I am wanting to build a similar engine I have an 86 l98 and want to run a tunnel ram with 2 490s on top mainly for looks not to race I would like to know what I can reuse in the stock engine or do I really need to build a 383 I plan on running A/C also so like I said mainly for looks because I have the tunnel ram it's going in 86 bird I have an old hood to cut hole in for bird catcher any help is appreciated

 

Same answer as the guy that randomly posted in this thread, his post is #9. See post #10 for the answer.

https://www.thirdgen.org/forums/engi...-l98-swap.html

 

i re-posted because i did not know if the other was still active but you all have convinced me to use something else all of us are not tech savvy just so you know we work with our hands

 

It's OK. It's not about "tech savvy", it's about common sense.

If all you care about is looks, don't bother with the cubic inches. Use the cheapest thing you can find that looks like what you want. If that means just use the motor you have without rebuilding anything, so be it.

 

The idle circuits on the 2 carbs will need to be recalibrated as you now have double the carbs on 1 engine. IFR and IAB's need resizing.. This involves drillibg out the stock pressed in orrifices and drill and tap the holes for screw in small holley type metering jets. .. The AREA of the IFR jets is now double and needs to now be reduced by approx ½. in EACH CARB. Area is not diameter of the jet hole. AREA is (π r)² (pi X radius) squared . r is RADIUS ( half of diameter). π = 3.1416. As you can now see its a but involved.

if you just throw the 2 carbs on it will run but will not drive right. as the idle and off idle throttle transition is quite rich.
Great at first on a cold engine but becomes way too rich once the engine and intake manifold warms up.

This is biggest issue and hardest for the novice guy to correct..
The 2x carbs will need different main jets as well.
Not such an issue as the required re calibration of the idle circuits in each carb.

Other than throttle linkage and (missing) availabke mounting brackets bolt holes for stuff like A/C compressor brackets its a breeze.

Can have poor drivability when cold as the tunnel ram height takes longer to warm up the plenum under the carbs. Longer engine warm up time.

Distributor advance curve will need mods.
With the tunnel ram the engine tends to want a lot more initial base timing at idle but the same WOT max mech timing 32-36 deg btdc.
This involves shortening the diz mechanical advance curve travel from 20-24 deg to about 10 degress. Some people simple LOCK OUT the diz mech advance curve. Timing is now fixed at 32-26 deg btdc all the time.

If you are willing and able to rework this stuff. have at it.

 

Any pair of carbs that were not specifically calibrated fir running on a tunnel ram or on a "dual quad" intake will need this same idle circuit re calibration.. The size of the pair of carbs does not really matter in this reguard. Its the calibration of the carb(s) idle circuit.
Even matching pair of "dual quad" carbs straight off a OEM dual quad intake will need at least some amount of idle circuit and main jet re calibration
to run correctly on a tunnel ram..
Once this critucal fuel metering re calibration is correctly done a tunnel ram car can run and drive very nicely on the street even on a mild near stock engine.

Even the Holley 2x4 tunnel ram specific application carbs will need final metering calibration.
EG the infamous Holley 660 center squirter 2x 4 carbs..
These tend to be much closer out of the box.
The main jetting on these 660 holley carbs tends to be very close but the carbs idle circuits will need at least minor tweeking to get it right on your street driven engines.

Edelbrock afb carbs can work well but again the idle circuits all need specif re calibration on a tunnel ram.
The 500's are no better than the 600 or 750 carbs in this reguard. The all work once correctly re calibrated (idle IFR and IAB modification mostly)

None of these are correctly calibrated out if the box.

 

The take away here is that ANY carb that was on a engine 2bbl or 4 bbl as a single carb its idle circuit will now be about 2x too big when combined with a addition carb or CARBS on a 2x4 tunnell ram engine.

