Upgrade from 625cfm carburetor

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1994redram

1994redram

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I replaced my slant 6 with a 5.9 magnum last year and bought a new 625 cfm street demon carb. Due to no tuning, used engine, and ignorance. I blew both headgaskets. The 360 came back out and get bored .030 over with 10.5 pistons, Hughes EQ heads with 2.02 and 1.62 valves. Hughes cam .539/ .549 lift and 218/222 duration at .050 and 110 lobe Sep. Stock crank and rods. Has an air gap RPM intake and headers flowing through a mandrel 2.5 inch dual exhaust. Shift kitted 727 and a 3.55 sure grip. The car starts, runs, and drives great. Drove it 200 miles roundtrip this last weekend without any issue. But it didn't seem to pull too great on the highway. I'm wondering if a bigger carb would help pick up some power. Demon makes a 750 cfm street demon. I'm not dead set on that carb but it's affordable, looks great, and I'm happy with the 625.

Would a 750 hurt the driveability on the street in traffic and such? How much, if any power should I expect?
 
Get a dual quad manifold and just get another 625 CFM! Then you can run it Progressive and have just as little or more than enough!
 
j par said:
Get a dual quad manifold and just get another 625 CFM! Then you can run it Progressive and have just as little or more than enough!
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I like the way you think. But a dual quad manifold would be nearly impossible to find for a magnum head. And, well, I'm too broke for all that.
 
1994redram said:
I like the way you think. But a dual quad manifold would be nearly impossible to find for a magnum head. And, well, I'm too broke for all that.
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Oh, I feel that broke part! I did spring for the manifold but one carburetor came with the car and the other one was free. You got to love those Chevy guys who think a brand new carburetor is going to solve every one of their carburetor problems with zero adjustment! LOL
Gave me his old one to put on the new one I'm sure if he didn't put 12 volts to the new choke it didn't work either! LOL
 
LMAO! Your no help Jpar! :lol:

1994RR
The 750 would be better all around. Just take the time to tune it well.
 
1994redram said:
But it didn't seem to pull too great on the highway.
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I think we need to understand better what this statement means. Is this being in 3rd gear and it would just not move out well when you floored it? If so, at what RPM were you starting when you tried to accelerate? Full throttle or part throttle? Is the trans downshifting properly? Carb throttles opening fully?

I can think of things like cam and ignition timing being issues before that carb size; you don't have a huge cam in there.

BTW, if you really have 10.5 static CR, then your DCR with that cam is probably in the mid 8's range. That is going to require some care in tuning to avoid detonation, especially on a cool, dry fall day. What pistons and cam model did you use so this DCR number can be more accurately checked?
 
Is it a safe assumption that you have that 625 dialed in perfect, and are getting all there is from it? And that you already have the Demon jet kit and have done all the tuning steps as stated in the Street Demon carb manual? In addition, I'd buy a wideband A/F ratio gauge/kit ($180+/-) and really dial in what you have. I bet you pickup more power that way vs. just buying a bigger carb.
 
I'm using KB 107 pistons. Already had them and they were new. The chamber is much smaller on magnum heads vs LA heads so the compression is pretty high. I don't have any detonation issues YET. I've been taking it slow with the break-in and slowly giving it more throttle for longer pulls to listen and check for signs of pinging. I'm using a colder thermostat to try and help with detonation and using 91 octane. The trans shifts great. And wasn't slipping. It has pretty good power at lower RPM. Not a race engine by any means, but pretty fun on the street. On the highway I was running from 65 mph and up playing around with my buddy in his new mustang. I just expected a little more pull from my car. I figured that 625 might be choking it a bit at wide open throttle. Carb is opening all the way. Fuel pressure is at 6 psi at WOT. Plugs look pretty good, I'll pull one and get a pic if needed. Kickdown works as it should. I'm happy with how it drives but if a $400 Carb will pick up power and still drive great, I'm in. I just don't have the money after the rebuild to throw at it and find out it didn't do any good. My wife still doesn't understand how I spent that much money on "Just an engine"
 
The 750 will not start making a difference until the last few hundred rpm.
The 218/110 cam is pretty small. But it should make gangbusters torque on the bottom.
The power-peak might be around 5000. With an auto that would want a shift point of about 6000, to hit second at 3540.
You don't say what chassis this is in so I'll guess a 94Ram. And I'll guess you have 29s on her. Well with 3.55s that 6000 would be 57mph in second....... and that's your problem right there.
However, I doubt you revved her out to 6000, but probably topped first out at 5500,right? Well that would then get you into second at 52mph and 3250rpm, and 65 will be 4070, and 5000 will be 80mph. Yeah so Ima thinking you need more rear gear.
A 750carb is not gonna make one bit of difference at 3250 rpm. Or even at 3540. But it will also not be a problem on the primaries. I've been driving a 750DP on my 367 since about year 2000
What's your vehicle weigh? and
what size tires are on it?

