I'm trying to squeeze out some "simple-free" Horses out of my motor.

This is what I've done to date:
Cold air intake (bugcatcher)
K&N Flow Stack (4 RWHP on the dyno!)
Adjustable Metering block (down to 0.5 jet size)
1/2" wood spacer/insulator
Dyno Tune - Air/Fuel ratio, timing (picked up 22 HP at the wheel!)
160 Degree thermostat (80/20 distilled water/coolant ratio)
Timing Curve Mods - Advance kit
Fuel Cool Can (water and ice mix)

The only thing I'll have left sucking manifold signal is the brake booster.
I have a 700 CFM mechanical Holley at the moment with the choke blade removed for better flow.
All vacuum lines are removed and plugged.

What else have you guys found that works for power, or even more efficiency?

Over the winter I'd like to add the following:
* PCV system to header collectors to free up more vacuum (need tips on this please) .
I see all the "big-boys" doing this at the strip. Is there any other benefit besides improving vacuum and rerouting blow-by gases?

* Fuel Bowl pressure equalization tube (what else does this do?)
A tube connecting the vent tubes of the fuel bowls, yet it has a cut-away directly over the venturis of the carburetor. Why?

Thanks for any explanation.

 

Ah, yes, carburetors. Something I feel I actually can post about in an "advanced" forum with some credibility (maybe not much).

You've done well so far. You're down to the short strokes now.

Here's the thing about carbs, and I say this with almost 20 years of expereince. Their WEIRD. That's a technical term. Let me define it for you. What works on a dyno may not work so well at the track.

It's not just getting the WOT A/F ratio right. There's a bezillion things that happen with carb tuning expecially in "transition" type situations, not to mention different altitude/air conditions at the track. Smack the throttle open real quick and what happens? WEIRD things. Let off the throttle and get back into it between gears and what happens? WEIRD things. Sometimes it wants to be pig-rich to get you outta the hole and then lean out up top. Sometimes the opposite. Acceperator pump tuning is CRITICAL if you want to lauch a car with good traction and steep gears well. On a street car with little traction it makes very little difference as long as you avoid a bog. Stretching it out in the upper RPMs some engines want to be squeaky-lean and others just gobble up the fuel and keep begging for more.

Many situations end up being a compromise. Jetting vs. accelerator pump tuning, balancing the benfits of one against the drawbacks of the other. No way to know without going to the track and trying different stuff. Every engine wants something a little different. How big a difference you ask? I've tuned 2 tenths out of a "perfectly" dyno-tuned carb at the track on a few occasions. And then again, I've also made them 2 tenths worse on a few occasions. Trial and error is the only way to dial things in dead-bang once you've gotten as far as you've gone.

So am I going to cop out and give you a useless "it depends" kinda explanation? F*** no!

I'm goign to tell you stuff that you may not like to hear. Kinda "keep your eye on the ball" basics and some other stuff:

Make sure you know your float levels. Don't guess. Set them properly and then DON'T CHANGE THEM. Tune from there but never ever change them or you've just changed a very basic overating parameter of your carb. Throw your old tuning out the window and start over. If yours are reasonably close now you will be MILES ahead by leaving them alone and doing all your tuning from where you are now.

Make sure you know your fuel pressure. And I mean KNOW IT. It must be in the range of 7 PSI for a Holley carb UNDER ALL CONDITIONS. The only way to know is to mount a gague where you can see it under actual running conditions. I often temporarily duck-tape a gague to my windshied and make a few runs. If fuel pressure drops more than 1 PSI at ANY point in the run you are outrunning you fuel delivery system. Find out why and fix it. Tunign around fluctuating fuel pressure is like chasing a ghost- you'll never get it right. NO DYNO IN THE WORLD CAN SIMULATE THE G-FORECES APPLIED TO YOUR FUEL SYSTEM UNDER AN ACTUAL LAUCH SITUATION. On a high pressure FI system it's negligible. On a carbureted system that runs at much lower fuel pressue the difference can be dramatic. Especially in the lower gears where the car is accelerating hard.

