Picking a timing curve

[Nitrojunkie]

Sorry for the long post guys:
Ok, so I got a '75 Tahiti with a 455 Olds, and a Legend pump clearanced at 0.050". I've yanked the 1/4" dished pistons, and added speed-pro flat tops w/ 4cc valve reliefs. I also went with a pretty timid Crane Cams marine cam, it's around 208/216 at 0.050". The heads are GM cast iron heads. And the intake and carb are edelbrock performer. Also, I run an A-2 impeller, so I top at about 4300 RPMs (she's very fuel efficient for a boat).
Shes quick enough considering I run pump gas, but the springs in the Mallory YL are are so worn out that it hits whatever full-advance I have set for at under 2000 rpm. It detonates like hell at low RPMs untill I either retard the timing so far it wont idle, or set the max total advance to be under 25 degrees (which still detonates a tiny bit and leaves nothing on the top end).
Now I have the curve change kit and can set any timing curve I want. In trying to choose a curve I find that boats are funny animals. The engine load increases drastically with RPM. I'm guessing that the small load near 1500 RPMs means I wont want much advance there, since less fuel and air will be in the cylinders than if it was a high-load condition like in a car. That also explains why I've been seeing detonation between near 1500 RPMs. This seems to mean that hop-out curves are right out, since the quick rise at low rpms is exactly what I don't want.
So my question is this... given my setup, what do the guru's think will be a good max total advance (it idles real nice from 10-12 initial by the way), and at what RPM do i want to reach my max total advance? I'm thinking 32-34 degrees at about 3000-3200 RPMs... What do you guys think? Is there anyone who thinks a hop-out curve is a better option for any reason? The max total advance can be changed easily, and I can guess and check that, but its the issue of when I want to hit the max that I'm not sure of.
Also, anyone know (ballpark) what lift the GM springs can handle? I'm thinking about going to adjustable rockers rather than the factory ones, and it would be a good time to up the ratio by a step. My cam's lift is .456/.484, and I'm not sure if the springs can take the extra lift.

[jetboatperformance]

Theres alot that still needs to be known here eg your altitude ,what the actual compression ratio What letter designation on the heads (G,K,J) what and what fuel your useing 87? 91?. But in general the motor shouldn't detonate with 34 to 36 total all in around 2500 .However the deep dish OEM pistons are around 8.5 to 1 OEM shallow dish around 10.25 so its hard to say what the flat tops are ?? I would give some serious thought to getting rid of the Mallory and replacing it with an MSD, they are easily programed,user friendly and offer outstanding performance. Also no offense but Ive personally never had any luck with Edelbrock carbs and dont use them.That motor would likely love a 750 or better dual feed double pumper with 7.5 lbs of fuel pressure . Just out of curiosity what brand AA and why the .050 pump clearance? Happy to try to help just need more info :idea: Tom

[SmokinLowriderSS]

