Manifold Vacuum Advance what am I not getting?
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.
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.