Watch out for the airflow through your carb. It should be STRAIGHT and free of obstructions near the top of the carb. You would think that ramming air straight down the throat of the carb would be the ideal way to go. Not necessarily true. Carbs respond MUCH better to EVEN, non-turbulent airflow into the venturis, even if you lose a couple CFM. Uneven airflow causes WEIRD things to happen- bogs and such that simply can't be tuned out. And you want to have enough "free space" over the carb. This is very important. If there's anything within 1.5" of the top of the choke tower or the venturis it's costing you power. A low profile air cleaner assy. would be a prime example. It might flow a lot of CFM but having the lid close to the top of the carb is gonna cost BIG power. Your K&N flow-stack is a good thing. The bug-catcher- maybe, maybe not. Depends how smooth the actual airflow into the carb is with it. Might work better with a large open element air cleaner, to be honest. Might not. Gotta try different stuff and see what works. There's a reason that big, round open element air cleaners work so well on old carbureted factory muscle cars.

A header-evac system is a good thing. Creates a low-pressure area inside the crancase which helps seal up the rings and prevent pressure from working against the bottoms of the pistons on the power stroke. Will work better with open headers than through mufflers. Relies on the negative presure pulses in the exhaust system between exhaust strokes of the various cylinders. Won't affect the carb at all except if you run a standard PCV system now through the carb- you'll have to speed up your idle slightly and lean it out a little on the idle circuits since the PCV valve is basically a "calibrated vacuum leak" and adds a significant amout of air at idle and light throttle situations. Won't affect WOT tuning much. These evac systems are not really for the street and you'll NEVER get through an emissions test with one, if that applies to your situation.

Fuel bowl vent tube.... lots of fugi-wachi here. Doesn't do squat, really, to equalize pressure betwen the bowls, it's real reason for existance is on REALLY hard-lauching cars. Fuel can acutally splash out of the primary bowl vent under hard acceleration and fall into the venturis, causing a rich-bog. The tube carries this splashed fuel from the primary side to the secondary side fuel bowl where it won't do any harm or cause a bog. If you've got a typical street/strip car that runs the 60 foot in 1.7-1.8 seconds its not usully necessary (although it won't hurt anything if you use it).

In general, tune the primary side of the carb FIRST. In fact, disconnect the secondary throttles COMPLETELY and tune the primary side on a bunch of 60' runs until you get it dialed in so it hits like a ton of bricks. THEN reconnect the secondary side and do it all over again. Jets, accelerator pump squirters and accelerator pump cams are your main tunign tools on a Holley, assuming everything is in reasonably good oeprating condition otherwise. If you get way way off of the stock calibration you've probably done stomething wrong. KNOW what the stock calibration is for your carb. Holley doesn't set them too far off right out of the box. If you're suddenly 10 jet sizes from stock calibration- something's probably wrong. Holley lists stock calibration specs for all their carbs in a PDF file downloadable from their website. It's always good to know a baseline "starting point" in case you get way out in left field.

Jeez, I've typed for almost 1/2 an hour and I feel like I've barely scratched the surface. I HAVE barely scratched the surface. Am I giving you what you need? Too basic? Too complicated? Way off from what you expected?

 

Dude, write some more that's a good post.

The air cleaner assembly will effect the carb no doubt. That is one of those things that on a complete motor should be there, air cleaner to headers every little piece makes a difference.

Also the spacers, a 1" might work better, a tappered spacer is always a good thing to try too. Try all of those out too.

Bret

 

That's perfect. Thanks for taking your time to write that extensive
post.

I'm now making sense of the "vent tube" which was improperly
explained to me as "pressure equalization". I couldn't understand the
relationship at the time; but you've set me straight now.

Emissions tests are not an issue, but a visual would be a problem
if I was to get pulled over. Let's hope the officer is a little ignorant
on EVAC/PCV alternatives!

I have a nice video from Holley which outlines accelerator pump
and squirter tuning. I find myself tuning this system as weather
changes. At the track, I stall high enough that the transistion
circuit doesn't factor in.

I rely on my jetting for 1/4 mile tuning mostly. I was told to install
a fresh set of plugs, take a hard 1/4 mile run, then shut down the
motor immediately and read the color of the plugs.

It takes a few sets of plugs to accomplish this, but it's the next
best thing to a dyno I guess...

