Holley Carb tuning!!!!

[MandarinaRacing]

HELP!!!! I don't even know where to start!

I've been reading a lot about the tuneability of Holley carbs. Swaping discharge nozzles, power valves, jets, etc. Being a novice at this, is tuning a Holley something necessary, or is it done to extract the best out of an engine?
For example. I've running my 350 Holley out of the box with no problems, even though the idle is rpobably a litte too high, but I've always thought it had to do with using a 280 Cliffy cam .....I have a 500 Holley that I want to use. Would I see any performance gains? or should I just tune the 350 ?????I know no two engines are alike but I want some sort of advice on where to start?????

Alex

 

[xctasy]

Use either one, and you'll be similing. The 350 is often harder to tune, and this is because a high lift cam can create a fuel standoff situation. The bigger carb is often easier to tune. But use the 350 cfm carb first, because under carbing is preferable for the reasons below.

With the log, the bottle neck is the 1.5 or 1.75 inch hole in the top. Going 500 cfm would, with the right intake figures, give you a potential 220 hp. The 350 cfm flat-lines at around 175 hp on a six, around 150 hp on a 2.3 four. There is more torque down low, because vaccuum is better. Where the 500 shines is with an intake that can really flow some air.

I'd stick with the 350 Holley, and look at making the fuel air mixtures best. To get the extra 45 hp would be, like mega bucks. The 500 cfm will work fine, it's just that it's not going to do any thing when you have two 1.675 inch throttles trying to dribble the go-juice into one 1.75" or less hole.

One of the best bookes I've seen is the Holley Carb book form the early 80's, an HP book??, can't remeber. Then there was a good Pommy book on 215 Buick/3.5 liter Rover V8's. Most Englishmen have never dealt with a Holley before, and it covered the set up very simply. If you want , I can post the info sometime within the week. As well, the David Vizard book , especially for 4-clinder Pinto OHC engines, covers some good points.

The idle resistrictor jet needs to be bushed down from the stock size if the carb was bound for a stock SBC or SBF or Mopar. I seem to remember 16 thou for four-cylinders, down from the stock 21 thou or so for a big banger vee-eight. Some where between that. That's where the richness comes from, in my opinion.

I'm back at work tomorrow, after 10 days annual leave. My forlorn boss, looking like a comander who just had a gun held to his head, arrived on my door step last week. Consequently spent a couple of half days called back on crisis control for our subdivision contracts. Punitive damages if we didn't have it have the pavement strength done to a performance spec. Potential cost were 600 Kiwi dollars a day, almost what I get a week. Title won't be issued on the properties until the results are on the big Engineers table. It lookes like more of the same for the next two weeks.

Glad I know pavements like the back of my hand. There are some real geeks around in the Engineering industry, one guy with an expensive sense of humour. More trained staff... embelish your resume, and get a job?

 

[MandarinaRacing]

Thanks "X" I believe there are a few things that can be done to help a carb to work with a long duration cam.

QUESTION I have a camshaft with a long duration and a high lift. It will burn your eyes at a idle. What can I do to fix this?

ANSWER The most common cause is an improperly rated power valve. You will first need to choose the correct power valve for your application. (see below) When you have a camshaft of this configuration it requires more air and fuel for it to operate efficiently. This in turn requires you to raise your curb idle screw to obtain the same given rpm that you had with a stock camshaft. If you are having a problem with the air/fuel mixture screws not making a difference then what happens is with turning the curb idle screw in farther this will sometimes cause the throttle plates to open too far and they will uncover the idle slots. This will allow it to bypass the idle circuit and start pulling fuel from the transfer slot. To correct this problem you can raise the secondary throttle plates a little with the secondary adjusting screw allowing more airflow into the engine. This will allow you to lower the primary throttle plates so you will be able to get back into the idle circuit. Another method would be to drill a 1/64" - 1/8" hole in each of the primary throttle plates on the side closest to the transfer slot. This will allow extra airflow into the carburetor also alowing you lower the primary throttle plates.

also...

A competition or race engine which has a long duration
high overlap camshaft will have low manifold vacuum at idle speeds. If the
vehicle has a manual transmission, take the vacuum reading with the engine
thoroughly warmed up and at idle. If the vehicle is equipped with an automatic
transmission, take the vacuum reading with the engine thoroughly warmed up and
idling in gear. In either case, the power valve selected should have a vacuum
opening point about 2" Hg below the intake manifold vacuum reading taken.

Alex

 

[Anonymous]

Alex,

Did you get my PM about the jet sizes??

If not, I started out with #73 jets then was running #70's then I swapped down to # 69's......Running much better now!!

The carb gets quite hot, being it is hanging right off the side of the intake over the header. I'm going to make another heat shield out of aluminum...That might also be somthing you might want to look into?

I have a bit of a bog when you punch it....Part of that is due to the lack of a stall convertor, but some of it can be cured by messin with the carb some more...Make sure to pick up a Holley tuning book. I got one of ebay for around $10.00 and it was well worth it!!

Thats also a good point about the power valves. Mine only pulls about 12" of vacuum at idle in park. I have to try mine in gear and compare the results wth the power valve thats in it.

Later,

Doug

 

[MandarinaRacing]

Doug thanks. I got your messages. I'll have to get one of those books. I'm justing trying to get an idea of the stuuf I need to buy. THX

Alex

 

[Anonymous]

I agree with Doug and EXECUTE. Go with the 350. I had a 500 on my car and it was way too much. I think the 350 is way better for the 200, that is what I have on my car now and I like it a lot more than the 500. And I would deffinatly get a good book on Holleys. I have one and use it often! There is a lot to know about tuning them just right. After you get into messing with it some you will not want to stop. I love Holleys! If you want to go fast you GOT to have one!

Dan

 

[MandarinaRacing]

THX guys!!!

How 'bout this one?

$15.95...not bad...!

Alex

 

[Anonymous]

Yes, Thats the one you want!

Later,

Doug

 

[Anonymous]

That is the same one I have! Great book!

Dan

 

[Anonymous]

I have that book (the one pictured) and was a bit dissappointed in it, I payed $40 for it , then found a mate who had another one, which is much better .
The excellent book I sourced from the aforementioned mate was a SA book, in a performance series.
volume 1 - Carburetion:
holley carburetors.

It reall is quite good, has seperate chapters for race carb tuning, street performance/ economy tuning and a chapter on turbo/super charging modifications (both blow and draw through), the book gives not just the methods but the logic behind the tuning operations as well.
I recommend this one. 8)