New 393C running, Won't Rev Beyond ~3500 RPM

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Casey72

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My Car
Two '72 coupes.
I got the 393C installed and running. It sounds amazing. Starts instantly, idles beautifully at 900 RPM, revs nicely when out of gear. However, on the road it loses power after 3500 RPM and will barely rev and accelerate beyond 4000 RPM, making the burbling noise and running less than smoothly. The PA 10" torque converter flashed at around 3500 RPM with the milder 351W the new engine replaced, but only flashed around 3000 RPM with the new Cleveland. The issue occurs both before and at WOT, so I doubt an issue with the FI changing modes. The converter isn't doing anything questionable that I can tell.

After some research and experimentation with timing and FI settings I've pretty much eliminated the Atomic EFI and torque converter as culprits, noting that both worked just fine before I pulled the Windsor. I'm looking at the valve springs which were installed on the heads by MPG years ago before they sat unused while I was busy with the GWOT. The springs were recommended by Comp for the slightly milder hydraulic roller I had intended to use for this build, but Comp also confirmed that they were compatible with the solid roller I ended up with, despite it being a stiffer grind.

393C engine specs:

4V iron heads, Comp springs (supposedly compatible with cam)

Weiand single plane intake

Hooker SC headers, Pypes 2.5" exhaust

Bored .030"

10.4:1 compression

Comp solid roller 242/248 @.050", 279/285 adv., 106 ICL, 110 LS, lash .016"/.018"

MSD Pro Billet Dist.

Atomic EFI (controlling timing) with HV fuel pump and return line

Timing 12 deg idle, 10 deg vac adv., 34 deg. total, all in at 2000 RPM

101 octane VP unleaded (for now)

So, am I on the right track? Anything else I need to check out before going for new valve springs?

 
I am no expert. I am shade tree mechanic at best who has done a lot of reading about engine building (both before and after the intenet was invented).

My understanding of mismatched valve springs for a big cam:

1. The spring won't allow the valve to open all of the way. The size and wind of the spring is such that it cannot be physically compressed any more. I doubt you have this issue since it tends to cause bent push rods and such.

2. The spring isn't strong enough to close fast enough between cycles. I would be suprised that you would see this at 3500 rpm even if the springs are mismatched.

What rocker arms are you using?

Have you confirmed that the Atomic EFI is properly controlling the timing?

 
I am no expert. I am shade tree mechanic at best who has done a lot of reading about engine building (both before and after the intenet was invented).
We're of the same background then. ;)

My understanding of mismatched valve springs for a big cam:
1. The spring won't allow the valve to open all of the way. The size and wind of the spring is such that it cannot be physically compressed any more. I doubt you have this issue since it tends to cause bent push rods and such.

2. The spring isn't strong enough to close fast enough between cycles. I would be suprised that you would see this at 3500 rpm even if the springs are mismatched.
The first point isn't an issue, as the springs were far from binding when degreeing the cam and installing the rockers. The second point is what I'm suspecting. The "kind of" correct springs with too little pressure as things start to happen quickly. The assembler could have always made a mistake with the order and required numbers, or the springs could be bad. Not likely, but possible factors. The issue is quite apparent by 4000 RPM, but can be detected at 3500 RPM. I'd expect it at a higher point in the revs, but I'm not a guy with a lot of samples to help me draw conclusions.

What rocker arms are you using? Have you confirmed that the Atomic EFI is properly controlling the timing?
Rocker arms are 1.73 CamResearch rollers sent with the heads from MPG. They checked out during assembly. The Atomic EFI worked perfectly with timing control on the former engine and is tracking numbers properly now, though I did lock the timing out and put a timing light on it again just to make sure it had a good baseline.

 
Is it possible you are running out of fuel? Not enough fuel pressure, kinked line, blocked filter? If you were floating the valves you should hear strange noises coming from the engine when they float. I would think about any spring would get more rpm than you are seeing. You said they were not bound when you installed. Can you put a fuel pressure gauge in the line you can see it when driving? Just my thoughts not an expert either.

