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Old 01-12-2020, 10:58 AM
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SBC power estimate

Time to upgrade my 357 SBC from hyd.flat tappet to full roller setup.
As the Promaxx heads are quite affordable with eveything assembled I'm looking to put together the following combo:

- 4 bolt 357 (0.040 over) stock crank bottom with forged rods and pistons
- CR 10.6:1
- Promaxx 200cc heads (281cfm at .600) milled to 62cc
- RPM Air Gap intake
- 750 Quickfuel VS carb
- Morel retrofit lifters 4602
- Howards HR cam 237/245, .560/.560
- 1.6 scorpion roller rockers, lift .597/.597
- Cloyes timing cover and button
- MSD HEI for now, may go later for ign.box

Promaxx will even upgrade the springs to PAC dual spring to suit Howards cam (talked to both and it's a good match), pressures 150lbs / 380lbs.

What do you guys think it makes hp/tq? At what rpm?
Also I know that forged crank would be next on the upgrade list but I just don't have the chance in the next few years. I may eventually do 383 stroke with the above parts. What do you think of the stock crank for now?

Thanks,
Dunig

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Old 01-12-2020, 11:43 AM
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Looks like a good combination, I'd say 460-480 HP at 6300 rpm or so.
It would go 500+ HP as a 383.

Only thing I'd suggest is a split ratio rocker set, the 1.6 on the exhaust side just seems to hurt torque and not really help HP at all. Run a 1.5 on the exhausts.
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Old 01-12-2020, 11:49 AM
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Thanks Ericnova72 for fast reply. What do you think of stock crank, no issues at this level? Also any advise if cam should be bigger, or just loosing street manners then?
Anyone have dyno reference of similar setup or experience with Promaxx heads?

Dunig
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Old 01-12-2020, 12:00 PM
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Stock crank will be fine. On that note, a aftermarket steel crank is only $200 and may end up the same price as having the stock one worked on if it needs it.
I'd also go 383 because the pricing is the same.
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Old 01-12-2020, 12:09 PM
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Quote:
Originally Posted by Dunig View Post
Thanks Ericnova72 for fast reply. What do you think of stock crank, no issues at this level? Also any advise if cam should be bigger, or just loosing street manners then?
Anyone have dyno reference of similar setup or experience with Promaxx heads?

Dunig
I would not worry about the crank either.

Bigger cam just trades street manners for top end power, you're right on that....but you would then be crossing into the area where you need the special high rpm short plunger travel lifters($800+) to be able to get up to that rpm....the lifters you've got are going to start to float above 6600 rpm or so.
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Old 01-12-2020, 12:30 PM
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Thanks for your advice, I can go 1.5 rockers on exhaust for sure.

What would be needed to break 500hp (crank). Not looking to spin to 7k though.

Anything else stick out as not a good match, or could be better suited?

Dunig
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Old 01-12-2020, 12:46 PM
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Quote:
Originally Posted by Dunig View Post
......

- 4 bolt 357 (0.040 over) stock crank bottom with forged rods and pistons

Also I know that forged crank would be next on the upgrade list but I just don't have the chance in the next few years. I may eventually do 383 stroke with the above parts. What do you think of the stock crank for now?.....

Thanks,
Dunig
I don't consider a forged crank as an upgrade, I see heads, intake, headers, carb or injection, even cam as upgrades.

The crank, rods and pistons lie at the heart of the matter, a failure here takes everything you spent your hard earned money on. Your top end plan portends a solid 420 to 450 ft pounds with an amount of horsepower in the 450-460 range at revs around 6200. This is a lot of stress on the bottom end.

I think you're building this too much with stars in your eyes draming of big power numbers while not appreciating the amount of structural strength it takes to contain those big numbers.

Factory cranks came in cast nodular iron or forged mild steel. Which do you have? A casting had a very narrow mold parting line, forgings a wide and smudgy parting line for a quick reference. 200 dollar aftermarket cranks are advertised as steel but when you look at the occasionally referenced alloy number it is actually nodular iron not steel. Not that you can't cast steels it just isn't much done and probably not at a 200 dollar price point. Low price aftermarket forgings start with low alloy 5140 steels you find inexpensive rods from this alloy as well. It's decent and a bit of a step up from GM's typical forging material of 1050 mild steels. 4340 puts you on the trail of a better alloy but the strength range is quite wide as this material allows some variation in the percents of alloying materials and in melt methodology. The low cost end jiggers the chrome to nickel content around a bit and is first furnace run, this stuff is pretty darn good but it gets better. Increasing the purity with remelts under carefully controlled processes like electric melt and that when combined with vacuum melt yields increasingly stronger and more expensive material. Vaccum degassed, electro-melt is the cream of these steels and is priced accordingly, this is true aerospace quality steel if you dont see those processes referenced it ain't aerospace quality steel regardless of the advertising and there is a lot of stretching of these terms in the daily advertising literature.

