Reply With QuoteDo you really have flat top pistons or just the ones with the smaller dish. Those are good for almost 10:1. What letter heads do you have?
Generally you want 36 degrees by 3000, so I think you're pretty close.
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.
Do you really have flat top pistons or just the ones with the smaller dish. Those are good for almost 10:1. What letter heads do you have?
Generally you want 36 degrees by 3000, so I think you're pretty close.
I'm sure you have thought of all this but It never hurts to ask. what fuel are you running?. If you can hear it.. that is not good in a boat... Any specs showing on the porcalin? what are the plugs looking like. I would start with the medium springs. Adv all in by 22-2400 start with 34 total to be safe. If you can richen her up a tad that might help some also. possibley a little colder plug. More information is needed about plugs. Any specs showing on the porcalin? what grade fuel? what lobe separation does the cam have. how hot is it getting. Jet sizes. mid range spring?
That Impellor is a big ax to swing. Between that and your cam (high cylinder pressure) it could be lugging it just enough at cruise to cause the detonation. Your RPM's are nice though for an A2.
I would get very conservative with low rpm advance, and start working from the other direction AFA the proper curve set up. but at the same time not retarding it too much as to let it get hot.. and again a dash of fuel for the safety factor.
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 Tom
i alos have a 75 tahiti 455 ,mallory yl dual point converted to unilite.i used mallory degree kit to put 16 degrees in the dist and all in buy 2800 , timed at 16 at the balancer this is with 9.6 comp engle 29/29 cam. the kit no is 209`15 from summit jegs ect
sorry i gave the wrong part no at summit maa- 29015 sry nyjetboat
i didn' cath the spring thing . i use harland sharp rockers and engle springs. i don't think gm spring will do the trick . my lift 544/544 had mill down the valve guides too . that much lift on stock guides they will hit i believe .
You may need some race fuel. With those pistons, your compression ratio may be pretty high.
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