Fuel Injection Vs. Carburetor

[Ray C.]

I received some new Corvette reading materials for the Holidays and there has been several passages referencing to a correctly adjusted carburetor or carburetors out performing a fuel injection car. That some of the higher performance ratings for the fuel injection cars were marketing driven rather than performance driven. Could someone please provide some additional information on carburetor vs. fuel injection performance?

Thanks! Ray

[Clem Z.]

Re: Fuel Injection Vs. Carburetor

the big thing is air flow as the std equiptment FI are set up for milage and emissions BUT the carb conversions go for HP not milage or emissions

[Ray C.]

Re: Fuel Injection Vs. Carburetor

Thanks Clem,

I should of added that the reference was in the C-1 section or when fuel injection was first introduced into a Corvette.

Thanks Ray

[Clem Z.]

Re: Fuel Injection Vs. Carburetor

those older FI were flow limited as places like gulf research and bill thomas tried installing dual air meters to try and up the air flow. the early ones we cut apart down the middle so we could open up the runners BUT i think we were air meter limited. i had some dyno sheets from gulf when they tried the dual air meters. ron mccafferty was the fellow in charge of the dyno work at gulf back then. i will check and see if i can find them BUT don't hold you breath as the was 40+years ago

[mike mccagh]

Re: Fuel Injection Vs. Carburetor

clem is correct. a 57 thru 65 rochester air meter is only capable of 580 CFM, whereas a pair of WCFB's will exceed that # significantly.mike

[Dave McDufford]

Re: Fuel Injection Vs. Carburetor

I heard or read the main advantage to the FI in the early Corvette was road racing. During the turn while road racing the fuel in the carb bowls would slosh around and you could lose power. I do not think it was a HP thing - but the Beach Boys liked it .

Dave

[Clem Z.]

Re: Fuel Injection Vs. Carburetor

it was for road racing to prevent carb problems when in the turns. when holley came out with the "le manns" float bowls for the ford GT racer for le manns race then the fuel injection offered no advantage. i stll have the invoice for the set i got early in 64 for use on my 365 HP corvette that i used for autocrossing and hill climbing. i think they cost me all of $35. i sold them on ebay several years ago for a lot more than that to someone building a 427 cobra kit car. he wanted them for the correct look. they are called "center hung float style" now, but they look very different. the newer float bowls have the needle seat vertical but the le manns one had the needle seat horizonal which made the carb very long

[Russ U.]

Re: Fuel Injection Vs. Carburetor

If you look at the road tests done by by magazines during the 1957 to 1962 era, I think the FI units outperformed the carb units.

You can see the road tests in the C1 Registry database. The test results vary from driver to driver and magazine to magazine. But if you read all of the results, you will come away with the impression that the testers clearly believed that FI outperformed carbs.

I created a table comparing the results awhile ago, which you can see below.

A road test by Car Life Magazine in 1961 pitted a 1x4, 2x4 and FI setup against each other. The best test of all. FI won. See the link FI versus Carbs below for the test results.

Russ




FI versus Carbs

[Duke W.]

Re: Fuel Injection Vs. Carburetor

FI has a more efficient manifold. It is essentially a single plane "tunnel ram". As a result, FI will make more top end power, but less low end torque than an equivalent long block with a 180 degree manifold and carburetor.

Carb or air meter flow rate has little effect on power as long as it is not excessively restrictive. The cylinders are the pump that moves air, and air meter or carb rated flow at 1.5" Hg depression is just a way of quantifying relative restriction.

Engine air flow demand can be estimated by using 80 percent VE at the redline for a production engine, which would be about 425 CFM for a 283 at 6500 and 492 CFM for a 327 at 6500. At the 5300 redline of medium performance SBs the air flow demand would be 347 and 401 respectively. The 2x4 manifold also has very short runners relative to the 1x4 manifold, and this will trade peak torque for top end power, but using the 283/230 and 283/245 as examples, I doubt if the difference in power is as high as the advertised 15 HP.

The biggest restriction on production engines is head flow, and if you improve head flow FI will begin to pull away on top end power, but will still be short on low end torque due to the manifold architecture. Pocket porting/port matching the heads an multiangle valve seats will improve redline VE to about 85-90 percent without effecting low end torque, which is why I recommend this procedure for engine restoration if you want "more power".