 
Is it possible you are running out of fuel? Not enough fuel pressure, kinked line, blocked filter? If you were floating the valves you should hear strange noises coming from the engine when they float. I would think about any spring would get more rpm than you are seeing. You said they were not bound when you installed. Can you put a fuel pressure gauge in the line you can see it when driving? Just my thoughts not an expert either.
Fuel pressure was 45 PSI with the old engine, bumped up to 65 PSI for the new engine. This is indicated both on the Atomic handheld and the fuel pressure gauge I installed in the center console. Yeah, I checked, even though I bumped it up to 65 as soon as the new engine was running. The Atomic, converter and transmission were all "known good" and providing good performance immediately before the engine swap.

As before, when the problem arises, the engine does "burble" or "growl" a bit when power drops off. The engine otherwise runs beautifully and is completely drivable in all gears and throttle positions until the RPMs surpass 3500 or so at more than 50% throttle.

ETA: When really getting into the throttle during the "burble", there has sometimes been the smell of gasoline present. Maybe blowing fuel back out the manifold while the valves are floating?

 
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Can you check the air/fuel ratio on the handheld? I'm like David, my first suspect is fuel delivery. Have you pulled a couple of spark plugs to "read" them?

As far as the valve springs, I'm like Will, I'd be surprised if they were too weak to be causing problems at 4K. One thing that does come to mind, though is installed height. Too much height will result in too little closed pressure. This could be caused by valves too long for the head, incorrect springs, or bad machining work on the heads. Your comment about having plenty of coil clearance may indicate this. It's easy enough to measure the installed height and compare it to specs. A little more difficult to check the seated pressure without the proper tool.

Your torque converter appears to be working properly, stall speeds are a function of engine power, lower power = lower stall. In any case the torque converter wouldn't cause your symptoms.

 
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Can you check the air/fuel ratio on the handheld? I'm like David, my first suspect is fuel delivery. Have you pulled a couple of spark plugs to "read" them?

As far as the valve springs, I'm like Will, I'd be surprised if they were too weak to be causing problems at 4K. One thing that does come to mind, though is installed height. Too much height will result in too little closed pressure. This could be caused by valves too long for the head, incorrect springs, or bad machining work on the heads. Your comment about having plenty of coil clearance may indicate this. It's easy enough to measure the installed height and compare it to specs. A little more difficult to check the seated pressure without the proper tool.

Your torque converter appears to be working properly, stall speeds are a function of engine power, lower power = lower stall. In any case the torque converter wouldn't cause your symptoms.
AFR indicated on the handheld has been good, though once the valves float all bets are off with AFR, but I'll get someone to watch it today while I drive in case I'm missing a trend. May be a chicken or egg kind of thing, but it wasn't an issue on the previous engine. I'd wondered about installed height, but failed to second guess the assembly and assumed parts and parameters didn't get mixed up between Comp or MPG.

I'm all but convinced that the valves are floating, but I've never dealt with it before.

 
Used to be you could depend on known brands for quality work. However, now days that isn't the case, everything has to be checked.
Apparently so, and not just with cars.

 
Anyone else with actual experience with valve float (unlike me) have a theory about what I'm dealing with? I'm assuming valve float, so I'm about to spend money.

And there's this thing here. Same issue, same RPM, same engine family, similar cam profile...valve springs fixed it, for kind of the same reason Don C discussed as a possibility.

Atomic EFI 351C not revving

 
plug the vacuum advance forever or until further notice.

warm the engine up

check the max advance

loosen the distributor bolt 1/16 of a turn

rev the engine to around 2000 to 2400 rpm

advance the timing 4 degrees and see if the rpm increases and it still runs smooth . if it does, advance it 4 more degrees and see what it does.

remove the front plug from each bank and post a close up photo of the end of it . white color is screamin lean.

post the plug part number

post timing results

 
plug the vacuum advance forever or until further notice.

warm the engine up

check the max advance

loosen the distributor bolt 1/16 of a turn

rev the engine to around 2000 to 2400 rpm

advance the timing 4 degrees and see if the rpm increases and it still runs smooth . if it does, advance it 4 more degrees and see what it does.

remove the front plug from each bank and post a close up photo of the end of it . white color is screamin lean.

