Vacuum Tuning

-
S

spinman_1949

Legandary Member
Legendary Member
Joined
Jun 20, 2012
Messages
1,167
Reaction score
50
Location
San Diego
New Quickfuel carb installed and now starting to do tuning.

Engine

340 X heads ported by Justin AKA Wild and Crazy.

Stock manifold but we are going to be putting on a LD340

Stock exhaust manifolds

Cam is an ISKY 777 650-750 CFM Carb. 10.5:1 compr. RPM-Range (3000-7000) Valve Lift (.480 .480) Valve Lash hot (.018 .018) ADV Duration (280 280) .050 Duration (240 240) LC 108

Carb is SS 680 with vacuum secondaries.


Here is my primary question.

Do you connect PCV valve for intial vacuum testing? The reason I ask is because so far I am seeing 10 to 12 inches at idle, and it bounces quite rapidly. If I block the PCV valve the idle will drop and the vacuum will drop to a fairly steady 10 inches. Is PCV valve supposed to allow pretty much open vacuum? Am I seeing such low vacuum at idle due to cam? If I open the throttle to say 2500 RPM the vacuum jumps way up to 30 inches or so. Quick blip causes a quick drop as expected.

Idle is pretty clean at this point with very mild lope. And exhaulst is no longer such an eye burner. Four corner idle is responding well, however I am gettting increase of vacuum failry deep into settings. Say 3/4 out. So I still may be a bit rich on the idle circuit.

Thx

Spinman

 
In general, a PCV is a controlled vacuum leak. You may need a different pcv to stay more closed at idle.
 
You didnt mention what your initial timing is.Try getting it up to 16/18/20 degree range.
Cams of this size(240*@.050), like idle timing and/or idle air. 10/11 inches vacuum at 900rpm with 18* seems about right. The 30* on decel you mentioned seems pretty high, leading me to believe that youre trying to get it to idle very slow, or the guage is off.IDK Once you get the timing up, the stink seems to go away. Or nearly so.More timing = more vacuum,to a point of course.
In bouncing up/down, that gauge is telling you whats going on in the manifold. Its not lying. That cam is allowing pressure pulses back up.It is after all on a 108*CL, and with 64* overlap. If you installed it with 0* advance, the intake wont be closing until about68*ABDC. Thats pretty far up, so compression could easily,at idle speed, be upsetting your gauge. Your gauge needs a damper/restrictor on it is all.And maybe,a bit more timing/idle speed.
Enjoy the new set-up.
 
AJ,

Thx for the feedback. Actually I am at 22 * initial. Mayby too much. You explained the fluctuation well so I will not concern my self with that. The reason initial may be a bit high is because engine seems a bit rough even above idle until I get to about 1700 RPM. I also need to check my distributer to make sure no advance in coming in until past that point. In a former thread I explained how I made the mechanical advance adjustable. Presently it max's at 7* on the rotor or 14* crank.
 
Every once in a while I come up against something like that:The engine likes a ton of initial, but likes less on tip-in. So it becomes a compromise.
-However, a lot of times its just the A/F ratio.Whats your idle speed? Are you running a PCV system?
-The position of the primary butterfly may not be syncronized to the idle transfer port.This is kinda critical.I usually take the carb off, flip it upside down, and eyeball it.I adjust the curb idle screw such that the idle transfer port is just peaking out on the manifold side.Then I close the secondaries completely.Then put the carb back on.From this point, I set the idle speed by manipulating the initial timing, or the secondary cracking screw, or occasionally, the float level.I leave that curb idle screw alone as long as possible.If I do adjust it I take a careful note of where it is before I move it, so I can get back to it if I need to.
-If the idle transfer port isnt syncronized, you will get tip-in problems.Im assuming your carb has idle feed circuits to the secondary side. If youre not using the PCV system, it gets harder to get enough idle air. That cam likes idle air.At least in my experience.
-As to the timing. I like a little less initial, and like to start bringing it in just off idle to maybe a couple of hundred above.Same goes for the vacuum advance; bring it in as fast as she will take it. It will vastly improve your driveaway.Just make sure that every thing drops out at idle.
-When I was running the 292/509 cam I had to build a 2-stage curve to run 87/E-10.I had to slow it down a bit, beginning around 2500.So it was fast in, then slow out, topping out around 3600.But I still brought a ton in with the can, and as fast as I could. Smoothes her out and makes some good torque.
-Happy tuning
 
crackedback said:
In general, a PCV is a controlled vacuum leak. You may need a different pcv to stay more closed at idle.
Click to expand...

This.... The are different part number P.C.V. valves, for a reason. The lower compression early 70's - 80's engines ,usually ran less vacuum.(The bigger cam, reduces vacuum similar to these engines) Do you have a part number stamped, on the one you are running ? Where I would start, with a good driver in seat (automatic trans) ,looking for peak vacuum in drive, about 900-1000 rpm. Timing at least ,14-18 initial,34-36 total. @3000-3500.(General information, makes my turds wake up)
 
Sir,

I suspect the chart relates to mild hydraulic cammed motors. I did some checking on my old Payback #2 thread and that thread relates to the motor before it was completely rebuilt. Even back then idle vacuum was jumpy. More so of course. That motor was in sad shape. Loose timing chain. Worn rings and valves. The only common denominator is we used the same cam. New lifters and springs of course, but the cam itself was undamaged. So the cam is what is causing the fluctuation. Makes sense. On the exhaust stroke with the overlap the intake is opening as well and that would in effect cause a slight stall in the intake charge to that cylinder which would cause a short drop in vacuum. As RPM's go up the air mass coming down the runners in the intake has increasing velocity which allows it to overcome the exhaust back pressure and of course the exhaust charge is also increasing in velocity which acts as a purging factor. I bet if i installed full length headers the fluctuation would drop even more.
 