For example, Dave McDufford's engine ('65 L-79 with massaged heads and LT-1 cam) made about 360 HP SAE gross @ 6500 on a dyno, which was equal to the top end power predicted by the Engine Analyser simulation program, and if the SHP manifold/Holley carb is replaced by a FI system the peak power prediction increases to about 400, but the bottom end is soggier.

A production L-79 is a lot closer to 300 honest SAE gross HP than 350, and the difference between the production carburetor and manifold and FI is marginal because the OE machined heads are close to choked at 300 HP, but open up the heads and a higher flowing inlet system will produce significantly more power, but may hurt low end torque.

Dave's engine's top end power is now limited by the 327 SHP manifold, not the heads. The same long block except for the substitution of a 30-30 cam retarded 4 degrees, an FI system with air meter flow improved to 750 CFM @ 1.5" Hg, and good race headers and open side pipes will make about 460 HP @ 7000 according to EA, but is pretty soggy below 3500. This would be a good engine configuration for a vintage racing Corvette.

The key to good engine performance - high top end power with accepatable torque bandwidth in the useable operating range (which is much broader for a street engine than a racing engine) is complementary components that work together rather than fighting each other.

Duke

[Duke W.]

Re: Fuel Injection Vs. Carburetor

It's tough to make and apples to apples comparison on 283s due to the many different configurations. The closest comparison would be the 270 HP and 283/290 HP FI engines, but the 283/290 FI engines had 10.5:1 CR versus 9.5 on the 270, and, of course, gearing has a significant impact on 0-60 times.

The '63 to '65 mechanical lifter engines are the best comparison, because the long blocks are identical, but the magazines always either tested FI or 300 versions and later L-79 versions. I don't recall ever seeing a test of a Corvette equipped with L-76, but a comparison of same year L-76 and L-84 with the same gearing would be instructive, and I don't think there would be much difference.

From my own experience of highway faceoffs with '63 to '65 FI cars my 340 HP engine was always their equal.

Duke

[Tom P.]

Re: Fuel Injection Vs. Carburetor

I have a SB400 in my 56 with an FI unit that is assembled from multiple components. The plenum is an early unit (58-59 style) that was cut in half by Bill Thomas, thoroughly opened up and welded back together. He was so good at doing this that it is nearly undetectable externally! The air meter is a 62, which flows more than the 57-61 air meters and the fuel meter is a 62. I have done various other mods to the FI unit and it runs great and is totally trouble free--------------------up to about 5500rpm, then it just falls on its face! The air meter just will not flow enough to feed 400+ cubic inches! If I could come up with a complete 62 air meter, I would modify my FI into a dual air meter unit like the one on Rich Mason's SR2. Not only do I believe it would allow the 400 to perform up to its potential, but if you have never seen the dual air meter FI unit on Rich's SR2, you've missed a real treat (that is if you are an FI lover as I am).
I've also added a 57 airox so that the FI unit is supplied with cooler air. Prior to adding the airbox, I was constantly fighting the typical FI heat problems. But with the 57 airbox, the engine is now much more trouble free (in regards to FI heat problems).

[Duke W.]

Re: Fuel Injection Vs. Carburetor

I doubt if the air meter has much to do with power rolloff. Long stroke SBs don't rev like short stroke setups because they run out of inlet runner flow at relatively low revs. A 3.75" stroke SB at 5500 revs is running at 3438 ft/min mean piston speed, which is equivalent to 6347 revs on a 327 and 6875 on a 283.

If you want a long stroke SB to rev like a short stroke SB you need more head flow, more inlet runner flow, and more cam, but due to the physical limitations of the size valves and runners you can pack into the 4.4" bore spacing, a big displacement SB won't make the power of a BB, which can support bigger valves and runners due to the larger bore centers.

Long stroke SBs are essentially "torquers", and if you system engineer them properly they will make respectable power up to about 5500, but rarely will rev useably beyond 6000 unless the cam is so radical that you kill the low end torque, which defeats the purpose of increasing displacement.

As discussed by Taylor, "similar engines" - identical except for stroke - will make about the same peak power at the same mean piston speed, so if you configure a 327 to make 350 HP at 6000, stroking it to 3.75" will result in about the same peak power at 5200, but it will have more average power across the rev range due to the greater average torque resulting from increased displacement, which is the primary advantage of increasing stroke.

Duke