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Old 04-08-2013, 10:36 AM
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Throttle response on blower motor

I have 2 750 Quick Fuel blower carbs on my hemi. I am happy with the way it runs and the power it is putting out, but if I crack the throttle off idle (not under load), it does not seem crisp like I would like it to be. Its not bad and I am being picky, but I would like it to be crisper.

392 Hemi (398 cubes)
cam = .570 lift, duration 270 @ .50, 111 center, solid roller
heads = flowed by Mondello, 2.08 intake and exhaust valves
6-71 blower, 8psi
pistons = Arias 8.25:1

Carbs: primary jets 74, secondary jets 84. bleeds 70 and 33. Squirters 37, pink cam #2 hole on linkage, #3 hole on cam.

Again I am being picky. Under load, this makes great power, but the throttle snap is just not crisp enough. Any suggestions?

Forgot to add: timing is locked at 34

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Last edited by 33Willys77; 04-08-2013 at 10:50 AM. Reason: more information
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Old 04-08-2013, 11:34 AM
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You have to realize that with that 270 deg cam, the motor needs RPM.
It will never be perfect below a certain RPM. below 3000 ish.
It wants to rev. That is the nature of a big racey cam.

Fine tuning , the idle and off idle transition circuits of the carbs may help.
A AFR gauge and a manifold vacuum gauge and a boost guage helps a lot.

Start by correcting the T slot throttle exposure at idle on all 8 barrels.
it must be in the sweet spot at idle or the transition fuel curve will be off.
remove the carbs and flip ove and look at all 8 barrels. aprox .020" T slot exposure at idle.
Get them all the same. Any throttle speed adjustment from this point to get more or less idle rpm
should be done evenly on all 8 barrels. To keep them all the same.
Get this right first, before messing with the idle circuits.

What driving mode(s) and rpm range and throttle positions are not responsive.

"throttle snap" from what rpm? what throttle postion?
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Old 04-08-2013, 11:47 AM
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Think what it would be without the blower.

Some people like it. They are not even racing. They want the biggest stampeed under the hood they can make. Sacrificing the under 3500 power for a added bit of rump.

You can shift the power around a bit, but doesn't keep the street manners.
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Old 04-08-2013, 12:13 PM
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It feels very good under power and will pull very hard (if the tires hold). The carbs are set right (transition slots). The only thing I would like a little better would be the off idle snap not under a load. This could be because of the cam characteristics. It idles nice at 1000-1100 rpm.

I have AFR, boost and vacuum guages. I did crack the back barrels ever just so slightly for idle adjustment - but this was just that you could barely even see the linkage move (very tiny amount). This could probably be solved with air bleed change? The idle mixtures are about 3/4 turn out. I do have the 1:1 linkage put on - it really liked that adjustment.

I have a 3500 stall so it works very well under load.

AFR - at idle (1000-1100 RPM) is showing 13.5-13.8. Cruize (2200 through 3000 RPM) - 12.8-13.0. WOT 10.5.

Vacuum at idle and cruize is 8.

Reasoning for his post: I set my dads 426 hemi up with a 8-71 and used 2 750 edelbrocks. I did some jetting/metering and put in bigger nozzles and that spins very fast compared to mine. His cam has more lift, but duration is about the same (going of memory here). He does have higher compression at 9.8:1. I took edelbrocks off mine (best thing I ever did for it).

I have also just installed water/meth injection. We still have snow here so it will be a bit before tuning can start with that.
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Old 04-08-2013, 02:33 PM
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I have AFR, boost and vacuum guages. I did crack the back barrels ever just so slightly for idle adjustment - but this was just that you could barely even see the linkage move (very tiny amount). This could probably be solved with air bleed change? The idle mixtures are about 3/4 turn out. I do have the 1:1 linkage put on - it really liked that adjustment.

Get all 8 barrels T slot exposure set up evenly at idle.
Do this first... Then,,, what happens to the AFR's when you slowly raise the idle speed rpm (slowly open the throttle up from idle)

is 34deg timing the amount of timing that makes best WOT power on a HEMI?
Do you have a boost retard box, so you can reduce the spark advance as boost builds?
Using this will allow more idle and non load/no boost/part throttle spark advance.
yet reduced spark advance under boost/as required.