I know what you mean about fine tuning at the track. As the
temperature drops, the jetting is increased. I also find that timing
can be tweaked a couple of degrees for best MPH trap speed.

THe only thing that concerns me about your post is the jetting
compared to a baseline factory setup. My secondary jets are
8 higher than factory, however I never thought this to be a problem
because my engine is quite a bit more than the average 355 CID Chev.

3 angle valve job, Sportsman II heads 220 c.c. runners, 72 c.c.
chambers, milled ~ 11.0:1 CR ( 235 PSI compression cold), Comp Cams 292H,
single plane intake.

With all that extra air flow, I'm good with the jetting I thought?
The dyno liked the increase as well. I'm pretty sure I grabbed
4-5 HP at the wheel by increasing the fuel.

Should I be looking into decreasing the jets, or does the above
combo warrant the extra fuel?

Thanks guys.

 

Give it a try at the track- forget the plug readings. You're beyond just getting it "roughly right." And killing the engine "clean" after a run is hard to do.... track officals don't take kindly to you stopping at the end of the track to yank the plugs and have a look-see. For starters try going leaner 2 jets at a time on the secondary side (sinde that's where you've done the most changes so far) and see if picks up or slows down. Keep making changes in one direction until it's obvious you've gone too far and then back up.

8 jets richer, eh? Wondering..... did your carb originally have a power valve on the secondary side that you replaced with a plug? If so, you'll usually have to go 8-10 jet sizes richer to make up for the fule the power valve used to supply just to get back to the same A/F ratio.

If not, one thing you might try after you play with the secondary jetting a little is to lean out the secondaries 4 sizes and go 4 sizes richer on the primaries. Might help, might not. The theory is that it'll give you a more even mix coming from both the primaries and secondaries when you're wide open. This helps even out the mix seen by the front cylinders vs. the rear cylinders. You don't want to be a little lean on the front and a little rich on the rear- both will end up making less than optimum power.

At every point you have to keep in mind how this affects your accelerator pump tuning. If you fatten up the primaries you might be able to give a little less pump shot or go with a slightly smaller squirter on that side. But don't go nuts trying to go wicked-lean with the accelerator pump tuning. It's far better to leave them a little fat than to live on the ragged edge of lean. Going too fat on the pump you'll be a smidge "flat" for an isntant, but it's not nearly as bad a trade off as going a little lean- that'll cost you BIGTIME ET and 60-foot, even if you can barely feel the bog from the driver's seat. In fact, it is often better to leave the accelerator pump stuff alone for a while (assuming it's working reasonably well and the car has no obvious bogs or flat spots). Do all your jet fiddling first until you've gotten everything out of it you can and THEN start playing with the pumps.

When you start playign with the pumps, follow the Holley information you already have- their procedures are valid and do it in the order they recommend.

Expect multiple "rounds" of tuning. It's an itterative process because any change you make affects a lot of other stuff and often at mutiple points in the run. Automatic cars that hit WOT at the start and stay there the whole run are MUCH easier to tune, especially on the accelerator pumps. Manual cars that are on and off the gas are FAR more affected by proper accelerator pump tuning since they come into play 2-3 times during the run (unless you power shift every gear).

Now here's the fun part..... once you get to a point where you're "stuck", don't know what to do next, could be a million things.... DO SOMETHING CRAZY! Throw logical "small changes" thinking out the window. Take that thing you are "pretty sure is right" but haven't played with much, and make a big change to it. Do it like you EXPECT it to slow down the car. You gotta test your own tuning assumptions every now and then. Everyone gets metal "blind spots" every now and then they just can't get past. Break through that. If you make the change and it slows down or does what you expect then you were right. HOWEVER, if it DOESN'T do what you expect...... OOOooooo, baby!....... your IQ will grow 20 points in about 10 seconds. That's where the big jumps in understanding of what works with your combo happen.

THink the primaries are plenty fat? Shove in 6 sizes larger jets. See what happens.
THink the secondary accelerator pump squirter is maybe too lean? Go down 4 sizes on the squirter or use a much smaller pump cam anyway. See what happens.

