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Engine falling off just past 4k - carb limit?
#1177533
02/13/12 02:02 AM
02/13/12 02:02 AM
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Joined: Jul 2011
Posts: 817
Eugene, Oregon
Secret Chimp
OP
super stock
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OP
super stock
Joined: Jul 2011
Posts: 817
Eugene, Oregon
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My new camshaft feels a good bit better than the old one around town, but the engine doesn't really want to go past 4200-4400 rpm or so now. It pulls strong up until that point, and it feels fairly consistent. It doesn't sputter or backfire or hit a wall, the power just kind of starts falling off and makes me have that "better shift now" feeling like when you start revving off the power band in a motorcycle or old car without a tachometer. It revs over 5000 sitting still in neutral no problem.
I started freaking on fuel pressure and replaced my slightly-kinked fuel hose and fuel filter before I started theorizin'.
The theory:
My loose old timing chain was letting the engine rev higher because it was pulling less air down due to delayed intake valve opening. My new chain and cam are "correct" yet worse because I'm actually hitting the airflow limit of my Carter BBD carburetor, which I can fix by installing the Performer and Carter AVS I have almost ready to go. My symptoms sound most the same as people reporting stuck secondaries on four barrels.
Reasoning:
The horsepower peak for my engine in 67 was listed at 4400 rpm. The biggest Carter BBD ever made was 325 cfm. The next step below that was 285 cfm.
Let's assume I have a 285 cfm since they also installed those carbs on 383s, and they'd probably use the biggest ones on the big blocks and save the smaller ones for 318s like mine.
If I use any one of the carb cfm calculators on the web (which all seem to estimate on the low side) and assume a terrible engine volumetric efficiency of 75%, they say I should need roughly 300-340 cfm with a max rpm of 4400. I.e., even though these are fudgy numbers, the stock engine and carb setup is peaking where it is because it's just about out of air to flow at that RPM.
Does this sound like a reasonable conclusion or am I talking out of my butt?
I just don't want to start chasing down nonexistent fuel or ignition advance gremlins if my "problem" can be fixed by adding on a carb with secondaries.
Last edited by Secret Chimp; 02/13/12 03:29 AM.
1967 Dodge Coronet Deluxe station wagon
1.03" T-bars, QA1 arms/rods, Cordoba/GM Metric/Volare brake & knuckle, XHDs, Hellwig rear sway, 318 Magnum w/ air gap, 727, 3.23s
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Re: Engine falling off just past 4k - carb limit?
[Re: Secret Chimp]
#1177534
02/13/12 03:23 AM
02/13/12 03:23 AM
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Joined: Jun 2006
Posts: 5,162
CT
GTX MATT
master
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master
Joined: Jun 2006
Posts: 5,162
CT
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What is the rest of the engine combo? It sounds like you've got a stock '67 383 2 barrel and just changed the cam and timing set. What cam? Speed Pro CS327?
If you've got a two barrel on your 383 (which is a big block) it probably wont spin much past 4000 rpm and keep making power, its a tiny carb. Don't get too caught up in the carb calculators, especially since they are geared for four barrel carbs which are actually rated differently. If I remember right when they flow test a two barrel they do it with 3 inches of vacuum, whereas a four barrel is flowed with 1.5 inches. So basically a 500 cfm two barrel is probably comparable to a 350-400 cfm four barrel.
I would recommend a four barrel on any big block, you don't have to go crazy. You wont suddenly have a race engine, but a factory four barrel intake with a Carter or factory Holley four barrel will give you some extra punch.
As far as the difference, if it revved out higher before it could be a couple things. For one the cam is almost guaranteed to not be the same specs, and the old one was worn. It is more likely that the new cam is bigger, so you should actually have more high rpm power. The old stretched out timing set would effectively retard the cam and may have shifted the powerband up. Its also possible that your new cam is too far advanced if you just installed it by lining up the dots. Does it have more low end torque?
You may also be encountering some valve float if the new cam is more aggressive and you're using stock springs.
Now I need to pin those needles, got to feel that heat
Hear my motor screamin while I'm tearin up the street
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Re: Engine falling off just past 4k - carb limit?
