BG MIGHTY DEMON VS HOLLEY VAC 2ndaries

[v80z]

Ok weel my last dyno run after putting on a single plane intake and BG Mighty Demon 750 was less than stellar. I am still in the 285 rwhp range. Peak did move up in the rpm range by about 1000 rpm and had some losses at the lower end which was expected. I am sick of this guess work. I have to make all the mistakes and follow all the wrong paths hopefully this will lead to a real understanding of internal combustion.

Question of the day.

 

I initially bought the BG to solve what I though was a lean condition on the high end. Well I got plenty of fuel now with this double pumper mech 2ndary 750. Yes I know it is toooo much cfm as a 355 at 7000 rpm only pulls around 600 cfm.

 

Should I:

a. swap this with a holley or clone 600 670 with Vac 2ndaries as the vacuum contol will only deliver the fuel you need?

b. Try to jet this carb down? there may be alot more than jetting involved to make this carb work well on this little cid.

 

355 300/.508 cam flat tops/ manual tranny/sportsman 2 heads/3.9 rear gear

 

 

Thanks

Joe

[280zwitha383]

PERSONALLY, I would sell both of them and get a 600/650 holley dp (mechanical secondaries). Maybe even a edelbrock. I think vacuum secondaries don't really work well with these light cars especially when combined with a 3.90 rear. And, 750 cfm is definitely too much carb for a 350. Can you punch it in your top gear and get smooth acceleration i.e. no bog or flat spot (which would be waiting for the engine to catch up or waiting for the secondaries to open up respectively). If you can then stick with your holley because you paid a lot more for the BG and don't really see much of a benefit from what you say. Have you played with timing?

 

Does 285whp seem a low to you. Seems to me a decent flowing head with 500 lift should get you anywhere between 350 and 400 at the crank. ASSume 25% loss (sorry for the math lesson but I had to think for a second how to figure this out):

 

X=crank power

y=% of loss

 

x-(x*y)=285whp

x-(x*.25)=285

x-.25x=285

.75x=285

x=285/.75

x=380hp

 

ASSume 20% loss:

x-.2x=285whp

.8x=285

x=285/.8

x=356.25hp

 

Point is you are probably making more than 350 horsepower at the crank and that's atleast close to where you should be. Maybe your drivetrain loss is higher than you might expect (and you're actually making 400hp but only getting 285 to the wheels) . Some people believe that you lose 28+ percent through an auto some say even in the lower 30's.

[lbhsbZ]

I think 25% is a bit much for driveline losses, especially with a manual. I could be wrong.

 

Did you forget the E-brake on while you were making your pulls? :biggrin:

[v80z]

Thanks for the input. It seems very strong. Maybe that is all I am going to get out of this mill. the displacement of an LS1 is 350 cid and soome make 330 rwhp. Is this just a different animal with optimized fuel and timing?

 

280zwitha 383. What kind of timing do you run?

 

I have mine locked out at 32 with 4 degrees mechanical on an msd pro billet backed by a crane cd brick.

[Pop N Wood]

the displacement of an LS1 is 350 cid and soome make 330 rwhp. Is this just a different animal with optimized fuel and timing?

 

LS motors have completely different head designs. The increased flow means more HP and bigger carbs then the first gen SBC.

 

GM recommends a 750 CFM carb with their 6.0L (364 CI) LS2 motors. But they also are rated at almost 450 HP. That would be a pretty wild first gen.

[280zwitha383]

I think 25% is a bit much for driveline losses, especially with a manual.

 

Haha, oops. Thought he said automatic.... Ok, so you're bordering on 350hp, probably below that. Something is probably a little off.

 

280zwitha 383. What kind of timing do you run?

 

Well right now I'm running 28 degress total about 8 initial just so I can run 87 octane (may or may not be productive in the end but my speedo is 5 mph over so a good gas mileage estimate might be tough). I'm running 10:1 on iron heads which should be right on the money with 91 octane.

 

You only have 4 degrees mechanical? What is your initial timing set at (at idle, no vacuum)? How much compression are you running? Unless you're running a lot of compression and you're trying to avoid detonation, I would start off at 34 degrees.

 

When you say 300/.508 I understand .508 is lift but 300 isn't the duration is it?

 

Did they tell you it was running lean or rich at the dyno? You could probably just take a trip to the track and get this thing tuned in a little better.

[v80z]

.508/.508 298/294 at close 251 248 at 050" 106 lobe seperation

 

Here is it and where I got it

 

http://cgi.ebay.com/5-Solid-Cam-Kit-sb-Chevy-IMCA-Hobby-Stock-508-508-Lift_W0QQitemZ180103500490QQihZ008QQcategoryZ33614QQcmdZViewItem

 

 

 

 

10.25:1 I got this dizzy from a friend. I need to look over the spring and the advance mechanism. I am not using the vac advance. It is a MSD pro billet.

