Carb experts...advice? HELP..

[stealth]

Gents I’d really appreciate some guidance. I posted about a dyno session that was a total flop. I’m thinking about running this engine again at a better place. I had a few carbs to try, but ended up just working with one. 975 cfm demon. ( gold sleeve with black 1.75 base)

while it was difficult to tell due to very suspect data acquisition this appears to go rich up top. Ended up with 85 front with pv, and 92 rear no pv

Anyway, with new dyno test I’m thinking should I buy custom carb for my combo or is this a good carb to stay with.

Here are the engine details:
448 ci
4.159 bore. 4.125 stroke
10.5 cp ratio pump gas
Solid roller 258 268 @50. 720 740 lift 112 lobe sep
Sb2.2 nascar takeoffs. Flow 380@.700
GM single 699 intake 4150
Try y headers nascar takeoffs

Hot street car that will see plenty of drag time

Made 738 hp @6700

So should I change carb for something better?
Demon carb any good? Hear mixed reviews...

Really like to hear what he experts have to say...

[FC-Pilot]

I have used a couple Demon carbs with good results. I always had to change the emulsion and air bleeds to get them in the ball park. The tip I was given was to look at old Holley setups that were similar in venturi size and throttle bore size and start there. On one of them I swapped the metering blocks to old holley 750 blocks and then with the air bleed change the fuel curve was dead on. I then setup the demon blocks to mimic the holley blocks as close as I could. I know it sounds like a pain, but it would save a grip of cash and is something that could always be changed back if you were concerned about it. Then if you wanted you could always grab a second carb to compare to. That is how I would move forward if I were you.

I am sure others will give better advice, so I am looking forward to their input as well.

Paul

[mag2555]

That cid motor with that level of head flow needs a 1050 Dom minimum!

[BradH]

It looks to be an earlier production Race Demon RS (screw-in fuel level windows, black base plate), so the odds are good it doesn't have the QC issues the later ones did. I think the pic shows at least one of the replaceable boosters isn't aligned properly and needs to be turned slightly to point straight down.

IMO, it's also a reasonable size carb for your dual-purpose application. However, the BG Claw & Demon carbs were notorious for having some horrible out-of-the-box tunes. Do you know if the carb's tune has been worked on already, with the exception of the jet change mentioned above? And do you know the complete tune as it is now?

I think it comes down to time vs $$$. I saw your original dyno post and don't put any faith in the AFR #s from it. Do you want to spend your time either on the dyno or on the road trying to dial in this carb, or spend the money for another carb that "should" be a closer tune to start with?

[PRH]

I wouldn’t show up for another dyno session with only that one carb.

If the tune is way off, you can waste the better part of a day and a bunch of pulls just trying to get the fuel curve straightened out.
And sometimes they need more than air bleed and jet changes to sort them out.
I’ve had some success sorting through Demon/Claw carbs....... but Ive also run into situations where they would have needed some major changes to work well.

If it’s within the budget, I’d have something built for it.
If you’re looking for something new off the shelf...... I’d probably look at the QFT Q-950 or Q-1050.

[stealth]

I wouldn’t show up for another dyno session with only that one carb.

If the tune is way off, you can waste the better part of a day and a bunch of pulls just trying to get the fuel curve straightened out.
And sometimes they need more than air bleed and jet changes to sort them out.
I’ve had some success sorting through Demon/Claw carbs....... but Ive also run into situations where they would have needed some major changes to work well.

If it’s within the budget, I’d have something built for it.
If you’re looking for something new off the shelf...... I’d probably look at the QFT Q-950 or Q-1050.

Thanks,

I think that's the plan. I'd rather start with something I know is built for the combination then continue to sort out this one.

Now.. what do I need and from whom?

Thanks,

[rustbucket79]

Did you try altering the fuel curve with a bleed change?

This time around since you presumably have all the break in and timing checks done you can focus purely on the carb. Make sure you have a few large carbs to test.

