ztheusa

My 67 Roadster has had an LS-6 crate motor in it for about 10 years. I am going to rebuild the original motor, a 390, and convert it to a 400 with tri-power.

The LS-6 has been a great motor, and even though it has higher compression it runs better (less pinging) than my 390 Hp "427" did on the crummy 92 octane unleaded available.

I want to rebuild the 427 into a smooth running no pinging 92 octane pump gas engine. Has anyone done this? Do I need to put in lower compression pistons, and if so, from 10.25 to what? Are there other tricks to make it smooth, drivable, and still make power? I'd like to leave the distributer curve alone. I have had terrible luck in getting the 390 to run without pinging by changing the timing curve. All that does is make the engine run worse and still ping.

Thanks,
Karl
1967 Marlboro Maroon 390-400 427 Roadster.

I am privledged to have ordered, from the factory, a 1967 Corvette back in 1967, brand new from "Fields Chevytown" in Portland Oregon. Is anyone else THAT old around here? :chevy :chevy :chevy :chevy :chevy

67HEAVEN

Yep!
Check out my site.



[Modified by 67HEAVEN, 11:33 AM 11/16/2001]

Kid_Again

....i'll defer to others who have done this stuff for years but i am in the middle of tearing down my #match 427/390 engine...the rebuild is .030 over, 781 oval port heads, tripower, probably 10.5:1 cr and i have yet to settle on the cam specs but it will be solid lifter, roller rockers............i've learned a lot and caution you to make sure the MkIV tripower manifold will fit your heads as you have a much later engine....you'll get that information here pretty qucikly - bunch of knowledgeable folks...i assume you're going square port? :cool:

StrayDog

I put on Endlebrock alum heads ....as the valves are larger & have undercut stems & hardened valve seats the 110 CC combustion chambers = 9.5 comp ratio ..NO pinging......however '' IF '' I could do it again the same heads are now available with 100 CC chambers this would be approx 10 to one & the 92 gas would be fine ......my 66 wieghs just 3080 ....Without me in it ...also removed spare & carrier assy put on mono rear spring , mini starter to reduce wieght.....turned 13.7 , with poor traction & 2.7 60 foot times with good traction 13.2 ET is possible, and this is with 3.08 rear gears........worked for me,,,,, :yesnod:


[Modified by StrayDog, 8:22 AM 11/15/2001]

MELVIN

I did the same exact thing you are doing to my 68 427 /390. I put an oval port tri power set up on it. You will run into no probs, except for two minor things: At least on the C-3's, you must change the accel cable. The qjet cable will not work, even though the end of the cables at the carb end look the same. The length and where the cable bracket mounts are different.

Also, at least on my 68, the 390 has a vapor return line, the tri power setups dont. I suggest still using the 390 remote fuel filter/return line, and do not use the holley internal filters. It doesnt take much to bugger up the treads in those tri power fuel bowls. All you have to do to run the 390 external fuel filter with a tri power is to modify the main gas line that goes from the top of the filter, to the first tri power gas line junction block. Its easy, they all have the same size ends. Just takes a little bending, or have one made for cheap.

TOM454 helped me in rebuilding my tri power carbs. There is no such thing as buying these things, and they dont need rebuilding. Go thru your carbs before you try to run it, or you will have a gas leaking, vacum leaking, dangerous POS. The metering plates are notorious for warping, thottle arms leaking air (see below), and old sour gas that was left inside just eats these old carbs up

You will hear folks say two things about tri powers. They leak like hell, or you cant get them to idle. Here is why:

If you cant get a solid idle, or you must set the idle high, its usually because the throttle arm bushings are worn, and are leaking air, causing a vacum leak. This is from wear, a very common problem.

If they leak fuel, its usually from either where the fuel lines go into the junction blocks, or where they go into the carb body fitting, or where the carb fitting goes into the carb body itself. In other words, everywhere the fuel lines go. It taks a lot of tourqing down so they wont leak, and its easy to bugger up the fuel line fittings, and especially the carb body fittings. You have to get them all very tight, Mine are, and they dont leak. But be very carefull. To get the line tight at the junction blocks, take a small adjustable wrench, and place it over the junction block while you tighten the fuel lines. That way, you dont bend the short fuel line that goes to the carbs, something Ive done more than I like to admit


Also, the secondaries take a little getting used to to adjust to get them right. But if you get a service manual, its very easy to set that, and all the other stuff, like the choke, etc. Dont let all this talk scare you. Its not hard, and if you email me, I gaurentee I can walk you thru all the fine tuning steps I went thru. But your carbs have to be rebuilt right in the first place

I went 030 over on my 390, a basic valve job, and nothing hot except a comps 260 cam. Still running OEM style pistons, still 10.25:1 comp. After getting the carbs right, it runs fine on #70 jets. I have no pinging probs.

Watch who rebuilds your carbs. Alot of so called experts/rebuiLding services out there dont have the experience. Even the holley rebuild kits dont have the right parts in them. DOnt even think about calling the holley tech line. They have lost all thier historical knowledge on their old 2300 series carbs. I was lucky to have a local man walk me thru it that knew the deal. I highly recommend you cruise the forum, and find a reputable place that knows these tri power carbs

I now have a gm 502/502 crate in my 68, that now has the tri power on it, and it runs like a scalded dog, with the same size #70 jets.

