1903 Won't Respond to Tip-In Procedure

[bry593]

Still waiting on the cam. 

In the meantime, I learned about the GM 395 cam currently in the HT383.  It was a Mercruiser marine cam originally.  Boat cams are designed to prevent reversion.  On shut down, reversion (due to overlap) will suck water back into the engine - hydrolock.  Generally, a marine cam has very small overlap and early intake closing.  That in turn makes the dynamic compression ratio extremely high.  This is why the HT383 has engine run-on and WOT detonation (87 pure gas).

GM 395 Camshaft Specs:
246/256 196/206 109 3A
49 ABDC 8.10 DCR 33 OL
.431/.451

Howard's Camshaft 180225-12 Specs:
260/266 207/213 112 4A
58 ABDC 7.69 DCR 39 OL
.450/.465

The new cam will delay intake valve closure by 9 degrees, reducing the dynamic compression ratio by .4 points.  Overlap is also improved by 6 degrees.    Hopefully this will be enough.

[tayto]

FWIW, you can get a GM roller cam p/n 94666492 for about $100 USD. While there are some various claims for duration, I recently put one in a friends boat as he was on a budget. My degree wheel said 202/210 @ 0.050” 112 LSA, 0.300”/0.310" lobe lift (.450/.465 w/ 1.5 rockers).

BTW, this cam has NO fuel pump lobe. An electric pump will be required

[bry593]

Thanks, I did see that.  It's a Crane Compu-Cam 2032.  Back in the day, this was a lumpy street cam for an L98. 

GM 492 Camshaft
270/276 214/220 112 4A
63 ABDC 7.44 DCR 49 OL
.452/.465" LIFT

Primary reason I did not use is lack of fuel lobe (plus a need for new valve springs).  However, the specs are decent for a street 383 with 9.1 CR.  The 383 would tend to mellow it out. 

[tayto]

Like I said, my degree wheel said differently. Don't know if they have changed the grind,but I was nearly 10* less than the 2032 grind on intake&exhaust. Hydraulic camshafts are $400 to $600 Canadian so its a no brainer for EFI SBC. It's a shame about the fuel pump lobe or I probably would have ran with with some 1.7 roller rockers for a bit more effective duration on my recent truck engine build. I ended up using a Howard's 180235-12 on my 10:1 sbc. Looks like 1 step up from the one you are waiting for. Anyway,goodluck!

[bry593]

Interesting.  How do you like that cam for a daily truck?

I considered the 180235-10.  It looked like a fair match with 49 overlap and 57 ABDC.  I saw Howards also offers a -12 (112 lobe separation).

[tayto]

I unfortunately only put 1000 miles on it last summer, had some issue with lifter bores and am only now getting it back together (waiting on exhaust). I will keep you posted during the next month.Hoping to get it on the road by the end of the month

[bry593]

Still waiting on the cam.  Continued disruption in supply chain due to C19 response?  Gates, Xiden and the CDC certainly want to extend the pain until everyone receives experimental gene therapy.  Incremental destruction of your constitutional rights.  Not to mention, I still don't have a cam.   

[tayto]

My vote is still for the GM cam i posted above even though a few less degrees duration. Than the howards cam. I have been waiting dor a missing piece from my flowmaster exhaust kit for over 3 months now. also helping a friend with his 1950 dodge and we bought a parts carb for it and have been waiting over 6 weeks. Yet my rockauto orders are delivered in 3 or 4 days.....

[bry593]

Finally received the cam end of June.  Got it installed and running a few days ago.

Before I removed the 395 cam, I measured cranking cylinder pressure.  It was 210 psi which is too high for 87 octane.  Why did one of those HotRod magazines claim it was 195 psi?  I haven't measured the pressure of the Howard's cam, but shutdown run-on and WOT detonation are no longer an issue (Cliff, thanks for recommending the cam swap!).

My guess is the new camshaft will slightly reduce gas mileage since I no longer have "beast mode" torque right off the line.  It used to almost always chirp the tires on initial acceleration.  I'd guess the power band has moved up about 500 rpm. 

New cam definitely idles smoother with 112 vs 109 lobe separation, even though intake and exhaust duration are increased about 10 degrees.

Exhaust note is quieter under normal driving conditions.

Still running the same carb tune and it seems okay so far.

Overall, I believe the 395 cam would work great in an EFI daily driver, but it is a poor match to an old school carb and HEI.

[Cliff Ruggles]

Good news.  You really didn't give up any power, it's making more power.  Wider LSA simply spreads it out some and doesn't spike it nearly as high.  This reduces octane requirements at the same time.

I'm NOT surprised idle quality is improved with more seat timing.  The longer seat timing and wider LSA will also make for a nice improvement in upper mid-range and top end power.

The engine may not "feel" quite as strong IF you are judging improvements by the "seat of your pants".  Do NOT be fooled by this.  Smooth/strong power over a broad RPM range will trump throwing all the power at you early and in a narrow RPM range. 

This simply happens because you are applying more power to the pavement on a full throttle run vs throwing ALL the power at you quick and early.  When this happens the quick blast of power "feels" like a shot of nitrous and we evaluate it as an improvement over the "locomotive" power from a smooth/flat/strong power curve.

I've tested camshafts on the dyno and it also reflects the same thing.  Tight LSA and shorter seat timing spike peak torque high and it happens early. 

One test I did back to back a few years ago was testing a Pontiac RAIV cam against a custom ground Comp cam with their "best lobes" (at that time). 

