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I have a new project, a 1961 with the 315-HP Fuel Injection. Many of the original engine parts are long gone and I am on the hunt for the correct components. I have just purchased the correct dated heads, but they have been ported to the 1.6 and 2.02 valves. How will the porting affect vacuum (I believe that vacuum is very important for the fuel injection to operate correctly)and is there any other areas of concern for building an engine for a fuel injection system?
The biggest infulence in the proper operation of the fuel injection is the correct camshaft. The cam determines to a large extent the manifold vacuum, along with engine tuning.
Don Yesacavage
Ray, As Don said the cam is very important for the proper vacuum to run your 61 FI. Put the wrong cam in there and you are in deep trouble. I know that there are a ton of guys on this DB that have their favorite cams that they say are much improved over the original 3736097 Duntov. That's good.
But in my humble opinion if you want to have a nice running FI with excellent vacuum use the correct style cam.
My 63 FI car engine was recently re-restored. I had the option of using a NOS 097 or my favorite cam. I choose the latter.
Here is what I recommend to many many 100's of my customers.
Speed Pro CS113R Sealed Power by Federal Mogul. It used to be the old TRW TP 113 cam that my friends have been using since day one because it was a lot less bucks the GMs cam. Wonderful cam. THey also have the lifters AT992 (?) Number escapes my mind. If its wrong I will chirp back. BUt there are better lifters on the market.
57 to 63 FI's,etc. used the same cam. At 900-1000 RPM my 63 has at least 16inches of vacuum. My car idles real nice now since it has a few more hours on it at 875. I didn't try and go lower. I actually like a higher RPM if I was planning on driving the car but for the ops check it should be in the 850 rpm range I believe. John
I ordered my cam from CarQuest.
I used to race a street legal 12 second "58 Chevy Del Ray. Had a 283+.060 in it using "Camel Hump" 2.02/1.60 valves. Never experienced any valve-to-cylinder clearance problems.
The .060" overbore yields a finished bore of 3.935". I'm still not sure if the large valve set will clear standard bores or a smaller overbore. This is somethilng that should be carefully analysed for each situation.
I'm not trying to rekindle the Great Camshaft Debate for the sake of starting an argument, but here's what I've learned from my small block system engineering projects over the last several years.
The 097 camshaft was designed for the early small port, small valve heads that have a "balanced" exhaust/inlet flow ratio of about 0.75. For what was probably a purely intuitive design by Duntov, it was a superb high output cam for a short stroke, small displacement engine with the early small valve heads, as long as you were willing to accept its driveability limitations like a rough somewhat unstable idle, poor low end torque, and the need for short axle gears.
When the big port 461 heads came along everything changed. The E/I flow ratio decreased to about 0.65, so the new heads had a restrictive exhaust port relative to the inlet port, which calls for more exhaust duration in the form of an earlier opening exhaust valve. Also, the larger inlet valve increased effective overlap, which is not favorable to the torque curve of a road engine with mufflers.
Chevrolet apparently didn't realize this for some time as the next two SB cam designs - the 30-30 and L-79 - were also single pattern designs (same lobe on both sides). It was not until the new for '67 base engine (327/300) that Chevrolet designed a dual pattern cam with more exhaust then inlet duration, and the extra duration was all on the "front end" in the form of an earlier opening exhaust valve. For unknown reasons the next camshaft design (L-46) was essentially a single pattern type. The inlet and exhaust lobes ARE different at the design detail level, but have essentially the same duration. This cam is effectively the L-79 cam with different detail lobe designs and four degrees retard relative to the L-79 cam.
The last camshaft design, the LT-1 cam, incorporated what Chevrolet learned over the years and has much more exhaust duration with a very early phased exhaust event. This camshaft incorporates two existing lobe designs. The inlet lobe was lifted from the L-72 camshaft, but was recast with a smaller base circle to slip past the SB bearing bores. The exhaust lobe is from the 30-30 cam, but is advanced four degrees relative to the 30-30. This camshaft was actually designed about the same time as the L-46 camshaft, and they clearly represent two completely different design philosophies!
All the above applies to OE machined heads. But when the heads are massaged for road use by pocket porting/port matching and a muliangle valve seat geometry there is a dramatic change - like night and day! These operations improve inlet head port flow by about 10-12 percent, but a dramatic increase occurs on the exhaust side - on the order of 30-40 percent.
The resultant E/I ratio increases to close to 0.80, which requires completely different valve timing to achieve the broadest torque curve. No OE or aftermarket camshaft comes even remotely close to being optimum because this new flow ratio works best with a cam that has a rather late exhaust opening with resulting less exhaust than inlet duration - on the order of about 20 degrees!
Fortunately, opening the exhaust valve too early does less harm than opening it too late, but a properly phased exhaust opening (not too early) will enhance low end torque without sacrificing any top end power. In this end I've come up with both hydraulic lifter and mechanical lifter camshaft designs that use lobes from various OE camshafts to provide more inlet than exhaust duration. These designs are specifically tuned to massaged heads and are TOTALLY UNSUITED to OE machined heads! I've discussed the hydraulic cam (special 300 HP cam) previously. This cam will idle just like an OE 300 HP cam (because it has the same effective overlap), but make considerably more top end power and useable revs with only a slight loss of low end torque. It uses the L-79 lobe on the inlet side and the '67-up base cam (929)exhaust lobe on the exhaust side. And I've recently come up with a "Special Mechanical Lifter Cam" that finally beats the torque bandwidth of my favorite LT-1 cam. It uses the 30-30 lobe on the inlet side and the Duntov lobe on the exhaust side. So the two cams that I least favor for massaged big port heads on a road engine combine to beat all others!
As previously stated, FI engines can be finnicky, and are very sensitive to idle vacuum. The less idle vacuum, the more problematic idle quality/stability and other operating characteristics can be. With a larger inlet valve that increases effective overlap with the same camshaft, the '61 FI engines can be expected to idle poorer than earlier FI engines with the small valves.
Once way to mitigate this would be to decrease overlap, which can be done by advancing the Duntov cam exhaust lobe by four degrees. As designed, the points of maximum lift are 108.5 deg ATDC on the inlet side and 112.5 degrees BTDC on the exhaust side. Advancing the exhaust lobe four degrees yields new POMLs of 108.5/116.5 and increases lobe separation angle from approximately 110 to 112 degrees. Idle vacuum at the same speed should increase about 1-2" - from 12" to about 13-14", or the idle speed could be dropped a couple of hundred RPM to the 12" point. Idle quality will be better (less lumpy - somewhere between the Duntov and l-79 cam), which may be "good" or "bad" depending on your perspective. Low speed driveability characteristics will improve as will low end torque due the less overlap. The earlier opening exhaust valve will negate some of the low end torque improvment due to less overlap, but it will improve top end power. The overall result is a flatter torque curve.
Another alternative is to grind the POMLs at 110/118, which effectively yields a mechanical lifter version of L-79 cam, albeit with less lift, but it will rev to at least 7200, assuming you upgrade the rods, which I recommend for for all early SBs.
Any cam vendor who has Duntov lobe masters can grind a "modified Duntov cam" to these (or any other) POMLs for about $125 more than the list price of an off-the-shelf cam.
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