I would take this motor, and my 427 when I had the tri power on it, on any 8 hr long cruise, and not have a doubt that it would make it.

Whats great also, is that the secondaries run on vacum from the center carb to kick them open. If you are on a long highway cruise, and want to save gas, just unplug the line on the center carb that feeds vacum to the secondaries, which takes about one second. (I carry the same size vacum line
thats plugged on one end, and place it over disconnected port when I do this so dirt doent get sucked in)

Believe it or not, you will then only be running on about 330 cfm like that (center carb only), but you wont have a prob with cruising, and yes, you will get 16 miles a gallon that way. Im living proof. But you wont have use of your secondaries. You wont believe the power though that you get just off the center carb

Another way to do that is to use very stiff secondary springs, but thats another story. I use very light secondary springs to get my umph ASAP, so gas mileage is crap. But if I disconnect that center vacum line, I get an average of 16 mpg. It worked on my 427, and it works on my 502. Its great while just cruisin the highway, or bopping around town

Flareside

I used to have a fuel line that's set up close to how MELVIN described it:


This is also an L36 coverted to 3x2.

-Joe

Steve439

I've got the GM (Edelbrock) aluminun heads on a 390 shortblock,
close to StrayDogs's motor. The 110 cc heads and a thicker
Felpro (#1027, I think?) gasket ended up giving me around 9.3
or so, which seems perfect to me. I really like those heads -
they pretty much bolted up with no foolin' around.

My previous motor was a 435, and I just went over to Chicago
Corvette Supply and bought a brand new set of carbs. It was a
little pricey, but I liked setting the dated originals aside and running
brand new carbs. If you're starting from scratch it seems like
the way to go...

I think the 400 manifold is the tougher thing to find?

ztheusa

Wow :crazy:

This is getting fun. The big truck arrived yesterday to take my old 390/427 to the re-builder. I have complete confidence in Del, from Del's Automotive in Squamish, Washington.

What I am doing is changing the complete complexion of my 67 roadster. The LS6, which has been a great motor will be coming out and Del will build the 390 into a 400/427 for a mild, smooth street engine.

I bought an almost virgin(supposedly) 400 oval port manifold off E-Bay. Turns out the seller was Chicago Corvette Supply, and Al there said it was the nicest he'd seen in 10 years. I am crossing my fingers. It WAS expensive. I also then bought, directely from them, brand new carbs, linkage, air cleaner Etc.

I have had this 67 since 1975. I have driven it 150,000 miles on the 390. About 1000 in the last 10 years on the brand new LS6 crate engine. The 390 was taken out when the timing chain broke and at that time I wanted more horsepower. Now I just want it to purr, have drivability, and I am sure it will still go plenty fast enough.

Since the driveline is pretty worn, I am also installing an overhauled transmission and rear end. I am going from an M-21 and 3:70 gears to an M-20 and 3:08's. So it will go from a "dragster" to a "highway cruiser." All the other driveline stuff will be replaced. Clutch, U-joints and whatever.

The nice thing about this car is that it has been lucky, NEVER hit. Almost unbelievable for a car that I drove every day for decades. Thanks to everyone who has had input and Happy Thanksgiving.

Karl
On my 9th Corvette
1967 400/427 Roadster
2001 Quicksilver Z06
:D :D :D :D :D :D :D :D :D

MELVIN

Flareside: Man thats, it!

Some other points: The center carb, like I said before, tends to develop a vacum leak over the years, due to wear in the throttle arm shaft. This can be somewhat remedied by what TOM 454 let me in on.

The wear is caused by the contant wear of the throttle arm being constantly pulled to the rear by not only the Acccel cable, but the return spring as well

To alleviate, make a very small, l shaped bracket, with a small hole that will mount like mine, on the rear, front carb base nut, drivers side. Now place your return spring on that, and attach it to the center carb: Now you have more or less equal fore to aft force for the center carb throttle arm, reducing its wear

olredalert

-----If your engine builder is good(and it sounds like he is)he will be able to build you a correct compression motor that wont ping.Make sure there are no sharp edges on either the pistons or the combustion chambers.Hot spots promote pinging.Camshaft selection is also very important for reasons your builder will elaborate on at length,I assume.Dist.curve is very important.I would humbly suggest that you break the engine in on a dyno,and set up all the parameters at that time.Tell him about your gear and trans selection as it will have a bearing on your choices.
-----Of course you already know that there is no return fuel line on any 67 no matter what engine option you have.The throttle linkage arm is indeed different than a 390.No 67 has cable style linkage like 68 up did either.Did Al supply you with a fuel line kit as well?Just noticed that I have a complete set of new 67 Tri-Power fuel lines laying around from an old project.Am trying to clean out my garage(see C-2 for sale stuff),and would sell the set for less than you can buy it for from any vendor if you are interested.............Bill S