That means that each of the now 2 carbs will need the idle circuit fuel metering orrifices (IFR) AREA. REDUCED BY APPROX ½ ( but not exactly half)
½ the AREA is a ball park start point.
You have to fine tune from there. thus the need for screw in swappable small holley metering block metering jets.
A AFR guage is a big help to get it all dialed in.
A simple narrow band type AFR guage 02 sensor gets it done on a budget.

 

So,,, if you are following along and have not given up and understand that each carbs idle circuut fuel metering jet will need the AREA REDUCED BY APPROX ½ for on a 2x4 carb tunnel ram set up
HERE IS THE MATH. SHORTCUT.

Instead of calculating the AREA of any fuel metering orrifice. "jet". to quickly get ½ the AREA and then recalculating the inverse of all that to find the new DIAMETER of the new required fuel orrifice.
You can substitute the rule of ROOT MEAN SQUARE for all this math.
Simply Divide the jet fuel orrifice DIAMETER BY 1.414 to get the requured new jet diameter that is HALF THE JET AREA.

EG: Origional IFR'(s) orifice diameter is .038"
Ballpark new IFR diameter is .038 /1.414 = .02687
or rounded .0269 to .027"
Thats your ballpark new IFR jet diameter size.
This is your idle circuit tuning START POINT to dial in the 2 carbs on a tunnel ram.
The final correct size will be +/- close to this..
The inverse of Root Mean Square math constant is 1/ 1.414. or simply .707

 

The engine. and the tunnel Ram manifold plenum and carbs must be FULLY WARMED UP to operating temperature to evaluate the idle off idle circuit transition AFR as you tune to dial in your carbs on a tunnel ram.. The air cleaners should installed too.

On a ¼ drag race car they get away with a lot of evils on their race only 2x4 carbs on a tunnel ram because the car only idles and drives for a very short amount of time. Just long enough to stage and drive back down the return road back to the pits... Plus for best ¼ drag ET they want the engine temp a bit cold and thus use a bit richer carb jetting..
This richer overall carb jetting used for ¼ drag racing will tend to soon foul your spark plugs when driving around on the street...
What works for ¼ mile drag racing does not work well for extended running normal street driving.
Thus the need to dial in the 2x4 carbs to drive well on the street.

 

This is just half the job of re calibrating a single 4 bbl carb for use on a 2x4 tunnel ram for street use.
But this is the hard part..
The other half if the job is dialing in the carb(s) main jetting and PVCR metering and relative reference for use on a 2x 4 tunnel ram.
The main jetting jet size will be close but not exactly the same.. The PVCR sizes will be Different..
The carbs MAB's will need to also be slightly different ..

The biggest part of the job is getting the carb's idle / off idle circuit re calibration right as on the street you do the majority of your driving around at slow part throttle using the 2 carbs combined idle off idle circuuts while driving around.

Otherwise you end up with a foul running engine that always needs new fresh spark plugs and constant attention to those 2 carbs.

Its all about correct carb setup.

 

An effective DIY method of reducing the flow AREA of a (any) internal metering "jet" orrifice in any carb is to insert a small diameter steel wire in the jet to reduce the net flow area..
You need to wedge in and or anchor that steel wire in the jet so it cannot get lost. Make sure it is securely anchored or wedged in there. You can use multiple wires in each jet to reduce flow AREA as required.
You can measure the wire DIAMETER and calculate that AREA to calc the new net FLOW AREA of the jet+ wire(s) as required.

Saves a lot of drilling and taping for swappabke screw in metering jets and aquiring all the jet sizes and precision size small drill bits.

 

Another DIY. Macgiver method way of cutting the AREA IN ½ OF ANY ROUND FLOW ORRIFICE " jet". is to cover half the jet hole opening. Now it is a D shape and approx ½ the flow area. Could be a lil sheet metal tab or.... Just make sure it is well anchored or it will likely get sucked in the carb and may end up in the engine. You can then tweek that tab over the hole +/- to fine tune the FLOW AREA for effect.

 

Had some free time on your hands, huh?
Great explanation though. Thanks.

 

very good information I never thought about any of that but at least now I have a clue