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.

[Moneypitt]

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.
TRW L2323F are 14.5 cc dish according to TRW.....And the numbers I quoted are for the "irrigation and marine" and "W30" heads at 69.8cc chambers. As I said, it aint the timing. Go get some race gas, time it to run, and enjoy it. Of course it would be a good idea to find out which heads he actually has....MP
PS, there are two threads running at once on this same subject...........

[malcolm]

PS, there are two threads running at once on this same subject...........
Some of us had noticed that when he first posted them. ;) This one seems to becoming more popular though.
We're all just guessing until Mr Nitrojunkie comes back and fills in the blanks anyway. I'll wait...

[SmokinLowriderSS]

I'm with ya MP, I just wanted to find out what heads he had to confirm my suspicions. My understanding is the Olds heads are all much smaller chambers than the chevy's, like Fords are. I was just trying to hit each of his questions while he dug up info to fill in the holes.
Hell, why race-gas, just switch to Alky, retune, and go like a bat outta hell. :D
Just half as far. :D :D

[Nitrojunkie]

I never got that confirmation email for the forum, so sometimes it lets me post, other times it says "I must be logged in to do that". Thats why the original double post...
At any rate... I'm not sure what the CC is for the heads, as for the compression ratio, I figured it'd be high, but I ran the boat on a stock GM HEI vacuum advance off of an old Trans Am w/ a 403... No detonation there. I retrofit my Mallory to a Pertronix breakerless and was able to bump up the plug gap over the HEI. This gave a noticable performance increase (I know some of you will scream and say as long as the gap is big enough to spark, it doesn't matter... I know... I used to race a 454 roadster on Methanol. I gapped the plugs for the magneto at 0.026" - but for whatever reason the boat ran better with a larger gap)
So assuming I don't know the total compression ratio, but I do know that a timing curve does exist that wont give detonation, I believe that the YL can do what I need it to do for now. It sounds like around 2800 RPMs is where I should target to hit max advance, and it sounds like 34-35 should be no problems... So we'll try that. As for the Edelbrock, they can PITA to tune, but I'm pretty familliar with their quirks.
by the way, I cruise w/ a skier at about 2200 RPMs for whoever was curious.

[Oldsquirt]

Nitro, go look for the large letter cast into the left end of your cylinder heads by the exhaust port. Come back and tell us what it is.

[Nitrojunkie]

Standing on the passenger side of the boat, reading the passenger side head from left to right, I see a giant "K" followed by a small "A".. Over number 8 exhaust port is "CFD", over number 6 is "413" or possibly "418". "191" is over number 4 and "233" is barely legible over number 2.
As for the 4cc dish, that is from memory from about 4 years back and is no way reliable. I did do head gaskets recently, so I know for sure that the pistons have valve reliefs and not a circular dish. Now that I think about it, I'm pretty sure it was the single pear-shaped relief, which will be much larger than 4cc. I was 99% positive they were speed-pro, but now I find myself questioning it... They had the nifty black teflon coating near the wrist pins which should make them pretty distinguishable if it helps.
I know the current Compression ratio is is higher than 9:1, But with how she behaves, I seriously doubt it's any higher than 11:1 (I would guess 10.5). The gaskets are standard fel-pro, not super thin or anything. Neither the deck nor the heads have been milled down. It is 0.030 over though.
Like I said earlier, the engine has run without the detonation on the original points in the yl, also with the GM HEI, and for a while on the breakerless retrofitted YL... This detonation started as the springs started to give out. Now the response to RPM is way too drastic. If I set total advance to 38 degrees, it will be at 38 degrees advanced before it gets to 2000 rpm (which it should NOT do), also as I stated earlier 2000 RPM is near where I pull skiers. This is the problem... It detonates at the worst place, but it didn't used to do it. I am quite certain my compression hasn't increased over time, so timing MUST be the issue.
Also, as I mentiond in my first post, I already run Premium. Although I am at sea-level, so the octane required is higher than it is elsewhere. Premium here is 93 octane. I've tried octane-boost (didn't help a bit), but never have filled it with racing gas.

