 rpm. So I'm wondering if even the 390cfm is over carbed for a street build? I know I see a lot of guys throw 750cfm carbs on a small block Chevy for the street, which makes more peak power but at the expense of power for everything below that. I'd rather have as much power off idle on up and a lower peak power, because for a street driven car that is just more fun. Am I better served with a 2 barrel? If you use a 2 barrel, should the stock truck intake manifold just be used?


After I posted this I saw CNC-Dude's post about producing a 2 barrel manifold. I suppose that might be an option too. Although it won't fit the Holley 2300 350cfm won't fit the initial manifold design. It also uses the 5 1/8" air filter. Since the factory thermal air filter uses a 10"x2" filter which the math says is plenty of diffuser for intake, and it has a cold air ducting already, I'm thinking it would be a budget friendly option to use it with a 2 barrel that will fit it. So the Holley wouldn't be able to use it. Unless a smaller 4 bbl Thermac air cleaner was used on V8s or V6s that could be swapped in and use the factory ducting. With the Camaro the stock air cleaner is just barely clearing under the hood, and there's not much clearance between the outside edge and the valve cover either, which really limits air cleaner options.

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I've thought about making a 2 bbl intake for the larger Holley 2300 series carbs also. There are so many possiblities for new and much needed intakes for these engines.



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 Originally Posted By: CNC-Dude #5585
I've thought about making a 2 bbl intake for the larger Holley 2300 series carbs also. There are so many possiblities for new and much needed intakes for these engines.


Yeah the Holley 2300 is an option, with the vacuum choke kit it is a little cheaper than the 4160 350cfm 4 bbl. I don't know much about the Rochester 2G and Dualjet to know if they are possible low buck alternatives too, but my stock air cleaner with ducting would fit them and cut costs further.

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I think I might be able to help with the idle problem. Idle speed is controled by the electric solenoid. The speed is set in two ways. One is the allen screw in the back and the other is turning the whole thing with a 9/16 wrench. One way is with it unplugged and the other with it plugged in. I forget which is which. Just unplug it with the engine running and find out. The idle MIXTURE screw pointing to the front of the car is for rich/lean at idle speed. Mine is loose and wore out. You also may have a vacum leak or two. Jay 6155

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 Originally Posted By: intergrated j 78
I think I might be able to help with the idle problem. Idle speed is controled by the electric solenoid. The speed is set in two ways. One is the allen screw in the back and the other is turning the whole thing with a 9/16 wrench. One way is with it unplugged and the other with it plugged in. I forget which is which. Just unplug it with the engine running and find out. The idle MIXTURE screw pointing to the front of the car is for rich/lean at idle speed. Mine is loose and wore out. You also may have a vacum leak or two. Jay 6155


Yeah, i hadn't played with the solenoid yet, that could be it. I swapped out all the vacuum hoses to get rid of any rot, and verified all the attachment points with the assembly manual (two were actually switched, as were the #3 and #4 spark plug wires). But there could be a leak elsewhere. I'd like to get a complete exhaust on it first before I tinker some more. I'd like to drive down the road in it with deafening pedestrians and myself.

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 Originally Posted By: intergrated j 78
Thats what under the hood of my Nova looks like! What kind of gas mileage do you get ? When you pull the head will you measure and post how far down the hole the piston is at tdc please? Jay 6155


I totally forgot to go back and check my project thread at NastyZ28.com:

http://www.nastyz28.com/forum/showthread.php?t=213364

1978 was the first year for EPA fuel economy ratings. The L6 Camaro with 3spd was rated at 18mpg city and 28mpg highway, with an average of 21mpg and $500 annual fuel cost. I calculated mileage for two fill ups and got 15.6mpg and 16.0mpg for all in town driving. That's with tuning it or a complete exhaust. I will probably have a new low flow exhaust on it in March, along with some ignition upgrades and tune up. I already have a K&N filter on it, and I need to flush the radiator and change the oil too. Then I can do some driving and see what the mileage is then.

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I gor 22.5 mpg driving the car home on the freeway. After plugs, wires,cap and a new water pump,pulley and the addition of a clutch fan I got 23.5 mpg mostly on the open highway. Jay 6155.

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 Originally Posted By: intergrated j 78
I gor 22.5 mpg driving the car home on the freeway. After plugs, wires,cap and a new water pump,pulley and the addition of a clutch fan I got 23.5 mpg mostly on the open highway. Jay 6155.


I was thinking of replacing my standard non-clutch fan with an electric. And I don't know how old the spark plug wires are, so I may swap them out for some new 8mm spiral core wires, and a new cap and rotor. Maybe a supercoil too, for best possible spark. Tune it, and then that should get me the best possible stock mpg. I noticed from Deuce's tests that the 390cfm Holley on an Offy intake did beat the monojet for fuel economy too.

