ฝัง
- เผยแพร่เมื่อ 11 ต.ค. 2024
- In this episode I decide to give my 1975 L48 350 SBC full manifold vacuum advance at idle to see if it improves it's efficiency and performance over the OEM spec'ed ported vacuum. In a previous episode I experimented with various mechanical advance springs to get the most optimum setup. This is a continuation of my quest to squeeze as much performance out of this mostly stock 75 350 SBC power plant. You can see the spring mods here: • 75 C3 Corvette HEI mec...
Thanks for watching.
email: volfandt1@yahoo.com
Great video and good review at the end. I have spent the last several months tuning my street '65 BBC. I have settled on 22* base timing but use the ported vac to add another 12* from 1500-3000 rpm to improve mid-range. Total mechanical at 36* at 3000 ROM and total timing at 48*. I am running 89 octane E-zero. No ping, good power. Manifold vac at 800 rpm idle is 16-17"
Altho they're mass produced, every engine has it's own individual sweet spot where it's hitting on all cylinders and is as smooth as a Singer sewing machine. Factory tuneup specs are functional but not optimal. Our reward for searching for and finally getting that optimal performance sweet spot is worth the effort in the end.
Thanks for watching and your great info, Carry On.
👍
Wow, I've been deep diving on this subject. Same here with my 454. Almost the same deg curve I've found works the best for my setup.
Love this video ❤️... Best Ported vs. Full Manifold video on TH-cam.
I'm currently running PORTED/TIMED vacuum at 12° BTDC and 36° total, and at a steady 21" of vacuum...runs like 💩, no power really and gas mileage sucks...using 91 octane in my 83-84 305 L69 SBC.
I've ran FULL MANIFOLD VACUUM before (on 87 octane) and got the same numbers as you did and like you, I assumed it was too much, but man she ran.
I love how you adjusted and showed the real time difference of vacuum and timing between PORTED/TIMED VACUUM and FULL MANIFOLD VACUUM.
I'm also having off idle bog and hesitation at take off. I thought maybe my Holley accelerator pump was misadjusted or my squirter nozzle was too small, but now after adjusting those things, and the pump cam, and seeing this video, I will do this routine and see whether or not I need to buy a bigger squirter or not.
How did it affect your miles per gallon from Ported Vacuum ?
Hey Tynan , thanks for the kind words. Everything improved on my 350 by going to full manifold vacuum. My MPG improved to nearly 17 mpg from 15. Ofcourse it'll drop when I can't keep my foot out of it, lol. And yes, make mote of your settings before for just in case but odds are you'll see improvement like I did. Probably do away with your bog too.
I just timed my Chevy 350 aswell with a new vacuum canister and I switched to manifold vacuum from ported aswell. My engine seemed to like lots of timing at idle... With ported (therefor no vac. adv. at idle) I sat around 25 degs advance, with total advance at 37 at 2500rpm. Now with manifold it sits at just over 30 degs at idle with total advance still at 37. Keep in mind that the vac. adv. essentially drops to 0 when you open throttle, therefor at 2500 RPM you only get initial + mechanical advance to get your total timing (so for me it was 25 initial + 12 mech adv. to get 37 total). At first I was shocked to see it so damn high at idle, yet when I changed the timing back below 28ish degs, it runs like crap. Therefor I just give the engine what it wants, not what I want.
It makes sense when you think about how lean the mixture is at idle or cruising speeds, therefor needing more advance to burn efficiently. And when throttle is opened, vacuum drops and timing is retarded for the richer mixture. The mechanical adv takes care of the timing needed in relation to the RPM while the vacuum adv. changes timing in relation to the amout of engine load.
Welcome to the new tune era, LOL. I cut my teeth on ported vacuum at idle and since thats how they came from the factory it just had to be right, right? We'll the 70's was the era of the EPA taking over the auto industry and inorder to make the new standards they detuned those muscle car engines from the 60's and walked that fine line between performance and compliance and compliance won...In any case full manifold vacuum at idle sure woke my L48 up. You might try the Moroso spring kit to get a little more quickness.
Thanks for watching Duddy Herold.
The problem I had with my '71 350 was a rough idle and bad cold engine idle running ported vacuum advance. To get it to idle properly I had to set initial timing to around 23 degrees btdc. I switched to full manifold advance. This allowed me to set initial timing to 10 degrees (with vacuum disconnected). When manifold advance is reconnected timing advances to around 24 degrees. I now have a smooth idle and better performance. This was a great video explaining the various advance methods. Thanks.
It's amazing how well the engine responds now. As I said it's against all I was taught but then again I was taught ICE & auto mechanics back when the smog controls got going in earnest and thats what they wanted us to learn.
