Need carb size and brand advice

[pparaska]

dr_hunt, thanks for that offer! I am afraid that my fuel system wouldn't be compatible with the alky though!!

[dr_hunt]

excuses, excuses.

[pparaska]

Pete, I'm running a Holley HP830 annular on my junk. It picked up 30 ft-# in the midrange compared to a 750dp, and about 20 hp on the top end. I've been very pleased with it.

 

John

 

Hey John, how happy are you with the throttle response with the HP830 annular? The ability to get a clean idle? Transition circuit action?

 

Thanks, Pete

[jt1]

It seems to be a litle fat at idle, but not terribly so. With the cam I can't do much with the LM1 at idle, so the idle circut screws are right where it starts to stumble if you lean it out anymore. Off idle the dyno shows a lean spike, but there's no stumble at all, so I haven't worried about it. Mid range & up it's great. The 830's come with the secondary blades drilled to about 3/16 or so, and an adjustable secondary stop screw, and air bleeds, so you have lots to fiddle with.

 

jt

[pparaska]

Interesting, thanks, John. I have 52 cubes more engine, and a bit milder cam. Big difference is the Victor JR, which I MIGHT replace with an air gap, but probably not unless I find one cheap (I know, not likely!) So I'm thinking this 830DP annular may work. Thanks, John!

[jt1]

Well......put a bigger cam in it.

 

Actually the next time mine comes apart it's getting a little smaller cam. Right now it makes peak hp at 6700, and I only turn it to 7, so I think I would go faster with less cam.

 

jt

[pparaska]

John, I agree - it needs more cam - maybe when the money for that falls out of the tree! Seriously though, the engine was very mild with that cam (Cam Motion Tight Lash Soild Roller, 244/249 @ .050", 274/279 @ .020", 0.577/0.570" lift (1.5:1), 112 LSA, using 1.52:1 rockers). I was surprised. The 331 (.030" over 327), with almost identical .020" duration, a lot less lift (.510/.501) and 236/244 @.050 and different heads (could have been part of it) was almost unstreetable. 75 more cubic inches in the 406, along with 53 degrees overlap at .020" versus 57 for the roller in the 406, and the 406 was just a ton milder. IT NEEDS MORE CAM!!! LOL NOt really sure - I think all it really needs is a tighter LSA, like about 105 or 106 LSA

 

I agree that if you are hitting 7000 at the end of a straight, then the power peak could be a few hundred rpm less than 6700, and the area under the torque curve would be probably greater with less cam (less duration, less overlap, etc.). Ever thought about using Vizards "chose an appropriate LSA and overlap, then let the duration fall where it may" method of cam selection?

 

Wow, I took my own thread off-topic! LOL

[pparaska]

I found a very slightly used (like it was put on an engine and run a few hours) 0-80511 HP series 830 DP with annular boosters, $425 shipped. It was taken apart and non-stick gaskets put on, inspected, put on the engine and run a few times in the garage. Then the guy got a 950 HP vac sec carb that he would be a better match to his 454 Chevelle race car, so the 830 is just sitting.

 

Now for some additional real-world input to my carb sizing question:

 

I was talking to a well know racer yesterday at the Burtonsville, MD shopping center cruise-in. This guy has been at this for nearly 30 years and has always had one of the fastest street cars in Maryland. I gave him the specs on the engine, and he said he would use (tada) an 830HP with annular boosters! He asked what the heads were, how they flowed, compression, cam, intake, displacement, gears, weight, tire size. That's about the 5th person whose told me the 830 will be fine, based on their experience with the 830HP annular carb on engines of this displacement, etc. Pretty wild that he suggested just the carb I had a line on!

 

BTW, he runs a 69 Camaro with a 447 ci SBC that has engine-dynoed right at 800 HP, or 1340 HP on the 3 stages of Nitrous. !!!! I didn't even ask what it ran in the 1/4.