There is one 4150 style carb that consistently "high marks" every other carb that crosses our dyno, and that's the Holley 950 UXP. 0-80805HBX

[stealth]

I wouldn’t show up for another dyno session with only that one carb.

If the tune is way off, you can waste the better part of a day and a bunch of pulls just trying to get the fuel curve straightened out.
And sometimes they need more than air bleed and jet changes to sort them out.
I’ve had some success sorting through Demon/Claw carbs....... but Ive also run into situations where they would have needed some major changes to work well.

If it’s within the budget, I’d have something built for it.
If you’re looking for something new off the shelf...... I’d probably look at the QFT Q-950 or Q-1050.

Thanks,

I think that's the plan. I'd rather start with something I know is built for the combination then continue to sort out this one.

Now.. what do I need and from whom?

Thanks,

only jet changes... I thought we should open up the high speed bleed, but didn't have time..(and well .. things didn't go well there)

How close out of the box for the holley? .. .I heard good things on this one before. At that price though should I be looking at a custom with annular as mentioned above QFT 1050.

Annular worth the effort or stick with down leg... I would like to have some street manors and broad power curve.

Thanks,

[PRH]

I’ll give a shout out for member NormS, Competition Fuel Systems, who built me a nice annular 850 Holley in about 1991.
It’s been my Go-to 4150 carb for all these years.

At that time I was working at an engine shop that did a lot of circle track stuff and we bought several carbs from him....... they always worked great.

We also had an 1150 Dominator done back then that one of my friends has been using all this time.

This was before the “HP” carbs arrived on the scene.

I’m sure he’s picked up a few tricks since then, so I have no doubt he could put together a nice carb for your application.

I’m sure there are plenty of other good carb builders too, but my experiences with CFS were all positive.

[BradH]

... I'd rather start with something I know is built for the combination then continue to sort out this one.

Now.. what do I need and from whom?

Hmmm... this might get interesting.

[BradH]

...
There is one 4150 style carb that consistently "high marks" every other carb that crosses our dyno, and that's the Holley 950 UXP. 0-80805HBX

Done any at-the-track comparison testing w/ that particular model? I believe it's better suited to a large(r) engine that can help offset the lack of booster signal and lazy shift recovery that doesn't show up on a standard engine dyno.

[PRH]

The Holley billet 1.600 x 1.750 wouldn’t even be on my radar for the OP’s build.

I know of a couple of horror stories with them.

[rustbucket79]

I guess we've been lucky so far, having sold a dozen or so of them and zero complaints as of yet.

As I said, it goes on 90% of the 4150 style engines on it's final pull, using the same sweep as I use with the customer's carb, and the graph mimics the previous test, at a larger number for both hp and torque. Occasionally I'll rejet if it's the correct application, but bottom line it's a well designed carb out of the box.

Even on a tunnel ram, a pair of them on my lil 406 has decent throttle response and loads nicely before the pull.

[JoePorting]

Why do you think you are having issues with the carb? Seems like the motor is topping off at around 5700. I'm thinking valve springs. What's your valve spring pressures and what's the recommended valve spring pressures for the cam?

What sort of power were you expecting? 738hp isn't too far off what I would think this combo could do. I'd think maybe 800hp or so would be really good.

[BradH]

I guess we've been lucky so far, having sold a dozen or so of them and zero complaints as of yet.

As I said, it goes on 90% of the 4150 style engines on it's final pull, using the same sweep as I use with the customer's carb, and the graph mimics the previous test, at a larger number for both hp and torque. Occasionally I'll rejet if it's the correct application, but bottom line it's a well designed carb out of the box.

Even on a tunnel ram, a pair of them on my lil 406 has decent throttle response and loads nicely before the pull.

That's why I asked about on-track testing vs other carbs, rather than standard dyno tests. I've had two carbs that made virtually the same HP & torque on the engine dyno, but you put them on a car and one carb (the one with smaller venturi and much better booster signal) ET'd .2 better and pulled 2 add'l MPH because of improved shift recovery. It's not that the dyno lies, but it doesn't tell the whole story.