Thread: Tunnel Ram Tuning HELP!

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I have a dual quad tunnel ram on my 289. I need some help or at least advice on how to go about tuning this thing. I consider myself to be quite experienced with carbs themselves but this is my 1st dual quat and it's throwing me for a loop. Here's my engine specs:

'66 289 bored .030 all stock lower end and approx 9.8:1 compression
'66 54cc 289 heads fully ported and Chevy valved (1.94/1.60)
Cam specs: 224/234 Duration @.050 with .496/.520 lift 107 int centerline and 112 lobe sep.
Crane full roller rockers and Edelbrock Rpm Valvetrain
2 holley 450 manual secondary carbs (4160 series)
Headers and full 2.5 Exh
Holley 'red' elec fuel pump

I can't seem to be able to make any ground in tuning it. Can anyone tell me how I go about doing this and with what tools? Any suggestions as to what the timing SHOULD be? I think my timing, last time i played with it, was well above 20 degrees initial and it really liked the advance.

I think you may have manyvariables here, so pick on e and start there....
timing...stock curve? vac advance? what vac readings are you getting? (should be nearly 14 in at idle at maniold.) is vac advnace plumbed in correctly? (shoud go to a carb port)

Jetting? any clue where you are or where to be or if you are close now?

Manual sec'y on carbs?
Auto or manual trans? Generally manual sec'y are for maul trans, vac sen on auto...but you can make it work either way.

Are your primaries set to open together, or progressively? if together, then you'll need 2 vacuum guages, one for each carb. you'll adjust the throttle plates so that both carbs pull equal vac. then you can set up the other linkages. - equal vac menas both are working evenly, and that is what you want.
if the linkage is progressive (the way trip 2bbls work) then it is kinda up to you as to what you want.

Since Ford and Chrysler both made dual quad factory engines, you could dig up the carb adjusting procedures for one of them and see what you can apply to your situation.

I have read about setting up dual carb VWs/Corvairs, and have 10 years with motorcycles as a pro mechanic (mult cab is common, but two stage is not).

Hope i have helped some....

HEY! Thanks for the reply.... It's REALLY hard trying to find someone who knows this stuff.

Ok to answer your questions, here we go-

I have a stock type points distributor. It's a reman so I have no idea how the curve is setup. I removed the points and installed a Pertronix unit and am using a Mallory Promaster coil.

I have the vac advance disconnected because I am getting a constant light draw from the 'ported' vacuum ports on my carbs and it's prematurely advancing the timing at idle. There is only one port on each carb to hook the vac advance up to and it's on the RH side of the carb on the metering block as a typical ported vacuum port on a holley would be.

As far as the inches of Vacuum at idle, I'll have to get back to you on that once I get a new gauge. It was less than 10 inches if I remember correctly. Also, i tapped the upper plenum for a PCV valve and tried running it. It seems to run fair but runs/idles better without the PCV valve hooked up. I'm also drawing vacuum for my power brakes and C4 tranny. The power brakes seem to work just fine.

I have jetted the primaries down a bit. It "seems" to be alright but is hard to tell without being under a driving load and able to measure the a/f ratio. I went from #58's to #54's in both carbs. The secondaries are stock as they are metering plates. (4160 carb with side hung float bowls)

I've also moved up the accelerator pump squirters from #31's to #37's and that seems to have taken away most of the off the line stumble. Also the powervalves were 8.5s but are now 3.5s.

The carbs I have are Holley 4160 model manual secondary (not doubble pumper or dual feed) They are 450 cfm carbs.

My linkage is setup to open both carbs' primaries at the same time. Hang tough as I'm going to have to purchase new vacuum gauges as the one I was using took a leap to it's death off my workbench the other day. I'll try and pick two Craftsman units up tomarrow since it's payday!

Finding info on a SFB with dual quads in a factory manual shows to be a real bear. THe only one I know of would have been the Shelby Mustang or the Shelby Cobra. Even then they were dual plane std. intakes and nothing like a tunnel ram.

YES! your info is very helpful. Please feel free to give me more pointers. I'll get back to you Fri or Sat with some vacuum and timing numbers to try to shine some light on this. Again, thanks for the reply!

Also- I have in my possesion: two accelerator pump cam kits and also two 50cc diaphram conversion kits that I have picked up.