Abody,

I think you are on to something. If I buy a PCV valve that is designed for the stock motor, then it would likely expect more vacuum at idle and thus restrict flow too much with the 11 inches I am seeing. That would explain the rich idle for sure. So how do I get a PCV valve that will be correct? I guess I should search for specs?
The one that I pulled from the motor has Made in USA and 2072 stamped on it.
 
-The PCV will not change the engine vacuum in any meaningful way.
-The base timing will make the biggest difference, followed by idle rpm.
-The PCV just makes it easier to get air into the engine, to get the A/F where the engine wants it, instead of resorting to other ways, such as a too-high curb idle screw setting or secondary cracking.Cracking the idle screw brings fuel with the air, and upsets the transfer port sync.and possibly the spark-port sync as well.Cracking the secondary screw may or may not bring the fuel along depending on design. The PCV does not bring fuel.I use it as a tuning tool.
-Your not understanding camshafting very well. The low vacuum reading, and the pulsing of the needle, is caused by the late closing intake valve, allowing some compression pressure at low rpm to turn around and head back into the intake manifold.AS rpm goes up, there is less and less time for this to happen and eventually the vacuum will stabilize at a high point, after which it will slowly begin to fall again.Try it in PARK and you will easily see it.
-Another thing that can happen at idle,if your primary butterfly is too far open is this;That pressure spike coming back from the cylinder can travel past it and past the idle port.Now that port, and maybe the transfer port too, dump fuel anytime it/they see air go by. It/they dont care if that air is going up or down. Then the next cylinder in the firing order pulls its charge which is now artificially enriched. Very hard to tune for that. Thats why with your cam its so important to close the primaries down as far as practical and still have sync with the transfer port.And thats why we introduce air in the secondaries, or the PCV, or in some other way.Like I said, that cam likes idle air.
-You havent told us yet what rpm youre targeting.I suspect it may be too low.If the engine is fresh it may have to be up nearer to 950,or better, until she breaks in some and loosens up.I know that cam sounds great at 700, but sometimes its just not practical.
 
Well it would seem that this subject has many threads on various forums. The results all seem to point to a faily low level of actual data to provide any real help. I did run across this. http://mewagner.com Some good information and of course they want to sell their product. But some logic as well. The wrong PCV valve could cause issues with idle and also offer long term problems. They even offer money back guarantee for 30 day trial. Even gave me an idea. I will take my F valve and drill a hole to install a vacuum bung and use their test to see if the valve opens at idle. I suspect it does.
 
spinman_1949 said:
AJ,

Thx for the feedback. Actually I am at 22 * initial. Mayby too much. You explained the fluctuation well so I will not concern my self with that. The reason initial may be a bit high is because engine seems a bit rough even above idle until I get to about 1700 RPM. I also need to check my distributer to make sure no advance in coming in until past that point. In a former thread I explained how I made the mechanical advance adjustable. Presently it max's at 7* on the rotor or 14* crank.
Click to expand...
Y
This is idle/off idle transion , circuits. How do you ,adjust idle speed & mixture? That affects everything, up to 2500-3000 on a street ride.
 
AJ/FormS said:
-The PCV will not change the engine vacuum in any meaningful way.
-The base timing will make the biggest difference, followed by idle rpm.
-The PCV just makes it easier to get air into the engine, to get the A/F where the engine wants it, instead of resorting to other ways, such as a too-high curb idle screw setting or secondary cracking.Cracking the idle screw brings fuel with the air, and upsets the transfer port sync.and possibly the spark-port sync as well.Cracking the secondary screw may or may not bring the fuel along depending on design. The PCV does not bring fuel.I use it as a tuning tool.
-Your not understanding camshafting very well. The low vacuum reading, and the pulsing of the needle, is caused by the late closing intake valve, allowing some compression pressure at low rpm to turn around and head back into the intake manifold.AS rpm goes up, there is less and less time for this to happen and eventually the vacuum will stabilize at a high point, after which it will slowly begin to fall again.Try it in PARK and you will easily see it.
-Another thing that can happen at idle,if your primary butterfly is too far open is this;That pressure spike coming back from the cylinder can travel past it and past the idle port.Now that port, and maybe the transfer port too, dump fuel anytime it/they see air go by. It/they dont care if that air is going up or down. Then the next cylinder in the firing order pulls its charge which is now artificially enriched. Very hard to tune for that. Thats why with your cam its so important to close the primaries down as far as practical and still have sync with the transfer port.And thats why we introduce air in the secondaries, or the PCV, or in some other way.Like I said, that cam likes idle air.
-You havent told us yet what rpm youre targeting.I suspect it may be too low.If the engine is fresh it may have to be up nearer to 950,or better, until she breaks in some and loosens up.I know that cam sounds great at 700, but sometimes its just not practical.
Click to expand...

x2.
Fix the problem - not the symptom. A single pattern, 240° @ 108, with stock exhaust manifolds was a mistake no matter what the static compression is. They don't flow well enough to stay out of reversion with it, and ports that flow well at low lift will exascerbate that. The only ways I can think of to attempt to overcome it is speed up the idle (cheap and easy), replace the camhsaft with a dual pattern and preferably on something wider than 110 (less coin than headers and exhaust - but might cause detonation issues with high static compression - you never said what you had there), or replace the manifolds with headers. I think #3 would be the most pricey but much better result overall.
 
Moper,

The original build was not done by me. Justin has often mentioned either changing the cam or installing headers. Once I do some more tuning I will take a video at idle. It actually runs pretty good as is. But to your point a big overlap cam and Justins porting has definitely brought out the worse from the stock manifolds.
 
You must log in or register to reply here.
-

Back
Top