yes

 

Cool, time to get out the timing light and vacuum gauge again...

 

Vacuum is a good indicator of engine efficiency, meaning the engines ability to draw air into the cylinders on the intake stroke.

If the ignition timing is off then the combustion process is either too late or too early. Both will leave pressure in the cylinder that is not evacuated fully on the exhaust stroke. That residual pressure means less air will be drawn in on the next intake stroke.

 

Hmmm, I know of engines that have very low manifold vacuum but VE is over 100%...
Manifold vacuum is just an indication of how tightly the throttle blades are closed, atmospheric pressure vs. manifold depresssion.

 

Hmmm, I know of engines that have very low manifold vacuum but VE is over 100%...

How is that possible at idle? The reason it's idling is because the VE is so bad (as regulated by the throttle) that it can't run any faster.

David

 

Hmm, does Joe Sherman linger in general tech? I'm currious on his take considering the varried answers. I know the cam plays a big roll, bit it makes sense that excess residual pressure would cause low vac numbers too...except that ignition timing and cam timing are somewhat seperate entities and the cam profile is what really determines when and how the cylinder fills and evacuates.

 

Hmm, does Joe Sherman linger in general tech? I'm currious on his take considering the varried answers. I know the cam plays a big roll, bit it makes sense that excess residual pressure would cause low vac numbers too...except that ignition timing and cam timing are somewhat seperate entities and the cam profile is what really determines when and how the cylinder fills and evacuates.

"varied answers" you got one correct answer for (timing affect on vacuum) for question .

different subject on What Makes Vacuum . lot of things can vary the vacuum reading .

 

from a stock engine to the highest n/a race engine ... it is a simple ..Yes.

sorry thats all there is to it .

 

Advancing timing CAN increase vacuum at idle and part throttle low engine speed.

Does it always increase vacuum @ idle? No.

The question was generic. The answers I've seen...some of them are too specific in relation to the original question.

The VE does have an effect on vacuum however, you can't go by that totally. Some engines are over 100% VE at wide open throttle but not very many of those have much, if any, vacuum at idle/low speeds. Mine is over 100% VE, as many of the little SBF's I've built over the years have been, but idle vacuum is pretty sorry. Current 438" only carries 4-6"/Hg at idle (1400 RPM).

 

question on base timing at idle , then replys to WOT . OK

what if the engine is check on Monday vs Friday

 

Well DanH...It may not be the "yes" You provided, which was helpful, but i appriciate the elaboration, as I'm trying to sponge as much from you all as i can.I dont know I'd your typing skills limit your responses or if you just know so much you've forgotten that learners are searching for the long answer sometimes. You all helped me, I'm thankful for that. ;-)

 

Nope. More efficiency and more VE (capability) will cause more vacuum from the same carb. If it was less, you'd never need a bigger carb as you made more power.. This is one way to verify effects at the track by watching your vac level and comparing to previous runs. It's a good tool as it helps you compare changes even if you had a bad run, missed shift, lifted, or whatever might pooch your MPH comparisons.

David

PS: unless we're crossing wires again on more/less/greater/less/increased/decreased/higher/lower vacuum. That's why I will stick with standardized manifold pressure - no vacuum confusion, and no change in units on the same scale..

 

Nope. More efficiency and more VE (capability) will cause more vacuum from the same carb. If it was less, you'd never need a bigger carb as you made more power.. This is one way to verify effects at the track by watching your vac level and comparing to previous runs. It's a good tool as it helps you compare changes even if you had a bad run, missed shift, lifted, or whatever might pooch your MPH comparisons.

David

PS: unless we're crossing wires again on more/less/greater/less/increased/decreased/higher/lower vacuum. That's why I will stick with standardized manifold pressure - no vacuum confusion, and no change in units on the same scale..

not crossing wires .
what makes pressure in a ... say a fuel system ? (positive pressure from a pump)
negiteve (vacuum) pressure , the same thing must happen
add .less manifold vacuum equals higher manifold pressure

 

I get what David said by increasing the efficiency of the heads and intake etc, that all other things that the vacuum would increase. It makes sense to me.

But What I don't get is why your cam profile only gives you the 6 inhg of vacuum. I mean that is the vacuum I'd expect in a Radical motor with a big overlap and a thumping loping idle.

What engine have you built that only gives you 6" Vacc?

 

Are you asking me? Or was that a general question? If your asking me, ill have to copy paste the build specs from another post when i get to a computer, its not easy to do from my phone...

