Vacuum on 1968 GTS 340 4 spd.

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Crazy update but a clue. My compression cold is 163 psi . I ran the car only a few minutes and tested again. 105 PSI ! Haven't done anything else yet I'm just too busy. Tomorrow I will test a couple maybe one on each bank cold, then warm up motor completely and and see what up. I've never heard of something like this before in my life. Might slowly dissasemble top end for clues and pull the whole thing and start from scratch by first having checked out at the machine shop. I'm at a loss since July and gonna have to throw in the towel.
 
Kendog 170 said:
Anyone have a stock 1968 Dart 340 4 spd ? I was wondering what your vacuum reading is. I bored mine 20 over with eddy heads KB pistons same cam . My vacuum is only 13 at idle. Engine Acts like it has a vacuum leak but cant find any. This includes putting a vacuum gauge on the dip stick tube after sealing valve covers and vacuum ports.I cant remember but I thought it should be around 16 In.
Thanks
Click to expand...
Fuel pressure?
 
Turn fuel pressure down to 4.5-5 and see what it does if you can/

There is something SERIOUSLY wrong with the engine or tune up with 8" unless it has a 508 cam in it
 
Ok, all the results are not in yet but The vacuum gauge I installed under dash (Digital) is crap. I used 2 different gauges and vacuum is at 15" . I used the new comp. tester. Cold test all real close to 165 PSI except #4 at 170. Test with engine completely warmed up. All around 160 but #4 165 PSI. Got my new balancer but want to use piston stop to confirm TDC as my Timing bracket is on LHS. I will mark TDC and use Timing tape then clearcoat. I'll keep you posted. I bored 20 over with KB pistons I'did not balance pins or pistons.A buddy of mine said I didn't have to. (IMO big mistake on my part.) Now I learned Cast are heavier than forged . Maybe I have a balance issue. If i still have issues after balancer change and timing adjusted I might start a new post on this.
 
Well, that is all good on the gauges! It really can be misleading with bad tools.

Hate to say this but your bobweight is now off by 150-200 grams with stock rods depending on the exact piston..... dang. Drop pan and timing cover, pull crank, and rebalance crank alone to a new computed bobweight would be the easiest way, and would be a decently good balance job. The piston/rod assemblies are going to be close to each other since the KB pistons are already closely matched.
 
Ok, same pistons and oversize in my son's 340. Bobweight with stock rods comes out to 2138 grams vs stock of 2308 grams. So it's 170 grams light.... that is a LOT. Factory balance tolerance is something like 20-30 grams. Something needs to done.....

Full balance fixes 2 separate balance factors. One is solved by matching up the piston/ rod assembly weights. The KB's being well matched out of the box takes care of most of that. So IMHO, you could be in decent shape to not do that part. It will be as good as the factory balance if you did not touch the rods.

The 2nd factor is fixed by balancing the crank to the bobweight. That part is waaasy off here and is what really needs the attention.

For sure doing it all is the best... But you could get by OK IMHO to just take care of the 2nd factor....

The good news is that you will just have to take weight off of the crank, not add any heavy metal.

But good compression numbers!
 
So you are sure on the bob weight of 170 grams off the crank is correct? So the heavier on the piston the lighter on the crank weight ?
 
I think I confused you with how I stated it.. sorry. What I meant to convey was that the pistons/rod assemblies are now so much lighter that the crank also needs to be lightened. And the changes with the KB pistons/pins means that the crank should be re-balanced with 2138 gram bobweights (unless any parts are lightened in the process). My stock 340 bobweight numbers are 2308 grams; so that is where the 170 grams difference is from.

I don't mind you asking for some confidence, and that is wise. I have gathered the info for 2-3 years now to go into SBM bobweight computations, and have a spreadsheet setup to run the bobweight computations, which are standard formulas that you can find, and which have been used for decades. I have also directly measured stock 340 rods, pistons, rings, pins, and locks, and the KB243 pistons, rings, pins, and lock. So all the numbers used have been directly confirmed at this end.

I have done balancing of the pistons/rods myself, and have gotten to the point of finalizing bobweight and just handing that number to the shop to do the crank balance. So I feel that I have the knowledge to give you the straight poop.

If you want the actual numbers I used so you can ask others, I'd be happy to send them along; this is not a trivial matter so IMHO taking it carefully is a good idea. The whole goal is to get any vibration issues resolved.
 
Are you kidding me ? Your a blessing to me . I am being careful is all. So when I go to the machinist I'm using I should tell them I need a total bob weight of 2138 grams on the crankshaft ?
Also I was confused because I thought I read that cast pistons typically are heavier than forged. But I trust your knowledge and am thankful for all the advice you've given me.
 
So the shop I've been using wants to balance everything and charge $450.Says it's a pretty standard He did say he can do just the crank for a lesser price but doesn't recommend it. I guess it depends on the lesser price. I don't have much cake or time before I have no garage . Decisions Decisions .
 
IMHO, he probably wants to be confident it is all right and is the best he can do for you. Not a bad thought......and it is the best way..... it just adds a lot of work and cost. What would be done is to match the big and small end weights on the rods (which is time and labor and skill intensive, hence the cost) and detail measure the parts to the tee. Then recompute the bobweight based on what are the final actual numbers, not jsut nominals. Since some weight would be removed from the rod ends to make them all match up, then the final bobweight that would results from that full process would almost always end up being under the computed nominal bobweight of 2138 grams for this combination.

The crank balance alone price is ususally much lower but you are in a higher cost area. Down hyar, I can get a crank balanced alone for a bit over $100 with spec'd bobweights and for removing weight; it would be pricier for adding weight, but you are in the remove-weight situation. And the crank-alone is going to solve what looks to be the gross error that exists.

How is this engine used? And what durability is desired?
 
Just a cruiser. I have my 500 stroker for beating. I agree Crank only with the info your supplying . I will get a price . I do know of another shop a relative swears by I can compare price with. I agree they want to make sure but as you said there's so much more involved going down that road. I am hoping to relocate in NC. once my wife is fully vested in her benefits. I hear there are a lot of reputable and lower priced shops closer to you than the Boston area.
 
OK, you get the point. For a cruiser, getting the gross errors fixed ought to be a BIG improvement. The nominal bobweight could be 10-15 grams off of your actuals with your rods, but that is a ton better than 170 grams!

It all boils down to what you think it is worth in effort and money.
 
The shop that machined my last 360 build told me I didn't need to have the bottom end balanced with the KB pistons if I wasn't planning on revving it over 4500 RPM... well that's a pretty low RPM limit to me so I said "do it!"

That engine ran great and was smoother than the stock 318 it replaced, put 25k miles on it over 6 years driving all over the state and made 9 passes down the drag strip. Then it finally gave up the ghost from too much pinging (my fault).
 
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