mopowers
Well-Known Member
Oh, mine has adjustable air bleeds. It just did come with adjustable idle feed restrictors or PVCRs or transfer slot restrictors.
Holley Transition Circuit Tuning
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mopowers
Well-Known Member
That's right. Thanks. I always get those backwards in my mind.
mopowers
Well-Known Member
Took the turd for a little test run. It's getting a lot better. It's still a point or so lean on the transition circuit though (low speed, low throttle position). Back to my original question. If I put larger TSRs that the slots themselves, will it even do anything? The slots themselves are equivalent to a .078" orifice, so if I put .086s in the TSRs, will I even see a difference?
Reason I ask is I'm at the limit for 8-32 hole size, so I need to order up some 10-32s to go bigger than .078".
I realize I need to get the idle and transition circuits figured out first, but once it's on the main circuit, it's great (according to the W/B gauge and my seat of the pants meter). Guess that means I'll likely have to go down on the main jets once I get the TSRs richened up. Also, I can tell I'll need a higher power valve. It's got a 8.5 currently, so I'll give a 10.5 a try eventually. The PVCRs will need to go down in size too. I decided to try .056s. I'll probably go to .048s when I get there.
Reason I ask is I'm at the limit for 8-32 hole size, so I need to order up some 10-32s to go bigger than .078".
I realize I need to get the idle and transition circuits figured out first, but once it's on the main circuit, it's great (according to the W/B gauge and my seat of the pants meter). Guess that means I'll likely have to go down on the main jets once I get the TSRs richened up. Also, I can tell I'll need a higher power valve. It's got a 8.5 currently, so I'll give a 10.5 a try eventually. The PVCRs will need to go down in size too. I decided to try .056s. I'll probably go to .048s when I get there.
TT5.9mag
Two atmospheres are better than one
Now you’re making me think and possibly change my opinion. If the slot is subject to pressure drop in the Venturi and the TSR is at atmospheric then the relationship of flow to size is not 1:1. Make sense? My head hurts.
mopowers
Well-Known Member
That does make sense and it seems pretty logical. I plan on ordering some 10-32s and testing that theory out. I appreciate the help.
Mattax
Just the facts, ma'am
Idle circuit = Low speed Circuit.
Test out the idle characteristics yet?
Somethin gon the order of this.
Fuel level of course, then find the number of tturns of the idle stop screw needed to reveal .10, .020 , .030, .040 of the slot. Put that in your notebook so you dont have to remoe th ecarb to know how of the t-slot is revealed.
Warm up the engine and set the t-slots at .025 and see what you can do with the idle mix screws.
If the engine is dying, you can try more throttle opening and later will see about working back toward .025 There's nothing magic about .025, its just a starting point. We know the somewhere around .040" will be too much exposed at idle and close to zero exposed will be too little.
Idle rpm can be adjusted using timing and/or air (through PCV, or 'idle-eze', or checking the secondaries or drilling the primary throttle plates).
Once its warmed up, see if you can get the t-slots to .025 or at least .035. Then run the idle mixscrews in and see if reducing the idle port fuel allows the idle AFR to be tuned. Set the idle mix for the best lean (running in 'til it drops or starts to falter and then richen an 1/8 or 1/4 turn)
If its an automatic, put it D and see if the rpms stay close to where they were in N.
You can try to change one thing and repeat, and with an automagic you can get a sense of what each of these changes have on idle in gear. When you've had enough of that for a while, then try a very slow accelleration.
I'm going to guess that this engine has so-so manifold vacuum at idle, and so off idle manifold vacuum increases resulting in a much stronger signal on the idle circuit. Off idle is IFR and IAB along with the t-slot. So work the IAB and the IFR along with the initial throttle position to improve the off-idle. Use the idle port to get the fuel mix at dead idle. Try bleeding a little more air in if the t-slot exposure is over .035. Try using a bit more initial timing if the rpms are too low and so forth.
Mattax
Just the facts, ma'am
Base the main jets on high speed steady state. 60+mph go as lean as the engine will tolerate.
Or what I prefer 'cause I'm chicken is set the jets for based on best time in quarter or Hp on the dyno.
Then set the primary jets for cruising. If it wants leaner for cruising, then richen the PVCR so WOT will be back where it was for best time.
Check the AFR curve in quarter or on the dyno. Is the AFR flat? If not correct with the HSAB. Trick will be figuring out whether primary or secondary sides are the ones to change.
When thats all done, see if the transition from low speed to high speed circuits needs some trimming with transfer slot restrictions.
