Manifold Vacuum Advance what am I not getting?
- Thread starter 340inabbody
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Well Turk.
Describe an engine YOU built that idles best at 12* & would not benefit from more timing at idle, whether by MVA, locked timing etc
Heads, cam, etc give us a parts rundown to all of these enquiring minds the magic numbers of this mystical engine. Now is the time to put or shut up.
Describe an engine YOU built that idles best at 12* & would not benefit from more timing at idle, whether by MVA, locked timing etc
Heads, cam, etc give us a parts rundown to all of these enquiring minds the magic numbers of this mystical engine. Now is the time to put or shut up.
Newbomb Turk
Well-Known Member
I’ve never said I build engines that run on 12 initial. Never said that.
What I’m saying is regardless of what Vizard said, jacking initial timing with MVA is the WRONG way to do it.
As for what GM did or does I do not care.
This started because a guy has an engine that runs like sour owl ****. And what do we know about it?
Well, it’s down in compression and some hero put a big, sloppy and probably a slow lobe as well cam.
That’s a recipe for junk. Can you hook it to MV and make it run? Sure. Is it correct. Not even close.
Even you know why they need a that timing.
It’s because the chamber has so little combustible mixture (plus a giant assload of EGR from the stupid cam selection) that it burns like old people screw.
Slow and incomplete (don’t bother to ask how I know that…my lips are sealed on that) so you have to add all that timing just to get some kind combustion done.
It’s a poor way to fix a hunk of **** engine build.
Unless of course you like engines with horrible low speed response. It just kills power.
And for the record (going off rough numbers because I’m not at home and I haven’t done all the measuring yet..you do measure what you build don’t you??) I’ll be between 12.1 and 12.3:1. If I can it will be 12.3:1 but I’m not sure if I can get that with the head gaskets I need. That’s because some idiot blew the chambers out on these heads and I’m going to live with it until I build the W2 stuff.
Iron heads. Tunnel ram with two 800’s. Real 800’s not some silly Carterbrock ****.
Quench will be .040.
SFT Racer Brown cam. 255/255 .620/.620 105 in at 105 but I may move it ahead two but I doubt it.
That’s it. I suppose 18-20 initial, maybe and maybe 24-26 at peak torque, which should be around 5000 rpm.
Peak power should be 6600ish and I’ll shift at 7ish.
It should be no more than 32 total. Of course in low gear I’ll add 4-6 degrees of timing and in high gear I’ll pull 3-4.
That’s on pump gas.
I’ll be dropping the fuel pressure at least 2 pounds at a cruise and I’ll run ported vacuum advance. I suspect it will only take 10-12 degrees of VA.
That’s it. Nothing special and it won’t rattle its brains out. It will idle down to 750 (probably slower) if I wanted it to.
Of course, I could drop the compression to a pump gas “friendly” 10.5:1 and jam a bunch of initial in it but to what benefit?
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Well that is your 'typical' engine that we see folks asking about the use of MVA:
- 12:1 CR.
- 255 @ 050 cam
- tunnel ram with 800 cfm carbs.
- revs to 7000ish
Yeah everybody has one, there are three in my street alone.....
That engine would STILL idle better with more than 20* initial......whether you are honest enough to admit it or not. One way to add idle timing is MVA. It is not the only way. But MVA is simple, inexpensive & reliable. And sure, you could run with 5* initial timing. But it would not be the most efficient, & the engine would run hotter. Why wouldn't you [ or anybody else ] want the engine to run as efficiently as it can???? One reason you are running 12:1 & 0.040" quench & not 10:1 & 0.125" quench is because you want maximum efficiency.
- 12:1 CR.
- 255 @ 050 cam
- tunnel ram with 800 cfm carbs.
- revs to 7000ish
Yeah everybody has one, there are three in my street alone.....
That engine would STILL idle better with more than 20* initial......whether you are honest enough to admit it or not. One way to add idle timing is MVA. It is not the only way. But MVA is simple, inexpensive & reliable. And sure, you could run with 5* initial timing. But it would not be the most efficient, & the engine would run hotter. Why wouldn't you [ or anybody else ] want the engine to run as efficiently as it can???? One reason you are running 12:1 & 0.040" quench & not 10:1 & 0.125" quench is because you want maximum efficiency.
Newbomb Turk
Well-Known Member
How do you know it will want more timing at idle? Are you that ******* arrogant that you think I won’t test it?
The last 4 or 5 Chryslers on my dyno, all 408’s and none over 11:1 didn’t want MVA at idle. I think the most one of those wanted was 24 and the cam was too big for the compression.
And guess what Geoff, I tested ALL of them with locked out timing and NONE of them idled better with 34-35 at idle. And ALL of them made more power with a curve.