[Re: GTX MATT]
#1177536
02/13/12 03:44 AM
02/13/12 03:44 AM
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Joined: Jul 2011
Posts: 817
Eugene, Oregon
Secret Chimp
OP
super stock
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OP
super stock
Joined: Jul 2011
Posts: 817
Eugene, Oregon
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Quote:
What is the rest of the engine combo? It sounds like you've got a stock '67 383 2 barrel and just changed the cam and timing set. What cam? Speed Pro CS327?
If you've got a two barrel on your 383 (which is a big block) it probably wont spin much past 4000 rpm and keep making power, its a tiny carb. Don't get too caught up in the carb calculators, especially since they are geared for four barrel carbs which are actually rated differently. If I remember right when they flow test a two barrel they do it with 3 inches of vacuum, whereas a four barrel is flowed with 1.5 inches. So basically a 500 cfm two barrel is probably comparable to a 350-400 cfm four barrel.
I would recommend a four barrel on any big block, you don't have to go crazy. You wont suddenly have a race engine, but a factory four barrel intake with a Carter or factory Holley four barrel will give you some extra punch.
As far as the difference, if it revved out higher before it could be a couple things. For one the cam is almost guaranteed to not be the same specs, and the old one was worn. It is more likely that the new cam is bigger, so you should actually have more high rpm power. The old stretched out timing set would effectively retard the cam and may have shifted the powerband up. Its also possible that your new cam is too far advanced if you just installed it by lining up the dots. Does it have more low end torque?
You may also be encountering some valve float if the new cam is more aggressive and you're using stock springs.
I have a 318 - I just edited my post, I saw how I made that kind of unclear originally
It has more torque - it's most noticeable outside of the stock stall/automatic shifting range. If I shift manually it really pulls stronger at RPMs beyond the stock converter stall, it just kind of starts giving up at 4200-4300. I'm using a Summit 6900 cam and new Comp 901-16 valve springs - nothing nuts. Again it revs willingly over 5000 without a load, if that makes any difference.
I just did a dot-dot lineup on the timing chain. I did note that the dot on my new gear was slightly further around from the key slot than the factory gear (by maybe a 1/2 tooth or so at most). I had the option to install it four degrees advanced via the crank gear but just left it in the stock location. I spun the engine around a couple times and the dots still lined up. The original timing chain was so loose I was within about an eighth of an inch from being able to pull it clear off of the teeth of the cam gear.
If this is a fuel pressure/supply problem, should the engine start acting up if I just run it up in 1st and stay on it for a few seconds? Or might it just hit a wall where I've just maxed out the available fuel flow without it leaning out enough to make any scary sounds? I don't have a fuel pressure gauge yet so I might as well try something out on my way to the store...
1967 Dodge Coronet Deluxe station wagon
1.03" T-bars, QA1 arms/rods, Cordoba/GM Metric/Volare brake & knuckle, XHDs, Hellwig rear sway, 318 Magnum w/ air gap, 727, 3.23s
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Re: Engine falling off just past 4k - carb limit?
[Re: Secret Chimp]
#1177537
02/13/12 04:14 AM
02/13/12 04:14 AM
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Joined: Jun 2006
Posts: 5,162
CT
GTX MATT
master
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master
Joined: Jun 2006
Posts: 5,162
CT
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For some reason I thought you had a 383 in one of your other posts too  Anyway your new cam is probably slightly larger than the original. What did it pull to before? I wouldn't really expect more than 4000 rpm still with that tiny two barrel.
Now I need to pin those needles, got to feel that heat
Hear my motor screamin while I'm tearin up the street
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Re: Engine falling off just past 4k - carb limit?
[Re: GTX MATT]
#1177538
02/13/12 10:11 AM
02/13/12 10:11 AM
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Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,040
Lincoln Nebraska
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T into the carb inlet & run a neoprene line out to a vac gauge taped to your windshield & take it out & see what psi you have at 4K at the in hg vac # that it acts up at in high gear. Most if not all of the round hand held vac gauges have a psi side.
live every 24 hour block of time like it's your last day on earth
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Re: Engine falling off just past 4k - carb limit?