post the plug part number

post timing results
All the timing is handled by the EFI and the distributor remains at a set point, though I can lock out timing and turn the distributor. I just can't run it in any other position in practice. AFR will still automatically adjust via the WBO2 sensor, so will this diagnose a WBO2 sensor/computer issue, because there's no indication that it isn't tracking at idle and part throttle until RPM comes into play?

 
sorry cant help you with computers but you need to plug the vacuum advance if you have one then somehow advance the timing and check the spark plug color.

 
sorry cant help you with computers but you need to plug the vacuum advance if you have one then somehow advance the timing and check the spark plug color.
I can lock it out in the system and twist the distributor. The RPM will rise in the process. Anyway, how does valve float sound as a possibility based on your experience?

 
sorry cant help you with computers but you need to plug the vacuum advance if you have one then somehow advance the timing and check the spark plug color.
I can lock it out in the system and twist the distributor. The RPM will rise in the process. Anyway, how does valve float sound as a possibility based on your experience?
i dont understand what you are telling me . if you can do the test exactly as i posted, just post the results.

im trying to see if your total timing is too low . just posting timing numbers won't help because every engine is different so they all dont use the same amount of timing.

we need the part numbers for your springs and the lift for your cam.

valve float at 3500 rpm is unlikely but if you have a big cam and stock type springs, it is possible, but my guess is that it is another problem and the spark plug photos will help.

we need information, otherwise we are blindly guessing.

.

 
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To me, this sounds like either ignition breakdown or fuel starvation. You say it revs fine in neutral and if I understand this correctly, it means that when out of gear it revs beyond 3500 RPM with no problem. I've never tested this statement, but I think valves would float at the same RPM range whether under load or not.

A failing coil or coil wire will behave as you're describing and eventually fail completely.

First, check the connections to the coil and ballast resistor (if you're running one). If you have a spare coil and coil wire available to swap out (one at a time), I suggest you put that into the mix of things you're checking. It would also be a good idea to verify your ignition power source is stable. You can do this by running a test lead from the battery directly to coil or ballast resistor. It could also be the ballast resistor, so keep that in mind.

 
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plug the vacuum advance forever or until further notice.

warm the engine up

check the max advance

loosen the distributor bolt 1/16 of a turn

rev the engine to around 2000 to 2400 rpm

advance the timing 4 degrees and see if the rpm increases and it still runs smooth . if it does, advance it 4 more degrees and see what it does.

remove the front plug from each bank and post a close up photo of the end of it . white color is screamin lean.

post the plug part number

post timing results
Controlled timing was 34 degrees at 2000 RPM. Locked out timing (including vacuum advance) from the electronics and had 15 BTDC on the balancer. Held RPM above 2000 and advanced to 19 or so BTDC, RPM increased and engine was smooth. Increased again to ~23 BTDC with the same results. Shut down after a bit and pulled plugs.

#1 Plug

PlugOne_zpsfj0no3bv.jpg


#4 Plug

PlugFour_zps1ydmub4b.jpg


Cam specs

393C%20Cam_zpsruvl5ibc.jpg


Springs are Comp Cams PN 924-16.

 
To me, this sounds like either ignition breakdown or fuel starvation. You say it revs fine in neutral and if I understand this correctly, it means that when out of gear it revs beyond 3500 RPM with no problem. I've never tested this statement, but I think valves would float at the same RPM range whether under load or not.

A failing coil or coil wire will behave as you're describing and eventually fail completely.

First, check the connections to the coil and ballast resistor (if you're running one). If you have a spare coil and coil wire available to swap out (one at a time), I suggest you put that into the mix of things you're checking. It would also be a good idea to verify your ignition power source is stable. You can do this by running a test lead from the battery directly to coil or ballast resistor. It could also be the ballast resistor, so keep that in mind.
I'll try your suggestions as I whittle away at each approach to the problem, but everything was pulled from an engine with no issues whatsoever and bolted directly to the new engine, except for the PS pump, which is a new addition. Nothing is impossible, but I wouldn't expect a couple days of swapping engines to turn "known good" parts into issues. Fuel pressure holds when the problem arises, so I'm hesitant place any blame there.

 
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