The Stovebolt Site Forums The Shop Area The HiPo Shop 383 Herky Jerky @ lower rpm

I might have identified why so many people go with a LS or with a automatic.

I am very pleased with my 383 5speed combo except for jerkiness when rpms get to 1500 and below to 1000. It bucks and jerks. I assume this is due to the more radical cam. I also assume this is why most people go with autos so the converter can act like a shock absorber with the bucking?

I am getting a little pinging so I need to back a little timing off which may help a little, but I doubt it.

I have the same issue but mine is with the carb. I use Edelbrock's and the one I had on was fine.
It would run idle in 1st with no bucking. Just creeping along. Like it should.
That carb fouled with some trash and I got another same carb and it bucked. I am waiting on
that original carb to be gone thru so I can put it back on.
Seems there is a port on the bottom side of the carb that if exposed enough will cause that bucking and lurching.
When the idle screw is adjusted to a point that port is exposed. Try raising your idle.

A 383 can be put together like any other engine, if it's got a cam that isn't compatible with the type of driving or application you intended then you may be unhappy with the results. Can you find out what grind the cam is and then make a better choice? swapping cam and lifters is not rocket science. Fuel/air delivery will also play a big part in drivability.

The two biggest mistakes some engine builders make is "too much cam" and "too much carburetor". Either one alone will make for difficult low speed operation. Put them together, and it's the stuff nightmares are made of. At 6K RPM, a 383 is flowing less than 600 CFM in real world numbers, so virtually every carburetor the hotrod shops recommend is WAY too much.
Jerry

I can get cam specs. Just have to dig a little. It's running a sniper FI so I may be assuming it's always happy with the o2? Maybe it's not.
I am going to see what leaning and riching does for it. It's not horrible, just not what I expected in the 1000 to 1500 rpm. Gassers aren't meant to run there. I may just need to get used to it.

You can datalog stats on the sniper.

What are your idle and cruise target AFRs?

Check your injector pulse widths at those RPMs as well.

Don't know how to do that yet.


Idle I think is 14.8 Cruise is 14.0. It smells a hair rich, but with no emissions its hard to tell.

Which model sniper did you get? The "normal" one has 4x 100lb/hr injectors.

I suspect you are falling off the bottom of linearity map for the injectors at that rpm/afr.

Next time you are messing with it, keep and eye on both the AFR and the Pulse Width while it is jerking.

I suspect you will find the pulse width is falling under 1.5mS and the AFR is going really lean before recovering and the jerkiness is it flopping back and forth between really lean and significantly richer than you target.

If so, I have a couple solutions to propose. If you are not seeing those symptoms, I don't want to muddy the water with unrelated suggestions.

So I mess around with this this weekend while making some of my wiring better. I have the "normal" sniper.

I also noticed it was pinging badly in 5th gear (which I can't use under 60) when I went for a test interstate ride.

I tried leaning out and then richening. Nothing changed with the jerkiness or the pinging.

I tried taking some timing out with the sniper and that didn't change anything. I suspect I will have to back timing off manually to eliminate the pinging.

AFR only goes yellow or red when I am off the fuel.

While it is bucking mildly my afr goes a little lean and the pulse drops to between 1 and 1.5 I believe.

Two problems to solve now......

Ok, that sounds like all the same problem.

Just for kicks, raise your idle AFR target to 14.0 and your cruise to 13.6. Those are richer than you want long term but will see if it helps your issues.

The problem is that below 1.5mS, the injectors become extremely non-linear and flow little to no fuel. With as large as the injectors are, it can't/won't meter the fuel properly at those rates.

If raising the AFR fixes the issue, that tells you the minimum pulse width is your problem. From there, you have two choices: run it rich and burn more fuel or swap out the injectors to smaller ones. Holley has 80# injectors that are physicallly identical, or you can get smaller ones in the same form factor from other vendors.

What would happen if the FI was told the engine was a 350 or 327 ci engine, not a 383? Would it act lake a smaller carb at all rpm ranges. May or May not help the low speed manners, but an interesting experiment I'd think. That is unless it leaned the mixture out to much. Plug reading is pretty much worthless these days so care would need to be taken if much time was spent at WOT. Idling around I'd think not a problem if low rpm manners were improved.

RonR

I will try that tonight as I am headed for a cruise night. I will put the programmed timing back to 36 and then play around with the afr's more.

I have wondered what would happen if you changed the set up to 350 or 400 too. I doubt it would have much impact on the afr as the o2 sensor would dictate what happens?

Setting the displacement only affects the base map. It uses the O2 sensor to adjust/learn from there.

There are limits to how far the learn is permitted to deviate from the base map, so you can get yoursellf into trouble by setting it too far out.

If you think you are hitting the learn limit, you can either widen that limit in the advanced settings or there is a function to apply the learned offsets to the base table to reset the % variance.

I recommend the latter option.

Here are all the numbers while at lower rpm cruise under little load

Those all look reasonable. The learn looks like it is maxed out at 20%, so you might want to update the base table with the current learn values.