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I had the Camaro out in the driveway today trying to tune it up as best as possible, but the battery won't hold a charge so I see a trip to the battery shop tomorrow. I had trouble reading my tune-up decal which seems to be bubbling between the two layers. I haven't been able to find a replacement from Camaro suppliers online. They don't seem to have L6 stickers for after '76 available. Anyone know a good supplier of reproduction tune-up/emissions decals for '78 inline 6?


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Here is a sample quick search on Ebay

http://www.ebay.com/itm/1972-Camaro-Nova...21827ee&vxp=mtr

MBHD


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Yeah, that's a '72. They seem to reproduce the L6 decals up to '76 and V8s only after that. The '77, '78 and '79 L6 owners are out luck?

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Lifeguard, here is the info off my 78 Nova sticker. Spark plug gap .035. timing, 10@600rpm for auto w a/c 10@550rpm for auto w/o a/c 6@800 rpm for manual. Base idle speed 425 with soloniod unpluged 2100rpm fast idle for auto trans 2000 fast idle for manual trans. I set my own timing at 12 @ ~600 rpm. Made sure it was idling slow enough not to start the mechinical advance. Hope this helps. Jay 6155

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here's a suggestion--have ol' "Intergrated j 78" take a digital photo of his "decal" (if its good) and email it to you and either make your own sticker or have the six-year old next door do it for you, and replace your old one. Keep it cheap, and inline.

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If Lifegaurd wants to try it I'll do my best. My sticker is not perfect but looks better than his. Jay 6155

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 Originally Posted By: intergrated j 78
Lifeguard, here is the info off my 78 Nova sticker. Spark plug gap .035. timing, 10@600rpm for auto w a/c 10@550rpm for auto w/o a/c 6@800 rpm for manual. Base idle speed 425 with soloniod unpluged 2100rpm fast idle for auto trans 2000 fast idle for manual trans. I set my own timing at 12 @ ~600 rpm. Made sure it was idling slow enough not to start the mechinical advance. Hope this helps. Jay 6155


Are those the auto or with AC specs, because mine is a manual and those are different from the specs I was able to finally read off of it a flashlight and magnification?

The sticker on my '78 is a silver finish with black ink printed on it and a clear layer on top, so at some point they changed from the old white decals that my '76 Camaro had. So to replicate with an ink jet is a little more challenging. The silver backing of the decal seems to be progressively breaking down. I could make a white backing decal on my inkjet and replace, and most people probably wouldn't know the difference. I also need to replace my cowl tag with a reproduction due to it rusting out. VIN number is still good and I haven't looked for the build sheet yet.

My tune-up instructions on the decal are to plug the HEI vacuum and adjust the timing to 6 degrees at 800rpm. Then to hook the HEI back up, unhook the EGR and Evap canister vacuum, and adjust the solenoid to 800 rpm idle. Since I couldn't get the engine to idle at 800 rpm while I set the timing, I just adjusted the solenoid with HEI plugged and was able to get to idle at 800 rpm with a look of turns of the solenoid. I'll try timing it with that change tomorrow (my battery has some bad cells and needs to be replaced under warranty, and I knocked my timing light clamp into the fan and shredded it, so I just got a new timing light). I took all my plugs out and gapped them to make sure they were .035" and while they are out I will try to find TDC.

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Oh, and there were two other steps. The third is to then adjust the 1/8" hex screw in the solenoid to 425 rpm. Then finally to bend the fast cam idle follower to get it on the high point of the cam at 2000 rpm, and then reconnect the EGR and Evap hoses. There are no instructions on the idle mixture knob, but my old Chilton's guide says to tighten it until seated and then back it off two turns.

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On my sticker there is no difference for manual trans with or without A/C. The only difference for auto trans cars is a slight increase in idle speed. I also looked up the the specs for the amount of vacum and mechinical advance for 1978 nova and camaro. Both cars use the same part number and advance curves for auto OR manual trans. In other words you could run 10 degrees of of advance to start and if it doesn't ping it would be no difference in total advance from an automatic trans car.(a very tame curve IMHO)

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 Originally Posted By: intergrated j 78
On my sticker there is no difference for manual trans with or without A/C. The only difference for auto trans cars is a slight increase in idle speed. I also looked up the the specs for the amount of vacum and mechinical advance for 1978 nova and camaro. Both cars use the same part number and advance curves for auto OR manual trans. In other words you could run 10 degrees of of advance to start and if it doesn't ping it would be no difference in total advance from an automatic trans car.(a very tame curve IMHO)


I was misreading your post, the numbers are a little confusing in sentence format versus table format. Yeah, that's what my numbers for auto and auto/AC are too. The change from year to year, but seem to be the same for all models with the same engine during a model year.

Working on timing today with my new timing light, after I pick up a warranty replacement battery. The timing marks on the L6 are a bit different. I'm working off the assumption that the widest V is the zero at the bottom with -4 TDC being the tip above and 4 is the tip below. So I'm adjusting for the bottom of the next cutout for 6 degrees. I should clean it up more to see if I can find some numbers.


