Holley 870 Street Avg vs Demon 850

[Higgins56]

Hi guys,

I purchased a 71 Mach 1 originally with 302 engine and now a 429 with Demon 850...Car is under restoration and I have never driven it.

The thing is that the DEmon carburettor installed ( brand new) is probably too big for the engine as it runs too rich with all settings. My friend recommended to change it to a Holley Street Avenger 870 as it - according to him - is more street friendly.

The Demon with a two inch air cleaner just fits the engine compartment when the hood is closed ( 1/2 inch free space ).

My Question: Will the Holley 870 fit there as well or is it physically higer than the Demon. - Has anybody an idea of this ?


Additional info:

The engine has Comp Cam 305 Magnum and Edelbrock performer RPM and also Hooker headers

 

[MotoArts]

How radical is the engine built?

Do you have the internal specs available?

First potential fitment problem that I see would be the intake manifold, not the carb. Does the engine need one that has the carb pad that tall?

Jetted properly, either 850 should potentially be fine for the 460 displacement. Again, depending on how the engine is built, a 750 would even do.

I would think that any carb's physical height would be within fractions of one another.

My 460 (iron, small valved C9 heads) has a Lunati Bracket Master cam (~5K RPM), and I am going to use a standard Performer intake with 750 carb. It will be purely a street engine, very limited track time.

 

[TommyK]

There should be no change in hood clearance switching to the Holley although if you are using a drop base air cleaner you may find clearance between the top of the choke horn and the air cleaner lid is reduced.

Frankly this is what I would use:

http://www.quickfueltechnology.com/carburetors-ss-series/street-carburetor-780-cfm-vs.html

 

[c9zx]

Physical dimension practically the same. The 850 isn't too big but it sounds like the calibration is not correct for your combination ( air/fuel volume screws, power valves, jets, air bleeds, float level, fuel pressure, etc.). While I agree with TommyK on Quick fuel carbs you may be able to calibrate the one you have and save some money. Chuck

 

[Higgins56]

Thanks for your comment.

The engine is to my understanding pretty original with cast iron piston rods and pistons, the Block casting is D1VE and the heads D0VE-C which are slightly ported.

My friend told me that the Comp Cam 305 may be too rough for street use and makes it impossible to adjust the engine with the current specs ay smal revs ( the running would get "normal" with high revs but under 3000 rpm will get the spark plugs black.

The main purpose would be street use and participating rallies...

How radical is the engine built?

Do you have the internal specs available?

First potential fitment problem that I see would be the intake manifold, not the carb. Does the engine need one that has the carb pad that tall?

Jetted properly, either 850 should potentially be fine for the 460 displacement. Again, depending on how the engine is built, a 750 would even do.

I would think that any carb's physical height would be within fractions of one another.

My 460 (iron, small valved C9 heads) has a Lunati Bracket Master cam (~5K RPM), and I am going to use a standard Performer intake with 750 carb. It will be purely a street engine, very limited track time.

 

[MotoArts]

Looks like a nice basic setup there, very similar to mine.

The block could either have been stock with flat top (I believe) or dished pistons. Take a look in a spark plug hole with a flashlight to see which pistons you have. Your compression should either be around 10:1 (plus a little) if flat top, or 9:1 (plus a little) if dished.

Your heads are basically the same as the C9's.

You may be right, intake reversion due to the big cam/carb may be making it hard to tune. They may be suited for 6500 rpm, but the rest of the package is not.

IF it were mine... your goals sound similar to mine...

Install a smaller profile hydro cam, a lower profile intake and 750 vacuum secondary carb, and have fun!

PS:

I would be very concerned with the amount of red RTV used... Hopefully during the rebuild/repaint that issue was addressed. The only thing all of that goop will do is clog the pick up screen (at best) when it squeezes out and falls off. My rule of thumb: if you can see it after assembly, it's waaay too much.

 

[BenD]

there's only 1 useful thing i can think of when putting a Demon to use, thats using it as an ashtray..

i have 1, converted it to an ashtray,.. its brand new but completly un-useable i used it for 130 miles ,

i could not get it to run good, it was always too rich no matter how i setup it up, turned out that the holes for the booster venturi's were different sizes on the same side, the holes for the emulsion tubes were different sizes and drilled at crooked angles, feul was pouring past the butterfly shafts into the engine and out of the carb..

those carbs are crap straight out of the box.. and customer service is something that they have never heard of at BG-carbs... called them, emailed them, no response what so ever... so its fair to say that i dont like mister grant and his crap carbs..

i now have a Quickfuel Q-series carbs,.. talk about quality....!!

you would be better off with a Holley or Quickfuel carb..

 

[cazsper]

Hmm.. I have a deamon 750cfm on my 393w and I couldn't be happier. Out of the box it was a little rich, but with a little re-jetting and a lighter spring for the vac secondaries, it's perfect. It idles well, has great throttle reaponse and makes great power. Though, I have used Holleys (and had great experiences with). I have only had bad experiences with Edelbrock (Carter AFB) carbs.

 

[BenD]

well, look at this.

i added the last measurement, but photobucket is somewhat malfunctioning , but that measurement reads 4.17 i believe..

all measurements in mm's

and thats just the booster venturi's inlet, you should have seen the meteringplates and the throttleplate, i sold those there was someone interested in although he knew what was wrong with it..

 

[cazsper]

Well, maybe I got lucky with mine. I heard they started making them again. Did they fix the issues they were having?

 

[Darkhorse71]

That's interesting (and good catch) BenD. I have two demons (mighty demon 850 annular and 750 Race demon) and their quality is superior to the two holleys I have, including a custom Biggs holley and a Street Avenger.

I don't know, as I've never seen or read about that, but how much difference could the diameter of those inlets be for the boosters really matter if the main wells on the metering blocks are restricted by the jet size and the booster diameters themselves are the same diameter?

Mine were rich also, but tuning/changing air bleeds, IFR's, fuel pressure and float levels I got them running very well..

 

[Darkhorse71]

Btw, to the original poster, there is no flow comparison between a 870 Avenger and 850 Demon. Holley dry flows, where as Demon wet flows. Demons flow WAY more than holley in their relative CFM figures.

 

[MeZapU]

I am especially fond of custom carbs. With the myriad of combinations of engine builds, no out of the box carb will ever be very satisfactory. Especially in high end, big crammed situations. I suggest going on to www.Prosystemsracing.com website and take a poke around. I am a believer as is everyone I know who has used their products and services. Every carb is custom built to the exact vehicle/engine combination so you get a fully matched carb out of the box. If you plan on doing the job right the first time, there is no other choice in my opinion. The only better thing is a EFI conversion.