[Moneypitt]

Try a compression test. Maybe you're burned between cylinders.......Or put the HEI back in it and see if the problem goes away. I read this info as a problem pinging that you had tried everything to cure. I also saw the "flat top" piston info, and went from there.......As did others.......Run the C/ test just for fun, and stick the HEI back in it........Did these fresh head gaskets ever run without pinging? What is the valve train?........MP

[SmokinLowriderSS]

Everytime it tells me I'm not logged in, and I enter my login info it double-posts!
When the login screen came up, I opened a new window to check and see if my post went through, it didn't... then I logged in, and bam! two posts. What gives?
Though you cannot delete duplicate threads (you can ask a mod to do it), you CAN delete duplicate posts. Just go to edit the post, and delete it.
Finally dug up some specs, on a standard 455, factory bore, 80cc chambers in the Ka head, a .039" fel-pro gasket, and I even sat the pistons at TDC 10-thou down inside the block, CR calculates to 11.26:1
.030" overbore makes it 11.40:1 .060" over = 11.53:1
Flush the stock piston (flat-top), 11.52:1
overbore that .060 = 11.80:1
Find the absolute slowest timing curve you can, you are IMO borderline to detonation, and tread lightly.
The MSD 8360 (of which I run an 83606 (marine) centrifugal timing has 6 posible curves. The most aggressive starts at about 700 RPM, and is all in by about 2400 RPM.
The slowest starts about 1400 and isn't all in untill about 5500 RPM
Also 4 different amounts of advance are available, 18, 21, 25, & 28 degrees.
Of course, on each curve, the lesser amounts of advance are reached sooner than the full ammounts I listed at RPM's above.

[Nitrojunkie]

I've had so many GM engines torn apart since I built this engine I have a hard time telling which is which. The worst part is that I had the bottom end built by the shop that bored it, so I never even handled the new pistons myself. But after some reading, and trying really really hard to recall, I'm leaning towards the TRW-L2323F30 as being the the most likely to be in the boat. With 80 cc heads (as someone was kind enough to identify for me, thank you for that) that would give 9.32:1, actually, slightly higher given the overbore. This is more like where I would imagine it, as the motor really isn't radical at all. The boat ran great on 87 with the original pistons, and ran fine on 92/93 with the new ones for a while.
The problem is a not really a ping, but a very subtle knock which is most pronounced at about 1800-2000 RPMs (sounds ALMOST like lifter clatter), and which is responsive to where I have the timing set. It has ran without the knock on these head gaskets, and it is clearly either all cylinders knock or none, so I do not suspect a blown head gasket. Unfortunately I no longer have the HEI lying around, so my only two choices are to get the Mallory in shape, or buy a more elaborate ignition setup. I have everything I need to set pretty much any curve I want in the mallory, So I don't feel spending tons of money on an MSD setup is justified. Sure it's easier to set they curve electronically, but I don't mind spending the five minutes to tear into a distributor to swap springs. I've had a lot of people go off on what I would call wild goose-chases based on compression ratios (granted I provided crappy info).... but I have enough experience with the boat to be certain that this is simply a timing problem. All I was wondering was what shape curve do I want, what number is good for max total advance, and what RPM should it be achieved? I understand compression ratios, cam profiles, intake/exhaust details, engine temp, altitude, fuel octane, etc. all play into this... but everyone got so sidetracked trying to tell me that my problem has to be purely compression ratios, or some other design flaw (basically trying to tell me that the fact that the boat ran great for a few years after the rebuild was an act of god or something).
From the few replies that stuck to the timing question, it sounds like the simple linear curve is in fact preferrable, as I had assumed. It also sounds like mid 30's are good for total advance, but I imagine the actual number will be dependant on the cam and compression more than anything else, I'll just do the old-school trick and tweak on the distributor while its on the floor and see where I get the most on the top end. It also sounds like max advance should be hit before 3000 RPM, which is what one would expect for a car, but still seems low to me for a boat, considering that the change in engine load from 3000 to 4000 RPMs is huge (which is not nearly as true for a car). This means that much less air and fuel need to be in the cylinders to maintain 3000 RPMs than 4000, so it seems like less advance should be required at 3000. Add to this the fact that at 4000 RPMs, the pistons move faster than at 3000, so you'd have to start the air/fuel burning sooner. Basically, I'm not yet convinced the optimal location for max advance is below 3000 RPMs.
Ultimately, i've gotten so many varied responses, the answer seems to be that I'll have to guess-and-check. I think I'm going to start with a max advance of 34-35 and have it reach that at 3500 RPMs and see what happens.

[malcolm]

That's where I figured you'd end up. So now we've come full circle. :D
So why haven't you just gone heavier and heavier on the springs till it works? I have the same setup but with a Comp 280 cam. Runs good on 91 but has a little high end rattle with 89. I have it set for 36* in by 3000.
Maybe try 34 with that smaller cam.

[CARLSON-JET]

Good choice. Just make sure to watch the temps. Retarding the timing to much ( as I think you know) adds alot of heat and if you are running aluminium logs can play hell with them. If you are going to run 87-89 oct. I think your idea of timing all in at about 3k is a decent starting point to work backwards to get the best performance. Good luck. R.B.