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That's correct. Sorry about the format. If you only knew my poor typing skills!! The lower down the tab the more advanced the timing. You can try both 6 and 10 and see what happens. Jay 6155

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Thanks, I'm indexing spark plugs right now to make sure they all face the exhaust valve for best ignition. New battery is in, and I tightened the alternator and PS belts. Just picked up oil, filter and coolant for fluid changes.

I'm going down the tune up decal to set timing, curb idle, base idle and fast idle. However, the instructions say to plug the EGR and canister purge hose. EGR seems easy, pull the hose off it and plug hose. However the Evap canister on the L6 is quite a bit different from the V8. There are two vacuum hookups with 4 hoses hooked to the manifold and carb. I'm not sure which ones to plug. The hose in the center is the return to the gas tank, so I can eliminate that one.


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I didn't think anyone (else) indexes plugs anymore, good job \:\)

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Something that I discovered with my L6 Camaro (I'm not sure if Novas or other Chevy's were the same setup) is that the duct for fresh air to the Thermac air cleaner is ducted on the driver side through the front support.



The inlet to this duct is located right behind the front left turn signal:







By removing the turn signal lens from the chrome trim bezel, I can turn this into a ram air duct for cool air. And to keep it street legal, offset the turn signal lens or use a smaller lens and bulb in the bottom of the fixture to let air by.



There's also plenty of room in the area by the passenger firewall for a cool can for fuel. So the carb can mixed cool air and cool fuel to maximize the mix.

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 Originally Posted By: 70Nova
I didn't think anyone (else) indexes plugs anymore, good job \:\)


Thanks, it's the little details that give you free horsepower and use less gas to make it.

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 Originally Posted By: 70Nova
I didn't think anyone (else) indexes plugs anymore, good job \:\)


I still do when I have the extra plugs to play with and I'm not in a hurry to swap plugs. ( track wise )


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I try to optimize everything to squeeze out every last drop of energy and mileage out of the amount of fuel burned, I don't feel like I should leave free horses on the workbench when labor is free.... and I don't have that many ponies to begin with that I could just give them away \:D And simple indexing doesn't require any extra plugs, I don't get that part. Anyone used crush washers with these washer-less plugs to allow for easy indexing? I only used NGK plugs on my other engines that used a crush washer. You get some room to play with them and additional adjustability with extra solid washers.

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 Originally Posted By: 70Nova
I try to optimize everything to squeeze out every last drop of energy and mileage out of the amount of fuel burned, I don't feel like I should leave free horses on the workbench when labor is free.... and I don't have that many ponies to begin with that I could just give them away \:D And simple indexing doesn't require any extra plugs, I don't get that part. Anyone used crush washers with these washer-less plugs to allow for easy indexing? I only used NGK plugs on my other engines that used a crush washer. You get some room to play with them and additional adjustability with extra solid washers.


I try the plugs in cylinders until i find a cylinder that they face the right direction in. Then I use spacers for the rest if I have to. I haven't finished putting the plugs in yet, so fingers crossed I can do it with just the plugs.

Another option is to buy extra plugs individually and try to get them from different batches or different factories, so that the machining on the threads will be different, and then you up your chances of finding plugs that will fit the different cylinders, and some left over for the next plug change. Back in the day when we were running leaded gas and had to change plugs every year, that made more sense then today when the plugs are supposed to go 100k miles plus.

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Extra plugs just means you don't always have to use the washers.
And I would pre-mark the plugs to the cyl.


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If the exhaust is done you could use a stock system from a 1981 Z28 - just install the pipe 42566 that is down stream from the cat flipped.

z28 exhaust

With all stock parts it should not cost too much.

With all the parts you want to buy so far - have you priced them out yet?

Last edited by efi-diy; 03/25/14 08:34 PM.

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 Originally Posted By: efi-diy
If the exhaust is done you could use a stock system from a 1981 Z28 - just install the pipe 42566 that is down stream from the cat flipped.

z28 exhaust

With all stock parts it should not cost too much.

With all the parts you want to buy so far - have you priced them out yet?


I can't make any changes to the exhaust manifold at this point, because it requires swapping out the integrated cylinder head, manifold and carb in order to install the Langdon cast headers or Clifford shorty headers (their full length headers are for trucks and will not fit the ground clearance of a 2nd gen Camaro). I can't afford that for some time yet. So I have to use a single exhaust from the stock manifold on back to a single muffler and tailpipe. Reversing the Z28 splitter pipe wouldn't seem to fit that exhaust. Since the header pipe was cut, and everything is missing, I don't have to replace the catalytic converter. The shop near my house said they could build a 2.5" signal exhaust for less than $200. I'm not sure what other parts you are asking if I priced?

Last edited by Lifeguard; 03/26/14 11:16 AM.
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Stock exhaust layout for the '75 to '79 L6 Camaros with integrated head/exhaust:

http://nastyz28.com/2gcog/tech/75796ex.jpg

Stock exhaust for the '74 and earlier with the non-integrated head:

http://nastyz28.com/2gcog/tech/70746ex.jpg

The earlier routing looks like a better flowing design going down the driver's side instead of going under the oil pan and down the passenger's side.

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