Thanks for watching James.
Had the same experience with my 400 Pontiac. At 10° was real lumpy, no vacuum, poor signals to carb, idle surge coming to a light. All of it. Read papers by Lars Grimsrud. Installed 15° can set to come in a early, plugged it into manifold vacuum and that 10.66/1 400 with the XE274 cam idles like stock once warmed up. Runs waaay cooler and gets 13.5 mpg romping around town. Idles like a champ at 25°. Cruises at 49° and romps back to 34° at WOT and blows the tires off shifting to third at 90 mph with no detonation on 93 octane. Ever. Unreal! It’s like an entirely different car. Very well behaved and street drivable.
On my 1985 454 build I got a little lumpier cam than I really have wanted or expected low vacuum poor brakes. So I set the initial timing up to 10 hooked to manifold vacuum that gave me about 20BTDC at idle I got good brakes the thing runs great.
Hi Dave. I adjusted my General Motors Holden 308 V8 here in Australia to 15 degrees initial, 15 degrees advance now at manifold vacuum and 35 degrees mechanical. At WOT total is 50 degrees at 3,000 rpm. Car idles and accelerates better and runs cooler. I use 98 octane and no sign of pinging
Great video
Hey Luis. Sounds good. Going to full manifold vacuum @ idle on my 350 really woke it up and I haven't had any negative repercussions since. Everything has improved. Sounds like you're seeing the same on your 308.
Have a good 'un and take care Down Under.
Dave
My Guy! Simply done 🎯💪🏾👍🏾
👍
Wolf-man: I know this is a 2 year old vid and only 186 thumbs up? WTH. It shows how little core knowledge most have of manifold vs ported vacuum advance. This is GREAT information budroe. Two questions tho if you're still reading comments. My 46 Chevy PU restomod is SBC 283 powered. Like you I don't care about burn outs . . . (most of the time ;-) Besides small tube headers and the too big Carter AFB carb its a stock engine with TH350 tranny. Vacuum advance on ported. Only had this setup for a few months. Hesitation/bog coming off idle circuit driving me crazy. Going to try manifold vacuum to see if it does anything. 1) No CAT or pump, only PVC funneled back to carb. Anything else to "de-smog"? 2) I'm considering simply switching the vaccum from ported to manifold and doing nothing else just to see if that 283 likes it. Not changing any timing at all. Am I exposing my babied 283 to any possible issues/damage.
Can hardly believe only 186 likes is 2 years. That's how indoctrinated most are today abut ported vacuum advance. KUDOs one old guy to another.
Hey Jay, thanks for the kind words.
Disclaimer -long reply.
1st easy check to make is to look down the throat of the carb with the choke wide open and engine off. Then fully open the throttle while peering down the primary two venturies. What you want to see is a steady shot (double shot in the case of a 4bbl) of gas being squirted down into the carb. This is to verify that your accelerator pump circuit on the carb is working. If it isn't and you know the carb is full of gas then theres your bog. If you do see a good squirt of gas then move on to step 2.
2) Set the maximum mechanical advance timing to 36 degrees BTDC (vacuum disconnected and plugged).
Usually this is around 3k rpm but the rpm could be higher. You'll need an adjustable timing light for this. Or you have the degree tape on the harmonic balance.
With the MAX timing set to this, don't worry about what the idle timing is (it'll be around 12-18 BTDC).
3) At this point you can connect the distributor vacuum back up to ported then test drive it.
If all seems well i.e. no pinging or bogging when accelerating and it cruises steady, you should be GTG.
BUT,
IF you want the best performance and effiency out of that SBC then you'll want to connect the dist to manifold vacuum. IF you do this you'll need to readjust your carbs idle speed down
to 600-650 but otherwise test drive it and see if you can tell the difference.
One thing, you'll also want to recheck your MAX rpm timing (both mechanical & vacuum) and make sure it doesn't exceed 51 degrees BTDC. If it falls anywhere below that you'll be GTG.
IF its at 51 BTDC or higher you'll need to change out the dist vacuum advance to one that limits the vacuum advance to less than 10 degrees. I had to do that to my 350. (Mine now adds 7 degrees advance)
IF per chance your engine pings after all this you may need to back down the MAX mechanical from 36 BTDC to 34 or lower, one step at a time.
36 degrees is generally the magic number for SBC's, SBF's & Mopars but each engine can differ based on wear etc so it's not unusual to have to fine tune.
