Was the Rochester system as effective as modern electronic fuel injection? No, not by a long shot. The film supplies a nice overview, and as a bonus, provides a detailed look inside a s-vintage carburetor lab and its fascinating equipment. Video below. The first 50 usable Fuel Injection units were lab assembled. However, in a pound Corvette test car, a pump with loose gears will cause the engine to fall on its face when you floor the accelerator pedal.
There really isn't a practical way to simulate every effect of car acceleration on the FI system other than to install it on a Corvette and drive it hard. My testing program includes driving every FI system I repair under real-world conditions.
To handle this testing I maintain three Corvettes set up to receive customer units and distributors. Two are convertibles. The third test car is a '57 model. I keep my own FI adapter manifolds installed on these test cars. That way I can quickly install and remove the customer's unit and distributor without draining any engine coolant.
My road testing includes some cold engine operation as well as some steady speed cruising after warm-up. The toughest test of any unit is hard acceleration through at least three gears, and I perform this test at least twice.
I've learned a lot through my road testing program. I doubt I would have ever encountered some of the FI problems I've had to solve if I had just kept using that stationary engine test stand. Let's face it… GM never did get the cold enrichment system design quite right for the '57 - '65 Corvette fuel injection units.
They did come close with the '63 - '65 hot air choke design, but even that one needs help to work properly. The design of the '57 - '61 cold enrichment mechanism was an evolving bad joke. It worked as well as a broken clock; it would be right occasionally, but not very often. The cold enrichment came on and went away in one distinct, rather than many gradual, steps.
Initially no vacuum would be sent to the fuel meter enrichment diaphragm. This kept the lever parked on the power stop during the first minute of operation.
The result was a super-rich mixture until the enrichment lever was allowed to swing over to the economy stop. This system used an electric heat coil to control activation. This coil usually cooled off much faster than the engine block. When an owner would re-start his car after letting it sit only 20 - 30 minutes, the cold enrichment system would be activated although the warm engine didn't really need an over-rich mixture.
The first true "choke" type FI cold enrichment system was introduced in It featured a butterfly plate in the air meter controlled by an electrically heated coil. While it did come on and go away in a gradual motion, it was problematic too. The coil still cooled off faster than the engine. A new problem was introduced as well. The steel butterfly plate was installed on a relatively flexible aluminum shaft. That shaft passed through four count-'em four holes in the pot metal venturi cone.
This would throw the four shaft holes out of alignment, causing the butterfly shaft to bind. Chevrolet finally went to an exhaust heat controlled choke in This made the choke activation more a function of true engine heat. The steel choke butterfly was still installed on an aluminum shaft. In this design it passed though "only" three holes in the pot metal venturi cone. However, the shaft would still bind due to changing dimensions after many heating cycles. A new problem was the over-rich mixture caused by the butterfly being too restrictive to air flow when the choke was fully engaged.
The kit included a choke pull-off piston to be installed in the choke housing, a more aggressive fast idle cam, and a weaker bimetallic spring on a new choke cover. This kit is best remembered for introducing a new problem. Engine manifold vacuum would move the pull-off piston down when the engine was running. This piston motion would hold the choke butterfly open against the closing force of the bimetallic spring until the spring warmed up.
There's really not much you can do to improve the operation of the '57 - '61 FI cold enrichment systems. I'll admit they will work adequately to get you down the road when the engine is cold. You'll just have to tolerate the over-rich mixture they produce initially. You can make several simple improvements to the '62 - '65 choke systems. First, you can align-bore the choke shaft holes in the venturi cone slightly over-size.
This will allow the aluminum butterfly shaft to rotate freely again. I would also recommend buying a reproduction butterfly plate and cutting a large window in it.
This will effectively lean out the mixture when the butterfly is closed without having to endure the hassles caused by installing a sticking pull-off piston in the choke housing. Until November, , I used pump premium gasoline for all my road testing and calibration work. Occasionally I'd get a bad tankful at some station and have to replace it with a different brand, but that happened only once or twice a year.
Unfortunately, I consider those years the "good ol' days". Today the pump premium sold here in Mobile is unacceptable for use in Rochester injected Corvettes. Now I use undiluted racing gas exclusively in my test cars. You're probably familiar with the poor idle caused by percolation boiling of pump gasoline in the distribution spider. You may not have heard about the problems it causes during cruising and at high rpm though. Those problems are a little tougher to detect. I was in shock by the end of that test session.
