gapimpin

im getting ready to install my new motor but im wondering what kind of carb i should put on it.

the motor is a 383 stroker lt1 out of a 95 firehawk with a 234/246 durration cam and a 700r4 with a 3500stall fuddle converter and 3.42 posi.

my current engine is a 350 vortec that i had a 600cfm edelbrock performer on it always had a pretty bad bog when going wot but i was looking at getting a new 650cfm edelbrock thunder avs carb for the stroker. anyone have any luck with these on larger displacement motors?

ive heard that with the cam and stall i have i should be using a mechanical secondary carb. since the thunder avs is an adjustable secondary will that work?

five7kid

iTrader: (14)

Why are you putting a carb on it? Many problems, no benefits.

"Mechanical" and "adjustable" secondaries are two completely different things. The Thunder is still a demand-based secondary system, even if the throttle butterfly is opened mechanically.

lukn4trbl

If you do intend to install a carb, I would start with 700 CFM based on
the duration and cubes of the motor. That cam should peak around
5800 RPM (give or take) depending on the rest of the engine components.

Mechanical, or vacuum secondaries will work. There's no set rule on which
type you should use based on your cam and stall.

Since the manifold pressure is generally higher with radical overlaps/durations
and the power curve really does not kick in until about 2500 RPM (with those specs) the vacuum secondaries will be open (even partially) for most of
the time in most driving scenarios.

The mechanical secondaries (MS) are best suited for track, street/strip because
the secondaries will open based on throttle angle, and not engine demand.
When you're down the track, it's pretty much 100% throttle except for the
launch.

The MS carb's are tough to learn around the street with auto transmissions.
Sometimes the engine will demand more air/fuel, the transmission will shift
and your foot is not in it enough.

With a vacuum secondary, they will open when needed.

I personally like the mechanical carbs much more. Once you drive around
a bit, you'll figure it out.

P.S. You should consider more rear gear with that cam and stall.

gapimpin

im doing a carb swap on it because i dont have the time or cash to replace the FI right now. my buddy already sold the intake wireharness computer and heads so im just gonna by the gmpp carb intake and a new carb for it. i would do more gear but ive driven a 350 lt1 with bolt ons and 3.73 gears and it killed a tank in 200miles. this is going to be my daily driver so im trying to make it as streetable as possible but still keep it a beast at the track.

xpndbl3

iTrader: (5)

750 holley HP, 750 proform or 750 quick fuel all mechanical secondaries.

4.10 gears would help as well

AutoRoc

iTrader: (1)

Pro-Systems can build you a custom made carb for the application. Go to their website and fill out the information form and they'll get a ahold of you with a price and product. Mine is dead nuts on and extremely driveable and the price was the same as an out of the box Holley HP from Jegs. Customer service is excellent as well. Goodluck! it sounds like a beast.

For what it's worth, he set me up with a 780cfm(wet flow) Holley HP with mechanical secondaries, power valve up front. He said it would give up a small bit of power over a larger carb but it would drive like EFI..and it does. Wideband O2 sensor and plug readings are very nice. This is a 396 stroker/solid roller, 11.5:1 auto/stall car.

five7kid

iTrader: (14)

Well, I know this is the carb forum and I'm a carb forum moderator, but I'd bet you'd be able to find a factory system, including harness and PCM, for less than it will cost you to put a carb on it.

Without a doubt, you'll get better economy with the factory induction.

gapimpin

probably better economy but less power plus the opti is cracked and i dont wanna deal with trying to retune it for the stroker and cam. carb just seems easyer plus i like how clean it looks with no wires in my engine bay.

