Changing Carbs over winter on roots blown SBC
 

I want to step up to a larger carb on my 400 sbc. I have a lot of info, but even more questions...I'll keep this as short as I can.

I knew from the time I installed the blower on the engine, the current Speed Demon 650 DP was on the very smallish side of what the engine needs. I added a new baseplate to the carb that allows the PV to be intake referenced, so I've always had that covered. Supposedly, this baseplate will also work on larger Speed Demon carbs...up to 850 cfm??

As some know, I have a small blower...a Magnuson MP-122H. Even the instructions that came with the blower says a 800 CFM carb (they use an Edelbrock 800 AVS carb) is recommended for use on a stockish 350 CI engine when using this blower.

I have also recently changed to the smallest pulley I can get for the blower, and I'm seeing 6-to-6.5 pounds of boost on my gage at WOT.

So - I've been running the car pretty rich to stay on the safe side until I get my carb situation figured out. Last thing I want to do is run too lean and detonate this thing because I know bad things can happen.

I actually have 2 other carbs in my garage...

-A 800 CFM Edelbrock "shiny" AVS carb I got off fleabay for cheap. Supposedly only ran once during engine break-in...who knows, but it does look brand new and came with the original box, CD etc.

-An older Pro-Systems HP750 CFM carb that I ran on the engine when it was N/A. I tried running this one on it after I installed the blower...I just can't seen to get it to run right at all. It'll backfire half the time when starting the car. I've tried minor tinkering with it...it needs more than that.

So here comes the questions/opinions:

-Should I go with the 800 cfm Edelbrock? I've already bought the high flow jets and hangers etc. that Edelbrock recommends for my roots blower. It also has an electric choke...and those that have a blown car know that's a plus..i.e no backfire on a cold lean startup...hopefully

-Tinker with the 750 cfm DP Pro-systems carb and hopfully get it to run right with that? I don't know where to start with that one...I'm guessing air bleeds and everything are wrong currently..

-Look for a good used 750/850cfm Speed Demon ..and put the baseplate from my 650 on it?

Whichever ones I don't use are going up for sale....at my loss of course..:(

In the end, I'm hoping to get whichever carb dialed in as best I can from my garage, and take it to a chassis dyno on a wide band to set the WOT A/F ratio properly. -So which one would you pick, and why?? :confused:

EDIT: And I really don't care if I lose 20 HP at the very top end...I just want the best overall streetable carb in the end...it'll make plenty of power regardless.

Re: Changing Carbs over winter on roots blown SBC
 

I would go with the 750 DP Pro-systems carb because it should have all the tunability you would need. If you give them a call they can probably give you a good starting point for a tune.

Nothing wrong with the Edelbrock carb, they work great on a stock or close to stock engine but seem to be more difficult to tune the more radical the combination is and for that reason I'd stick with the 750DP.

Re: Changing Carbs over winter on roots blown SBC
 

I called Patrick at Pro-Systems a while ago. The ONLY thing they would tell me is to send them my whole carb to get modified and re-jetted for the roots blower. They wouldn't give me any starting point or anything since they want to make the $$....

And with what they want to charge to do that, I'm very close to what a brand new Holley HP supercharger carb will cost me. So they're basically telling me I'm on my own...

As far as radical...the cam I'm running with the blower is much more tame than what I ran when the engine was N/A....but you're right...I am a lot more familiar with Demon type carbs than E'brocks..

I just have a bad feeling this PS carb is gonna take a lot of changes to make it work remotely right....more than I've ever done before...

Re: Changing Carbs over winter on roots blown SBC
 

I prefer the 4150 carbs but it may be worth trying the edelbrock depending on how much pro-systems wants to charge you to re-tune it. I wouldn't have a clue how to tune a carb for a blower. Pro-systems used to be great but I think they have gotten too big and the customer service has suffered. My last experience with them wasn't very good but up until then they had always been great.

Re: Changing Carbs over winter on roots blown SBC
 

Well, considering my limited experience with carb tuning, I had it tuned fairly well with the 650 Demon, but I think there's more power to be had with a better matched carb. My primary jetting on my 650 is 82 and the secondaries are 94 I believe...somewhere close to that..I have it written down somewhere.

My understanding is it's not much different from an N/A setup with the exception of referencing the PV. Pro-Systems customer service is horrible nowadays...they want 600 bucks just to modify and tune the one I have - not gonna happen.

I do still have the original flow sheet they provided me, so I know what size air bleeds, jet sizes and everything that came with it. I do know the base plate isn't Holley...it's a red anodized one..don't know if it's a Pro-Systems brand or what....of course, the "sheet" doesn't mention what brand that is.

Re: Changing Carbs over winter on roots blown SBC
 

After researching the web, running the Eddy 800 I have definately seems possible....just the tuning is going to be different. I think I'm going to give the thing a shot....I guess if I don't like it, I can just sell it and try tuning on the Pro-Systems carb....:nod:

I'm also adding a newer MSD 6 with Multiple spark...I only have straight boost retard without M-spark presently. Also changing the timing curve/springs in my unilight to come in faster, and all in by 2800-3000 rpms. Then I'll start carb tinkering...oh joy...