I found that pump gas burns much leaner at idle below 1, rpm and high rpm above 4, rpm during hot weather than racing gas. Oddly, pump gas burns richer than racing gas at legal cruising speeds 30 to 70 mph. This means that although your car will run on pump gas, it won't run its best.
In the summer your idle will be rough after the engine is fully warm, and you'll foul spark plugs more frequently while cruising. This also means sustained engine operation many minutes at very high rpm above 5, rpm with pump gas could burn a hole in a piston or a head gasket on a hot day. Here's my opinion why modern pump gas doesn't calibrate properly in old Rochester FI units.
Gasoline is a mixture of many components. These components have different physical characteristics. Some are "light ends", and vaporize at relatively low temperatures at atmospheric pressure.
This fact isn't apparent when you see the published vapor pressure of gasoline because it's an average for the entire mixture. At idle, the distribution spider is only pressurized to 0.
This is very near atmospheric pressure. A significant portion of modern pump gasoline will vaporize inside a hot spider hub at such a low pressure.
It may not be lean enough for your seat-of-the-pants gauge to detect, but I can certainly see it going lean using the dynamometer control panel instrumentation. The high speed lean-out is caused by a similar, but different, vaporization problem. All pumps need a certain amount of feed pressure on the suction side to maintain liquid flow at the design pumping rate. If this feed pressure called "Net Positive Suction Head" is less than required by the pump design, the incoming liquid will partially vaporize into bubbles.
It's called "pump suction cavitation" when this happens. Bottom line: a pump that doesn't have enough Net Positive Suction Head will not flow as much liquid as it should. The amount of Net Positive Suction Head is determined by the height of the liquid feeding the pump suction side and the physical properties of that liquid at pumping temperature. Hot modern pump gasoline less than 3" high in the fuel bowl doesn't provide squat for NPSH. I can see exactly when gear pump cavitation starts while watching the dyno control panel.
On a hot day, it can occur as low as 4, rpm under full acceleration load. It only gets worse as the rpm's climb. By 5, rpm, cavitation can cause a mixture as lean as That's lean enough under load to hurt some engine parts if you don't take your foot out of it.
Pump gas burning richer than racing gas while cruising is not a big shock to me. After all, octane rating is often indirectly related to volatility. Because it is more volatile, I believe modern pump gas burns more efficiently than racing gas at relatively low engine compression ratios. Running slightly rich while cruising won't cause any major problem. You can run LL Aviation gasoline, but it won't help much. Those special gasoline additives sold at auto parts stores don't have any significant effect either.
I've tried the thick, one-piece, plenum to baseplate gasket too. It was no help. Insulating the gas line from the engine fuel pump to the FI fuel meter makes things worse. I use the VP brand with a octane rating, but other brands of racing gas would probably work just as well. This is due to pump gas boiling in the spider. I know of no FI modification or fuel additive that will completely prevent pump gas boiling at very low nozzle pressures during hot weather.
They tried several fixes to eliminate the problem. The first was to isolate the spider from the heat radiating upward from the adapter manifold. This proved ineffective, so the large gasket wasn't used on later units. The next cure they tried did have some positive effect on the idle percolation problem, but it also introduced a new problem that wasn't apparent for several years.
In the gas feed circuit to the distribution spider was made into a loop. Instead of getting gas that had been through the spill valve, the spider now received only some of the gas from the gear pump while it was on its way to the spill valve. This new design circulated more of the relatively cool gasoline from the fuel meter through the spider hub.
I think that's why the '63 - '65 FI units don't have as bad an idle percolation problem as the pre-'63 units. The down side of this spider feed loop arrangement is its effect on the spider hub design. The anti-siphon needle valve was gravity operated from through In though, gravity could not be used. The spider hub had been inverted to make more room for the fuel supply loop. This made the hub needle valve "upside-down". A tiny spring had to be installed below the needle valve to shut it when the engine was turned off.
This new spring-loaded needle valve design had an unintended effect: it provided a variable resistance to gas flow into the spider. The more the spring was compressed by gasoline flow, the more backpressure it produced.
The previous gravity-operated needle valve had a constant amount of backpressure essentially equal to the weight of the needle valve. I believe this variable resistance of the spider hub spring was properly considered in the engineering design of the '63 - '65 FI units.
However, the manufacturer of these tiny springs probably didn't have good quality control. I believe that many spiders were assembled with springs that had significantly different spring rates than what GM specified.
I call this my "freefall-to-idle" test. It works like this. I then suddenly take my foot off the accelerator pedal and disengage the clutch at the same time. This allows the engine speed to freefall from about 5, rpm to an rpm idle without any resistance from the drivetrain. At this point the FI system has been running full rich for about ten seconds.