The CL compensation is only 1%, so the learn is probably pretty close, but I like to give it some more flexibility.

So were you getting driving issues with it at that point? That is at the top of the range of the RPM band that you said you were having issues.

The learn will go as low as 10ish but jump right back up again when throttle input changes or you shut it off and re start.

Was not having bucking when I took pics.

The learn is filling in a table with RPM, Throttle Position, and Engine Load dimensions.

You will expect the value to vary as those things change.

I may have gone too much.

Compression Ratio: 10:1

Camshaft: Hydraulic Roller Cam - .544 Intake / .555 Exhaust Lift and 230 Intake / 236 Exhaust Duration @ .050 - 110° LSA

Pistons: Hypereutectic

Crankshaft: Steel Crank - 3.750" Stroke and 5.700" Rods

OK. I started backing the timing out of it until the pimp went away.

Took 11 degrees out and am ow at 25 degrees of total timing according to the sniper. Detonation is virtually gone now.

Went to blueprint website this morning. They say I should be able to run 34 degrees total with 91 octane.

No so in the little blue truck so far.

There is more to "34 degrees total timing" than just that number. Mechanical advance can be adjusted to come in at a specific RPM.
Vacuum advance can be modified as well. Initial timing plays in the formula as well.I am not going to pretend that I have any idea where your issue is or what to try, but 34 degrees total timing is pretty much the golden standard to get the most out of a V8 Chevrolet.

Like Carl, I don’t pretend to know much, I’m just along for the ride, hoping to learn something. Just curious how the cam was timed. Degree wheel or lined the dots up. If degree wheel what was it set in at?

RonR

Good point Ron. There may be some additional "timing" built in that won't show up on a timing light. I am guessing that degreeing a cam at the timing sprocket makes the engine behave that way, Where's Hotrod when you need him?!
He's likely mowing the back forty looking for that other Bridgeport that he remembers buying a while back.

Camshaft timing and ignition timing are two different, but equally important procedures when you're looking for a few extra horsepower, or extra torque at a particular RPM range. 34 degrees of initial, plus centrifugal timing is about right, although some of the round track engines I've built liked up to 36 or even 37 degrees. Staying a bit below that range is prudent, because one degree too much ignition timing robs as much power as 5 degrees retarded from the maximum power number. Vacuum advance can add as much as 10 extra degrees of advance for improved gas mileage, but only under light throttle cruise conditions.

Cam timing can be advanced or retarded a few degrees with offset woodruff keys at the cam or crankshaft gears, or extra keyways in the gears or sprockets. Advancing the cam from its "straight up" position adds low end torque- - - - -retarding the cam adds top end horsepower. Computer controlled timing cam do both with the same cam, but that adds a whole extra layer of complexity to the equation.
Jerry

Thanks for jumping in Jerry. My thought was along the lines of peak cylinder pressure increasing from advanced cam timing and the effect that might have on a 10.0 engine with todays gas.

RonR

Agreed on every piece. I have reached out to BP to see what they say. I do have it running well with no detonation currently.

Ron, one way I've been able to dial in a couple of extra degrees of advance is to polish the combustion chambers. That pressure rise you refer to can cause a thing called "preignition" which generates a second flame front across the combustion chamber from the spark plug. A rough combustion chamber can lead to carbon buildup, which glows from combustion temperature and acts like a second spark plug. When the two flame fronts slam together, that's the "spark knock" people hear on an uphill pull. It can be extremely destructive- - - -causing a hot spot on the piston crown and blowing big holes in pistons. A totally smooth chamber resists the formation of those deposits. Those kinds of modifications should be reserved for competition engines with a projected lifespan of a few hundred miles- - - - -not for street engines that are expected to run tens of thousands.
Jerry

Hook up a vacuum gauge, see what kind of vacuum you’re getting at steady throttle in the rpm range where you’re having trouble. I would think if you have too much overlap (Too hot of a cam) it would pull weak vacuum at low speed. It’s easy to check out & not expensive. It’s something I’d want to know if I were trouble shooting your issue

Radical cams only work well in one operating range- - - -high speed and big throttle openings. Everywhere else, a stock camshaft makes more, and smoother power. It's sort of like being the woman who had an ugly baby- - - - - -"You grew it- - - -you'd better learn to love it!"
Jerry

With apologies to Flip Wilson.

OK. May have solved this??????????? Well I didn't. BP may have solved it.

Got a BP rep on the line. Explained everything and how I was struggling with AFR and timing needing to be pulled out due to detonation.

He asks if my vac advance is connected. It is. Received engine with it connected.

Should be disconnected. If connected I am adding 10 to 15 degrees of timing on top of sniper program.

So I go home, disconnect, cap, put timing back to where BP wants it on the sniper and take for a drive.

Completely different engine.

Throttle response is now manageable
No detonation
much less bucking at low rpm
Exhaust is significantly quieter through entire rpm band. This is very confusing to me?

Now I have to start playing with it again adding cruise timing for some fuel efficiency.