Good luck
Dave
Hi Dave, funny thing is I have been living and breathing this topic for about a week you will find 80+% think ported vacuum is the way to go, but I’m here to say the more research I do , I am coming over to the manifold vacuum side, I like the idea of having more initial vacuum via the manifold vacuum from off idle, and a better burn off the get go, I’m also playing with how much vacuum advance is there when all in, which when on manifold vacuum you shouldn’t be over advancing because vacuum should fall off under hard acceleration so I’m thinking I could very likely run 36degrees, anyway I just started your video, and I’m sure this time I will get way more out of it, I will report back later😀😀
If 36 BTDC causes pinging you might back it down to 34. I don't know how modified your 350 is but some higher perf SBC's prefer 34 over 36 so trial & error isn't unusual. Another mod you might try is to go with lighter springs in the distributor. Moroso makes a kit with 3 different spring sets and weights. All the springs are lighter than the OEM. I ended up going with the silver springs and it made a noticable difference. The engine spins up faster. Unless the OEM weights are worn I wouldn't replace them, just the springs. I've read where the OEM weights are much better anyways.
Good luck and have some fun my friend.
Ok Dave just finished watching the video for the second time, you did a great job of explaining everything, really good to get your final numbers, these engines love a lot of advance, so now that you have had some hot days do you still like the performance, it should run cooler, and I do have a spring kit for the distributor and a metering rod and step-up spring kit for the Edelbrock carburetor, if I should happen to get to much overall timing I am wondering if I will take care of that with a stop bushing in the distributor or limit my advance in the vacuum advance canister because mine has the adjustment spring, 27 degrees at idle sounds crazy but like we said they love there advanced timing, I am heading back over to the 55 tomorrow so pretty excited to make these changes, talk soon my friend 😀😀
Hey Mike, my temp gauge has never gotten above 200, even on a 90 degree day in bumper to bumper traffic.... So I got one of those infared temp guns and took measurements all over the engine, radiator and associated metal surfaces. It appears my temp guage is accurate enough to trust... Even measuring at the thermostat housing reads close to the gauge. So yes, I'm VERY happy with the performance...
IF your vacuum adv is adjustable I'd adjust it that way. Altho that bushing a tried on my unit did work but I decided to order a new vac adv that limited adv to around 7-8 degrees. I know what you mean about the timing at idle but the funny thing is, it works, LOL...
Let me know how it goes. And like I said in the vid (i think I said it) take readings before so it can be put back "just in case".
@@Volfandt 👍👍ok, yeah going to play with the springs too, thinking one heavy and one light
With low initial timing the emissions they were talking about made the engine heat up faster.
Hey Dave, your right about the ported vacuum on the old Chevy 216 and 235 theres no doubt that distributor is advancing, i'm about 10 minutes into the video, i will have to come back in the morning to watch the rest, just wanted to say hi and have a great evening, catch you in the morning
Same back at ya.
Ur settings are the same as mine. Runs smooth plenty of power.
Great video! Thanks for sharing 😊 I watched every single minute - all observations and conclusions appreciated 👌
Hi Dave took me longer to get back to the video than I thought, looks like you got it dialed in , yes with the cost of everything now a days you can’t afford to burn rubber,lol, good video, talk to you soon
Hey Mike, yes she's dialed in pretty good but some new unrelated problems have cropped up. Unfortunately there'll be no shortage of future Corvette repair vids, LOL.
@@Volfandt Cool Dave i look forward to them
Nice Wheelhorse!
If it works it works. Give your engine the timing it wants. Ported or manifold? When it comes down to it, every combo is different. Glad to see you got your Chevy running good. Great video. Cheers
Vacuum advance is only really there for part throttle economy. Once you open the throttle, there is no difference between ported and and direct manifold vacuum advance. It adds advance when there is low load on the engine, takes it away when there is a heavy load and modulates for loads in between. The only real advantage to manifold vacuum on the advance is full advance at idle. If you have a bit of a cam, it can make for a smother idle. Which ever one you use, your initial timing should be set with no vacuum on the advance. I don't know why you want to limit it at idle because as soon as you open the throttle the manifold vacuum will drop and take away some of the timing. I only limit the vacuum advance if I get spark knock on light acceleration. You can get an adjustable advance unit that has an allen screw inside the vacuum port for adjustment.
With that said, I am running full vacuum on the advance in my hot rod. I've got a Buick nailhead with a little hotter than stock cam. With full advance at idle I get a pretty smooth idle.
Good post. According to the literature I read, you want to limit total vacuum advance to no more than 12 degrees advance. Mine was at 21 before I limited it. It's approx 11 with the limiter.
Thanks for watching Joe.
@@Volfandt Yes, 21 is a lot.
Back in the '70s and early '80s they also had Transmission controlled spark and what that did even on the manual transmissions it wouldn't allow any vacuum advance until you reach top gear.
