Road-Racing carburetor modifications for 350 V8

[bjhines]

I would like to know more about differences and similarities between carby modifications for drag VS Roadracing.

 

This comes up because a friend and I BOTH need new carbys for our track cars(one engine is an early 327 and the other is a TPI 350). We run on road courses like V.I.R and Road Atlanta. My buddy is looking at $800 Holleys that, IMO were modified for drag strip use or circle track cars that turn left only. We turn both directions and must be able to get on the power smoothly and consitently under a wide variety of conditions. Throttle blips for downshifting need to be consistent. Power must come on smoothly past the apex. full power under hard cornering must be dead on consistent. Neither of these engines is putting out more than 350HP, so CFM flow/size is not the problem.

 

I have heard of capping the powervalve off entirely, or installing a cup that places it's pickup lower in the bowls. I don't think we need four corner idle, or fancy throttle plates, or high flowing venturis. What we need is a carby that works under extreme G-forces in all directions, and a carby that is VERY predictable and consistent under widely varying circumstances.

[johnc]

I would like to know more about differences and similarities between carby modifications for drag VS Roadracing.

 

 

 

While I don't know the differences, I have a CMC Challenge competitor as a customer and he went with this modified Holley from SMI and is very happy with how it works on his converted TPI 350.

 

http://www.smicarburetor.com/products/sfID1/7/sfID2/5/sfID3/41/productID/53

[John Scott]

I've heard.. PVs both ends, connect vent tubes together to avoid slosh. (Google)

Drag applications have jet extenders in the rear and no rear PV.

Edited April 6, 2010 by John Scott

[hoov100]

me and my brother used a box stock holley 750 in our unlimited auto-x car for 2 years without any problems, we now run a proform 750, with with a machined choke horn, billet metering plates, billet base plate and a few other mods, which I'm not at liberty to say. As long as your floats are adjusted correctly and they are full width floats, you shouldn't have any problem, unless your turning like 2 g's for seconds at a time.

[JMortensen]

I've settled on this one for my 5.3 (325): http://cgi.ebay.com/ebaymotors/Barry-Grant-650-Race-Demon-2282010RR-Road-Race_W0QQcmdZViewItemQQhashZitem5ad7509b62QQitemZ390159440738QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

 

They make a 750 which might work too for the 350.

[David K]

Nice to see this thread. Ive been helping a friend out this week whos AI intended fox body keeps sputtering on riverside and sweeper turns at Buttonwillow, and on turn 2 and the sweeper at Willow. We suspect the carb, but we havent been able to mimic the behavior even when we jack the right side of the car up and run it full throttle in the garage. The carb is an unmodified 4barrel 670cfm Holley Street Avenger, so i would stay away from those until i find out why the engine is sputtering

Edited April 6, 2010 by David K

[bjhines]

I have a short list of mods that may be useful... or not.

 

double wedged floats for primary and secondary

secondary jet extensions

billet adjustable metering blocks

higher quality inlet needles

install cup over power valve inlet with hole in bottom

install dual power valves?

[grumpyvette]

the guys have you pointed in the right direction, but Id add that knowing how to read plugs and tune is mandatory, and use of a quality return style fuel pressure regulator and an # 6-8 lines and a fuel cell is a huge help, and road racing is mostly about power to weight,a stiff roll cage,the correct tires, a low centrally located center of mass, big brakes, effective cooling and properly set up suspension, not max peak power

a really light weight car, with huge effective brakes, and a stiff roll cage, the good tires etc can do well against cars without those factors that have far more horsepower

 

http://forum.grumpysperformance.com/viewtopic.php?f=44&t=773

 

http://forum.grumpysperformance.com/viewtopic.php?f=63&t=1755&p=4431&hilit=+suspension#p4431

 

http://forum.grumpysperformance.com/viewtopic.php?f=55&t=1961

 

 

http://forum.grumpysperformance.com/viewtopic.php?f=55&t=733&p=6470&hilit=fuel+cell#p6470

 

http://www.jegs.com/p/Barry-Grant/Demon-Race-Demon-Carburetors-Road-Race/762590/10002/-1

