BBC thoughts

General engine tech -- Drag Racing to Circle Track

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F-BIRD'88
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Re: BBC thoughts

Post by F-BIRD'88 » Fri Oct 06, 2017 10:44 pm

Here is the math:

cam card 29 + 180 +65 = 274 274 / 2 - 29 = 108 deg intake C/L

your readings from degree wheel 31 + 180 + 64 = 275 275 / 2 -31 = 106.5 106.5 deg installed intake C/L

This assumes that you did the TDC right to position the degree wheel and are not getting angular or defection error from the dial indicator.

SET up the dial indicator on Cylinder #6 and repeat. Do the intake and the ex .050" timing.

What does the whole cam card say?

Does the cam lobe lift measurement agree with the cam card? Do the .050" in/ex valve events match it. other than cam position.

if the Degree wheel positioning (TDC) is wrong the valve events will be wrong too.

i would start with the cam in well advanced 102 to 104 deg intake C/L as a start point..
Start out with the cam advanced. For you ,,, it is going to be better.
My Magic Squirrel says so.

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Re: BBC thoughts

Post by F-BIRD'88 » Fri Oct 06, 2017 10:56 pm

This test stall transmission need not cost a lot of money or be hard to do.
You can perform these short time duration quick read max stall stationary stall rpm tests using your trans in your car
if you take basic care to avoid overheating the transmission.
Short time tests with cool down time and (forced) air flow across a trans cooler cooler between tests.
And time to allow the trans fluid to recover.

You will get better answers to the questions that matter using this tool than asking people on the internet.

You will get the car tuneup 97% race ready.
Last edited by F-BIRD'88 on Fri Oct 06, 2017 11:22 pm, edited 1 time in total.

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Re: BBC thoughts

Post by F-BIRD'88 » Fri Oct 06, 2017 11:16 pm

You could even buy a Used racing torque converter that "has a broken sprag"

Crap I think the sprag broke, My ET is off"....
The car launches lazy now....

This common when the converter has a broken sprag
it is usually repairable....

When the stator sprag "breaks" the torque converter becomes a 2 element fluid coupling.
It still functions but does not multiply engine torque at stall on launch.
The stator just free wheels inside. Bad for drag racing...good for doing stationary stall tests.

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Re: BBC thoughts

Post by prairiehotrodder » Mon Oct 16, 2017 5:31 pm

there is no turning back now! I'm just about ready to fire this baby up with the new cam and the tunnel ram. I bought the 750 track warriors that i posted earlier and its looking pretty serious.
Brian

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Re: BBC thoughts

Post by prairiehotrodder » Mon Oct 16, 2017 7:22 pm

float levels were way to high but other than that it fired right up and sat and idled awesome. I backed off the main idle screw a 1/2 turn on each carb (they are calibrated for single carb use) and now it idles at a nice low rpm. After it warmed up i blip the throttle and it seems to have amazingly quick response. Just to run it a little i set the valves at .018 cold. It calls for .025 or .026 hot i think. I didn't put the stud girdles on because i am going to be taking the rockers all off for winter storage. To bad there are no race days left here. We have a couple nice days right now so i may drive it around town a little and show off all that jewelry.
Brian

one question : is it better to put the throttle return springs on the carb linkage or down on the main linkage?
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Re: BBC thoughts

Post by F-BIRD'88 » Tue Oct 17, 2017 8:56 am

Lookin' serious now. .

The springs go on the carb.

The idea being if the linkage fails the spring on the carb
will close the throttle.

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Re: BBC thoughts

Post by prairiehotrodder » Tue Oct 17, 2017 4:39 pm

to start the tuning process on this set up i removed the rear jet extensions. Warmed up the motor and checked idle vacuum at about 6". I ordered a pair of 3.5 power valves. Stock is 6.5 on these carbs. Am i going in the right direction here? Idle AFR is about 13:1 on my gage right now. Shooting for 15 maybe? Then i'll play with main jets at the track from there on if it seems to be close at part throttle.
Brian
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Re: BBC thoughts

Post by statsystems » Tue Oct 17, 2017 5:04 pm

prairiehotrodder wrote:
Tue Oct 17, 2017 4:39 pm
to start the tuning process on this set up i removed the rear jet extensions. Warmed up the motor and checked idle vacuum at about 6". I ordered a pair of 3.5 power valves. Stock is 6.5 on these carbs. Am i going in the right direction here? Idle AFR is about 13:1 on my gage right now. Shooting for 15 maybe? Then i'll play with main jets at the track from there on if it seems to be close at part throttle.
Brian
Don't pick power valve opening by idle vacuum. Mark has linked many times a video where he proved that idle vacuum has nothing to do with PV opening.

