I think my carb is too small ??

mgbv8 · Post by **mgbv8** » Tue Oct 23, 2012 6:54 pm

I did some testing with the new engine a while back. And my AFR logs showed up a potential problem with the carb.

Brief history:
Old engine was stock 4.6 with large ring gaps for nitrous use. This engine was a bit of a heavy breather. Stock heads. In fact stock everything apart from forged pistons and a real steel blower cam.

Back in July she ran our new PB of 10.2 with 12 psi of boost + some nitrous.
AFR's on gas and boost were stable at 11.5's.

Then I broke it.....

The new engine is bored and stroked to 4.8 litres. The heads are ported, O ringed, mech cam and solid lifters + a Ken Belle rocker set, and it has big valves fitted. The cam is unknown. All I have been told is that is for blower / nitrous use. The forged pistons have total seal rings fitted, so I guess thats why it doesnt puff much out of the breathers now.

So I now have a much more efficient air pump eh?

First test passes on 4 psi of boost showed no issues.
Second test passes on 7psi were good and showed AfR's in the mid 11's again.
Then I upped it to 12psi and it went slower. Logs showed AfR's running down as low as 9.9 at highest rpm on track (around 5300 rpm)

I then dropped back to 7psi and took 2 degree's out of the timing and she ran even quicker.

So my thought is that the 800cfm carb cannot flow enough air?

I've used the forumla I have and at 7psi I should need 762cfm

For 12-13 psi it says I should need 960 cfm.

So I assume when using more boost the carb air flow was restricted, but the vac under the carb dragged more fuel out anyway??

I guess its time to start saving for a bigger carb?

In the mean time I'll play with setting the engine up to max it out on the 7psi on the dyno. Then I can add some nitrous and fuel under the carb for more power and to adjust AFR to suit on track.

Does that sound right?? A 1000cfm carb for a 4.8 engine using 13psi of boost??

Perry

DEVONMAN · Post by **DEVONMAN** » Tue Oct 23, 2012 8:12 pm

Assuming that you will be splashing out for a new carb in any case, at the risk of trashing the 800 cfm carb I would gradually increase the size of the high speed air bleeds to see if the mixture is more stable at high revs.

Cheers Denis

kiwicar · Post by **kiwicar** » Wed Oct 24, 2012 9:29 am

Hi Perry
You were saying on Sunday you had a second mechanical secondary carb for this engine sitting on the shelf in the garage, rather than giving it to Jay why not make up a plenum/ adaptor to fit a second carb above the blower? As you already have the carbs the only real expence would be an ally plate top and bottom and about 2 square foot of 3mm sheet ally to make the sides with, and getting it welded up! It would look dead cool

Best regards
Mike

Eliot · Post by **Eliot** » Wed Oct 24, 2012 10:11 am

Something doesn't seem right to me - an 800cfm carb is pretty big.
a 4.8L @ 6krpm and 100% ve only requires a 500cfm carb, so in theory again at 100% ve and 1 bar of boost you need a 1000CFM carb.
Allow for heat gain and not 100% ve and you have 800cfm i would say.
If you was running a little 390 or 500 carb - then yes maybe it is too small - but an 800 doesn't seem overly small to me.

My throttle body is 4x41mm which is around 800cfm - which feeds my 5.7 at 10psi - but then again i dont have (or need) a venturi.

Some narative i wrote down at the time:
http://www.mez.co.uk/turbo9.html

Cobratone · Post by **Cobratone** » Thu Oct 25, 2012 10:17 am

Eliot wrote:My throttle body is 4x41mm which is around 800cfm - which feeds my 5.7 at 10psi - but then again i dont have (or need) a venturi.

Does that mean my 8 50mm Jenveys are too big for my 383 chevy then??

Pel can't you borrow a 1000 carb to try or check for airflow restrictions like your filter causing the low afr's??

kiwicar · Post by **kiwicar** » Thu Oct 25, 2012 10:46 am

Hi
No 50mm ia about spot on,
at a guess you have about 1/2" of lift on your cam, a 2" inlet valve . . . gives avout 3 1/8" curtain area fof the valve at full lift. . . a 2" throttle plate has about 3" area fully open (allowing for shaft and plate edge on) so pretty good match maybe should be a little bigger but it really depends on the state of tune.
Best regards
Mike

mgbv8 · Post by **mgbv8** » Thu Oct 25, 2012 3:34 pm

Cobratone wrote:
Eliot wrote:My throttle body is 4x41mm which is around 800cfm - which feeds my 5.7 at 10psi - but then again i dont have (or need) a venturi.
Does that mean my 8 50mm Jenveys are too big for my 383 chevy then??

Pel can't you borrow a 1000 carb to try or check for airflow restrictions like your filter causing the low afr's??

