I have read here that a 650DP is a good size.

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick

I just had a carb built when I decided to make the switch to E85 and the carb specialist recommend around 500cfm for a boosted slant. What most people don't understand and where blow through guys have all there problems with tuning a carb is getting the venture boosters to draw properly.
And I've read hundreds of posts on the turbo forums about this subject and this is from some of the blow through guru's that a 750 holley is to big for most engine combinations other then the higher horsepower motors. I've actually heard them recommend 650 for 400cid and under combos and the slant even a big bore slant doesn't come close to that displacement.

I've actually tired a 750 holley with annular boosters, 650dp and the carb that's on right now is a 390hp holley with annular boosters that's been flow tested at 530 cfm.
I couldn't for the life of me get the 750 to work the 650 wasn't bad and it actually ran pretty good. The 390 is amazing though all I have to do is bump the throttle and it is so responsive and starts drawing through the boosters at a unbelievable rate.
This is just one man's opinion who's been tuning his own boosted slant. So take it for what it is BOOSTED MOTORS ARE DIFFERENT FROM NA AND THE MOST IMPORTANT AND HARDEST THING FOR BLOW THROUGH IS TO GET THE CARB TO DRAW THRU THE BOOSTERS

I wasn't even going to comment on this because me and Bill have had this debate for over a year. But I couldn't sat on the sideline any longer
Aaron

CFM = Displacement X RPM X VE / 3456 Example: CFM = 350 CI X 6000 RPM X 90% Volumetric Efficiency / 3456 CFM = 350 X 6000 X .90 / 3456 = 546.87 CFM = 546.87

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[quote="Joshua Skinner"]

18° initial timing might be fine at idle and under considerable boost, but even a drag engine operates in between these extremes.

It seems to run very well with no vacuum advance, and part-throttle driveability is good.
How much manifold vacuum do you have?

Sixteen inches on the gauge... at an idle. Varying amounts at part-throttle.

You might do well to connect vacuum advance directly to the intake manifold. The vacuum advance will disappear under load, but will ignite the less dense part-throttle mixture at a more advantageous time and increase part-throttle torque. You'll probably still have to put some mechanical advance in the distributor so that you can use less initial timing to offset the vacuum advance at idle, but I feel the effort would pay handsomely. Better yet would be a full normal timing curve for a low compression engine with a relatively large cam (lots of initial timing and restricted to 28° total mechanical and generous vacuum advance) with boost retard.

Given this car's extremely limited active life, which will involve, almost exclusively, trips down the drag strip and not much more, I think all that complexity is unnecessary in terms of payoff for time and money spent. If it were a street car, even a small part of the time, I can see the benefit of what you recommend, but driving it around in the pits and down the strip under power (and nowhere else,) it would seem to be time and money spent unnecessarily. If I had a dyno to test engine output with various spark-curves on, and unlimited time to fine tune the ignition vagaries, I probably could realize something positive from such activity, but for now, I don't think it needs vacuum advance or a mechanical curve, considering that most of its life will be spent ieither idling or at full throttle.

Stalling it is the current fly in the ointment, and there are several ways around that, the best and simplest being (probably) to replace this probably too big 750 carb with a smaller one; maybe a 600.

I am not married to the idea of a 750 carb on this engine. I'll tell you how it came to be there: Before I bought it, I reasoned that it takes a certain amount of air to make 500 hp, which was my goal. I have a 360 Magnum with a Vortech supercharger on it (one 750 Holley dp 4bbl, 10 pounds of boost,) and it works really well, with little or no driveability issues, making a little over 500 flywheel hp (445 at the rear wheels.)

Since the two engines were (ostensibly, at least,) going to BOTH be making virtually the same hp, I though they'd be using the same amount of air.

So, I figured they needed the same size carburetors. The 750 worked well on the Magnum, soooooo....

Add to that the fact that Ryan Peterson's and Tom Wolfe's engines (the ones I tried to copy) both seem run out of steam at much over 5,000 rpm, I just thought it MIGHT be the fact that they both use carburetors in the 600-650 range. I thought that might be a bottleneck.