You need to measure vacuum at a cruise and set PV opening by that.

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Re: BBC thoughts

Post by MadBill » Tue Oct 17, 2017 5:18 pm

A 13:1 idle is relatively lean as big-cam motors go. Just tweak the screws for fastest idle at the desired RPM and then open them up maybe 1/4 turn for better warm-up manners and see where it lands. Tip: Many people turn the screws slooowly. I find it easier to detect a change by making abrupt 1/4 turn adjustments.
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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Re: BBC thoughts

Post by F-BIRD'88 » Tue Oct 17, 2017 6:19 pm

prairiehotrodder wrote:
Tue Oct 17, 2017 4:39 pm
to start the tuning process on this set up i removed the rear jet extensions. Warmed up the motor and checked idle vacuum at about 6". I ordered a pair of 3.5 power valves. Stock is 6.5 on these carbs. Am i going in the right direction here? Idle AFR is about 13:1 on my gage right now. Shooting for 15 maybe? Then i'll play with main jets at the track from there on if it seems to be close at part throttle.
Brian

First: DO NOT SET the idle according to a AFR reading. Set the idle mix screws for best idle quality
once the car is WARMED UP ! Let it warm up.
Then set the idle mix screws. Carbed cars idle at a AFR that is richer than 14.7:1 EG : 12.5:1 or 13:1
This is Normal. Give it what it wants to idle the best.

Power valve: for the best idle quality without decay over time and to keep the plugs clean over time
use a power valve that is vacuum rated just lower than the vacuum at idle when idling in IN GEAR.

EG: if the manifold vacuum at idle IN GEAR is 6" use a 3.5" or a 4.5" rated power valve.

The AFR meter does not matter. What matteers is that the power valve is CLOSED at idle when idling in gear.
If consistant tune and plug life matters on this street driving car than this MATTERS.

Get it right.

A drag only car is not that critical but a street driven car that idles a lot MATTERS.
Check the vacuum idling IN GEAR. If it is 5" use a 3.5 or a 4.5 power valve.

Ignore this and expect shorter plug life and inconsistant car perf OVER TIME.

Let the car warm up and set the idle . Ignore the afr gauge at idle.
It wants what it wants. Idle quality will improve when you let it warm up.
With a tunnel ram that takes a little longer.
When the engine and manifold is cold keep the rpm up say 1500rpm to avoid fouling.
Let it WARM UP. then set the idle. it will idle all day at about 800-900rpm.
and near that in gear.

Now open the throttle to raise the rpm and WATCH the afr gauge. 1200 to 2000 rpm point>
" off idle transition" What does the AFR gauge say? You want it in the 14:1 zone at this rpm/throttle point
13's is good. The manifold must be warmed up. Warmer is better.

When you get it out and drive it you can check the PART THROTTLE cruise afr ( main jets) at 45+ road rpm.
14.5 ish+/- is good.

Anything below this road speed will be idle off idle transition until the point of primary main start up.

A Afr in the 14 range is good at this point. You can tweek the primary idle air bleeds to adjust this +/-.

If the power valve is OPEN at idle when idling in gear then eventually the tune quality will degrade
OVER TIME ( street driving time) requiring more frequent spark plug changing and fiddling with the carbs idle set up. What the afr gauge says is not relevant.
if you want it to stay clean and consistant get this right.

Let it warm up. During warm up keep the idle rpm UP.
My buddy with a tunnel ram uses a lil block of wood in the throttle linkage to raise the idle rpm during Cold warm up. (1500+rpm idle when cold)

WOT afr should be in the 12.5 to 13.4 is range at WOT.
Tune the WOT jetting for best track MPH not a preconcieved WOT AFR
But this is the range.

Again set the idle mix for best idle not AFR. Give it want it needs to idle best.
15:1 is way too lean at curb idle.

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Re: BBC thoughts

Post by F-BIRD'88 » Tue Oct 17, 2017 6:42 pm

Street driving is DIFFERENT than drag racing. On the street you want to let the car FULLY warm UP
to get some HEAT in the intake manifold for best idle quality and "drivability"
The more HEAT the better the fuel vapourization at idle and low speed driving.