I might be able to borrow a dominator to try. I dont use an air filter. Just the bug catcher !!

JP. · Post by **JP.** » Thu Oct 25, 2012 3:49 pm

In my experiance, more boost with a "roots" compressor means more drag.
There's a point that the engine needs more bhp to spin the blower.
6-71's can rob a lot of horsepower.
Got 2psi out of my blower which increased my ET last year.

Get it back on the rollers M8 to see where your issue lies.
Have you ever considered a Predator carb ???

Darkspeed · Post by **Darkspeed** » Thu Oct 25, 2012 7:21 pm

What pressure drop is the 800CFM rated at as not all the manufacturers rate at the same datum making some carbs seem like they flow more than others.

stevieturbo · Post by **stevieturbo** » Thu Oct 25, 2012 8:43 pm

Are you blowing or sucking through the carb ?

From your first post it just sounds like the engine/carb is tuned well for 7psi. And is not tuned well for 12psi ?

I doubt one set of jets and tuning will be optimised for all levels of boost.

And a simple test....assuming suck through.

Stick a vacuum gauge between the carburettor and blower.

mgbv8 · Post by **mgbv8** » Fri Oct 26, 2012 9:52 pm

stevieturbo wrote:Are you blowing or sucking through the carb ?

From your first post it just sounds like the engine/carb is tuned well for 7psi. And is not tuned well for 12psi ?

I doubt one set of jets and tuning will be optimised for all levels of boost.

And a simple test....assuming suck through.

Stick a vacuum gauge between the carburettor and blower.

Its a suck through setup Stevie.
At 7psi she runs sweet. When I increase the boost I get the manifold pressure, but then it runs rich as hell. My assumtion is that the carb venturi's are struggling to keep up with air flow demand which is creating a bigger vac under the carb so fuel is being sucked out of the carb due to the increased suction. I can try reducing jet sizes to get the fuelling back in line on the higher boost pressure I guess.

Next time I run the car I'll stick a vac gauge on the servo tapping under the carb to see what it reads. Wont be until Xmas though.

Eliot · Post by **Eliot** » Sun Oct 28, 2012 8:34 am

Cobratone wrote: Does that mean my 8 50mm Jenveys are too big for my 383 chevy then??

The size of individual throttlebodies would be larger than a single throttle body feeding all 8 cylinders - because only one cylinder requires air at any given moment.

kiwicar · Post by **kiwicar** » Mon Oct 29, 2012 2:01 pm

Hi Perry
the 4 barrel carb, being fixed venturi will get richer the more you ask it to flow even if it is sized correctly, the amount of depression (suck) at the jet goes up with the square of velocity through the venturi, so pull twice the air through it and it wants to draw 4 times as much fuel through the jet. . . that is why it has air bleeds, these dilute the fuel with air bubbles as more depression is seen at the venturi so it tends to ballance things out.
An SU gets around this for 95% of it's funtion by having a variable venturi in this design the velocity through the carb remains consant so with just an open jet tha amount of fuel drawn would be fixed so the mixture would get leaner as more air is drawn through the carb, that in turn is why the needle is attached to the piston and makes the open jet area bigger as the carb piston lifts. You always need to rejet/reset the air bleeds when you change the state of tune of the engine/ supercharger drive ratio. at a guess you probably want to go up on jeta and air bleeds, don't go down on jets, the reason is that you will eventually reach a limmit on what any jet can flow and the mixture will go richer. . richer. . suddenly rapidly lean at the very top, a bad thing! if you go for a richer jet and a bigger air bleed you should not run into this situation, I ould probably do most of the up jetting, bigger air bleeds on the secondarys aswell, just to give a bit more progression as you open the throttle.
I could always lend you a 4 barrel throttle body and a megersquirt controller (the one I made a few years ago, not the new one)

Best regards
Mike
Best regards
Mike

DEVONMAN · Post by **DEVONMAN** » Mon Oct 29, 2012 4:56 pm

Hi Perry,
Following up my previous suggestion to increase the size of the high speed air bleeds, I note Mike has suggested it also.
I have used a Holley with screw in air bleeds and playing with these worked for me in the past although it was a blow through set up.

Regards Denis

kiwicar · Post by **kiwicar** » Mon Oct 29, 2012 5:19 pm

Eliot wrote:
Cobratone wrote: Does that mean my 8 50mm Jenveys are too big for my 383 chevy then??
The size of individual throttlebodies would be larger than a single throttle body feeding all 8 cylinders - because only one cylinder requires air at any given moment.

Hi
Hilborn stacks (throttle bodies) for the SBC are normally around 2 3/16 to 2 3/8, in the region of 55mm to 60mm probably at the larger end of the range as they are really only intended for racing, but not silly big!
Best regards
Mike