 

 

http://forum.grumpysperformance.com/viewtopic.php?f=44&t=773

 

http://forum.grumpysperformance.com/viewtopic.php?f=50&t=1853&p=4848&hilit=ratio+meter#p4848

 

 

 

Edited April 11, 2010 by grumpyvette

[BAINATSEABOARD]

OK guys, I have been through this part and will say that a stock holly 5160 hp carb will not let you down on the road course. The power valve issue just keeps you from needing larger jets. I have a 383 stroker in my road course car with stock floats and just needed to tune for af ratio. I do have a 750 cfm holley with road race floats and cannot tell any difference with standard floats. I did have to rejet the carb 10 sizes to get the correct a/f ratio

 

Good luck,

 

Bain

[bjhines]

Thanks Bain. It is good to know that the HP series works so well. I don't really want to BUY a modded carby like the HP series. I wanted to know what to do to a base model double pumper like the 4777 I already have.

 

I have a 4777 double pumper that needs a new baseplate and a rebuild. I was really going to do this myself, So I wanted to know about the mods that can be made to improve a base model carby instead of paying for the "full boat" treatment $$$.

 

I have a Chinese lathe and mill and a creative mindset. "If you didn't build it yourself, it's just a rental." is my motto. I also have experience with several sets of various Weber designs and their counterparts on motorcyles and cars. I have dual narrowband guages installed.

Edited April 8, 2010 by bjhines

[BAINATSEABOARD]

Thanks Bain. It is good to know that the HP series works so well. I don't really want to BUY a modded carby like the HP series. I wanted to know what to do to a base model double pumper like the 4777 I already have.

 

I have a 4777 double pumper that needs a new baseplate and a rebuild. I was really going to do this myself, So I wanted to know about the mods that can be made to improve a base model carby instead of paying for the "full boat" treatment $$$.

 

I have a Chinese lathe and mill and a creative mindset. "If you didn't build it yourself, it's just a rental." is my motto. I also have experience with several sets of various Weber designs and their counterparts on motorcyles and cars. I have dual narrowband guages installed.

Did I say 5160? I meant 4150 hp. I am really hoping to see your car at the track soon. Do you have any pics of the whole car that you can post?

 

Bain

[jt1]

john, the main differences between a hp and a standard 4150 are:

 

1: The choke tower is milled off, and the top of the main body is nicely radiused

2: The rear metering block has a PV and is set up for 4 corner idle

3: Both low and high speed air bleeds on the primary and secondary are drilled and tapped and use the replaceable bleeds

4: the secondary has an adjustment screw for the transfer slot exposure, and the rear blades are drilled

 

There may be a few others, but those are the main ones I can think of right now.

 

jt

[BAINATSEABOARD]

john, the main differences between a hp and a standard 4150 are:

 

1: The choke tower is milled off, and the top of the main body is nicely radiused

2: The rear metering block has a PV and is set up for 4 corner idle

3: Both low and high speed air bleeds on the primary and secondary are drilled and tapped and use the replaceable bleeds

4: the secondary has an adjustment screw for the transfer slot exposure, and the rear blades are drilled

 

There may be a few others, but those are the main ones I can think of right now.

 

jt

[BAINATSEABOARD]

Boy, I will say this.....That JT fellow sure does know his racing stuff. I will attest that what he tells you is very reliable. I think he has a very good knowledge of racing engines and race stuff in general. If I could afford to pay him, I would have him build all my racing engines....

[bjhines]

I own 2 books on Holley tuning and modifications. They flat out suck balls. They make absolutely NO mention of HOW to do any of the typical modifications beyond painting the float bowls and installing off the shelf parts.

 

 

This reminds me of the fact that NO manufacturer has EVER printed a dimension for a distributor cap and base.

 

What the F@#$ is up with Chevy tech? Thank god we have some smart fellas around here.

[jt1]

Posted by BainatSeaboard

 

If I could get him to actually do something, instead of just talk about it, I would have him build all my racing engines....