 

I also spoke with a crew chief and engine builder for several fastest street car competition teams, one of his past efforts won the competition in recent years. He has 30 years of experience in building competitive racing and hot street/strip engines professionally, Cobras, etc. So I tend to trust his judgement. He feels that the 830HP annular may not have quite as crisp a throttle response as a 750, but would work well on my combination and need little if any tuning beyond jets, idle mixture and speed screws, and possibly squirter size.

 

I am capable of handling that but can call on his expertise if needed since he is local.

 

He felt confident that it was not too much carb. In fact he pointed out that the annular boosters, due to reducing the WOT flow over downleg boosters, would possible steal a few HP. He felt that for my purposes it was a fair trade as the mixture quality from the annular boosters would help part-trottle distribution over the downlegs. I certainly will be using it in a part throttle mode most of the time! The last 5 HP aren't important to me, but the last 20 or more are.

 

So I suppose my mind is made up to go for the 0-80511 830HP with annulars, especially since the seller is willing to take it back if it doesn't work out, and is selling it for what I feel is a reasonable sum.

 

Part of the equation is price. $425 for a basically new 0-80511 is not a bad deal to be able to get into an HP carb. If it's close to optimal and tunable without a bunch of machining, etc, then I am ready to go with it. I also have a good assortment of tuning parts for Holleys, and that figures in as well.

 

I plan to use my Innovative LC-1 WBO2 sensor to optimize the circuits for idle, cruise, acceleration. What I'm looking for is a decent place to start with carb selection that may not be the OPTIMAL carb (tough to say what that is with a tri-purpose car!) that is tunable and affordable.

 

Now for the geeky stuff:

 

I hate reading nomograms! So I took the one on the second page of this Holley document on carb selection (for sizing mechanical secondary Holleys):

http://holley.com/data/TechService/Tech nical/Selec...

...and fit a second order polynomial to the inputs (minimum WOT rpm and displacement in cubic inches):

 

cfm = 0.000886*WOTrpm*disp + 0.000159*disp^2 + 0.0000066*WOTrpm^2 - 0.345*disp - 0.0554*WOTrpm + 439

 

Checking it against that nomogram, it seems to reproduce the results of drawing a line across from WOTrpm through disp to find the cfm needed, within +/- 6 cfm, when I checked it against the 10 data pairs I used to read off the graph and then check it. It all really hinges on how closely you read that graph to come up with the input data, and how accurate the data they used to make the graph with actually was. I used a graph that I tweaked to exactly match each of the 3 axes of the original graph, zoomed in to very precisely read the data off of lines I drew from several WOTrpm/displacemt pairs over to the cfm axes, as well as one graph for each axes to find the non-linear formula for each of those. Like I said, I geeked out.

 

Here's a spreadsheet I made up to do the calcuations:

http://alteredz.com/tools/HolleyMechSecSizing.xls

 

All of my work is shown on the calcs sheet, if you want to check my math

 

Just put the WOTrpm (minimum rpm for WOT use) and displacement (in cubic inches) in the yellow boxes and the answer appears in the magenta cell to the right of those.

 

Watch out, we have no idea how accurate the data in that original graph is! Also, they make no mention of whether the type of boosters are, the throttle plate size, or the venturi size. I believe all of these play in the selection.

 

Also, note that Holley and BG may have a name for a carb based on a cfm number but the total cfm they flow can largely differ. BG's 650 DP Mighty Demon flows 830 according to them, so if you pull 830 off of that graph, you'd want a 650 Mighty Demon!

 

Anyway, for minimum WOT rpm of 1500 and my engines 407ci displacement, I get an answer of 798cfm. So an 830 might be a touch big . Or not. (See discussion above).

 

For a 355 at 1500, it's 740cfm.

 

So that's where I am - kind of sold that an 830 HP with annular boosters will work well for my application, but with the ability to sell it back to the guy for the same money if it doesn't work. I'd then probably go to a 750 HP.