I can suggest, that you can fatten the idle mixture about a 1/4 to half turn and lower the idle setting to 600rpm with the transmission in drive. If you're noticing it idling high in park like 800rpms, lower the throttle idle setting and richen the idle mixture till there is barely a bump in the idle when putting the transmission in and out of gear. Your mechanical advance can go to 37°. Just make sure you don't get any pinging in the valves.
Great information, I think I’ll give it a try in my’57, simple switches what do I have to loose, thanks Dave.
Hey David. Definitely don't have anything to loose and possibly alot to gain. If you choose to accept this assignment, let us know, LOL.
Great video. I have a chevy small block engine with mild cam. I have set timing for 35 degrees advance and 12 degrees initial timing. But the best vaccum I can get is 14 pounds of mercury on vaccum gauge. Any suggestions would be appreciated. Thanks.
If it's a mild cam it should make more vacuum than 14". Your timing looks right. Try the following:
1) adjust the idle mixture screw(s).
I usually will turn one idle mixture screw IN at a time until the idle gets rough then turn it out where it evens out then move over to the next idle mixture screw(assuming you have a 4 bbl).
You can also do this while monitoring the vacuum gauge, turn the mixture screw(s) out to get the highest amount of vacuum.
If the above doesn't do it, check for vacuum leaks.
Good luck
Dave
14 Inches at idle is not terrible but some cams just won't get you more even if you are set up OK. Follow Dave's advice below to make sure your carb is set up OK. What is your idle RPM and what kind of carb are you running? What is your vac advance adding at idle? 12* base may not be enough
I've heard drag racers professional drag racers say that they can tell when it's detonating by the color of the exhaust with open headers.
Hi there again , what you think so far after 1 year any problem?
No problems. Starts, idles and runs great. I recently took a 40 mile interstate cruise to an area that was bumper to bumper traffic (Vette show in Pigeon Forge TN.) and the Vette performed great.
have a ?.....i gave a 81 with 350 and auto trans... would this help on mine
Absolutely. Mark down your present settings before you change them for just in case. I doubt you'll go back, LOL.
Pre pollution info from the 60`s ......Worth a full read ......
Beaware that many a automatic equipped vehicle distributor that was manifold connected had to the vac line removed and plugged when setting base timing . Often there was a a connected spec as well but a very wide spec so better to remove line and do base idle as its more accurate .
The manifold connect was for more timing at idle . Especially for automatic trans vehicles that dropped the specified idle speed . Extra timing was for idle quality and to help the engine pull upto the convertor stall .
The manual /stick vehicle used ported
Remember ported and non ported act almost the same except for idle
***Emissions
Slowing of advance and retard during deceleration was common
*** short comings of a mechanical distributor and better methods
Original can style vac advance cannot be tuned perfect ,, which is why the ECU spark advance systems were invented . Some were on just b4 the emissions laws and worked well . Unfortunately majority were thrown away by monkey mechanics.
***Modern day are programable ignition boxes that have manifold connection [load ] by MAP sensor . Vacuum advance can be set to whatever u need and when ever Start about 500$ thru to FULL ECU 2000$
Very nice video , what is the safe timing at idel in degree with vacuum advance connected .
Good question. Ideally you want the vaccum advance to add a max of 10 degrees or less. When you set to max mechanical adv to 35/36 degrees BTDC, generally the idle mech advance will fall in around 12 - 18 BTDC. So add in a max vaccum of 10 degrees to get total advance at idle.
Total mech & vac adavnce should never exceed 51 degrees BTDC.
You can add lighter flyweight springs to speed up the mech advance and either add a limiter or change out the vaccum advance unit to one that doesn't exceed 10 degrees advance.
Good luck.
@@Volfandt just to be clear at idel it safe to be between 20-22 degree with vacuum advance hooked
@@ahmadaldahoud1719 Yes... I know it goes against traditional factory settings but it works.. Take a road test with no music so you can hear the engine and put it through it's paces. As long as you don't hear pinging under heavy load you're GTG. If you do hear pinging you can back the advance down until it stops.
As an FYI, my engine doesn't ping at those settings.
You should notice a quicker engine that runs cooler and "may" give better MPG.
@@Volfandt thanks for replying and your time.
Full emission controls camshaft ,, cylinder heads ,, compression,, carburettor ,,inlet manifold ,, distributor advance [mechanical and vacuum ],, exhaust design ,, aircleaner design . The poor old distributor never had a chance .
timing light trouble might be due to faulty plug wires. ask me how i know
Valve pinging. It's called cylinder pinging. You can hear pinging but you can't hear detonation.
Mines is idling up high then it will get low wen I come to a complete stop it will shutt off wya dave
I have the same timing gun and it loves to turn off the blinky blink till i switch it to RPM them it flashes. Dumb cheap tech.