People in the pits will think you're nuts becuase you're excited about shaving off 3 hundredths from an odd-ball change that shouldn't have worked. They won't get it. You WILL. Thousandths become hundredths. Hurdredths become tenths. Tenths become seconds.

 

Damon,

My Holley does not have a power valve on the secondary side.

I like your idea about evening jets sizes front to back. I tend to
forget my street setup isn't going to work on the track!

I've saved this information...unfortunately I can't put any of it
to use until Spring 2004. I'll be under the hood for most of the
winter preparing for my break into the mid 12's!

God willing, I'll post my results in the Spring. Hope you're here
to see the benefits of your post! On top of that, here's a "bow"
to Bret and Rich for saving my *** with oil pressure problems and
other advanced topics.

 

A power valve is going to even out the jet sizes for you. Going to that will be a big help in tuning.

Bret

 

Good call Bret, I could always swap metering blocks on the secondary side to add a PV.

What value of power valve should I install? My primary is 2.x as my
idle reads ~ 5 in./hg.

For secondary, how to I determine the value?

 

I own an Ls1 and my brother has a 1984 Camaro Z28 I've been wanting to put fuel injection in. However due to cost, I'm going to try and get some tips on using the carb and see if I can't get used to living life with a carb. I've messed with his carb and get it to run semi good, but I don't think it runs what it could. Here is the setup:

Mildly Ported w/ 2.02 1.60 valves Protopline Vortec Heads
Comp Cams 240/246 .50x1/.51x Lift cam (Xtreme Energy 280? I think)
Edelbrock Performer Rpm Intake
Holley 750 Double Pumper
T-56 6 speed
Compression ratio with speed pro forged flat tops is around 9.8:1.
3.42 gears

Right now the idle is at 1000. The car feels really smooth accelerating and has quite a bit of power all the way to 6600-6800 rpm or so. However, it has a real bad tendancy of fouling plugs. It also seems to swap back and forth between rich and lean on occasion, depending upon how it feels.

I have a decent idea on what I need to be doing, but have never had it exactly explained to me the best procedures are step by step and in what order to tune a carb at the track.. What would you suggest I do to start with to get the thing dialed in. I'm going to the track either this friday or next friday, and I want to try and get some gains out of it. What are some of the basic things I should be trying, and perhaps some of the more advanced things as well Also, how do you tune a carb for nitrous and n/a? I very badly want to put a 100-150 shot on the beast, but I'm really worried about running lean and how much timing I should take out, as well as how the carb should be tuned.

I really learned alot from this thread already, but a few things were over my head as I don't have that much experience.. I don't want the last horsepower for now.. But I feel that I'm really selling the car short of at least 10 horsepower the way it runs right now.. because I know it isn't as close as it could be due to my ignorance..

Thanks,
Josh

 

See if I can offer some advice:

If you haven't already done so, insulate the carburetor from the
intake manifold. Keeping the carburetor as cool as possible will
help atomize the air/fuel mixture.

I used a 1/2" laminated wood spacer to improve my scenario.

It's like dropping a brick from 40 ft in the air, as opposed to 5 ft. -
It will smash up into tiny little pieces....so shocking the charge from
a cooler carburetor to a smoking hot manifold helps break up the
mix.

It will also do wonders for the fuel bowls allowing the fuel to stay
cooler and I would guess prevent the gas from expanding due to
heat.

Another "cool" trick is to block the heat passages from the cylinder
heads through the base of the intake manifold. You may have noticed
two steel plates that come with your intake gaskets. Place these
between the head and gasket to block the exhaust gas from
flowing through.

Aside from that, jetting and timing are your friends. I have replaced
my accelerator pump cams and squirter nozzles to really make the
car launch hard.

It went from a bog/studder to a neck snapping acceleration machine.

You can buy these kits from a local pro shop. They include about
10 different cam profiles and squirter sizes.

The cam determines when the pump shot is delivered and how much
of that fuel gets to the squirter.

The squirter nozzle handles the duration of the pump shot. A small
squirter bore (smaller # value) will distribute the fuel slowly over a long period of time.

A large squirter bore (higher # value) will spray it much quicker.

I've also had great results by removing my vacuum advance on
the distributor and changing the advance curve with lighter springs.