The factory cam was 304/316, 231/240 @ .050" and 113 LSA.

The Comp cam was 284/296, 240/248 @ .050" and 112LSA.  We had it ground to replace the Crower RAIV clone cam thinking the bigger Comp cam would make more power and improve both ET and MPH at the track.  The Comp cam also had a LOT more lift than the short lobed RAIV cam.  Even with high ratio rockers the RAIV cam was only .516" lift.  The larger lobe Comp cam was over .560" lift. 

EVERYONE involved in the swap was betting the "modern" Comp cam would outrun the old RAIV grind everywhere.

Well, NOTHING went well that day on the dyno.  We made great power with the RAIV cam, right at 500hp/570tq.  Once we dialed in the best numbers I installed the custom ground Comp cam and we fired up the engine.  Immediately I noticed that the engine idled a little better and it was really "snappy" right off idle whacking the throttle quickly.

I checked the timing and we started making pulls on it.    My heart sank as it printed out the first dyno sheet.   The larger Comp cam was down nearly 15hp and 25ft lbs torque....WTF?

I made several adjustments, timing, fuel, and it just got worse.  I even went in and move the cam ahead 2 degrees and then retarded it 2 degrees and things just got worse with everything I did.

I finally returned everything to the initial settings and fine tuned it to perfection and we were still off 10hp/22ft lbs torque that we just were NOT going to get back.

The engine also made peak power 400rpm's LESS in the RPM range with the larger cam, or done at 5200rpm's vs 5600rpm's with the RAIV clone.  I just couldn't figure it out?

So I changed directions and installed a HR cam (we had one ground just in case this testing didn't go well). 

The HR cam was a LOT bigger than in lobe lift than either of the flat cams.  It used .361" Comp XFI lobes, 284/296 @ .006, 230/242 @ .050" and 112LSA.


........Continued

[Cliff Ruggles]

I was expecting MAJOR improvements moving up to the roller.  We did end up making a little more power with the HR cam, after spending the entire weekend on this testing it netted 3hp/4ft lbs better than the Crower RAIV cam we started out with.

Wow, I got a lousy 3hp/4ft lbs improvement spending well over $1000 on a cam swap...bummer.

Even though the HR cam made a few more HP it actually made it at 200rpm less than the flat cam.  All of this baffled me for a while until I thought about what the engineers were doing with cams back then.  They were using longer seat timing and wider LSA but not much lift.  They must have known that the time the valves spend off the seat can very quickly trump shoving them around with greater authority and much further off the seats. 

I suppose it all makes sense if you sit back and think about it.  When we have 30-40 or even more cycles per second happening longer seat timing provides a considerable more amount of time for cylinder filling even if we aren't shoving the valve open as far or opening/closing them more aggressively.

Just some food for thought here as it's pretty rare to see anyone actually doing any back to back cam testing with no other changes.  The folks selling cams ALWAYS change several items in the mix so their product wins the prize.......FWIW.......Cliff

[old cars]

Wow that was a long read. Cliff was right when he said the carb should have been installed with stock calibration. Even if changes to calibration were wanted/needed you at least no where you started.

Unless you are as experienced as someone like Cliff, making more than one change at a time can be counter productive. Making single changes enables you to learn how each circuit works/reacts to changes.

.040" in the hole pistons and thick? head gaskets is a disaster for detonation/timing issues. Not sure I ever saw what the total ignition timing was. That led me to wondering why it wasn't backed off a bit.
Vortec heads do not require or like as much total timing.
Cliff's comments on ignition timing/curve, vacuum advance were on the money.

[Cliff Ruggles]

Thanks.  Correct about timing, and fuel requirements follow that as well.

The better you do with the engine build in terms of compression, quench distance and cam choice the LESS timing and fuel the engine will need to be happy.

If you were doing this thru the "smog" years you found out first hand how these things work.

Back in those days I owned and drove a 1970 Chevy Impala with a 350/300hp engine.  It was powerful, idled smooth, great throttle response and nailed down 20-22mpg's on the highway w/o overdrive.  It was stone stock with the only modification being dual exhaust, H pipe and a set of free flowing mufflers. 

Those engines had flat top pistons, small cam, and excellent flowing small combustion chamber 041 heads.

I also owned and drove a 1970 GMC 3/4 ton truck with a 350 in it.  Even with 4.10 gears it got pretty decent fuel economy and made great power.  The engine started using a lot of oil and needed valve guides and seals.  Back then I had no funds for this sort of thing.  I was offered a set of finished heads from a local hot-rodder in trade for the heads from my 350 engine.

He just told me there were 1.94's and would work fine.  Well I did the head swap one weekend using parts store "rebuilder" thick head gaskets.  That move knocked what felt like about 100 horsepower out of my EXCELLENT running 350 engine.  I couldn't believe it.  What a "turd" it was with the new heads in place.  Then I started looking at the heads I removed and noticed smaller chambers and much larger ports.  I pulled the heads I was given back off the truck and compared them closer.  The heads he had given me in trade were "882" castings. 

I gave him back that junk and did a few side-jobs and got my 1970 heads rebuilt and put them back on the truck.  I also used .020" thick steel head gaskets just like came off of it because I wanted everything right back the way it was.  Power was restored and I logged that experience deep in my memory banks. 

LOW compression, thick head gaskets and chitty flowing heads KILL power in these engines.  The smog years were not good for the industry and they didn't get it figured out till almost 2 decades later........