BBC build- Don't tell the Admiral

[Scott Danforth]

The cam is supposed to be an HP500 clone grind from Arizona Speed Marine. it is a custom Comp Cams grind



Here are the old crane specs for the HP500 EFI.

292*/298* @ .004"
230*/236* @ .050"
.352"/.359" lobe
.598"/.610" valve
114* LSA
109* ICL
119* ECL

Here are the HP500 carb’d cam specs

284*/292* @ .004"
222*/230* @ .050"
.576"/.598" valve
110* LSA
105* ICL
115* ECL

The comp-Cams custom grind version thru AZSM

277*/287* @ .004”
228*/232* @ .050”
.588”/.596” valve lift
114* LSA
110* ICL
VOI 4
VOE 54
VCI 44
VCE -2

I verified the cam when I installed it using a degree wheel, dial indicator and a tubing wrench as a piston stop. I checked cylinder 1 and 3. The actual valve lift I was measuring was .586”(.344”) and 0.594” (0.349”) using checking springs ($3 from ace hardware). cam valve events match the cam card

I measured 3 times prior to buttoning up the timing cover, bolting the heads on for the last time and

Per recommendations of AZM and Comp, the cam is installed straight up.

 

[Scott Danforth]

Something isn't right....

agreed. however everything tells me the cam should be fine. However the more digging I do , plenty of HP500's have reversion, and plenty of stainless marine exhaust systems have reversion. Could I be the one lucky guy that has the perfect mix of both gremlins?

I may have to live with a high idle if the H-pipe doesnt work, or go dry pipes.

 

[alldodge]

Don't know what the spec is on my cam (530HP) and I'm using EMI with +3 and long runners. The EMI and Stainless is pretty much similar design manifolds. If you have to run 1100 at idle that is going to be a real bear to dock, and may be an issue in the no wake zone.

Maybe post a question on OSO to see if they have any insight

If you do I can help post the pics if needed, but you can also link from iboats

 

[Scott Danforth]

AD, will probably post a thread on OSO. What is your member name on OSO?

 

[alldodge]

AD, will probably post a thread on OSO. What is your member name on OSO?

Same as here

 

[Scott Danforth]

Same as here

here is the thread I just started on OSO https://www.offshoreonly.com/forums...uild-having-reversion-issues.html#post4609909

 

[Scott Danforth]

OK, so I have access to a few tools...... modeled up the +6 elbows and how I am going to cut them apart, and re-weld. with all the elbows I have, I am aproximately 50" prior to exhaust gasses mixing with water

I superimposed an EFI 8.1/Bravo motor in the picture for reference, even though I am running carb, etc. plenty of room, and I can most likely still run my selectable exhaust

 

[Scott Danforth]

per advice on OSO, I backed off the base timing from 14 BTDC to about 8 BTDC. that significantly reduced the amount of reversion.

Here is a video of the motor running
https://www.youtube.com/watch?v=Lxw0OhrSc4A

here is a shot of the starboard manifold after yanking the elbow.

next step is longer jacketed elbows

 

[Bondo]

Ayuh,.... Lookin' Great,.... That's a huge package of pipes to get stuffed into that small space, for Sure,.....

 

[alldodge]

Sounding better, but still at 820 rpm, do you think you can get it down around 700 or less?

 

[Scott Danforth]

Ayuh,.... Lookin' Great,.... That's a huge package of pipes to get stuffed into that small space, for Sure,.....

the welder I have working on this just finished a '72 vette with LS conversion and twin 80mm turbos. so he is good with stuffing 90# of stuff in a 1 # box. the tubes will be cut down 2.5" (see below)

Sounding better, but still at 820 rpm, do you think you can get it down around 700 or less?

not sure on 700 or less, will know after I get the longer tubes AND cross-over made. I may in theory be able to hit 650 RPM without reversion and may be able to pull my timing back up a bit.

I think I now know why the Merc HP500's had reversion, even at 8 degrees of base timing. cam overlap and base timing have a lot to do with reversion.

I also learned that videotaping a motor running in a garage directly under a tin door makes casual observers make comments on rocker noise, etc. sound wave harmonics and cheap phone mics make for bad video.

 

[Scott Danforth]

So, just a few shots over the weekend. Hired one of the fabricators to swing by my buddies shop to work on the exhaust. We have most everything cut, here are a few shots as the new tubes are going back together. the H-pipe cross-over will be 3" diameter tube. originally we were going to put a flange there, however thought it best to simply make these one-piece. Should be fully jacketed by this coming weekend

 

[alldodge]

Nice fab work

 

[Scott Danforth]

same guy I had weld up my oil pan.

 

[Maclin]

Very nice so far.

 

[Tail_Gunner]

I may just keep the idle high. However since the elbows head to within 6" of the transom, i will be welding a 90 degree elbow on the ends with a 6" tail. Both inner and outer tubes.

Should be about another foot of run

A thought... I seem to remember it takes 1000 rpm for reversion to entirely clean up, I do not know what can be done with your ignition timing below 1000 but if you can advance it that too will clean up reversion. Its been so long I cant recall for sure but I do think one can program the module to advance 3 degrees below 1000. Your engine sounds very tight very good that sharp lope is indicative of a good throttle response. Very nicely done that boat should pull like a tractor on rail's.

Opps your way ahead of me lol I see you have done as much already...

 

[Scott Danforth]

I have to tweek the advance curve a bit. the dizzy is old-school..... springs and stops. can set that up anyway needed, however for now just adjusting the stops. my fabricator sent a pic of the cross-over pipe in place a few minutes ago that he fitted over lunch. its coming together quite nicely.

 

[Scott Danforth]

a bit of progress with the exhaust since I have been working mostly on the new fence and roof at home. last bit of water jacket was welded today. will be polishing it back up this weekend.

 

[Bondo]

Ayuh,..... B e a u t i f u l work Scott,.....

 

[Scott Danforth]

getting back to the exhaust (we just painted the house and had it re-roofed)

started polishing it.



managed to burn out 3 harbor freight tools during the process, then I simply dug out my old craftsman right angle to finish buffing