[Re: OrangeProwler]
#1177541
02/15/12 02:51 AM
02/15/12 02:51 AM
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Joined: Jul 2011
Posts: 817
Eugene, Oregon
Secret Chimp
OP
super stock
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OP
super stock
Joined: Jul 2011
Posts: 817
Eugene, Oregon
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The hoses kept popping off of my gauge (really exciting with gas and exhaust manifolds) so I'm doing a solid run tomorrow after I get some little hose clamps from the hardware store. The nose-off does seem to be inconsistent - last night I got over 4500 before it started slowing down, earlier today I wasn't even past 4000. Sometimes it feels abrupt, sometimes the engine just climbs slowly without seeming to fall off a cliff. A new Airtex fuel pump plus an adjustable regulator (I understand those pumps run around 7 psi, which is too much for the AVS I have going on soon) will hopefully be a cheap solution to this if the gauge confirms my suspicions at this point.
1967 Dodge Coronet Deluxe station wagon
1.03" T-bars, QA1 arms/rods, Cordoba/GM Metric/Volare brake & knuckle, XHDs, Hellwig rear sway, 318 Magnum w/ air gap, 727, 3.23s
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Re: Engine falling off just past 4k - carb limit?
[Re: Mr.Yuck]
#1177543
02/15/12 03:07 PM
02/15/12 03:07 PM
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Joined: Jul 2011
Posts: 817
Eugene, Oregon
Secret Chimp
OP
super stock
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OP
super stock
Joined: Jul 2011
Posts: 817
Eugene, Oregon
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Quote:
do sb have heat risers in the mani's like a BB? if so maybe yours is stuck sending all the hot exhaust back up into engine... I also wonder if your ECM box is getting hot. You should in no way need an electric pump for that car. I'm going to guess it is not a fuel delivery issue.
I have my MSD box mounted up on top of the inside of the fender next to the battery, not back on the firewall. I don't think it's an electronics problem.
The engine does have a heat riser, but it's not stuck (I actually have a problem with it rattling before the engine's warmed up all the way, time for new springs). When I start the car cold I do get a lot more vapor dumped out the left pipe than the right, but it looks and feels even to the back of my hand once it's warmed up a little.
If I end up finding a pressure or supply problem I'm replacing it with a new mechanical pump, not electric.
1967 Dodge Coronet Deluxe station wagon
1.03" T-bars, QA1 arms/rods, Cordoba/GM Metric/Volare brake & knuckle, XHDs, Hellwig rear sway, 318 Magnum w/ air gap, 727, 3.23s
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Re: Engine falling off just past 4k - carb limit?
[Re: Secret Chimp]
#1177544
02/15/12 03:39 PM
02/15/12 03:39 PM
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Joined: Jan 2003
Posts: 24,562
Brookeville, Md
Mr.Yuck
Not enough dumb comments...yet
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Not enough dumb comments...yet
Joined: Jan 2003
Posts: 24,562
Brookeville, Md
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Quote:
Quote:
do sb have heat risers in the mani's like a BB? if so maybe yours is stuck sending all the hot exhaust back up into engine... I also wonder if your ECM box is getting hot. You should in no way need an electric pump for that car. I'm going to guess it is not a fuel delivery issue.
I have my MSD box mounted up on top of the inside of the fender next to the battery, not back on the firewall. I don't think it's an electronics problem.
The engine does have a heat riser, but it's not stuck (I actually have a problem with it rattling before the engine's warmed up all the way, time for new springs). When I start the car cold I do get a lot more vapor dumped out the left pipe than the right, but it looks and feels even to the back of my hand once it's warmed up a little.
If I end up finding a pressure or supply problem I'm replacing it with a new mechanical pump, not electric.
is your gas tank vented? Maybe the vent is clogged???
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Re: Engine falling off just past 4k - carb limit?