Or what I prefer 'cause I'm chicken is set the jets for based on best time in quarter or Hp on the dyno.
Then set the primary jets for cruising. If it wants leaner for cruising, then richen the PVCR so WOT will be back where it was for best time.
Check the AFR curve in quarter or on the dyno. Is the AFR flat? If not correct with the HSAB. Trick will be figuring out whether primary or secondary sides are the ones to change.
When thats all done, see if the transition from low speed to high speed circuits needs some trimming with transfer slot restrictions.
mopowers
Well-Known Member
This is a lot to take in, but I'll try to reply to everything.
Fuel level is just under midway on the sight windows with the engine running and pressure is regulated at 6.5 psi.
Primary transition slots are almost dead square at the preferred idle speed - so I'd say right at .030" give or take a few thou. which puts in-gear RPM at 750-800 at operating temp. With this configuration, small turns in the mixture screws do change AFR quite a bit. To get low-mid 13's at hot idle in gear, mixture screws are right at 3/4 - 7/8 turns out from bottomed out.
Highest vacuum in gear at idle I can get is about 12 in-hg which puts cruise vacuum at 17-18" in-hg.
Right now, it goes lean off idle and slow cruise. Since it has the original sized IFRs (.027"), I'm going to throw in some .030"s in to see if it improves while I wait for more set screws to arrive in the mail. I suspect with that change require the mixture screws to be in quite a bit to maintain a clean idle, so it may require larger IABs (70s in it now).
Timing is where it's been for years. It wants a lot of initial timing (26*) and total is at 34*.
At cruise (60-ish mph), AFR is right around 14, which seems good for E10. PVCRs definitely need to be smaller and open sooner. When I mash it from a 60 mph cruise, it goes lean very briefly until the PV (8.5) opens, then really fat. This is with 68/76 jets and .056" PVCRs.
I appreciate the input.
When you say it goes lean, if you are using an AFR, don't sweat it so much. Going lean and pulling back is fine. Put a larger squirter on it and possibly change the pump cam to a more aggressive profile if the lean condition is problematic when mashing the throttle.
I've run my stuff at 15.5 cruise afr and higher. As long as the plugs are good and the engine isn't surging, tying up, overheating, etc. lean is mean...
I've run my stuff at 15.5 cruise afr and higher. As long as the plugs are good and the engine isn't surging, tying up, overheating, etc. lean is mean...
Mattax
Just the facts, ma'am
Some one hasn't said it yet??
The AFR number is just a number. Don't aim for AFR numbers. They are just a reference that can be helpful in seeing what a change is doing.
Yea sure the FSM in the very end of the 60s is concerned with AFR at idle because of the need to reduce CO - hence the lean idle mixes targetting 13.9 or 14.1 or whatever it says.
Thats a huge amount of initial timing unless the engine is something like very high overlap and somewhat lower compression. Over 240 at .050? With a cam like that 750-800 in gear is pretty good in my opinion, and with 12"Hg thats about where my engine is with roughly 230 degrees at .050. Except its timing is around 17 to 18*
What is the idle rpm in Park or N?
IFR of .027 and .070 IAB isn't crazy
Going from .027 to .030 should be a noticible change.
The AFR number is just a number. Don't aim for AFR numbers. They are just a reference that can be helpful in seeing what a change is doing.
Yea sure the FSM in the very end of the 60s is concerned with AFR at idle because of the need to reduce CO - hence the lean idle mixes targetting 13.9 or 14.1 or whatever it says.
Thats a huge amount of initial timing unless the engine is something like very high overlap and somewhat lower compression. Over 240 at .050? With a cam like that 750-800 in gear is pretty good in my opinion, and with 12"Hg thats about where my engine is with roughly 230 degrees at .050. Except its timing is around 17 to 18*
What is the idle rpm in Park or N?
IFR of .027 and .070 IAB isn't crazy
Going from .027 to .030 should be a noticible change.
mopowers
Well-Known Member
Thanks! I hear ya. At slow cruise (barely off idle), AFRs were in the mid 16's and more importantly, the 340 wasn't liking it at all. It was not running well. On the power valve, it had a very slight lean bog when I leaned into it. Maybe that will go away once I sort out the idle circuit. But yeah, I've got plenty of pump cams and squirters to play with too. Right now, It's got a red cam and 31 squirter.
I'm definitely not aiming for an AFR in particular. I'm using those numbers just to see if the changes made are doing anything and where, and to confirm what I'm hearing,seeing, and feeling in the way it's running.