Given those two (flawed) theories, I went for the 750.

Now, I think it was most likely a mistake, and the 750 needs to be repaced by something smaller.

But, I have the 750 on the engine and will test it at the strip, for reference, at the earliest opportunity.

If it still shows a propensirty for working poorly, I guarantee you, I'll replace it with a 600, or 650. But, they will be Holleys because I am somewhat familiar with their systems, now, and can tune them more easily than I could a Carter or anything else.

Thanks for all the good advice; I am flying blind here, but the good weather is just around the corner, and maybe we'll learn something about what this ol' tub likes, at the drag strip!!!

Bill

Aaron, do you think the 600 Holley is too big for my motor? If so, what would you recommend?

Bill

It's not the carb that is a slant bottleneck, it's the small bore! Also, the 600 should be good for a boosted slanty. No need for any more than that, no matter how much hp you're going for.

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yes the combination has a lot to do with the size of carb your engine likes. the math adds up to a smaller carb but if you don`t try you will never know what is best for you engine.

Bill yes I believe a 600 would be a good fit for your needs.
My builder said he probably wouldn't have to hard of a time to get the 650 had I too work for what I needed but since I had the 390 he liked that choice much better.
Bill the offer still stands on the carb I actually could put together a 600 if that's what your after I believe a have enough parts here for another complete carb
Aaron

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Terry that statement is so true like I. Stated in a previous post I've tried 3 different carbs and will still see what happens because I'm still a long way off myself.



In my opinion and this is based from my own experience and how my carbs are setup because I do most all my own work other then the e85 conversion and that's only because this carb has a extra circuit that this particular builder is his specialty. My 650 has 72 main jet and 82 rear and is rich into boost at 10.5. Bills 750 has 92 in the primary that tells me he's carb is not responding properly thru the discharge boosters or boost referenced power valve but i no he's using Shakers brpv which I'm using his design but home brewed by me and enless its not referenced correctly I don't believe that's it.


Aaron

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Aaron,

Thank you for your generous offer to loan us that 650; I think I will take you up on it.

One other thing... our F.A.S.T data-logging, wideband A/F meter wasn't woking properly, so I ordered a new oxygen sensor for it (it just unscrews from the bung and plugs into the wiring) so, we can get the mixture monitored correctly. It got to where it would display a number and stick there, no matter what was going on, We have been running some low-lead 100 av gas and I think that maybe the lead has poisoned the sensor/probe. Is that possible?

Anyway, send me a blank email to Billdedman@Hotmail.com and I'll send you a mailing address for the carb to your email box.

We sure appreciate it!!!

Bill

o2 sensor`s and lead don`t mix. just like deposites on sparkplugs

I had heard something like that, Terry, but wasn't sure. So, what kind of lead-free high-octane gasoline would you recommend we use? The only alternative I know of is a 110-octane racing gas sold at the local speed shop for $10.00 a gallon. There's got to be a cheaper alternative than that...

Isn't there??

Bill in Conway, Arkansas

VP sells an unleaded racing fuel for the newer cars with O2 sensors and Cats....

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There's no such thing as too much cam....only not enough engine!

the cost is $19.00 gal up here that is why I went with water injection.

Well, Terry, I have water/alky injection ( Stage I Snowperformance Boost cooler) and an Intercooler, but I still don't trust 93 octane.

CNC Dude, I'll check out the unleaded VP for sure; thanks for the tip!

Bill

I usually mix 110 (some lead or not) when I have my '68 at the track, just for insurance. 93 octane and 1/2 of 110 octane gets you 101.5 octane, which is probably overkill. Just a few gallons of 110 shift the octane quite a bit. If you are not using much lead, your sensor will last quite a while. Mine has shown no signs of failure and I've run 4-5 drag weekends with some leaded gas in it.

Personally, I don't think the 750 CFM (the flow rating) is the problem, but some other part of the carb tuning or boost referencing. Many NA Slant drag racers use 750 carbs and they work fine. I just put a 650 DP on one of my NA motors and will be tuning it soon, but it works OK out of the box.

Lou

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