But I gotta tell ya, a TRUE 10.5 Scr is waaay too much for that cam AND iron heads. In at 106* the ICA comes to 57* and;
Static compression ratio of 10.5:1.
Effective stroke is 2.95 inches.
Your dynamic compression ratio is 8.83:1 .
Your dynamic cranking pressure is 181.64 PSI.
V/P (Volume to Pressure Index) is 165

You need aluminum heads to run full timing with those numbers.

I think you outta pull the plugs and check the electrodes. While they are out I would highly recommend a compression test.

With theses numbers 182psi/165VP4 it should be smoking the tires to past 60 mph, like mine did ,lol.
 
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It's a 1970 dart with I believe a 26" inch tall tire. I have some BF Goodrich radial T/A tires. They worked great with a slant 6. But traction is awful with the magnum. Haven't done any serious racing so I let her spin and slide through 1st and spin a bit in 2nd then hook and rev it to 5500-ish and shift to 3rd. But the highway pulls this last weekend were all done in third.

Spinnin' ain't winnin' but it sure is fun.
 
All done in third lol, well that is the problem then, you need more gear for sure.

65 in third is about 3130 with 26s, so there's not a lot of pulling power there yet, hang on that's a roller cam then right?
Hang on post #11 was worked out with a flat tappet. Here it is for a roller
Static compression ratio of 10.5:1.
Effective stroke is 2.79 inches.
Your dynamic compression ratio is 8.40:1 .
Your dynamic cranking pressure is 170.32 PSI.
V/P (Volume to Pressure Index) is 146

So not quite as bad but still, with a tight-Q 165psi is generally considered max psi for iron heads and 91pumpgas.

So now if the engine goes into detonation, it will loose a lotta power.
And if you dial back the ignition, it will also loose power, just not quite as much.
And if you run her cold, the energy goes into the cooling system instead of into the crank.

Now getting back to gears;
in a lightweight 70 swinger with 26s, those 3.55s are a great gear, and especially with that cam............ just not so much at 65=3130,lol.
Now in third; 80 will be about 3850 and so again; you were just in the wrong gear. You would need a stroker to make a good showing at those Rs.
But now; in second from 65 to 80 would be 4320 to 5320, so that's in the powerband for that cam.

The cures for the over-pressure are
2 to 3 more ccs of total chamber volume; bringing the Scr down to 10.0, or
aluminum heads or
a bigger cam with a later opening intake,or
4* of cam retard. This will soften your low-rpm power, trading it for another 100 or so operating rpm at the top,and adding a couple of horsepower up there. The hit is pretty heavy tho,dropping your VP from 146 to 135,ouch, but so will a 230cam, which is what you would need to drop the pressure to 165.
or
some combination of these.


So again, the compression test should be top priority, and a look-see at the plugs.
 
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rumblefish360 said:
LMAO! Your no help Jpar! :lol:

1994RR
The 750 would be better all around. Just take the time to tune it well.
Click to expand...
I already know...
12many said:
Is it a safe assumption that you have that 625 dialed in perfect, and are getting all there is from it? And that you already have the Demon jet kit and have done all the tuning steps as stated in the Street Demon carb manual? In addition, I'd buy a wideband A/F ratio gauge/kit ($180+/-) and really dial in what you have. I bet you pickup more power that way vs. just buying a bigger carb.
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Also anytime this wideband O2 sensor is brought up I always remind people to make Summit price match eBay or whatever I was able to get mine for just under $150 back when they wanted 209 for the innovative.:thumbsup:
 
AJ is partially right about the bigger carb.
Perhaps, one day he will try the two and see that he is wrong.
The difference is small and it would be a small gain vs the money spent. You should notice a minor gain in the low end and a small gain up top. If it were myself, I would not bother swapping carbs at the cost factor. From a starting point, I would have gotten the bigger carb. Neither here or there now. But I have done this.
 
agree with comment above that 750 will help only on the top end
through a vacuum gauge on it and see how much vacuum up top
esp with a short cam
have you spent enough time on the ignition curve
save your money on the carb
double pumper on his build- you gotta be kidding
also agree with rumble but we can't start over did op say holly, AFB, AVS?
 
