Engine doesn't pull as hard as it used to between 4000-5000 rpm. Been running the same combo for 10 plus years. Same timing, same jetting. Freshened up the motor last year and put a new block in it with a steel crank and new rods. Everything else is the same. Heads, cam, intake, carb, pistons, everything. Replaced valve springs as well. It just doesn't pull as hard. Any ideas?
Deck height is similar. Old block was zero deck with a thick head gasket to have some piston to head clearance. New set up is in the hole a bit with a standard head gasket. Compression ration is slightly higher, had to shave the heads a bit to clean them up.
What would the difference be? used to run a .051" head gasket at zero deck. Now running a .041 head gasket with the pistons down .010 in the hole. Seems to me the quench would be the same.
That could be, been running the same 700R4 since 2001 and converter since 2003. No apparent slipping in the tranny could the converter sprag be toast? It just feels like the engine is working harder than it should. Almost feels like it's climbing a hill or pulling a trailer.
Information for those misinformed, deck height differences. This relates to quench distance, the distance of the flat area of the top of the piston to the flat surface of the head with piston @ TDC.
Usually, stock early engines ran the piston dow in the bore .020 to .025. The head gasket was steel shim type, compressed thickness of .18 to .020. Adding, this would give a quench distance of .040 to .045.
Zero deck engines have the piston flat at the deck @ TDC, level with the gasket surface. To attain the correct quench of .040 to .045, a simple compressed thickness gasket of .040 to .045 is used, done, same outcome as the dropped piston in the bore and shim gasket.
The reason for that quench distance is to make sure the mixture gets disbursed and held in the correct places in the chamber, and to assist flame propagation over the top of the piston equally across the piston top. too large a quench distance, mixtu5e migrates out of the quench areas, leaning those areas to possible detonation, too small, reverse occurs, mixture is blown oout of thiose areas, with the same lean burn consequences.
Just info for those that didn't already know it, and those that were misinformed by incorrect info.
you say new block.. was this a new crank and rods also.
If the power loss started as soon as you went fresh I would bet the old engine had less internal friction ..more so the piston to wall clearance was looser on the old one.
I have tested .0015 piston to wall vs .0035 and .0045 piston to wall with all the same piston Cast 345NP.
The tight build .0015" was down 1000 rpm on shift points.. nosed over 1000 rpm earlier... just too tight.
I went straight to .0045" on the next build.. Awesome power.. cooler running also.
Then tried .0035" acted the same as .0045".
.002" here and there or even .001" will start stacking up and take power out of it.
The old block was very loose. Like put the micrometer away and use a tape measure loose. Was using a litre of oil for every tank of gas before the rebuild. Old block was a production '010' 350. New block is a GM bowtie sportsman with the splayed caps. New steel crank and H-beam rods. Old pistons were re-used as they were still good.
I believe most builders prefer to zero deck the block and set the quench height with the gasket. I believe they state that the combustion process gets a little messed up by the walls of the block if it's not zero decked. It creates a less efficient burn so slightly less torque and power. I know I know it's not too informative. I'm tired, you can probably find the exact/scientific answer with a google search.
If the new crank & rods are heavier the motor won't build rpm quite as quickly. I don't know it for a fact but I've heard it rumored that people will turn a crank down just to lighten it a little.
Perhaps weight is part of it?
no. there is no "cylinder wall height". There's the deck height of the block, the deck clearance between the top of the piston and the top of the block and the quench distance which is the deck clearance plus the thickness of the head gasket. It's considered good if this distance is about .035. It's really a product of choices made before and during the build. Tight quench is widely believed to promote good combustion and fight detonation by inducing chamber turbulence as the piston approaches the head and fights detonation the same way. Additionally, detonation is avoided by by cooling of the part of charge furtherest from the spark plug, keeping it from exploding and pinging. We don't normally think of the top of the piston as being cool, but it is when compared to 1,000 degree combustion gasses.
tight quench also contributes to bad emissions the same way. cooled fuel/air charge in the quenched area may not burn fully, leading to exhaust of unburned hydrocarbons. This particular type of emission is bad because because it generates the brown cloud seen over many cities as the HCs break down in sunlight. always compromises.
Nope, not everything. Let's shoot for a .035 quench. What's the difference between a piston down .010 in the bore and a .025 gasket, or a piston at deck height and a .035 gasket?
Fel-Pro sells an excellent .039 gasket you can buy about anywhere cheap. Thinner gaskets are expensive AFAIK. There used to be cheap GM tin gaskets, but I don't even know if they're still available. I never did like using them. Really need fresh deck, fresh cut heads and iron/iron. I think they were .021. Worked out pretty good with a .020 deck clearance.
Personally, I'd rather zero deck the block and run the good Fel-Pros. Or, .004 out of the hole and .039 gasket. But, I like cheap and easy to find parts along with standard machine practices.
BTW, there's nothing sacred about the .035 quench. I've worked on engines that had way less than that. One guy I worked with was running a USAC midget. high winder too. He thought .022 was good. Left prints in the pistons, every time. He didn't see anything wrong with that, he didn't want to lose any compression. I told him he was off his rocker, he asked me why he didn't have busted up engines. He didn't. I had no answer for him. He should have at least had some stuck top rings, but no. He had all of his old engine parts for 10 years stacked on shelves. He was having his pistons made by Venolia and the top rings weren't close to the deck like they are now.
