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| Turbo Base Tune https://mail.slantsix.org/forum/viewtopic.php?t=52666 |
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| Author: | RyGuyTooDry [ Mon Jul 08, 2013 8:46 pm ] |
| Post subject: | Turbo Base Tune |
I'm picking up a rebuilt slant this week, I'll be running a blow through Holley 2300 500cfm on a super six manifold, aussie permaseal head gasket, stock exhaust manifold to j-pipe to a hx35, intercooled. Couple questions.... 1. Will the 500cfm be too much? Should I use a 350 cfm one? Hoping for around 12 psi. 2. What should the timing be set at for first start? 3. What jet size should I run off the start, and should I even drill out the power valve at all with that little boost? If so, how much? Thanks for all the help you guys have been giving me, hoping to have the car down the track a couple times this year. Should be pretty quick. -Ryan |
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| Author: | billdedman [ Mon Jul 08, 2013 10:47 pm ] |
| Post subject: | My ideas.... and just mine, only... |
1. "Will the 500cfm be too much? Should I use a 350 cfm one? Hoping for around 12 psi." In my opinion, I'd go with the 350 just to keep flow velocities up, through the carburetor. Might make it easier to tune... 2. "What should the timing be set at for first start?" The timing on our engine is locked in at 18 degrees before TDC, with no centrifugal advance mechanism and no vacuum canister. It runs great tuned like that, but we drive it very little on the street. Perhaps a vacuum advance unit would garner us better gas mileage (and maybe a cooler-running engine, but ours never climbs above 180), but for a new engine just starting up, I think our system is simple and effective, Our distributor is a Lean Burn unit that is completely devoid of advance mechanisms of any kind, and the engine doesn't seem to miss 'em at all; starts on the first click of the key and never kicks back on the starter... 3. "What jet size should I run off the start, and should I even drill out the power valve at all with that little boost? If so, how much?" Get yourself a wideband A/F ratio meter and tune the carb for an A/F ratio of about 11.5:1 under boost and 13:1 for cruise. I am not saying it is actually impossible to tune a forced induction carb without a meter like that, but I just can't imagine even trying it. When you buy it, ask them about the effects of leaded gas on the O2 sensor. i ruined ours with av gas and had to replace it; got to where it wouldn't work at all. Twelve pounds of boost is enough to THOROUGHLY wake up a slant six, beiieve me! you will be astounded at your new-found power... I sure was at ten pounds!!! Insofar as a cam goes, for a project like this one I'd think that the most practical thing to do would be to obtain, one way or another, a set of 1.6:1 (high-lift) rocker arms and run them on the stock cam. Over on FABO, there is a thread that shows some Buick Straight-Eight 1.6:1 rockers that are touted as fitting a slant six with minimal work done to them. I made a set (of 1.6:1 rockers) by modifying some stock rockers, and it wasn't very hard to do. Tom Wolfe showed me how. If you'll send me your email address, I'll fill you in on all the details regarding the process. Send a blank email to Billdedman@Hotmail.com. I'll deliver the details. Good luck with your project!!! Bill, in Conway, Arkansas |
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| Author: | Charrlie_S [ Tue Jul 09, 2013 8:19 am ] |
| Post subject: | Re: My ideas.... and just mine, only... |
Quote: 1. "Will the 500cfm be too much? Should I use a 350 cfm one? Hoping for around 12 psi."
I don't agree. Don't forget a 2bbl card is not rated at the same pressure drop as a 4 bbl. A 500cfm 2bbl is not the same flow as a 500 4 bbl. A 500 2bbl is only about 354 cfm when compared to a 4 bbl.
In my opinion, I'd go with the 350 just to keep flow velocities up, through the carburetor. Might make it easier to tune... |
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| Author: | billdedman [ Tue Jul 09, 2013 11:24 am ] |
| Post subject: | |
That's right; I totally forgot about that stupid "schizophrenic" flow rating system Holley uses for the 2bbl/4bbl comparison. I've never understood the rationale behind that. Charrlie is right, I think, after considering the situation regarding the dual rating system. He usually is...  Bill in Conway, Arkansas |
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| Author: | DusterIdiot [ Tue Jul 09, 2013 12:29 pm ] |
| Post subject: | Not totally holley.. |
Quote: I totally forgot about that stupid "schizophrenic" flow rating system Holley uses for the 2bbl/4bbl comparison.