It has fully compressed the spider hub needle valve spring during that period. Now the FI system has to immediately create a much leaner mixture.
The spring has to extend to the precise location necessary to support an idle. Evidently that's a hard test for an imperfect spider to pass. If the spring is defective, the resulting idle mixture will immediately be too rich or too lean and the engine will die. Almost all fuel injected Corvettes with starting problems are equipped with Cranking Signal Valves. When the engine starts and the plenum vacuum jumps above 19" water 1.
Well, that was the original theory anyway. However, it almost always failed partially open. This allowed too much plenum vacuum to the main diaphragm, making the FI unit run very rich. A Corvette engine with the "Duntov" camshaft can make as much as 22" water vacuum in the plenum while cranking IF the throttle is fully shut and the Cranking Signal Valve is not bleeding off any vacuum to the main diaphragm.
However, these aren't normal cold starting conditions for FI engines. Usually the throttle is partially open because of the fast idle cam position.
Also, the Cranking Signal Valve is letting some of the plenum vacuum go to the main diaphragm. Under typical cold start conditions for FI engines, the plenum vacuum varies in rapid pulses between 4" and 7" of water. This lower-than-optimum plenum vacuum is actually a good thing. If the plenum vacuum was over 14" water, the Cranking Signal Valve would be essentially shut and should not pass enough vacuum to make the FI unit squirt any fuel from the nozzles. You're probably wondering how much vacuum is required at the main diaphragm for an FI unit to shoot a steady stream of fuel from the nozzles at cranking speed - rpm.
The answer may surprise you; it certainly did me when I tested two units to find out. That's not very much vacuum. Engine manifold plenum vacuum is normally measured in inches of mercury. One inch of mercury vacuum equals This means that plenum vacuum during a normal cold start is less than. My tests of Cranking Signal Valves were also a little surprising to me. I found that the plenum vacuum passed by these valves is extremely low. Hard to tune. Go with a fiteck system.
Work good looks like a carb easy to install runs great. Done a few no problems. Truckdoctor Andy likes this. My opinion. Joined: Jul 20, Posts: 8, Profile Page. Here's one in the classifieds , spendy and you need to find someone that knows how to tune them.
Parts aren't exactly easy to come by and cheap if they are missing. Joined: Oct 22, Posts: 5, Profile Page. Rochester was kind of like Hilborn but the added a throttle valve that used a venture like a carburetor to set up a vacuum signal to control the fuel valve to sense load. Hiborn only knew throttle opening and RPM.. Equal fuel distribution was the big advantage. Deuces and 6-bangertim like this. Joined: Sep 25, Posts: 5, Profile Page.
Except in the Corvette circles, many more were taken off rather than installed over the years'. Deuces , Nitroholic and belair like this.
Joined: Mar 9, Posts: Profile Page. My Hobby Shop landlord has a 62 fuelie that I rebuilt the injection on. Loves it, runs like a top. AngleDrive , Aug 22, Deuces and kidcampbell71 like this.
Joined: Jan 22, Posts: Profile Page. I have owned a '65 unit since the early '70's and ran it on different cars over the years. The above picture is on my Roadster I completed in They are not that hard to tune once you understand how they work and use an wideband air fuel ratio meter.
I finally gave up on it after having idling problems while sitting in traffic. The new gas is so thin and with the ethanol the small lines going to the injectors would vapor lock. If you go the websites of the people who rebuild them they all tell you they won't work correctly with the ethanol gas. The unit never stranded me on the road and I put many miles on it.
Joined: Jan 21, Posts: 1, Profile Page. I have had a few, still have 2 63 units. It really hauled. I stay away from the ethanol stuff. Not sure what you have to deal with in Illinois.
If I wanted to run one now,I would they were really great , just keep a spare drive cable in the glove box. Roger O'Dell , Aug 22, Joined: Feb 23, Posts: 4, Profile Page. Ran one on my 57 Fuely for 38 years. Sold the car a couple of years ago. Never had any major issues. Rebuilt it a few times and a few for other guys as well. Did a 40 that had a 65 unit on it--still running great. Always carried an extra drive cable-never broke one.
We drove that car everywhere. Wife ran it at the drags as well a few times. It did not like the new ethanol gas as stated above. Joined: Nov 9, Posts: 5, Profile Page. Ran a '62 Fuely for 13 years, rebuilt it a few times, street raced it a few times. I was smart enough back in the '70's to buy all the rebuild kits and consumables I could find for future use. Was happy with everything about it.
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