You can go for .035 quench if you want but I wouldn't use any oddball parts or machine work to get there. Nobody will ever be able to tell the difference between .039 and .035 even with a well controlled dyno setup, much less a real car, which is what we drive.
I can't even image how many of these I've used. $21. Use these and the small block version long enough I might be able save up and buy a $400 oil pump that doesn't do anything the $34 stocker won't do. Except be the latest cool thing out. Naaahhhh.
In my mind it is not the quench.
It is everything else you did.. all fresh and snug.
I did the experiment on piston to head distance on a 350" My own daily driver.
Pistons down the hole .013" and I ran .015" steel shim gasket for a piston to head of .028"
Flat tops 5cc reliefs and 54cc chambers.. bunch of compression to say the least.. cam was 224@ .050 comp magnum 270H.
I could run 93 octane and 41 total timing was the sweet spot.. 91 octane and 38-39 total timing depending on weather.
After 35,000 miles (about 6 months) I went with a .038 head gasket.. Now I have .051" piston to head.
Everything was the same other than a head gasket..
It ran exactly the same..
Now there was one thing i noticed. It was not as finicky on the tune when the weather changed.
Same ET and same MPG.. I used it to pull trailers also... No difference there.
Buddy had a stock rebuilder 350 with some miles on it..cast gm crank stock rods and cast pistons.
3000 lb 76 stepside 411 gears 3500 stall 800 DP hurricane intake.. 4" borla's.
I built him some 601 heads 1.94-1.60 valves.. 252@ .050 .566" lift Solid flat tappet with 1.6 rockers 106LSA.
It ran 11.5's in the 1/4 .HE Put too much timing in it 70 degrees total ate some bearings.
He got a wild hair and wanted more power.. He got an eagle rotating assembly and stuck the cam in 4 degrees retarded.
It would not pull near as hard.. he shifted at 7000 normally.. said it used to pull super hard past 7.
Now just before 7000 it was nosing over.. ran 12.20's
He advanced the cam back where it used to be and said it still did not have the pull that the old setup had.
got some 12.0's and then got stupid and wrecked it.
You changed too many things to expect it to run like the old loose one.. I still do not think it is quench.
I would start with trying different timing curves and different jetting.. Re tune for the new combo.
It may pick up some more power after about 40,000 miles.
Swapped carburetors. Running my spare 650 Holley. Car runs great now. Now I need to tear apart my 750 and see what's going on with it. Just ran a 12.21 with the small carb. Far from my best ET but at least it pulled all the way down the track.
#2. Exactly what is the weight difference between the old crankshaft, and the new one?
...another thing that may have changed is the distributor shaft. If there's wear then you might be getting ignition timing fluctuations. Especially during high RPM operation. That alone can make some of the engine power go bye bye.
What about the more simple things such as plug wires, and plugs? Did you replace them or are you still using the old ones? If you threw the old plugs in the new engine, did you check the gaps first? And if you re-used the old plug wires, can one or two of them be arcing against the exhaust? Take a look under the hood with the engine running in the dark to check for arcing off the plug wires. I always look for the easy things first.
What about your fuel filter(s)? Have you switched them to another type? Or perhaps if you re-used the old one, perhaps it's partially clogged. Just some ideas.
I agree that the wideband with the O2 sensor would be a great tool for tuning the carb. I also can't help but wonder that despite your ignition upgrade, it can be possible that the ignition timing curve you're using now is not as good as whatever it was you were running before. So perhaps you need to re-tune the present ignition curve as well. For instance, do you know at what RPM your total timing is coming in at?
Also be fully aware that the only MSD compatible timing lights can be used with MSD ignition boxes, ( learned that the hard way). If you're using a timing light which isn't MSD compatible, it will still work and provide timing readings, but they can be the wrong readings, and very inaccurate, ( I know this from experience with MSD ignition boxes).
My timing is all in at 2500 RPM, 18* initial, 36* total. I have a fully degreed balancer and use an old school timing light that plays nice with the MSD. My dad has an expensive MAC tools dial back light that does not play nice with the MSD. I've tested my timing light against my machine shop's expensive MSD compatible timing light and it's bang on.
What I really need is more time to wrench and tune. Hard to find time for the car with 3 kids under the age of 6.
Thanks! It's definitely busy. In a few years I'll have a pit crew. And a few more years after that I'll be the pit crew for 3 more cars. The two older ones already love to ride in "Daddy's blue race car". :thumbs:
Or "Mommy's monster truck".
Yeah, fam makes it hard to find time, at times but we wouldnt have it any other way.
Anyway, it has NOTHING to do with compression, a tighter engine, maybe the cam phasing is different? And I have had a 750DP lose its tune sitting on a fence rail during an engine swap. Actually something must have clogged an air bleed. Now, I saran wrap the carbs when they come off an engine. That one was a PITA to diagnose.
It may be time to upgrade to a new carb. My 750 DP I've had for 12 years and I bought it used at a swap meet. It may be older than me! I've gone through it pretty much once a year. Probably time to retire the old fuel leaker and go shopping for a new carb, I hear good things about Quick Fuel from some friends. That might have to be my birthday present to myself this year.
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