Ok, so I got a '75 Tahiti with a 455 Olds, and a Legend pump clearanced at 0.050". I've yanked the 1/4" dished pistons, and added speed-pro flat tops w/ 4cc valve reliefs. I also went with a pretty timid Crane Cams marine cam, it's around 208/216 at 0.050". The heads are GM cast iron heads. And the intake and carb are edelbrock performer. Also, I run an A-2 impeller, so I top at about 4300 RPMs (she's very fuel efficient for a boat).
Sounds pretty decent for spinning a "AA" with a pretty conservative cam, but I don't really know how aggressive/conservative that is with an olds. It just strikes me as pretty darned mild.
Shes quick enough considering I run pump gas, but the springs in the Mallory YL are are so worn out that it hits whatever full-advance I have set for at under 2000 rpm. It detonates like hell at low RPMs untill I either retard the timing so far it wont idle, or set the max total advance to be under 25 degrees (which still detonates a tiny bit and leaves nothing on the top end).
And "low RPM" is what? My Chevy seldom cruises below 3,000 RPM and by then, my full advance is in anyhow.
I'm thinking you may be miss-reading the detonation, or are WAY TOO HIGH ON YOUR COMPRESSION RATIO.
What heads? 4cc dish, OK, any overbore dimentions or dead stock? Any ammount the piston sits below the deck top at TDC? How thick of a gasket? Give me enough info and I can work up a compression ratio.
Now I have the curve change kit and can set any timing curve I want. In trying to choose a curve I find that boats are funny animals. The engine load increases drastically with RPM. I'm guessing that the small load near 1500 RPMs means I wont want much advance there, since less fuel and air will be in the cylinders than if it was a high-load condition like in a car. That also explains why I've been seeing detonation between near 1500 RPMs. This seems to mean that hop-out curves are right out, since the quick rise at low rpms is exactly what I don't want.
I'm running 25* advance in my MSD dist, with 10* base, full advance should be in by 2800RPM, total being 35*. I'm still running 8.?:1 CR tho and have few fears of detonation as anemic as that is.
So my question is this... given my setup, what do the guru's think will be a good max total advance (it idles real nice from 10-12 initial by the way), and at what RPM do i want to reach my max total advance? I'm thinking 32-34 degrees at about 3000-3200 RPMs... What do you guys think? Is there anyone who thinks a hop-out curve is a better option for any reason? The max total advance can be changed easily, and I can guess and check that, but its the issue of when I want to hit the max that I'm not sure of. On a Chevy I'd tell you 34 to 36*, on an olds, I will defer to others with practical experience, and they ARE on this board.
Before 3-grand should be perfect in a jet boat, IF your CR is not too high.
Also, anyone know (ballpark) what lift the GM springs can handle? I'm thinking about going to adjustable rockers rather than the factory ones, and it would be a good time to up the ratio by a step. My cam's lift is .456/.484, and I'm not sure if the springs can take the extra lift.
Adjustable rockers are a good thing IMO. Get springs in a kit from the same maker as the cam, their reccomendation. That way you are least likely to get into a coil bind condition, but tripple check anyhow to make certain.

[Moneypitt]

Too much compression, need bigger chambers with flat tops.........MP

[squirt'nmyload]

get ahold of timinator on the boards.....he wrote an article in hot boat last month about timing

[Moneypitt]

get ahold of timinator on the boards.....he wrote an article in hot boat last month about timing
It aint the timing...........Small chamber head, large chamber piston........MP

[squirt'nmyload]

It aint the timing...........Small chamber head, large chamber piston........MP
alrighty then

[Moneypitt]

TRW L2323F 9/64ths cup stock replacement: MUST be used with 96 octane or higher.
TRW L2390F 5/16ths cup for use with 91 or lower unleaded fuel.
The .145 cup above (9/64ths) was 10.95:1 standard bore, and 11:1 at +.030.
This cup is 14.5 cc, and he put flat tops in it!!!!!!
AND that is with the performance 70cc heads!!! Imagine what he'll have if the heads have been whored to the low pro ones!!!!!!!
Can we say 12. to one??????? That is just a wild guess, but the performance factory deal at 11:1 with the shallow dish........AND 96 octane minimum seems to be telling us something........MP
ADD: I just re read the post, and he had the 1/4 dish, (well, they were .312 not .250 as he stated), so he is off the scale with flat tops. Race gas is the only cure, period!!

[Taylorman]

If you have stock iron heads with JE pistons with 5cc piston dish which i believe is what they are, your above 11:1 compression. Depending on all the other measurements, could be as high as 12:1.

[Taylorman]

Wait a minute. My brain is in left field. Im reading speed pro and thinking srp(JE).
Now im in the ballgame. Speed pro 4cc dish pistons????????????????
Where did you get these? Ive never heard of such a thing? I have the 2323F and they have a 18CC dish.
Stock heads are between 80-84cc.