So there you have it. And none of those engines are exceptional or odd. And most of them had more cam than the compression wanted but to get the rpm out of them that’s what you have to do.
And still NONE of them wanted all that idle timing.
Try again.
Newbomb Turk
Well-Known Member
If that’s a success you don’t even know what failure is.
I’m not sure who the author is, but it’s pretty hard to read on my phone. I’ll read it on the laptop tomorrow.
None the less, a poor build requires all that idle timing. Doesn’t matter who did it.
Guys please keep in mind the following:
This engine is what it is. I bought it already built “as is”. I didn’t specify the build nor know what cam was in it or compression. Only after installing it I bore scoped it and read the stamping on the piston as TRW 12:1. I pulled a lifter and determined it was a solid lifter. Say what you want about the motor ..shitty build, pig what ever. THATS NOT THE POINT. I am just trying to make the best of it for now until I can afford to rebuild it.
The engine gave me the most vacuum and ran smoother with ~40° of timing. Some folks here (and not here like David Vizard) are not surprised that it wants that. OK so where do we go from here? Accuusing me of not knowing how to use a timing light ain’t right for two reasons. One I didn’t use one when I set that timing purposely. It was hooked up but I used my ear the vacuum gauge and my oil pressure gauge (to some extent) to GIVE THE ENGINE WHAT IT WANTED.
Also the first time I used a timing light was in 1978 on my 1973 Z28. I am not saying I know everything but I am always willing and desiring to learn more. I am forgetful and don’t always understand what you write. I am human and make mistakes. You guys are a great resource for me and others and I appreciate the help sincerely. But denigrating me ain't cool for me or this site or the future of this hobby! Intimidation WILL keep the younger ones away. It will I guarantee it. It will also shut good peple down and they will stop interacting here and disappear.
The hobby and this site faces serious challenges in the future and we are the stewards of its future.
This engine is what it is. I bought it already built “as is”. I didn’t specify the build nor know what cam was in it or compression. Only after installing it I bore scoped it and read the stamping on the piston as TRW 12:1. I pulled a lifter and determined it was a solid lifter. Say what you want about the motor ..shitty build, pig what ever. THATS NOT THE POINT. I am just trying to make the best of it for now until I can afford to rebuild it.
The engine gave me the most vacuum and ran smoother with ~40° of timing. Some folks here (and not here like David Vizard) are not surprised that it wants that. OK so where do we go from here? Accuusing me of not knowing how to use a timing light ain’t right for two reasons. One I didn’t use one when I set that timing purposely. It was hooked up but I used my ear the vacuum gauge and my oil pressure gauge (to some extent) to GIVE THE ENGINE WHAT IT WANTED.
Also the first time I used a timing light was in 1978 on my 1973 Z28. I am not saying I know everything but I am always willing and desiring to learn more. I am forgetful and don’t always understand what you write. I am human and make mistakes. You guys are a great resource for me and others and I appreciate the help sincerely. But denigrating me ain't cool for me or this site or the future of this hobby! Intimidation WILL keep the younger ones away. It will I guarantee it. It will also shut good peple down and they will stop interacting here and disappear.
The hobby and this site faces serious challenges in the future and we are the stewards of its future.
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FYI I posted this quick plot on a specific thread not to confuse the matter asking if this made sense. Since this post is still active I will repost here. I asked if this plot made sense or if I am incorrect. I have not seen any indication it’s incorrect from anyone. But the idea of a better burn at idle and cruise is why I am thinking MY Engine would benefit from MVA not ported.
Newbomb Turk
Well-Known Member
I’m not denigrating you. You didn’t build it.
I said many posts ago to hook it to MVA because you only have what?? Four options?
Hook it to MVA. Cheapest, quickest way to get it going. But you just can’t hook it to MVA without doing something with the curve. IIRC you had Halifax send you a distributor so you have that done too.
You can change the cam. That costs considerable money and it’s a good bit of work. Will it run better with a smaller cam? Probably. But it’s still a bunch more work and expense.
Change the pistons or keep what you have and get the compression up. The MOST expensive, work intensive way to fix it. But and it’s a huge BUT for me if you do get the compression up it will run on the same cam you already have unless it’s hopelessly wrong and make the most power with higher compression and the big cam.
The problem is (and I’ve known this since 1980 when my dad gave me the first Direct Connection “Bible” I had and then I read it, cover to cover) that the majority of engine builders are so GM centric that anything that doesn’t fit how GM does it they won’t do it.
And what they won’t do is machine the deck of the block so that the deck of the piston is out of the deck of the block. Most just won’t do it, even if you show them the book.
I run the EXACT same piston you do, except I am 12:1 and I milled a TON off the top of the dome to get there.