BiGgE

iTrader: (2)

What compression are you running in your stroker? It is impossible to size a carb without knowing how much air it will move...

gapimpin

heads spec out at 273 cfm on the intake at .550 lift compression is 11.5:1

BiGgE

iTrader: (2)

Sounds like most everyone is on the right track-a 750 would be pretty close. I am running a similar set-up (11.2:1 scr and 9.2:1 dcr 388 ci) and I am running a 750 mech. secondary demon. The only advice I can give is that you should always put the smallest carb on your engine that will fit your needs. ie. an 11.5:1 motor with your cam, and aftermarket aluminum or ported iron heads should approach 100% volumetric efficiency when you are up in the power band, so if you plan on spinning it 6500 rpm for instance it will require 720 cfm. So if you put an 850 monster on it, you will not get much if any gain on the top end, and your throttle response and fuel economy will suffer mightily. Also, for my money, if I am running an auto car, I would stick with vaccuum secondaries as they are much more forgiving to drive when you don't have the luxury of a clutch. Anyways, hope this helps.

five7kid

iTrader: (14)

I can't say I agree with any of your observations. Compression doesn't have anything to do with airflow, except for matching it to your cam (which will affect RPMs, which does have an effect on airflow). You don't want too little compression with a wild cam, and you don't want too much compression with a mild cam.

The smallest carb per the calculations should only be followed for a street-only engine. The calculations are based on single-plane intakes, which don't need as large of a carb, since a dual-plane splits the carb in two, while the single-plane uses the entire carb. A healthy, high-flowing, high-reving 383 will want a lot of carb with a dual-plane intake, and 750 would be marginally large enough.

So, what are the intake choices for carb'd LT1?

A vacuum-secondary carb is rarely the right choice, regardless of transmission. If you are in a traction-limited situation (such as street tires at the track in a chassis-modifications-limited class), then you can use the vacuum secondaries to hold back the power until you're moving enough to get around the traction problem. Even then, the only sane set-up is to use the Quick Fuel adjustable secondary housing, which controls opening by effectively orificing the vacuum signal, which will allow eventual full opening of the secondaries using a weaker spring without causing WOT bog.

And, I still think the prerequisite for putting a carb on a poor innocent LT1 is a lobotomy.

gapimpin

I know your against it but thats how it is and it looks damn good....lol
the only intake made for an lt1 carb is the gmpp dual plane intake but im sticking a 2" open spacer on it to get some extra plenum volume wich should take some demand off the carb and help it idle a little better.

lukn4trbl

I have to disagree with the comments on compression and how it relates to
carburetor selection.

The trick to making power is presenting the most charge and pressure to the
cylinder at every RPM based on the fuel limitations.

If the VE curve + dynamic compression ratio allow for safe combustion at torque
peak, then cylinder pressures above and below the TQ peak RPM are
not producing as much force, nor concerned with detonation from compression alone.

Working backwards from that equation, if you were to install a larger
carburetor on the same engine, the VE curve would shift the torque peak
UP, and produce less air/fuel charge from idle to TQ peak.

The reduction in charge would benefit from a higher static compression
to restore the pressure and ultimately power produced by the motor.

As for installing a carb on an LT1, it's not so bad, but the heads were
initially designed for fuel injection. I've heard of issues at high port
velocities where the fuel cannot negotiate the port.

I don't know if your motor will experience these problems, but some port
work might be in order if you're looking to maximize your combo for a
carburetor.

gapimpin

heads have been fully worked over so i dont think that will really be a problem atleast i hope it wont. I had no problems with my old vortec heads running a carb and from what i know the vortecs are baised off the lt1 designe

five7kid

iTrader: (14)

Alright, great. Nice general theory.

Now, you've got one choice of intake manifold for this conversion, a relatively mild dual plane. You've got maxed-out port heads. You need a bigger cam to take advantage of the heads than a manifold is available to support it - the spacer ain't gunna change the basic characteristics of the manifold enough to band aid that. That makes CR moot. With the intake mismatch, the size of the carb is also moot.

This is a bad, bad move, no matter how you want to gild it with theory.

lukn4trbl

I agree that the open spacer on a dual plane intake is just wrong. I also
would recommend a single plane design.

My comment was directed to airflow, cams and compression.

gapimpin

intake has been ported out as well.... not just portmatched but the inside was ported and knife edged. I think the combo will work fine we shall see this weekend when i fire her up