Check out the Edlebrock 3702 cam specs, this is about as far as the stock computer can be pushed, even at that Edlebrock makes available a chip in case the stock computer pukes at it.

What's going on is the stock computer has a bit of learning space for accomodating different enviroments around the country and for normal wear. So with a functioning O2 sensor it will pick up some moderate changes in cam timing as well as headers and duals. The factory learn function is rather slow at making the changes permanent so you have to go through that cycle a few dozen times before all the hicups go away as it relearns on every start up for a while.

I don't see any reason for the straight timing, installed exhaust systems always need a little more timing on that side to accomdate the plumbing beyond the manifolds or headers.

Bogie

 

Moving to Electronic Engine Forums

 

The 3702 from edelbrock is quite a bit different than the summit 1102. I always thought they were the same thing due to the part number.

The 3702 looks like the 300hp cam from GM but with less ramp perhaps.

What is it about the larger cams that the computer doesn’t like? Seat duration? 0.050 duration? LSA?

The 3702 is probably a good upgrade for the peanut 305 cam, especially the super anemic truck 305 cam but it doesn’t look like it would be the pick-me-up that the 1787 is.

As to the single pattern I chose it for a few reasons.

- I couldn’t find a decent dual pattern with a 110 LSA in the price range I wanted. I didn’t want to go to a 112 LSA in an attempt to get the torque curve in as fast as possible.

-Being in a truck, I didn’t want to shift the torque curve any higher with the additional exhaust timing considering how much duration i was already adding.

- And comparing to the summit 1102 (price range) it has 28 degrees less seat duration on the exhaust and 18 degrees less on the intake side.

So that’s why I went with the single pattern. Right wrong or indifferent. Mostly it was that I couldn’t find what I wanted in a dual pattern.

Being that the 1787 iis so much bigger than the 3702, you think it probably wouldn’t work with the TBI?

 

The TBI computer likes high maniflod vacuum, thus the relatively short duration high LSA cams.

The way this system works is manifold vacuum (what GM calls Manifold Absolute Pressure or (MAP) isin fact mapped in the chip to RPM and throttle position ( hence the Throttle Position Sensor (TPS). There is a mapped relationship of these three things that the chip uses to compute what's called an address. The mapped address contains fuel and spark instructions for the engine loading that the exists there. When a change is made to the camshaft now the relationship of MAP to RPM to TPS generates a different address that has instructions for fuel and ignition timing that are different from the preprogrammed load instructions, so you get the wrong fueling and ignition instructions for the conditions. The mapped instructions are fixed in the chip, there is a little learn for environmental change and normal aging so if you stay close to the Edlebrock it usually picks it up unless the engine is really tired and has used up the available band width just trimming for outside events. If the engine is fresh and configured in stock form, then you have a chance of picking up the changes.

Bogie

 

Thanks for the info. Sounds like Standard speed-density programming with some room to learn. Makes sense to me.

I should get a vacuum gauge and see what kind of vacuum I get at idle. It idles pretty smooth.

 

The Eddy 3702 is about the same as the base level 327/350/400 car 194/202 durations cam...

Several comments under the 1102 / K1102 cam say it worked well with their stock TBI... and can't beat the price with lifters:

https://www.summitracing.com/parts/sum-k1102

Same as Eddy 2102 cam (except price):
https://www.jegs.com/i/Edelbrock/350/2102/10002/-1
.

 

The Eddy 3702 is about the same as the base level 327/350/400 car 194/202 durations cam...

Several comments under the 1102 / K1102 cam say it worked well with their stock TBI... and can't beat the price with lifters:

https://www.summitracing.com/parts/sum-k1102


Same as Eddy 2102 cam (except price):
https://www.jegs.com/i/Edelbrock/350/2102/10002/-1
.

So this is the crux of my question. If the 1102 works with the TBI, shouldn’t the 1787 also work?

The 1102 and 1787 have very similar overlap despite the different LSA.

Upon retrospect, I was confusing the eledbrock 3702 with the 2102.

 

Assuming that both the 1787 and the 1102 both are measured using the same seat-to-seat(advertised duration)measurement spec(i.e. ,004", .006", ect lifter rise) the 1787 actually has 19° less overlap.

1787: 260/260° dur, 110° LSA = 40° overlap seat-to-seat.

1102: 278/288° dur, 112° LSA = 59° overlap seat-to-seat.

This calls into question whether both cams are rated at the same seat-to-seat spec point. ???

If we look at it from a intake valve closing After Bottom Dead Center(ABDC) standpoint and how this relates to intake vacuum/more mixture trapped/less shoved back out by reversion as the piston is rising.....
Using listed advertised duration and LSA, and had to assume a 4° ground in advance to generate intake lobe centerlines(ICL) since no cam card information was available and no ICL was listed for either cam...

1787: intake valve closes at 56° ABDC

1102: intake valve closes at 67° ABDC.....11° later than the 1787.

If both cams are in fact rated for advertised duration using the same spec point then the 1787 is going to develop more vacuum.