Yes, I said it correctly. I milled the dome DOWN to get the compression where I wanted it. Because when the piston is out of the hole like it SHOULD be, the dome is big enough to damn near hit 14:1 with a 3.313 stroke!
IMO, that’s the correct way to build these engines. With a POSITIVE deck height. You can get quench very easily. Then reducing the dome does several things.
It makes the piston lighter.
It has helps flame travel but it’s not as critical on these engines as it is on engines with a poor spark plug placement.
It also puts the radius of the dome on the spark plug side of the piston much closer to the head, so in effect you are getting at least some quench on the plug side of the piston.
The FIGHT with some of these builders to use a positive deck height is stupid. But they don’t learn. It’s not worth the fight and if a machinist you chose won’t do it, find one who will.
Fixing the compression issue is by FAR the most expensive, labor intensive way to fix it. I’d bet everything we both have, all of what @RRR has, and all the stuff @PBR, @boosted, @Magamopar, @lead69 has and anyone else who’s **** we had access to and then a bunch of stuff we don’t have that your pistons are not out of the deck. In fact I’m betting the deck is down the hole .030-.040. Or more.
That means the engine has to come out, get disassembled, cleaned, machined and put back together. That’s HUGE money and labor and IMO it’s just not even close to worth it.
I’m only saying this so that down the road if you want to fix this engine or build a new one you’ll have an idea of how it should be done.
The last fix is pretty spendy but it gives you more ability to tune the timing curve and that’s the Progression ignition distributor.
You can change the curve to about anything your mind can imagine (within reason) with a phone. Or a laptop with Bluetooth I would imagine.
At this point I’d hook it to MVA and start tuning on it.
And since I forgot to mention it earlier, if you don’t have a positive deck height on the pistons you aren’t anywhere near 12:1. To that end you do not want or need any fuel other than pump gas.
Any more than that and you’ll hurt power and you’ll just make it harder to tune. I’ve seen this so many times I can’t count it.
No Avgas (great fuel but you don’t need it and it’s cheaper than race fuel by a long way), no 110 or even 100 octane race fuel. You don’t need it and it’s a waste of money. And it hurts power.
Don’t get discouraged. Just because someone writes a book doesn’t mean they know what they are doing.
If you like to read and want to learn more (and especially if you like math or if you are like me can use a scientific notation calculator you’ll love these books) you can get the Obert book (I forget the title of that one), the Larew book (see above) and the two volume set of Taylor’s “Internal Combustion Engines” book published by MIT.
Those books will help with carburetor tuning and how these engines really work.
The Taylor books are by far the most comprehensive and technical but if you read them and only learn 3-4 things on the first read it’s worth it.
Also, if go search for NACA paper 49 it gives a great explanation of air bleed and emulsion.
Self education is the best way to understand this stuff. Then you won’t get burned by glitzy books, magazine articles and websites.
Edit: I forgot to mention you can mill the heads to get the compression up but machining a bunch off the heads can make it prone to hurting head gaskets even if you only rattle it a bit.
I had a friend mill the hell out of his heads for compression (his engine builder at the time wouldn’t deck a block that far because…well because the guy was so bull headed he couldn’t understand it) and it would knock a head gasket out of it.
He finally brought it to me. The guy used the thick Felpro gasket so he had to take another .020 off the head when if he used a thinner gasket to start he wouldn’t have had to mill the heads (or block if you’re doing it correctly) and the deck of the pistons were .060 in the hole. Same piston we are using.
So the dude had to mill another .080 off the head just to compensate for the thick gasket and low deck height.
I threw the heads away and fixed it. I’m not now nor have I ever been a fan of milling the heads to death for compression.
If you are going to do it, at least angle mill the heads so you don’t have mill them so much. And that opens up a new rabbit hole to see if you can find the bottom of.
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Woodys_Cuda
Ontario, Canada
I've been following this thread and see how little I know, it's gotten quite in depth. It'll be interesting to see what the OP's compression test reveals.
Turk,
I know your engine would idle better with more timing because I have doing this for so long.
It is called experience.
Pay for my plane ticket & I will come & dial in the idle timing..... the proper way.
One engine I tuned, close in specs to your engine: 11.3:1 CR, alum heads, headers, T2, Holley 950, 254/262 @ 050 HR cam, 108 LSA. It idled best with 48* which was done by giving it 18* init + 30* MVA.
You keep going on with your NONSENSE about people picking the wrong cam/pistons. Maybe some do, but they decide to live with the decision [ & save $$$$ ] & tune the engine accordingly.
How many people reading this are building engines with 12.3:1 CR, 255 * @ 050 cam, tunnel ram with dual 800 carbs???????? Only one I know is YOU, especially if it is to be street driven.
The facts are the facts. Factories & racers alike use[d] MVA. And they didn't have 12:1 CR.....
Below is another one, four cyl engines...