 

There. Fixed that for you.

 

jt

[bjhines]

JT1 and his white 240Z were inspiration for me to get into this. The thing I love about that car is the fact he could return it to bone stock, factory trim in a weekend. He approached that build with an attitude of getting the absolute most fun for the money. That is something that takes years of experience to figure out.

 

 

I would like everyone's opinions on a few points.

 

1. Float bowls; Do I need to change things in the float bowls like extensions, baffles, foam, various wedged, slanted, slotted, weighted or otherwise non-sock floats? I will inspect and weigh the existing floats and most likely buy replacements, stock or otherwise.

 

2. What about power valves?

a. Do I really need a power valve at all??? Obviously rejetting would be required. Would the mains circuit need entirely different emulsion wells, ducts, and/or boosters to work without a power valve?

b. What about 2x power valves(primary and secondary)??? That seems like it would make a mess of tuning and jetting the thing. I am using a 4150-4777 with mechanical secondaries. Is the secondary power valve a magical fix for someone who just likes to floor it all the time?. This gets back to DRAG racing mods IMO.

 

3. Milling the choke horn? I really only need more clearance to my aircleaner. The choke horn is within 0.25" of the lid. I have read that removing the horn entirely/improperly will change airflow around the air-bleeds. Should I just remove 0.5" and leave it at that?

 

4. Bowl vents? should they be baffled or modified in any way?

 

5. Four-Corner Idle ports????????? WTF? It is a race car. All I need it to do is idle without smoking up half the spark plugs. It is not a street car that needs to idle flawlessly with an emission probe up it's tailpipe.

 

6. Throttle plate??? I have a worn-out, rusty-bladed throttle plate. I could buy a replacement for ~$150. I never liked the way Holleys set IDLE speed like a lawnmower carby. How do they deal with transition ports when they never know where the blades will be to begin with. Why drill the plates? is there no way to add a balancing/air-bypass needle? Do you drill the plates to set idle speed, that seems like a lot of trouble to get idle right at a set blade opening.

I am sure I can come up with more Q's.

Edited April 9, 2010 by bjhines

[jt1]

John, you are way over thinking this. For a while on the white car, I was running a 4150 700 dp that I cut the choke tower off with a hacksaw and die grinder. It had no other mods, just rejetted and changed the PV's, tuned on a chassis dyno. I'll post some pics of the tower mod if you like. At that point, the car was making about 400 at the wheels. Later on I bought a 830 annular hp, new in the box, from Matt King, the magazine guy. He was going formula racing and selling off his V8 stuff. On the dyno, we went up 2 jets all the way around, and down a size on the HS bleeds. I changed the PV's to 3.5's to clean up the idle. The 830 was worth about 20 hp on the top and about 35 ft-# in the middle. The white car would only pull about 1.35 -1.38 lateral g's, and at that point the carbs didn't need any special mods. Get your carb together and spend some time tuning it with a wideband, and you will do fine.

 

jt

[BAINATSEABOARD]

There. Fixed that for you.

 

jt

 

Well, That's exactly what I was thinking, but you know I am much too nice to say it. at least where you would hear about it... lol

 

Bain

[John Scott]

A novel way to help stalling under hard braking image from rockcrawler . com:

 

Also might want to search the Barry Grant website. They aren't shy about posting every spec, jet, and bleed hole for their carbs. Compare the difference from drag carbs to road race.

 

http://www.barrygrant.com/demon/default.aspx?page=29&specs=29n

 

drag race:

 

http://www.barrygrant.com/demon/default.aspx?page=29&specs=29o

 

I have one of the less common 4150 HP 750s that came as a road race 80528-2, PVs front and rear, square jetted.

Since I prefer more of a drag carb, I swapped out to jet extensions, notched floats, plugged rear pv. like the 4150HP 80528-1.

If you are willing and as mentioned, spending time with a wide band will be priceless. I'd also recommend learning what all the restrictions, emulsion holes, high and low speed bleeds do, and be willing to drill, tap, make you own bleeds, and experiment.

Edited April 9, 2010 by John Scott