This is why you can't just look at LSA and say that a larger LSA means more vacuum.

 

I don't doubt that with the cam appearing to have quicker ramps and it having less overlap, along with the higher velocity heads that your 305 is stronger on torque. Sounds like probably a pretty good combination for what is involved.
The original 350 would probably respond in the same fashion with swirl port heads and appropriate cam spec.

 

You have to watch LSA, it really is nothing more than the exhaust to intake lobe centerline angle divided by 2. The actual event timing is what counts both overlap and intake closing.

The LO3, 305 head's intake valve pocket is not as impeded as that of the LO5, 350; the 305 pocket ramp being quite a bit smaller. But either head is more responsive to duration increases than to additional lift. However, that said using a 1.6 rocker speeds up the entire lifting event so the valve sees more lift sooner that lasts later in the lobe cycle, so there is a benefit as long s the overlap of the cam isn't too long.

When you look at GM's TPI (TBI didn't get as many choices even the TBI cop cam is no big shake) cams, that are mostly rollers, you see the durations really hang pretty close to the old 300 horse flat tappet cam (which also works very well with TBI). The LSA's of these cams travels a spectrum from 112 to 117 degrees. Since these cams don't wander very far apart in duration with them the LSA is a pretty good comparator of overlap and intake closing event.

- The L30 and L31 Vortec cam uses 191/196 at.050 with almost a 112 LSA, this cam works well with LO3 and LO5 TBI engines as do the next examples I'll provide except for the HT383/Ramjet cam.

- The HT383 and Ramjet 350 cam at 196/206 on a 109 LSA is an example of a cam that would need a chip as that's how the LSA change of 3 degrees and adding a little duration affects TBI. If you stick with 1.5 rockers and the engine is in super good shape there is a chance the self learn will cover that pesky 109 LSA. That LSA gives a nice oldtime idle, but given the total timing its telling the world a lie about having a hot cam in your motor.

- The Production LT1 cam times 201/208 with lifts close to .45 inch with an LSA of 117 degrees. TBI engines just love his cam especially when mixed with 1.6 rockers without a chip change.

- The production LT4 cam times 203/210 with lift again about .45 inch on a 115 LSA. This cam runs on stock TBI tune very nicely with 1.5 rockers, generally it will take a 1.6 rocker if the engine hasn't stretched learning out for wear or oddball environments.

For comparison the old 300 horse flat tappet cam of years gone by timed 195/202 at .050 with lift really close to .4 inch on an LSA of 111 degrees. As with the slightly milder duration but with a tiny bit more lift than the L30/31 Vortec cam. Keep in mind the old 300 horse 327 and 350 was rated at SAE gross power (no functioning accessories nor installed exhaust system. The L31, Vortec was rated 250 hp SAE net power which is working with operating coolant pump, spinning but unloaded alternator and some regular production exhaust system.

So when shopping for a cam that works with production TBI engines use this as a guide to arrive at not having to spend hundreds of dollars on a custom chip and all the PIA of dialing those things in that often comes with them.

You can see that GM engineering has dinked with the problems of dialing EFI power in with the confines of how these early EFI systems are designed.

Since these cams are quite similar in gross timing the LSA does shed a little light on overlap quantity.

Bogie

 

Awesome response Bogie. A lot to digest there.

Thank you.

 

So reading between the lines, the 1787 might be outside Of the learning capabilities of the stock TBI.

This is based off the 0.050 cam timings. The only thing I don’t know is the seat-seat timing of the various production cams. If that matters, that is.

The LT4 cam being the most interesting and comparable to the 1787. With the LT4 being one of the biggest you can get to work, it gives me doubt that the 1787 would be OK because Of the 110 LSA.

That said, the 1787 has relatively short ramps compared to comparable factory HFT grinds.

And I keep going back to the 1102 cam from summit. That supposedly works fine. The 1787 has less overlap than the 1102 despite the narrower LSA. So based on that, it should work. But compared to the LT4 0.050 timings - it looks like it won’t.

So confused......

Perhaps looking at overlap from a 0.050 + 20 degrees will shed some light on it. As that seems to be another common way of comparing cams on a calculator.

 

I've never used one, but always thought the SUM-1787 (204/204, 260/260, 110 LSA) cam would be a good upgrade to a stock cam in a low compression engine. Looks like it might be made by Crane, because specs are the same as the old Energizer 260H.

The intake valve might actually get closed a little sooner than the stock cam's if you use ADV duration to determine closing angle. And even though overlap @ .050" is a little more, it may be less than stock if you use ADV duration. Wouldn't surprise me if manifold vacuum was at or near stock levels.

Not sure what kind of air/fuel movement is getting by the intake valve below .050" tappet rise (.075" valve lift) on the compression stroke, but it's at least some. So that stock cam with it's slow ramps may actually let more cylinder pressure blow by the intake valve than the SUM-1787.

What say you guys? Maybe great for a carb but not so good for an archaic EFI computer?