[280zwitha383]

So you take it up to 7k? What intake are you running?

 

This is not the right cam and I'm assuming you're running a single plane intake. It gives you an idea of what you should be making at the crank. Again, if you were losing 20% at the crank (which you're not) and making 400hp you should be putting down 320 to the wheels. I think for where you are with tuning and such you're really not far off.

 

[v80z]

Thanks for the dyno . Interesting. Yes Single Plane. So I think you are right. Alittle more tuning and that will be that. Shift light at 7000 rev limiter at 7200.

 

You have prolly seen this vid but here is what it sounds like.

 

[280zwitha383]

YEEHAW.

 

The comment is only slightly ironic....

 

Could you post your actual dyno? I would be curious to see how the curves of DD coincide with reality. I'm sorry if this has gotten away from your original question btw.

[v80z]

[280zwitha383]

Peak at 4800rpm?!?! That should be encouraging I think because you know something's not right. With that big of a cam you should peak around 6k. Since you're not you know you have a lot to gain.

[v80z]

Yeah it is trying to find the magical combination to get it up there.

[Tech @ BG]

v80z,

 

Can you post a complete list of your engine/driveline specifications?

[v80z]

355 cid 1970 vintage stock crank rods and flat tops 10.25:1

cam 508/.508 298/294 at close 251 248 at 050" 106 lobe separation

heads: world product sportsman2 64 cc chamber 200 cc runner 2.02/1.6

1.5 ratio roller rocker

solid lifter

Single plane

bg Mighty Demon 750 not sure of jetting but my guess is stock 75/83 or 86

 

block hugger headers

Muncie m20

3.90 rear

255/50/16 tires

Running at sea level

[Tech @ BG]

v80z,

 

 

Although your heads may be a touch big for the combination, and the camshaft less than optimal the numbers seem way off. This type of combination should be able to make in the 400 HP range without too much work. A manual transmission shouldn't loose much more than 10% so the numbers are way out of left field.

The carburetor is probably a touch big, which can lead to some drivability, and performance issues, however shouldn't be this far off. The other thing this is extremely puzzling is your Peak HP RPM. This would indicate something either severely out of tune (possibly going extremely rich or lean) or something mechanically wrong long a clutch slipping.

Do you have any dyno pulls before you changed the carb and intake? Also do you have any of the AFR numbers off of the dyno?

[280zwitha383]

I don't think your problems have anything to do with fuel delivery unless you're just not getting enough fuel from your pump. IMO there can only be a few other things that would cause what you're talking about. I would check valve timing, maybe you're off a tooth on your cam.The only other thing I would think is something with your ignition. Maybe your coil is weak or maybe your ignition module is bad. Really you should keep both carbs until you figure out what your main problem is. Then see what kind of gain the demon carb will give you and make your decision from there.

[v80z]

As far as valve timing when I assembled I lined the timing marks up bottom and top not advancing the cam at all. How does one check the strength or weakness of an ignition coil?

 

BG when you say heads are too big and cam less than optional I am not sure what you mean. This application is an 80% closed circuit car and 20% goof off on the street. It is is no way meant to be a street car/daily driver. I have a 5 speed tranny on the way and need power at the top end. My initial goal was to give up some on the low end, as this is a fairly light car, in order to gain on the top end Down the front straight. I understand the carb is big/too many cfm for a 355 mathematically speaking. However this carb came to me for $300 and I could not pass it up as every built holley I looked at was $550 - $800. So here I am.

 

Now where to go from here. I probably wont go too deep into the motor until this winter. Ignition and valve timing. HMMM.

 

BG where can I buy jets? Do Holley jets fit and where can I find info on how to change them and tuning? The manual that came with the carb does not have very much information and if there is info on the BG website it is buried some where cause I have tried to find it.

 

I am not familiar with holley's or BG style carbs. Have workedon weber side drafts and edelbrocks. Just need a good manual.

[Tech @ BG]

When you say closed circuit what exactly are you doing with this combination? Is this an oval track car or are you Road Racing? A 200cc runner with only 10-1/4 compression would seem extremely big on an engine of this size. The cam profile doesn’t really make a lot of sense with this size runner either.

All of your tuning parts will interchange between ours and Holley’s. The instruction manuals have a fair amount of information, you may be better off actually buying a carburetor tuning book.

What did the AFR look like on the dyno? For your power to peak, and roll where it did there almost has to be something seriously wrong with your combination.

[v80z]

BG Forgive my ignorance.

What doesnt make sense about the cam/ intake runner combination. Explain it. Road Course. I did not have afr's measured. I sure tham not going about this correctly. But there has been no clear path.