Dan,
Without the vac advance hooked up you will not get enough advance and the car will fall on its face during acceleration. Been a long time since i studied up on advance curves, but assuming a car can take 36 degrees total, 8 is initial (set with timing light), some is mechanical (and in your application should be in by 2500 to 3000 rpm. stock curve dist may not come in until 4000 or more). Perhaps 1/3 to 1/2 total advance is from teh vac advance unit. adjustable ones are available too. I would think you could hook up tht evac advance right to the intake port without a problem (i have done it...on a stock motor i never noticed muchif any difference). with only 10 in of vac you'll never get full advance.

total vac - a bit low (not for this motor, but in general). that is why the PCV is making things unhappy, it lets the total vac drop too low. not sure it this is related to the vac port on carb issue or not.

900 CFM is a bit of overkill for a 289. at 5000 rpm, at 80% volumetric efficiency you can only get 670 cfm into the engine (math available upon request). The carb/intake you have will be dog down low, but will scome into it's own up high in the rev band. assuming you have a proper cam for hat, and trans setup and rear gears...

when an intake valve opens and hte piston creates a vacuum, a column of air begins moving. it is still moving when you close the valve (momentum works on air too). for torque down low, use long small intake runners (see modern cars). your intake has your carbs a long way from the intake valves, so the column of moving air (with the fuel in it) has a ways to go, takes a while to get started, etc. I would suspect that there would be some throttle lag, particularly at low speed and off idle. This is probably why the accel pump change has worked. (you open throttle, but it takes a bit gor the engine to rev so the air/fuel it is sucking is still lean, by the time the good stuff gets there you are out of the throttle.)

I will be out of town this weeknd, but early next week i will get back to you...i will see if i have anything on tuning a tunnel ram (i still have my car mags rom the 80s...)

Low vac will somewhat confuse the carbs too...have to look up the fix for that. you could try using a vac canister (an air tight can hooked to intake port (brake vac line?) and then run the vac accessories off that.). Also, a vac pump is sold that runs off a belt like a water pump/alt, etc. not that this solution would help the advance issue, but if the brakes are run off that the vac left may be enough for pcv? Never used one of the aux vac pumps...perhaps they plumb into the intake too? see Summit or Jegs and then check the manufacturer's website.

Chris-
Hey guy, thanks for being so patient. I havent' had a chance to go fiddle with it to get those readings yet. The weather has turned for the worst so I have had to turn to some other outdoor projects to finish them up before the " real nasty " is here to stay. (if you live in the NW you know what i mean!) I'll get them posted up here ASAP!!! Thanks!!

I haven't forgotten about you either, but as you say, winter is coming. I hate to say that as summer doesn't oficilly end till next week sometime...

had to get teh camaro mobile, and a few more bits on it. Want to get my lincoln sprayed before it gets too cold (late oct)...still have WAY too much to do on that.

Wife is due to deliver a baby on Oct 9...

Grass is getting DEEP here.. and the Huirricane will rain in us by the end of the week...

I have old car carft mags that i think will have info you can use..just need to find the time to go through about 7 years of them...

Well I hope the weather hasn't been too harsh for ya so far. It's getin colder and darker every day here. I've already managed to catch strep throat now so I'm out of work for today. I guess it takes a real nasty sickness to finally slow me down! I've been on a roll these last few weeks gettin all my projects squared and Now I'm dead ended with a nasty sore throat. I hope you've been able to catch up on your things!

Anyhow, down to business. Last tuesday I was able to sneak out and toy with my car a bit after work. I took th timing light, 3 vaccum gauges and a wad of handtools to my "Moto Shade" shop I have erected in my backyard for the time being. (Looks like tent-city in Saudi -lol) Anyhow here are my setups and also my findings:

I have 2 holley #9776 Manual Secondary 4160 model carbs without choke setups. Each carb only has 1 vacuum port which comes from the primary metering block on the passenger side of the carb. This vacuum port seems to read manifold vacuum rather than "ported" vacuum as most all holleys do. I have a C4 Auto trans. The carbs are currently jetted with #54's (came with 58's when I got them) and of course has a non-adjustable metering plate in the back. I increased the accelerator pump squiters to #37's to take care of an off idle stumble. (was originally #31's) The powervalves are 3.5's which I didn't do, the previous owner i bought the setup from changed those from 8.5's. The setup was only a few weeks old when I bought it. Not really sure WHY he took it off his car. All i know is that his engine was built even bigger then mine not to mention a 302. The carbs are set up to open together. I'll also include pics of my setup to help you to "see" what we're dealing with.