[Re: Secret Chimp]
#1177545
02/16/12 02:28 PM
02/16/12 02:28 PM
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Joined: Mar 2008
Posts: 5,746
Ontario, Canada
Dodgem
master
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master
Joined: Mar 2008
Posts: 5,746
Ontario, Canada
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To keep accelerating (with any life) a carb has to have say 25% more cfm available than needed at a specific RPM. so at 4200 at 100% is 386 cfm + 25% = 483 cfm so a 500 cfm carb would be kind of minimum to get to 5000 with some decent pull! Just my  600 eddies and holleys work good on a 318!
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Re: Engine falling off just past 4k - carb limit?
[Re: Dodgem]
#1177546
02/16/12 05:38 PM
02/16/12 05:38 PM
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Joined: Jul 2011
Posts: 817
Eugene, Oregon
Secret Chimp
OP
super stock
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OP
super stock
Joined: Jul 2011
Posts: 817
Eugene, Oregon
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Quote:
To keep accelerating (with any life) a carb has to have say 25% more cfm available than needed at a specific RPM. so at 4200 at 100% is 386 cfm + 25% = 483 cfm so a 500 cfm carb would be kind of minimum to get to 5000 with some decent pull!
Just my
600 eddies and holleys work good on a 318!
If I don't find any fuel problems this afternoon, I'm just going to finish putting my AVS together and get it on before I mess around any more. This carb ran perfectly up to high RPMs and this isn't that crazy of a cam, maybe cfm is all it is.
1967 Dodge Coronet Deluxe station wagon
1.03" T-bars, QA1 arms/rods, Cordoba/GM Metric/Volare brake & knuckle, XHDs, Hellwig rear sway, 318 Magnum w/ air gap, 727, 3.23s
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Re: Engine falling off just past 4k - carb limit?
[Re: Dodgem]
#1177549
02/17/12 10:48 AM
02/17/12 10:48 AM
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Joined: Mar 2008
Posts: 5,746
Ontario, Canada
Dodgem
master
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master
Joined: Mar 2008
Posts: 5,746
Ontario, Canada
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If the cam is to advanced the power will fall of quickly too. Why is the chain loose? did you not change it? DID you roll the motor over a couple more times to see if it is in fact lined dot to dot rolling and bring the dots back will show when you thought you were straight you were in fact not! 
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Re: Engine falling off just past 4k - carb limit?
[Re: Dodgem]
#1177550
02/17/12 11:29 AM
02/17/12 11:29 AM
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Joined: Jan 2003
Posts: 24,562
Brookeville, Md
Mr.Yuck
Not enough dumb comments...yet
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Not enough dumb comments...yet
Joined: Jan 2003
Posts: 24,562
Brookeville, Md
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Quote:
To keep accelerating (with any life) a carb has to have say 25% more cfm available than needed at a specific RPM. so at 4200 at 100% is 386 cfm + 25% = 483 cfm so a 500 cfm carb would be kind of minimum to get to 5000 with some decent pull!
Just my
600 eddies and holleys work good on a 318!
Ummm you should be balke to put a 2bbl on that thing and have it pull to 5k.
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Re: Engine falling off just past 4k - carb limit?
[Re: Dodgem]
#1177551
02/17/12 05:44 PM
02/17/12 05:44 PM
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Joined: Jul 2011
Posts: 817
Eugene, Oregon
Secret Chimp
OP
super stock
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OP
super stock
Joined: Jul 2011
Posts: 817
Eugene, Oregon
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Quote:
If the cam is to advanced the power will fall of quickly too.
Why is the chain loose? did you not change it?
DID you roll the motor over a couple more times to see if it is in fact lined dot to dot rolling and bring the dots back will show when you thought you were straight you were in fact not!
The chain WAS lose, the new chain isn't. I was wondering if the old chain had done something to the cam timing that changed the rev range vs the new cam and new chain. And yes I did roll it over and the dots came back in line. The fact that it's never the same RPM whenever it hits the wall has me fairly certain I didn't goof up the installation.
1967 Dodge Coronet Deluxe station wagon
1.03" T-bars, QA1 arms/rods, Cordoba/GM Metric/Volare brake & knuckle, XHDs, Hellwig rear sway, 318 Magnum w/ air gap, 727, 3.23s
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post any new info