It's like the brakes are dragging or something, while on the gas it feels great, tons of torque and power - but as soon as I let off the gas the engine slows, like it's not spinning freely, this is in 3rd or 4th going 45-50 - if I don't give it significant gas it slows, like a car does when downshifting. Also when driving along in 4th the car every now and then stutters slightly, like it slows down with a barely noticeable jerk bip bipbip then continues on. I don't feel any of this while in neutral. This might be unrelated.

It's a 340 bored .040 over w Keith Black 10.5:1 pistons. The cam is a Hughes 3038; dual valve springs, J heads, stock rods, forged crank, windage tray, RPM Air Gap, BG 650cfm, drop base air cleaner. The rest is a Passon stock 4 speed, new 3.55 8 3/4 suregrip gears (just put in), moser axles, older green bearings, new wheels and tires, new front discs, new rear drum pads, new plugs and wires, replaced a leaky exhaust manifold gasket. It has dual exhaust a mix of 3" and 2.5" pipes with an X-Pipe and big mufflers. I also changed the valve covers, EGR valve, and the wiring is all new. (Busy summer)

It used to have a 2.93 highway gear in it - which I put in for a honeymoon trip 10 years ago, and never took out. So I drove it that way for 30k miles. It was awesome on the highway. Maybe the engine got broken-in at the speeds that old gear settled into, and now it's tight in the new rpm ranges?

It also feels like it's really working on the highway; don't have a tach wired up yet but I think it's 3000+ rpm at 60. It was getting hot during highway driving, over 200 deg so I put in a 160 deg stat and now it goes down the highway at around 190 deg. I previously took off the heater hoses so there's no heater core in the loop which is prob why it's running hotter.

Recently I had the distributor out to get at the oil pressure sensor, think I got it back in more or less where it was. It was pinging a bit on 87 gas so I switched to 92 and it ran great. That got expensive so I went to 89 gas and bumped the timing back a couple degrees. Haven't properly timed it yet but no pinging and acceleration is still good.

Maybe this is just how a high compression motor acts, it's hard to know how it should feel, not having driven another car like mine. I feel like it would go great down the track but for driving around it feels like the engine is either pulling or pushing, no in-between sweet spot, no coasting. the 2.94 gear sucked in 1st gear but it felt okay the rest of the time, not like this.

I guess the rear is probably still breaking in but it's been over 500 miles and it's not loud. I suppose it could be an out of round drum or disc or stuck caliper or something but I feel like that would pulse. Coasting seems fine.

Would engine timing cause any of this? Or is something going bad in the motor like crank bearings? Or maybe I just need to drive it more and it will loosen up?

And yeah I get that I basically changed everything but what are the likely culprits?

3:55 will drag you down a bunch more than the 2:94.A high comp motor will drag some too. See what happens when to got into neutral as you slow down.

Pretty much what Stumpy said

Sounds normal to me to. I have a 68 Dart with 3.91s. my tach is at 3500 at 75, and yes when the gas pedal is released, it feels like the brakes are on. I think some of it too is that new cars have better bearings with less resistance.

Sounds normal. A 10.5 iron head engine running on 87 or 89 octane and too much timing is probably causing your overheating especially if it's running a bit lean at cruise too. Your heater core has nothing to do with it. You should be running down the highway at thermostat temperature or only slightly over, if you're 30 over, your cooling capacity isn't there or something else needs attention.

I don't think it's going to loosen up. Also, newer cars are calibrated to keep the throttle or idle speed motor open during decel to limit engine braking during normal in-gear coasting.

New cars have the torque convertor that are unlocked when in high gear too which helps reduce engine braking and increasing fuel mileage.

Heater core has no effect on over heating. IMO if it gets hot on the highway with plenty of air going through the radiator then I would bet it's lean creating the over heating issue. Put 180 or 195 thermostat in there, 160 is no good and could actually hurt your motor in the long run.

If your used to 294s then it'll take a while to get used to 355s. I drove these old cars with 411s in them for many years on the street. It felt like 276s to me when I went to 355s.

I drove 5 different 340s over the years on the street.

Engine braking is stronger with higher compression engines. Lower gear ratios are like a longer level applying more torque. This "lever" works both ways- when you let off the accelerator the vehicle will slow quicker than it would with your previous gear.

Great info, thanks all!

With the 2.94 it had a sweet spot at 2700rpm / 70mph, and it ran at 180deg all day, even in traffic, running the a/c. I think I just never revved it this high for an extended period. Now just one exit on the highway @ ~65mph and it climbs to 190-200. At least with the 160deg stat it takes a little longer and doesn't go quite as high.

There's still a big aftermarket a/c condenser that covers the whole radiator surface. I never worried about it before but now I wonder if it's blocking airflow. I also just changed radiator hoses so I guess I could have a suction issue. It has an early steel fan shroud and a thermal clutch fan.

I'll put the 180deg thermostat back in and work on the timing. Plus I have to get the tach wired in, and a new speedo pinion, so I can really see what's up. With the old speedo pinion it shows 90mph and I'm in the slow lane. Considering an overdrive gearset from Passon, or switching to a 3.23 gear. The 3.55 really wakes it up, though. [spins tires]

200* isn't really hot.

How tall are the back tires? Short tires and 3.55's will definitely turn some RPM's compared to your 2.94's.

Play around with this RPM calculator, it will tell you what gear ratio suits you the best for the highway.
https://www.ringpinion.com/calculators/Calc_RPM.aspx

One thing is for sure, when the engine is broken in it's broken in. It doesn't have to be broken in for a specific rpm range.

With a 27.5" tire you'd be 2600rpm at 60, 3.55 gears To get 3000rpm would mean a 24" tire.

Sounds to me like you will have to address the compression issue. Here's why...retarded timing will increase engine temps, but incipient knock can raise the rate of heat transfer to the combustion chamber and cause overheating as well.

The slight "blip" you mention at highway speeds could be lean surge. Being lean can cause overheating. When you changed the rear ratio you probably changed somewhat the part of the fuel curve the carb is on at cruise.

I'm thinking water injection would help.

R.

Whats your piston part number?

This is the first thing I would check. A friend raced his car for awhile with the timing set at 50 accidentally, moving it back to 36 made it run perfectly. Set your timing correctly, or have someone do it for you, at least get the total timing set for now, for example 34 degree's total when you rev it. This alone could solve your overheating and engine surging.

Tire height will tell us more about your gearing.

10.5to1 compression will not make it run weird or do weird things, should be smooth as silk. You may have a carburetor issue.