The engine definitely wants that much initial advance. Compression ratio is 9.6:1 and cam is 231@.050, .561" on a 108. RPM in P or N is maybe 25-50 higher. There's barely any rpm drop in gear. It surprised me too when I played with the timing, but anything lower than 20* and it was not as happy.
Thanks again for your help. I appreciate it.
Are you using an O2 sensor? If you are, ignore the numbers and make it drive like it should. Then look at the numbers.
You can get down a rabbit hole really quickly.
mopowers
Well-Known Member
It rips now. I swapped out the .027" IFRs with 31s, played with the IABs a bit (I think I ended up with 73s) swapped the 8.5 PV with a 10.5 PV and ended up with 68/75 main jets and .046 PVCRs. It's a happy little cruiser now.
TT5.9mag
Two atmospheres are better than one
How’s the off idle lean condition? Were you able to richen it up without throwing a bunch of squirter at it? And how much more control of the idle mixture screws do you have now with the larger IAB?
mopowers
Well-Known Member
Off idle is good now. Slow cruise doesn't have the lean surge (if that's what it's called) it had before. The larger IFRs seemed to have worked well. Squirters are 35 primary, 31 secondary with red cam in primary and pink in secondary positions. I may try the original 31 on the primary side and see if it likes it, but it seems to like what's in it now.
Mixture screws ended up at about 5/8 - 3/4 turn out from bottomed and seem to have the same amount of control over the idle mixture. I'm liking how it's running though. There is zero rpm drop at idle going from P/N into gear and fires right off when starting.
mopowers
Well-Known Member
I have a related question, so I figured I'd toss it in here.
When would you restrict the t-slot feed vs idle feed? In other words, what's the difference between a t-slot restrictor and an IFR? The was I understand it, the IFR restricts the fuel to the idle mixture channel in addition to the transfer slot, but if you needed a leaner transfer circuit, when would it be advantageous to reduce the t-slot restrictor vs going to a smaller IFR?
When would you restrict the t-slot feed vs idle feed? In other words, what's the difference between a t-slot restrictor and an IFR? The was I understand it, the IFR restricts the fuel to the idle mixture channel in addition to the transfer slot, but if you needed a leaner transfer circuit, when would it be advantageous to reduce the t-slot restrictor vs going to a smaller IFR?
If you need a leaner transfer slot, you reduce the IFR. The amount of air coming from the T-slot does not change, but fuel is reduced=leaner. The idle mixture screws will likely have to be backed out slightly because the idle cct is also leaner.
This is true IF the T slots are Holley OEM width and length.
Almost every single throttle plate that isnt Holley has the T slots too wide and too long.
To clean that up you can’t just reduce the IFR.
In that case you install T slot restricters.
mopowers needs to measure his and see what he has.
If it’s an aftermarket throttle plate they are probably a bit off. If it’s a billet plate it’s probably off a good bit.
mopowers
Well-Known Member
Mine's running great. I was just curious and couldn't find the answer after doing an online search.
Under what condition would one trim the T-slot restrictors vs. the IFR? Seems like a t-slot that's too wide and/or long would make for a fat low speed cruise. Wouldn't trimming the IFR's address that?
The question becomes how far can you trim the IFR and not get into idle issues.
The T slot as it came from Holley before the era of massive emulsion and other trickeration was narrower and shorter than what we get today.
The length and width of that slot controls how much fuel gets through it and for how long it’s adding fuel.
You end up with too much fuel (width) and you get that fuel too long (length) with they way it’s being done now.
At this point I take a quick measurement of the T slot to make sure it’s not old Holley dimensions and then I install restricters.
To tune them, I look at what the transition is like, and what the engine “feels” like at a cruise.
When you are on the T slot and not yet on the booster if it’s fat I pinch down the T slot restricters until I get it a bit in the lean side.
Then I go about .003-.004 bigger and leave it.
You can be too big on the IFR so that needs to be corrected first.
And the IFR should be in the bottom of the metering block and not the top.
Lowering the IFR should be the first modification done to a Holley or clone.
mopowers
Well-Known Member
I think I'm getting it. It sounds like you tune the IFR for a decent idle and as close to a clean transition as possible and if it still 'feels' rich at that point (for instance), that's when you trim the t-slot restrictors? Is that pretty much it?
I'm just wondering how would a normal person know when to reduce (or enlarge) the t-slot restrictor instead of the IFR? I realize if the IFRs are too big, the mixture screws may need to be screwed in too far to get a clean idle. I'd imagine the oposite is true as well? Does that help determine when the t-slot restrictor size is changed?
It's really hard to find information on transfer slot restrictor tuning online. Do you know of a decent source to find information on the topic?
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