Re: Changing Carbs over winter on roots blown SBC
 

Try what you got, see if it works. Wouldn't be the first time somebody stumbled upon a "magic" combination that works better than the sum of it's parts would indicate. You'll want a step-up spring that is 6-7" or higher (since it can't be boost-referenced) and start your tune about 4 jet sizes over stock front and rear, no changes to the rods. That would be similar to the changes I did to my QJet when I changed over from N/A to sitting on top of the huffer and it seems to work well.

$600 to re-tune an existing carb? Yikes! I'd buy a new blower-specific carb before I'd shell out that kinda dough.

If you were anywhere near Philly I'd say stop by and we'll try some stuff (including bolting my 800 CFM Qjet on top), but, unfortunately you are "geographically challenged" being out in cheese-head territory.

Please tell me this car already outruns your SRT Challenger!

Re: Changing Carbs over winter on roots blown SBC
 

Well, since I'm clueless about tuning on the Eddy's, I called them months ago and gave them details on my setup. Nice enough guy...he suggested a few things I should get, which I have on hand:...
-A step-up spring assortment kit which ranges from 3-8"HG depending on which spring I use. He said I'll probably end up using one of these two:
1. the yellow (7"HG) or
2. the plain one (8"HG)

-He also recommended a high flow needle and seat (.145")

-However, he did recommend two different rods to try:
1. Edelbrock 1416's
2. Edelbrock 1442's
He based a lot of it off the Eddy 800 that comes with the E-Force blower kit they sell.
So overall, it sounds like your recommendations are very simular to his!!...I'm not suprised!
My thoughts exactly...made me wish I'd never bought the first carb from them...
Wish I could too! But our beer temps are quite simular... :lmao:
Since this is a Thirdgen board, I'll just say this...
The SRT's with the 392's are routinely running anywhere from 12.3/12.8 seconds in the 1/4...bone stock....It'd be a good run. It's a beast!! :nod:
BUT - there's no N/A car that can beat the instant torque and acceleration of a roots blown motor!! :waveman:

Re: Changing Carbs over winter on roots blown SBC
 

Good. Sounds like you're in the realm of sanity with trying the 800 Eddy.

Do yourself a favor- make some runs with the existing carb and have a vacuum gague (a big one you can read while driving) hooked up. See how much vacuum you're pulling under the carb. Then do the same thing with the Eddy.

My 800 CFM Qjet I can still pull more than 1" of vacuum, nearly 1.5", under it and I'm just using a little 142 blower making about 470HP at the crank (390 at the wheels). You have more headroom for making power with your newer, much more efficient blower.

Carb restrictions are much more detrimental to power production with a roots blower than on a N/A setup. And remember that the carb isn't just feeding enough to make the flywheel HP it's also feeding the power that's consumed by the blower itself. So the actual flow demanded from the carb was more than I even expected.

Re: Changing Carbs over winter on roots blown SBC
 

"Realm of sanity"?? Haha...so you initially though I wasn't very sane in trying a Edelbrock carb? Don't blame you....I've heard a few guys saying I'll lose 40-50 HP by putting one on. I don't buy into that.

I'm running around 390 RWHP and 495 lbs. RWTQ right now...with the Demon 650 on it. So I figure a bigger CFM =better breathing carb and better flowing headers/exhaust along with better ignition might get me even more. I'll see!

I don't think the vacuum gage will tell me much with this particular blower since it has the "bypass" valve built in while I cruise at part throttle. It doesn't close till I hit 5"hg intake vacuum when mashing the throttle, then it's instant boost...but I have a nice big gage and vacuum hose...I can try it.

Re: Changing Carbs over winter on roots blown SBC
 

No, insanity is putting a QJet on top of one like I did! ;)

What I mean about vacuum levels is how much vacuum it pulls under the carb AT WIDE OPEN THROTTLE. Not in the intake, under the carb. I.e. a way of measuring how much restriction the carb is presenting to the blower's demands when you call for "ramming speed". The bypass valve won't make any difference at WOT since it's closed and operating just like my no-bypass 142 at that point.

If you want to see something silly, try disconnecting the secondary throttle linkage and make a BRIEF full throttle pull with the vacuum gague hooked up. Around 3500 RPMs your jaw will hang open when you see that vacuum gague start to rise.... and rise.... and rise!

Re: Changing Carbs over winter on roots blown SBC
 

Thanks for all the advice Damon...

Unfortunately, I doubt I'll get the opportunity to do the vacuum test...looks like the first snow of the year is here. The car doesn't see snow, and especially salt...it's seen enough of that in the past.

Got a long weekend off next week...and I have A LOT of new parts that's been sitting around ready to install. None of it is interior though...:(

It's the weakest part of the car...and really hard to find anything decent nowadays. I should have started stockpiling interior stuff years ago, but as they say..hind sight is 20/20...

Re: Changing Carbs over winter on roots blown SBC
 

Go enjoy some ice fishing and report back in May!