1994redram said:
I'm using KB 107 pistons. Already had them and they were new. The chamber is much smaller on magnum heads vs LA heads so the compression is pretty high. I don't have any detonation issues YET.
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With 62 cc chambers and those pistons a few thousandths out of the block, you static CR will be right up there at 10.5-10.6 even with a .050" thick head gasket. Being at 1200' around OK City will help a little on DCR. I'd not try to push the ignition timing too far....BTDT many moons ago.

I would be doing those pulls in 2nd as pointed out to be in a better operating range of the curve and just have more wheel torque. 2-3 shifts in the 80 mph range would seem about right.
 
I didn't want to slam into 2nd since I was worried about pinging. It's all still untested. So while I was running it hard to nearly top speed, I wasn't hitting it TOO hard. I'm using a stock 340 electronic distributor with an HEI module and MSD coil. I have NO idea what the curve is or what it needs. That's all out of my grasp of knowledge. The 625 was purchased for the stock magnum and only had a few miles on it, so it got used on the new engine. The fel pro gaskets are .054 compressed thickness. I calculated it to 10.5 static.

I wanted a bigger cam for several reasons. But this cam is the largest allowed with the springs that came installed on the heads. Ran out of money quickly.
 
I disagree on the 750 not making a difference till the upper RPM's. The DP Holley will drop the 60 ft time and pull better the whole rpm band if it's tuned properly.
 
1994redram said:
I didn't want to slam into 2nd since I was worried about pinging. It's all still untested. So while I was running it hard to nearly top speed, I wasn't hitting it TOO hard. I'm using a stock 340 electronic distributor with an HEI module and MSD coil. I have NO idea what the curve is or what it needs. That's all out of my grasp of knowledge. The 625 was purchased for the stock magnum and only had a few miles on it, so it got used on the new engine. The fel pro gaskets are .054 compressed thickness. I calculated it to 10.5 static.
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OK, understood on the circumstances, and I agree with your CR numbers. I'd expect less chance for detonation at higher RPMs... less time for the combustion pressures to build up before the piston is dropping and lowering pressures. (All other conditions being equal like AFR.) BTW, I have been assuming 4 degrees of ground-in advance in the cam. That is common these days and looks correct from the website. I have also assumed that the lobe ramps to intake closing are fairly fast; that may or may not be true, and you can't find that data from the Hughes website. But it won't make DCR a whole lot lower with medium speed ramps.

These are all things going in the right direction to avoid detonation:
- At least you have small chambers going for you in regards to detonation; that is a definite help versus open chambers.
- And you are probably getting at least a bit of quench benefit with the flat-top pistons a bit out of the hole.
- Plus the smaller carb may actually help you; with a smidge more restriction, cylinder fill ought to be a bit less and thus DCR a bit lower. (It's like being at a slightly higher altitude.) So keeping the small carb is likely a help... and you can do a lot with 600 cfm on the street and around 350 cubes.

Going with more and earlier on the ignition timing will push towards detonation-land so that was the point on that matter. With your high CR, there is little (or less) need for a lot of ignition advance anyway.

1994redram said:
I wanted a bigger cam for several reasons. But this cam is the largest allowed with the springs that came installed on the heads. Ran out of money quickly.
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Roger.... FWIW for future thought, cams can be had with less lift and more duration; that is the easy direction in which to go...if this becomes a problem.
 
perfacar said:
what kind of fuel pump are you running? what size fuel line? filters?
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Summit brand version of the holley red pump. Using the 5/16 OE line. Pretty large Napa gold fuel filter after the pump. Fuel pressure looked great under WOT and oil pressure has been great. Those gauges were temporarily zip tied to the cowl. I removed them since all has been well.
 
you still have the sock on the inlet in tank? food for thought, 5/16 line does not provide enough fuel to feed your engine. it's running out of fuel. also, if it's in stock location, it runs next to the headers, also has many bends in it. bigger carb will not solve the problem. I've been there.
 
I doubt 5/16 line is the problem. I went 12.35 at 107 with a 440 Satellite with 5/16 line with a Holley blue in the back pushing through a stock mechanical up front. Maintained 6 psi through the traps. Not that I would recommend it (5/16) if starting from scratch.
 
I can beat that I did 11 5 with my slant 6 stock fuel line.
You said you didn't know much about the timing curve and I would definitely look into that first cuz that would be like free and I've never heard of a Mopar in the size you talking about not benefiting from a bigger carburetor in the yippie Department. I remember when I first got my dual quad tunnel ram it was on my 318 and everybody laughed. But I laughed last when I dropped a half a second in the quarter mile.
 
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