After it was all up to temperature, it sat at idle of 1200 rpms in "park" and the timing was at that point already set at 30* initial. Yeah I know....ALOT of advance. It never pinged, not once. I had 13" of vacuum at each of the carbs and 12" at the back of the upper plenum which is where my vacuum taps are at for my power brakes and tranny modulator. The PCV valve is tapped centrally into the underside of the upper plenum between the carbs.
Now when i put it into drive, the vacuum drops to 7" at both the carbs and the manifold. It refuses to idle steadily at all below 800 RPMs. It will literally "thrash around" if you try to go less than 800 kinda like you'd be starving it of air.
Now I tried playing with it a little bit. It doesn't seem too picky at the initial timing setting. So I rocked it back to 14* and idled it back up to 1050 Rpm rather than 1200. It was pulling 10" at both the manifold and carbs while in "park." I tried to put it into drive and because it was below 800 Rpms, it was idleing erattically, as in from 500-800 bouncing continusly and occasionally trying to die which I'd tap the gas a bit to keep it alive.
One thing I did notice is that when I put the timing light on it, I ran it up to 3500 to see how far it's advance..... well.... it didn't even move.... yeah BIG red flag there. Tore of fthe cap, anf the mech advance is operating fine. moves freely and springs back. So I put it back together and tried it again.... same crap. So i took the distributor out only to pull the damn ARP drive rod out as well (SON-OF-A-@&*@$&) and took the distributor into the shop and took out the Pertronix unit and pulled the advance plate out. All the weights and springs all look good and function fine....what gives there?
Oh and another thing I noticed, when I was throttling the carbs (i had two of the exact same vacuum gauges, both only weeks old bought at Sears on the carbs) the front carb made a more erratic sweep on the gauge than the rear carb and the rear carb was sluggigh to return to the front carbs' idle vacuum when letting go of the throttle. The rear carb didn't have nearly the vacuum sweep the front carb did at all. Kinda weird isn't it?
Also i played witht he PCV being hooked up or not. I really didn't have any effect to the manifold vacuum (at idle in park) when it was hooked or unhooked and plugged.
You made a note of hooking up the vac advance on the carbs. Well when i do that it seems to prematurely advance the timing and when it's throttled it actually retards momentarily. I think that method only works for Chevys and a few Mopars but on fords, you just can have the vac advance pulling at idle.

Anyhow, it's a load of info. I hope I gave you some useful stuff to help you help me on my carb nightmare i'm having! No real rush on the reply. I can understand you're a busy man. Just when you get a chance, perhaps on a cold rainy night.

I have to teach myself everything I know about cars, but it seems that vacuum advance would retard the timing under acceleration. When you floor it, the engine is still not spinning very quickly, so it would have a tendancy to detonate. Also, manifold vac drops significantly, so if the ignition is advanced as vacuum increases, then it would retard as vac decreases. Also, that is why carbs have an accel pump - to keep the engine fed with fuel until it regains enough vac to draw from jets. So, not having the vac advance hooked up shouldn't make the engine fall on its face unless it were to advance so quickly that engine began detonating badly. And finally, vac advance is like a precurser to electronically controlled ignition and knock sensors. If your spark is advanced enough to give you any kind of bad knock, manifold vac would drop, retarding the timing.

Plz tell me if any of my thinking is wrong.

Originally posted by 76GMC1500
I have to teach myself everything I know about cars, but it seems that vacuum advance would retard the timing under acceleration. When you floor it, the engine is still not spinning very quickly, so it would have a tendancy to detonate. Also, manifold vac drops significantly, so if the ignition is advanced as vacuum increases, then it would retard as vac decreases.
Well... yeah it would "in theroy" retard when vacuum is decreased but actually it would just return to zero vacuum advance and use the initial 'set' advance. The thing is, in order for it to actually 'retard' the timing you must have a diaphram setup to allow that. Ford does have dual diaphram vaccum advance units that allows this. Mine is not. You can either set it to be at full advance (mech/vac) at idle by manifold vac., or set it for no advance untill throttled (mech advance until throttle plates are opened) known as ported vacuum.

Also, that is why carbs have an accel pump - to keep the engine fed with fuel until it regains enough vac to draw from jets. So, not having the vac advance hooked up shouldn't make the engine fall on its face unless it were to advance so quickly that engine began detonating badly. And finally, vac advance is like a precurser to electronically controlled ignition and knock sensors. If your spark is advanced enough to give you any kind of bad knock, manifold vac would drop, retarding the timing.
Yeah that sounds about right. I'd explain it a little differntly but I think you have it there.

Plz tell me if any of my thinking is wrong.

Here's another pis of my setup for reference

And last one. I'd put all three pics together if I knew how to. Just never seem to have the time to sit down and figure it out.