MILD THUNDER

iTrader: (3)

Yes! Good advice on the mounting bolt torque.

Shibby1485

Bringing back an old thread.

Finding myself at a crossroads, I have a 420 mega that was converted to 3 lobe helix rotors. Updraded shaft etc etc.. I currently run it on my 515" with dart pro 1 325 heads. Just under 7psi made 830 hp. I have a superchiller on it. And a pair of Holley 750

I'm thinking of stroking my setup, would be a 548, and Sending heads out to go 355cc.

I'd like to see 950+ HP on pump gas... We'll see how much I have to overdrive the 420 to make boost.

MILD THUNDER

iTrader: (3)

I think you could make that power with the 325s. Best bang for the buck, would be porting the chambers, bowls, good valve job etc. Simply going bigger on the intake port, may not do a whole lot for flow below say maybe .600 lift.

Guess what im saying, is a 325cc port , that flows as good or better from .200 to .600, will result in a better running combo than the same flow from a 355cc port. The 325 will simply have better airspeed in the rpm range.

Shibby1485

Makes sense... I will consider that input when it comes to sending things out for machine work.

I'm 8% overdriven currently with my 515" setup to make almost 7psi with chiller. Worried how much I'm going to make the modded helix 420 megablower work and spin to make the power and boost and keep the intake temps down. How far overdriven until the temps get silly even with a chiller to just suck it up and change my blower package... I would love to keep my setup, I'm rigged for it.

Mild Thunder.... Thoughts on going to 8mm belt drive instead of the spring loaded tensioner and 16 rib I'm still running?

MILD THUNDER

iTrader: (3)

Thats right about where i was on the dyno with my 420 blower. 8% over, made right in the 7-8psi range. Currently, they are at 16% over now in the boat. Seeing about 8.5-9psi according to my data log map sensor readings.

Ive alwaya kinda said , that i felt blower size/overdrive, was more HP related, than cubic inch related. My overdrive, and your overdrive, were both showing similar boost levels, and also making similar power (mine 818hp). I have 468ci though. Makes sense to me, that an 800hp simply needs to move xxx amount of air , and it takes xxx amount of overdrive to get there.

I dont know how much the 3 lobe will help, but i do think, the 750 carbs, arent gonna help much. You want maximum available cfm available to the blower. They do not "suck" very well on a roots. I know several guys over the years here, who have went to larger carbs on their roots setups and gained boost, with no other changes to the blower ratio.

I have twin 850s on my setup, and kinda wishing i would have just bought a pair of quick fuel 1050 cfm 4150 carbs.

I dont think youll have an issue with your current 16 rib belt setup. But, going to a cog certainly wont hurt.

mike tkach

iTrader: (3)

imo you would gain power by going with bigger carbs.also i don,t know your compression ratio but if it is 8 to 1 you could up the boost a pound or two.i don,t think opening the intake runners to 355 would be a good move.i had a pair of 548s with dart 320s,8.2 to 1 c/r and a crane 651 cam,8.71 blower and a pair of 850 carbs making 950 with dyno headers. add in,it had superchiller,s.

Shibby1485

Great information guys, thank you for the input. Also great comparing notes on our builds.

My 750 carbs have choke horn tunnel still on them also. I guess rather than getting them milled off I should consider picking up bigger carbs, I've been told that already a few times also, just been cheap about buying new ones.

My cr I believe is about 8.8:1 but if I'm moving forward with a 4.25 crank I will have to order pistons also and appropriately change CR. Quick reason I'm thinking of upgrading... Been running stock GM balancer that was externally balanced(ughh) to the GM crank I'm still running... Well I have rubber in my bilge past few runs, from the balancer coming apart, and coincidentally a massive splattering oil leak in the vicinity. I already have Carrillo 6.385 rods in my 515. Figured a crank and pistons and get it all internally balanced to run an ATI or fluidampr.

I run a Daytona ignition system with a MAP sensor... My data logging shows about 6.8-7.2 psi typically.

I just figured the 3 V accessories being driven, with a stock GM balancer, with 16 rib blower belt and that spring loaded tensioner keeping it so tight, there's gotta be some forces on that GM crank with all that hardware on front, hence I was thinking 8mm drive be a little looser and free in my silly logic.

MILD THUNDER

iTrader: (3)

Yep. Replaced my GM balancers, for that exact reason you are seeing. I went with the Powerbond / Dayco balancers. They make them internal or external, and dual keyway for SC applications. Nice units for the cost. Icdedppl been runnign them on his blown 540's for a while now. The ATI and Fluid dampers are very good, but at a higher cost of course. Hard to go wrong with those.

If I recall right, the dart 325 Pro 1, is the casting they use to make the "335" pro 1 cnc.

Intake
325 335 cnc
.200 166 174
.300 245 245
.400 297 306
.500 330 353
.600 355 383
.700 370 401

Exhaust
.200 127 136
.300 170 178
.400 211 235
.500 244 265
.600 267 282
.700 282 296

Going to the 335 CNC program, would net you on average, 17cfm on the intake side, and 16cfm on the exhaust side, from .200 to .700 lift. Doesnt sound like a lot, but , it kinda is. The other thing, is , most of these off the shelf heads, aren't designed for a 4.500+ bores. Not until you get into the Dart "race" series heads, that they offer different bore size options. It wouldn't be very cost efficient, for them to have various bore size programs for each engine application. So, they kinda have to offer a head, that fits a 4.250 bore 454 , as well as a 4.600 bore 565ci. If the head was designed for a 4.600 bore, and you went and put it on a 454 with a 4.280 gasket, well, there'd be problems obviously.

Theres some airflow available, by working the chambers, to unshroud the valves. Esp when you start getting into a 2.300 valve. One of the big bonus's of larger bores, is the flow. Thats why guys would notch the bores on the 454 blocks. But when you get into the 4.500, 4.530, and bigger, you can start taking advantage of the larger chamber. Thats why on the custom race series heads, dart used to offer them ported to various bore size engines. The down side, the chambers start getting big, volume wise.

Back in the dart 320 Race series casting days, stock intake volume was actually 327cc. Going with darts factory "full port" on those heads, the intake runner grew to 346cc. Chamber volume, went from 119cc, to a whopping 139cc. If there was no power in chamber work/porting, they certainly wouldn't go in and remove 20cc of material when porting.

My local head guy that Dan and I use, wants the head gasket you're using , when he's porting your heads. He wants to take advantage of any available real estate , within reason of course. My point, is there may be more available flow from the heads, working on the chamber, throat, and valve job, than simply porting the actual ports themselves in some scenerios.