They cannot all be wrong be wrong.
I know your engine would idle better with more timing because I have doing this for so long.
It is called experience.
Pay for my plane ticket & I will come & dial in the idle timing..... the proper way.
One engine I tuned, close in specs to your engine: 11.3:1 CR, alum heads, headers, T2, Holley 950, 254/262 @ 050 HR cam, 108 LSA. It idled best with 48* which was done by giving it 18* init + 30* MVA.
You keep going on with your NONSENSE about people picking the wrong cam/pistons. Maybe some do, but they decide to live with the decision [ & save $$$$ ] & tune the engine accordingly.
How many people reading this are building engines with 12.3:1 CR, 255 * @ 050 cam, tunnel ram with dual 800 carbs???????? Only one I know is YOU, especially if it is to be street driven.
The facts are the facts. Factories & racers alike use[d] MVA. And they didn't have 12:1 CR.....
Below is another one, four cyl engines...
They cannot all be wrong be wrong.
Newbomb Turk
Well-Known Member
I just fixed an engine yesterday because some guy listened to bull squirt like this.
The guy that owns the car is lucky he wasn’t beating on it.
Too much idle timing and it ran like ****.
So how do YOU think adding MVA to it would make it run better?
Are you saying I should reduce initial to say 14 and then add in 10 with MVA to get it back to 24?
That would be idiotic.
Buy your own ticket, fly your *** here, bring cash and I’ll show you how to build engines that don’t need MVA in the first place.
I’ll say it again, you are crutching a bad build.
You are what happens when you read books and don’t think and test for yourself.
I mean, the books you read tell you to set power valve opening off idle vacuum and the main air bleed affects both ends of the fuel curve the same.
Part of that is your fault. Your reading comprehension lacks.
"Guys,
I know this subject has been beat to death. So if you don't want to partake in this discussion that's fine I don't blame you. All I ask is that you be respectful as I want to better understand this and would much rather do this in a pub over a bear with a pencil and paper. I want to know if this makes sense or if there is wrong logic etc. I am just trying to learn."
Quote from post 1
I know this subject has been beat to death. So if you don't want to partake in this discussion that's fine I don't blame you. All I ask is that you be respectful as I want to better understand this and would much rather do this in a pub over a bear with a pencil and paper. I want to know if this makes sense or if there is wrong logic etc. I am just trying to learn."
Quote from post 1
Newbomb Turk
Well-Known Member
Nothing stopping anyone from respectful, civil or even polite.
This is a crazy subject and after reading other threads about it here for years, I wouldn't ask the question. I would just run what I thought was right or what worked (not even best) and not bring the subject up. No reason to argue about it or get your panties in a wad because someone brought it up again. Obviously you brain surgeons of manifold vacuum are piss poor teachers because most of the fellows here don't understand it. Oh, I have a question for you. What oil filter is the best? 
I agree it's not worth all the insults, but it's ok for you to whoop in here and insult people over it? Being a moderator and all? That's not being a very good representative for the site, Mike.This is a crazy subject and after reading other threads about it here for years, I wouldn't ask the question. I would just run what I thought was right or what worked (not even best) and not bring the subject up. No reason to argue about it or get your panties in a wad because someone brought it up again. Obviously you brain surgeons of manifold vacuum are piss poor teachers because most of the fellows here don't understand it. Oh, I have a question for you. What oil filter is the best?
Mike you’re always changing the subject! But WIX for sure!
Newbomb Turk
Well-Known Member
This is a crazy subject and after reading other threads about it here for years, I wouldn't ask the question. I would just run what I thought was right or what worked (not even best) and not bring the subject up. No reason to argue about it or get your panties in a wad because someone brought it up again. Obviously you brain surgeons of manifold vacuum are piss poor teachers because most of the fellows here don't understand it. Oh, I have a question for you. What oil filter is the best?
They can’t teach it because in MOST applications it doesn’t work.
Although one guru says he can make it work in anything.
Of course, no one who actually gets paid to build and tune engines believes crap like that.
I know I sure don't. I take every combination as a complete new case, because even if exact builds, their tuning needs could still be different.
Engine should be tuned in gear with auto transmission. set carburetor first then timing, once timing is set fine tune carburetor.
I'd start with 18 initial, give or take. Probably end up 20 -24 degs initial
More likely the carburetor is the problem
If what you really wished you had was less compression, without rebuilding the whole motor, what about some thicker head gaskets to drop it a point or so?
I question the engine having 12:1 compression. Let's not start changing gaskets. lol
Newbomb Turk
Well-Known Member
I’d bet it ain’t near 12:1. He’d be lucky if it’s 11.5:1.
So far, all he's told us is what he "believes", not what he's "verified". I think I asked somewhere in this thread for compression test results. That would help greatly.
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