 

Dan - a diesel has the same effect. Although there is no vacuum in a traditional diesel and it's 'throttled' by the amount of fuel, the effect is exactly the same - just in fuel instead of vacuum. So, in a diesel idling at 800 RPM, if we increase the engine efficiency, the RPMs will rise. To idle again at 800 RPM will show a reduction in fuel due to the efficiency increase, rather than a reduction of manifold pressure as seen with a gasoline engine. Same thing, same effect, different medium.
:tup:
David

 

Dan - a diesel has the same effect. Although there is no vacuum in a traditional diesel and it's 'throttled' by the amount of fuel, the effect is exactly the same - just in fuel instead of vacuum. So, in a diesel idling at 800 RPM, if we increase the engine efficiency, the RPMs will rise. To idle again at 800 RPM will show a reduction in fuel due to the efficiency increase, rather than a reduction of manifold pressure as seen with a gasoline engine. Same thing, same effect, different medium.
:tup:
David

David don't use the diesel for example . think of what your saying .

hint , diesel anywhere needing more air from the ports/manifold at idle.

remember , engines are a pump , both pressure and vacuum

 

I like you guys. I'm missing something that I clearly know the answer to, and it seems you know I know, you know? LOL, when I have a chance, I'm going to re read this whole thread and then head to my chero with the timing light, vac gauge and hand tools. I'll try to document the process with some pics and videos so i can share if your interested that is. Might be a few weeks as I'm working two jobs and my wife is rather partial to my one day of,Sunday. I'll let yall know!

 

Dan and David are two of my favorite persons here on FM to read. Dan because it's easy to understand and written plainly without elaboration, and David becasue he don't miss a beat in a text book style explanation.

Scr, what is the approximate HP of your engine and what's your valve overlap specs on your cam?

 

When one assumes himself to be devoid of interest, he may simply be fooling himself into delusion by disallowing the context of his thoughts to permeate his frontal lobes.

 

But if one is able to disallow ones self the permiation of context while believing themself to be devoid of interest, one may not be delusional, but simply a bore, who's interest are limited to pondering the depth of ones lack of interest in seemingly interesting topics.

That said, tdc overlap is 12 degrees.hyd flat tappet. 108 lsa 228@.050 .544 lift. 408 (m) cubes, 9.75 comp ratio hooker comp headers, edelbrock performer rpm heads, performer rpm airgap intake, 750, edelbrock carb, 1 inch open spacer, adjustable roller rockers.I'm hoping for 400 plus HP to the wheels. I'm faster than cars that are capable of low 13 to very high12 second Quater times, I'm shooting for mid 12's myself when the trans is set up.

 

You are killin me with you Fill-o-soff-y! LOL

Um.... You cannot have 12* of valve overlap at TDC...

But having 12* overlap in your cam and your 400-ish HP, I'm thinking that you can have as much as 14-16" of vacuum at idle. That's not a lot of overlap and I'm thinking it would cause only slight gauge needle bouncing.

What do Dan and David have to say about that?

 

You are killin me with you Fill-o-soff-y! LOL

Um.... You cannot have 12* of valve overlap at TDC...

But having 12* overlap in your cam and your 400-ish HP, I'm thinking that you can have as much as 14-16" of vacuum at idle. That's not a lot of overlap and I'm thinking it would cause only slight gauge needle bouncing.

What do Dan and David have to say about that?

;-)

When i called the cam MFG they seemed to think i woldnt seen much more than 6-8 inhg...

 

Well I'm telling you that I (who don't know nuthin) think you can get MORE than 6-8!

And if your exhaust is seated at 2* ATDC then there shouldn't be too much reason why you can't have more vacuum at idle in my opinion.

Someone please splain to me why he can't get more than 6 or 8" of vacuum at idle. This is making me hiccup!

I think carb or vacuum or timing issue.

 

I ave been doing NOTHING but talking about vacuum and timing....

 

@.050 on my cam.

 

Just cause I can't help myself, I'll say that if you use a vacuum gauge to set your initial timing then adjust your system to be all in when you want it all in, I think all the rest will fall into place, and I think you'll get 10" or better of vacuum at idle.

 

WOW, I almost hate to jump into this thread! Let me start off by saying that I've never had an engine with a lumpy cam like yours, so there may be things that are different there I guess.

It seems to me that if vacuum is going up when the throttle is opened up, that that does indicate richness at idle. The increased airflow creates a better mixture, which causes more vacuum. Vacuum should be at a maximum at idle.

Don't shoot me for asking - but your vacuum gauge isn't hooked to ported vacuum is it?

From my experience, if you set initial advance by a vacuum gauge, you end up too far advanced. At idle with no load, an engine can handle all sort of advance that would never work under load.

 

From my experience, if you set initial advance by a vacuum gauge, you end up too far advanced. At idle with no load, an engine can handle all sort of advance that would never work under load.

Good stuff (seriously), but this last part is assuming the use of a stock curve and limit. That's not what we are shooting for here. All of the timing will end-up custom to fit the specific engine, from idle to WOT to cruise, so it will be most efficient at every rpm and every load.

David

 

Mmmmkay.... I'm missing something then. If the neutral idle timing is set to max efficiency you would see best vac and therefore best peak cylinder pressure angle. To continue the process, the in-gear (if auto) idle or loaded idle (manual clutch slipping) timing is done the same way. You are establishing your timing plot. The curve (graphically a timing table) will continue to be plotted for all rpms and loads from there for max efficiency as well.

I'm not sure how 'too advanced' ties into this scheme, as the point is to be properly advanced. Tell me what you're thinking or what you have observed so I can get your angle on this.