If your just didling around on the street for your own amusement this build is probably fine. If you're planning on hammering the motor a lot, give more consideration to what happens if a crank fails.

Bogie
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Old 01-12-2020, 01:02 PM
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Thanks for reply Bogie,

This is purely street, well probably I have to get a 1/4mile time someday.
It's going in a second gen camaro with 4.1 rear and 200-4r transmission.

Like said at the moment no chance to upgrade crank, it's original for what I know. I have had engine builder to inspect and re-assemble bottom already.

I am not shooting for a certain number, just asking if the parts are in balance or something way off. Dyno will show and engine will be tuned properly. Thanks.
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Old 01-12-2020, 01:13 PM
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I think a solid roller would be a much better choice. Did not know promax heads had that much flow. Also with the dual plane intake I think an 850 as a better choice for carb size.
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Old 01-12-2020, 01:26 PM
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Hyd.roller is the sole reason I'm going this route. Street only and want to have low maintenance and quiet motor with useable power. I already have the expensive morel lifters, bottom end, intake. Cam is pretty much settled if there is nothing greatly wrong with it. I will choose still rockers (Harland), heads (ProMaxx) and carb (Quickfuel).

I have talked to AED and Cstraub and both tell me to stay away from any 850 holley based carb (even AED's own). They have a large venturi that will kill velocity. Even 950 is better suited cause that is actually equipped with 750 venturi or something like that.

I have read some good feedback of the ProMaxx heads. They were originally Patriot heads that I hear were sold to or turned into ProMaxx brand with upgrade in their machining/cnc. They confirmed they'll provide even PAC springs so it looks like a decent head for a bargain.

Br,
Dunig
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Old 01-12-2020, 02:00 PM
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Quote:
Originally Posted by Dunig View Post
Thanks for reply Bogie,

This is purely street, well probably I have to get a 1/4mile time someday.
It's going in a second gen camaro with 4.1 rear and 200-4r transmission.

Like said at the moment no chance to upgrade crank, it's original for what I know. I have had engine builder to inspect and re-assemble bottom already.

I am not shooting for a certain number, just asking if the parts are in balance or something way off. Dyno will show and engine will be tuned properly. Thanks.
The parts are probably not in balance. Aftermarket rods and forged pistons are generally different than GM production. Little imbalances when just sitting still get to be really big as the revs grow. I highly recommend going to a balance shop it is something that does a lot to help an engine stay together as the revs get high.

Bogie
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Old 01-12-2020, 10:28 PM
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Hello,

Thanks for all the advice. The discussion is slightly going off topic due to my poor choice of words. With balance I meant if listed parts are suitable to compliment each other in this combo so I am not leaving anything to table for no good reason, and not balancing of rotating assembly. The bottom end is already done by a machine shop and in a 400hp engine turning 5800rpm without any issues.

I am changing the upper end to roller setup and asking for parts selection and possible questimates for powerband, and if there are better options for parts.

Promaxx heads allow .600 lift, and I will follow the advice to use a bit less lift on the exhaust side. Are there even better cams out there?

I have no previous experience on Promaxx heads or Howards cams so anyone have a word on those? For example are Howards 111815-10 grind "old" or is there something better for a 357 sbc?

Br,
Dunig
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Old 01-13-2020, 08:04 AM
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HP expected

If you have access to the Motor Trend channel go to Road Kill series and find the Engine Masters dyno runs comparing 650, 750 and 850 Holleys on a 383 ci sbc. Very interesting, 474 hp with 750 seems to be combination.
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Old 01-13-2020, 09:13 AM
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Thanks for the link!

I only see a preview on youtube but found the results listed online.
So for 383 it is recommended either 750 or 850 but more leaning towards 750 for good response.
I think also in this 357 build as I have 'better' heads and will go with a slightly larger cam, then also in this case 750 is the ticket. Thanks!

Any more opinions on roller cam, is Howards good? Don't want to go to custom grind this time.

-Dunig
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