A supercharged motor (especially with a big cam) has a REVERSE spark timing curve requirement. More timing at idle, than when under boost WOT.
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Old 04-08-2013, 03:32 PM
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Yes I have a MSD 6BTM. Hemis dont usually require a bunch of timing as the spark is in the middle of the piston and the flame does not have to travel across it. I could see what it would do at 36 and pull enough out with the ignition.

I will pull the carbs again, but I believe they should be set fine.
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Old 04-08-2013, 08:26 PM
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is 10.5:1 A/F good for a blown engine or was that set up that way as a safety?
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Old 04-08-2013, 08:42 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
[I

A supercharged motor (especially with a big cam) has a REVERSE spark timing curve requirement. More timing at idle, than when under boost WOT.
Yes, this is pretty true.

Using a e curve I chased the timing all around. It worked best at any boost 4-8 psi I tried. Give it the fastest curve with max timing 36. Then pull out 2 per pound. I haven't had much luck setting the btm to cut less or more than 2 degrees. If the fuel is good , it is . I started way rich on the edelbrock diagram. Then leaned the mixture balancing a safe margin till detonation.

My AF wot ended up around 12:1 .

This, is not that. I got a 305, you have something slightly meaner.
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Old 04-08-2013, 08:49 PM
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Vinniekq2: 10.5 might be a little rich, but it doesn't smoke and runs hard, so I am ok with that. I will see what happens when I put water/meth with it.

So, I looked at the slots and they are set good. I put them back on and adjusted the linkage again and started everything back at zero. Now, before I ever so slightly opened just the back barrels to get the idle up a little. I now watched closely and opened the primaries and seconaries on each carb just the same. You can hardly even see the linkage or blades move, more of just having the screws touch to open them. It idles nice at 1000 RPM, AFR is at 13.3-13.5 at idle and seems just fine. The primary air bleeds are 75 and the idle screws are about 1 turn out. So then I let it warm up and all was good. I then cracked the throttle and BAM, right there it was. It caught me off guard and it really responded nice! So I am happy. I think if I had the primaries open the same as the secondaries, I would have been ok. The smallest adjustments sometimes make a world of difference.

As soon as the weather decides to act like Spring, I will take it out for a test. A new set of plugs should help too. I might try moving timing to 36 and pull 1 degree per pound of boost when I tune the water/meth. I would think that cam should like that as well.

F-Bird - thanks for you postings. I have taken your advice on your other forced induction posts and learned all I could before I got into this forced induction stuff, so I thank you for all your postings you have done over the years. It sure helps doing all the research before just jumping into it and then trying to fix it later on.
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Old 04-08-2013, 08:58 PM
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Spinn: I am not so sure I would like this engine at 12:1 at WOT. I would be a little nervous. I drive this thing sometimes 8-9 hours to a show, so getting good fuel is always a good guess (reason for adding water/meth too). I don't have a timing curve - it is locked at 34 presently and I pull 1 degree per pound of boost, so at 8 pounds, total timing is/should be 26. Plugs have been looking good.
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Old 04-08-2013, 09:13 PM
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I run the btm to provide the margin for detonation protection. It was designed to work with the 93 octane pump gas at 2 degree per pound. Looks like you found a mix thing for custom setting. Good for you.

When I tune, it starts rich when tuning and do timing first. Get the timing right and lean it to best power without detonating. Give it what it wants to get the job done, let the numbers fall where they fall. No manifold vacuum for the advance pod, go ported on FI, if at all.

When it is right it is right. You don't need to touch it again. Only wait for something to break.

Last edited by spinn; 04-08-2013 at 09:31 PM.
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Old 04-08-2013, 10:00 PM
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I am glad it responded well for you.

You are so right sometimes the smallest change in adjustment makes all the difference.
(especially on how the carbs idle circuit works)

Thanks... for the thanks....
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Old 04-08-2013, 10:08 PM
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33 the thanks I posted to you is for making FB a new and much improved FB
inside joke kind of...sorry FB for poking fun at you,,,
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Old 04-08-2013, 10:55 PM
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Yes, thanks are in order. Good post with good feedback and ended up with good results.
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Old 04-08-2013, 10:55 PM
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Quote:
Originally Posted by vinniekq2 View Post
33 the thanks I posted to you is for making FB a new and much improved FB
inside joke kind of...sorry FB for poking fun at you,,,
Thats ok.. When guys like this have success with what I suggest to solve their tuning problems , It makes it all worth while.
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