Holley isn't the only manufacturer that did the dual scale. Mostly it's because the scale on the pressure drop on bigger carbs won't hold up 100% against a 1 barrel or small 2 barrel on a max flow test (the smaller restrictive carbs on a test engine may not drop to the 0"hg limit (since most carb companies derive from a chevy 350-putting a Carter BBS on one would probably keep about 6" hg at WOT due to the restriction of the small venturi), but a 600 cfm AFB most likely would show 0" at WOT.One of the local performance shops (now closed due to economy after quite a few decades of family business), use to blueprint and test carbs and they always tested the carb (regardless of size) at 0.5"Hg, their "theory"was that during racing or hi-performance, the only real flow that matters is what the carb will flow close to WOT since the rating system for carbs even at 1.5"Hg is not even close to the position of the plates at WOT...also they felt that at that flow rating they could compare the flow values of various types of carbs more reliably and be able to compare them without the "conversion" issue. 2 cents there. On another note, for a mechanical governed distributor, I've researched and ended building 2 for board members for street use, Mechanical timing I kept conservative limiting out at 18 degrees- which means either running 0 initial and a 9 governor, or welding it up so you get some initial and a very short slot, depending on your method of delivery you may or may not be able to use vacuum advance, but there again I dialed a short advance can with a tight limit on the advance to be conservative. -D.Idiot |
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| Author: | RyGuyTooDry [ Tue Jul 09, 2013 5:35 pm ] |
| Post subject: | |
So Charlie and D.I are saying I should use the 500? Does the 500 come on a car from the factory? I know some fords have the 350, but I don't even know what car that comes off of... |
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| Author: | DusterIdiot [ Tue Jul 09, 2013 5:55 pm ] |
| Post subject: | Are you.. |
The next real question is...are you going to hog out the super six holes? The 350 cfm carb throttle bores/adaptor plate when compared to the BBD bores in the manifold need to be enlarged to make them even, the 500 has somewhat larger bores than the 350...although the turbo is going to push the mix right on through I can't see a "shelf" between the adapter and manifold being a good thing for flow. The 500 would get the most flow out of the 2 barrels, but you might get better signal and metering out of a smaller 4 barrel...before you get all hands on I'd suggest getting a Holley book that shows the mods for boost so you have a reference (there will be some drilling involved, and depending on the year or part number your 500 was made, you may not have the correct passages to do the mod (if it's a modern 4412, you're in better shape). I think Holley also used to recommend a mod for the throttle shaft bores since a blow through can push air and fuel out the ends of the shaft, I think they used to use a drilled plate under the carb to divert the fuel... I can't recommend a jet size, someone more experienced in that lineup will make that call, I do have lots of experience with Holley's and will say you need to get a book and read, then line up the parts for the mods, and it can go faster if you score a second carb of the same type to swap as you make changes. |
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| Author: | RyGuyTooDry [ Wed Jul 10, 2013 3:44 pm ] |
| Post subject: | |
Yes, I'm going to open up the holes more. But, I have decided to only use the 350 for now. This engine is going to be my beginner one, so I'm going to make it simple, and build a forged low comp engine on the side. I'll end up tossing my little brother this engine in a year or so anyways. |
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| Author: | Rust collector [ Fri Jul 12, 2013 7:39 am ] |
| Post subject: | |
I have no clue about this, but I would say you can stuff a lot of air thru a 350 under pressure, and if it makes it easier to tune..... |
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| Author: | Turbo Toad [ Fri Jul 12, 2013 8:44 am ] |
| Post subject: | |
6.5 pv 72 to 78 main jet 6 to 7 psi on fuel pressure but remember fuel pressure has a direct affect on air to fuel ratio I'd start with a gap of about..28 on your plugs and adjust according About 18° on your base timing Just my opinion Aaron |
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| Author: | billdedman [ Fri Jul 12, 2013 9:40 am ] |
| Post subject: | Fuel pressure variations as regards the A/F ratio |
Aaron, your comment about fuel pressure having a direct affect on the A/F ratio was puzzling to me. Within the confines of the needle and seat's area of its ability to control the opening and closing of the orifice, the float level should remain the same, even at varying pressures, shouldn't it? If that's true, then the float level wouldn't change even though the incoming pressure varied, as long as it were still within the design parameters of the needle and seat's ability to do its job, i.e., not become overpowered and letting in more fuel than was called or, thereby raising the float level. That would manifest itself in a situation wherein it would be possible to change the incoming regulated pressure from, say, five to seven pounds without altering the float level (and the A/F ratio) at all, wouldn't it? What am I missing here? Bill |
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| Author: | Turbo Toad [ Tue Jul 16, 2013 6:59 pm ] |
| Post subject: | |
Bill We know that the fuel pressure must raise 1:1 with boost or for every psi of boost the fuel pressure needs to increase which should be handled by the fuel pressure regulator. And that statement was made with regards if the fuel pressure doesn't rise when one is in a boosted situation the motor will run lean! I've also found that for whatever reason when I set my rear float I must set it just under were fuel trickles out the sight plug and the fuel pressure needs to be set at 6 psi if I try and set it where fuel trickles out the sight and at 7psi, fuel will come out the vent tube. I hope this helps explains my statement Aaron |
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| Author: | Dart270 [ Wed Jul 17, 2013 6:35 am ] |
| Post subject: | |
I would use the 500, partly for flow, and partly because it has the big 50cc accel pump. Lou |
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| Author: | billdedman [ Thu Jul 18, 2013 12:07 am ] |
| Post subject: | |
Quote: Bill
Aaron,We know that the fuel pressure must raise 1:1 with boost or for every psi of boost the fuel pressure needs to increase which should be handled by the fuel pressure regulator. And that statement was made with regards if the fuel pressure doesn't rise when one is in a boosted situation the motor will run lean! I've also found that for whatever reason when I set my rear float I must set it just under were fuel trickles out the sight plug and the fuel pressure needs to be set at 6 psi if I try and set it where fuel trickles out the sight and at 7psi, fuel will come out the vent tube. I hope this helps explains my statement Aaron What I had issue with was the statement "6 to 7 psi on fuel pressure but remember fuel pressure has a direct affect on air to fuel ratio" you made in a previous post. I am sure we are on the same page here, but if we talk boost-referenced carburetors, we have to assume that if the boost is, say, 10 pounds, then the fuel pressure must also rise 10 pounds, or the air pressure in the carb will be pushing gas backward into the line. So, the raised fuel pressure (orginating from the boost-referenced regulator,) must increase an amount equal to the boost pressure. Now, we have hopefully, the same air/fuel ratio we had with NO boost, because the increased fuel pressure has exactly offset the boost pressure it's having to act against. I think you and I were saying the same thing, just using different words. Thanks! Bill |
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