David

 

he said , could be advance more than what would be acceptable with a load . thats for vacuum gage timing .

locked out dist. are not a great idea for pump gas , street use . same idea

 

That's what I read into it also, but that is the opposite of what I am saying we're after. There is a conflict of concept here. Let me give an example of an engine that would have the timing tuned the way I am describing.

You've set your initial idle mixture and RPMs approximately to begin the tuning process and want to move on to tuning your ignition. We need to know the extremes of the timing table to establish our timing range, so by testing, we find the WOT timing at peak HP - say 34°. To find our mechanical limit of range, we disconnect vac advance, load the engine heavy at low/idle RPMs and vac test (or dyno) again, turning the distributor to juggle timing against vac and and mixture for best timing - say 14°. This 14-34° range (20°) is our mechanical. We set our mechanical range at 20°, reset total timing to 34°, and it should be idling at 14°.

Now we test for timing at neutral idle by best vacuum, juggling timing, mixture and re-setting RPMs as we go - say 22°. The degrees and noted pressure difference from our loaded 14° to our neutral 22° (8°) is our idle vacuum advance, and rate. Reset the total timing to 34° again. Choose or adjust your vac canister for 8° from the loaded idle vac reading to the neutral vac reading. Plug your vacuum canister into manifold vac.

Bingo - first level done. The effect is that the engine will neutral idle at 22° (perfect), and when placed into gear the manifold pressure will rise, decreasing advance to perhaps 18°. As soon as your foot begins to move the throttle, the pressure rises instantly with the load, and timing drops to 14°. Perfect again. From here the remaining RPM and load areas and final curve are established with more tuning using both mechanical rate and vacuum advance along with carb adjustments in appropriate areas to create the final product.

While there naturally are compromises with a mechanical system (compared to infinite electronic timing control) - such as vac advance at idle and vac advance at highway cruise - the concept is still to provide the perfect timing at all loads and RPMs. All. If it were a drag car, we could lock it out at 34° and be fine, as we don't care about off-idle timing for pump gas, highway mileage, increased cruise heat, long-term valve seat recession, etc. Likewise, a true road-racing engine could get away with just mechanical timing for the same reasons. Street timing is the most complex of all - never too much or too little - for peak efficiency, power, performance, response, longevity and mileage under all conditions, all the time.

Does that make more sense to everybody?

David

 

Do any of your adjustments effect the engine idle at all? Or am I reading too far into your post?

 

Do any of your adjustments effect the engine idle at all? Or am I reading too far into your post?

Very little if at all. I only had about 5 minutes to screw with it before i open up the thread again, so i did not have time to search for vacuum leaks. I simply wanted to test the theory. The only thing that seems to change the vacuum is the engine speed. I didnt have the tach with me to see what the idle speed and "off idle" speed was though. Sorry, i know...

Im moving this month, which means driving the chero 40 miles one way, which has never been a problem. Ill have to make time to check it all out though so i can have an otherwise uneventful buzz up I-5, HW18, hw410, and lake tapps parkway...Its made the trip several times, but now im freaking myself out that something is wrong that needs to be corrected before a longer drive that has several long hills...After that, ill be back on the performance search.

 

Hmmmm.... I'm missing something....

If your AF screws effect idle only very little, then the carburetor is not performing as it's intended to. Once you CAN effect the idle with them, it will ensure that any future adjustments are going to be more predictable in what they do to your engine.

 

Hmmmm.... I'm missing something....

If your AF screws effect idle only very little, then the carburetor is not performing as it's intended to. Once you CAN effect the idle with them, it will ensure that any future adjustments are going to be more predictable in what they do to your engine.

I missunderstood your question. I thought you were asking if they affected idle vacuum at all. They distinctly affect idle quality and speed, but do not seem to affect idle vacuum.

Also, someone asked if i was testing at the ported vacuum. I did once, but realized it quickly, and switched to the direct vacuum port, with the results i've been discussing here. Fair question though

 

Thanks PSIG! There's even more to the picture than I initially thought...You should come tune my ranchero for me and teach me a few things I got to the rig last night! I improved the vacuum! I made some videos! As soon as i figure out how to post them, i will. Improved to 13-14 inhg at 850 rpm and 15-18 degrees of initial timing.

 

you got the max timing set also . what is it set at ? all in at what rpm ?

 

Cool. :tup: Now you've proven the theory, but as NBalcom pointed-out with over-advance off-idle, it's useless if you don't reconfigure the distributor and curve to make use of it. That was the point of my last post.

There are some FMers around here that want to do various FM Project Days, and tuning may be one to consider next winter when it's too cold to wrench but fine for indoor tuning. Perhaps the morning covering concepts and some manual tuning techniques, and the afternoon at the chassis dyno to do an initial drag tune, a road-race tune, and finally a full street tune, using the dyno to speed the remaining process. Cap it off with pizza and beer and quick coverage of how to tweak the tune using on-pavement results. Whatcha think?

David

 

I would happily supply my time, vehicle and all the beer if we could use my rig as the test baby I've also got two digital cameras, a dvd camcorder, and a smart phone that records hd video,so we could document the process.