Thats why you let it warm up fully, then set the idle mix screws for best idle on a WARM engine.

This also allows for leaner carb MAIN JETS for best cruise AFR.
Again the more manifold HEAT the smaller main jets can be.

DRAG racing is DIFFERENT. Usually for best drag race ET MPH you want the engine a bit colder.
(100 to 140 F ). This COLD idle set up requires richer idle mix adjustment and a Bit richer (Bigger main jets)
to get Best power jetting. If you stage and run it "cold" to get he best drag ET pay attention to this...
As this richer COLD engine racing jetting will be too rich for best STREET driving
on a Fully warmed up motor and extended driving.

2 different driving conditions.... 2 different jetting requirements.

You can make up the difference a lot by the power valve channel restriction size choice
to find the happy medium overall jetting.
Jet realize that with the tunnel ram it takes Driving time to get heat in the manifold and when it does
get fully warmed up it will be happier with a bit leaner jets than when DRAG racing it (Cold)

Fuel vapourization quality improves a lot with HEAT in the intake manifold.
Good fuel vapourization likes a bit smaller jets. (fully warmed up engine)

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Re: BBC thoughts

Post by F-BIRD'88 » Tue Oct 17, 2017 7:04 pm

If you are going to street drive it consider getting a vacuum advance for the distributor
and USE IT. ( on your car) Locked out distributor with a vacuum advance of about 10 to 15deg of added vacuum advance added at street and hiway cruise. The tunnel ram will love this.
At the track , for drag racing you can just disconect the vacuum line if you want.
As it is not really needed. But on the street you want the benefit of vacuum advance at Cruise.
ESPECIALLY WITH A TUNNEL RAM. on the street.
Most all racing distributors ( MSD etc) can have a vac advance added. The vac adv mounting boss in the disz may not be drilled and tapped but it is there.
Take the diz cap off and look for it.

if your ignition box has a "high speed" retard function you can sue that too to good effect with the tunnel ram
at the track for best launch and ET.
The High speed retard is a bit of a mis nomer as really you add some extra static advance for the stage and intial brief car launch and engage the ignition "retard" to reduce the spark timing just after launch.
This gives a cleaner DRAG launch with a "COLD staged tunnel ram.
The amount of extra dialed in staging/launch spark advance and thus the amount of "high speed" retard dialed in has to be found by drag testing. usually in the 30 ft to 300ft range up to to top of first gear range.

Usually BBC's like about 38deg WOT spark advance but it may want and can benefit from more timing at stage and initial launch 40++deg Briefly. Using this ignition retard box function can really help a auto trans tunnel ram car launch harder. You don't need it for the street.
if you don't understand it don't use it.

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Re: BBC thoughts

Post by cjperformance » Tue Oct 17, 2017 8:16 pm

As said , dont set pv by idle vac, pv's have NOTHING AT ALL to do with idle anything, why dont people get this !
You want to get the cruise jetting right, check the cruise vac & have a pv that starts opening where the cruise mix just gos lean as you slowly acellerate away from cruise, then with correct pv you will have a clean transition from cruise to power and a nice cruise mixture.
For idle , where the engine is happiest is best, forget the A/F gauge at idle.
Yes Bill, totally agree, a quick change of idle mix posi is the quickest way to hone in on the idle mix screw posi that is in the sweet zone, then a gently play from there.
Lookin good.
Craig.

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Re: BBC thoughts

Post by F-BIRD'88 » Tue Oct 17, 2017 9:42 pm

With that much valve lift and valve spring pressure I would avoid running the engine without the
stud girdles installed. Thats a lot of force on the BBC rocker studs and BBC aluminum head stud bosses.
Even at low rpm....

Good idea to loosen off all the rockers for extended winter storage. Some engine storage lube spray in the ports and cylinders is nice too. Be sure to put some anti seize on the spark plug threads.
Storage is actually really hard on a car..... Brakes etc etc. Moisture/Humidity is evil.

Put your Mickey Thompson drag tires in Garbage bags and store away from a furnace. ( Ozone and UV light really kill the rubber )

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Re: BBC thoughts

Post by prairiehotrodder » Wed Oct 18, 2017 5:16 pm

Thanks for all the advice guys. I'm gonna be reading through it more next spring as i tune this setup. I allready installed the 3.5 Power valves and suddenly the idle screws actually became responsive. They didn't seem to do anything before. I'll revive this thread next spring when racing starts.
Brian
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