Twin 350's to twin 383's

In process of replacing my twin 350 4BBL roller cam Vortec (310hp @5000) engines with 383 strokers.
Blocks bored .030 oversize,
blocks clearance for stroker cranks
GM Vortec PM rods used
Scat ProStock 9000 cast cranks

Questions I have, in my attempt to build a "quench" built engine I've come down to either a:
KB135 piston (18cc dish),
KB121 Piston (28cc dish)or a
Speed Pro H890CP (23cc dish).
Deck height is still stock at 9.025" but can be cut to as little as 9.015" in order to maintain a quench of .035 - .042.
9.68:1 for the KB135's,
8.9:1 for the KB121's
9.17:1 for the Speed Pro's.
Head gasket is .028.
Cams will be stock roller marine GM grinds

I'm leaning on going with the SpeedPro's as the compression is about where I want it. I spoke with Scat and they are claiming their cast 9000 series cranks are 25% stronger then the cranks used in marine GM 350's are will be fine in a marine engine build. Anyone have thoughts on what I'm building please chime in.

Careful, You have just opened a CAN OF WORMS..............

Wait and you will see........

Have you CC'd your heads yet? If so what are they? 64, 62, ??, cc's?

This is what you need to start with for piston choice so I assume you have. What is the CC of the heads?

Anyway because you are not building a high performance build, using what you want here will be fine. I think the 9.68 would be a good choice!! Just need to be careful of max timing!! keep it below 32 degrees total!! Because you will not be over revving this then a higher compression will give a better quicker torque response in my opinion.

Also because of no high performance the scat crank will be fine. Many marine cranks are cast and are also two bolt main caps.....Anything running under 6000 rpms and moderate compression typically do not need 4 bolt or forged. Its when you really plan on beating on the engines when you need to upgrade the assemblies.

You will rarely go over 5000 rpms the cast crank will do just fine. They last hundred of thousands of miles in light duty trucks which tow ect.....

JV, you can safely pay little or no attention to the Nay Sayers.
What you propose is the correct way to build a SBC for Marine use.
There is no can of worms if you educate yourself prior.

Take a few minutes and read this thread starting at post #7.

JV Sr. said:

In process of replacing my twin 350 4BBL roller cam Vortec (310hp @5000) engines with 383 strokers.
Blocks bored .030 oversize,
That is correct...... the 4.030" bore w/ the 3.750" stroke = roughly 383 cu in, or roughly 6.3L.

blocks clearance for stroker cranks
GM Vortec PM rods used
Yes..... 5.700" would be a good choice. There is no need to increase the length for the SBC Marine Engine.
(the longer rods move the wrist pin up, causing the need for shorter skirts.... and we do not want that!)

Scat ProStock 9000 cast cranks

Questions I have, in my attempt to build a "quench" built engine I've come down to either a:
KB135 piston (18cc dish),
This piston deck mirrors the Vortec cylinder head quench surface..... and would be good choice re; piston deck!
However, you will want to run your numbers using an On-Line Static Compression Ratio calculator.
Vary the dish volume until you come up with a comfortable SC/R.

KB121 Piston (28cc dish)or a
This piston deck better mirrors the earlier style SBC Non-Vortec cylinder head quench surface.... and would not be a good choice w/ the GM Vortec heads.

Speed Pro H890CP (23cc dish).
This profile offers a quench band only...... IMO, not the best choice for a SBC Marine build.

Deck height is still stock at 9.025" but can be cut to as little as 9.015" in order to maintain a quench of .035 - .042.
9.68:1 for the KB135's,
Nothing to fear if the Quench dimension is tight!



Head gasket is .028.
You would want to know your piston deck height first.


Cams will be stock roller marine GM grinds

I'm leaning on going with the SpeedPro's as the compression is about where I want it.
No Full Quench with this piston.


I spoke with Scat and they are claiming their cast 9000 series cranks are 25% stronger then the cranks used in marine GM 350's are will be fine in a marine engine build. Anyone have thoughts on what I'm building please chime in.

Click to expand...

kghost said:

Have you CC'd your heads yet? If so what are they? 64, 62, ??, cc's?
Heads are 64cc, Vortec versions 062 castings.

Anything running under 6000 rpms and moderate compression typically do not need 4 bolt or forged. Its when you really plan on beating on the engines when you need to upgrade the assemblies.

You will rarely go over 5000 rpms the cast crank will do just fine. They last hundred of thousands of miles in light duty trucks which tow ect....
Engines are propped for a max of 4600RPM..

Click to expand...

Appreciate the feedback.

RicardoMarine said:

JV, you can safely pay little or no attention to the Nay Sayers.
What you propose is the correct way to build a SBC for Marine use.
There is no can of worms if you educate yourself prior.

Take a few minutes and read this thread starting at post #7.
Excellent read and learned a bunch.

Click to expand...

I purchased a copy of Dennis Moore's SBC Marine Performance book ($120) off Amazon and have read it extensively. Very well written and full of information.

I will be using stock GM PM 5.7" long rods

Stock 062 casting Vortec 64cc heads

KB135 pistons

If I deck the block to 9.020

Use a .030 head gasket

Have a deck clearance of .012

SCR of 9.64:1 which seems a little high for me.

and a quench of .042

Makes me wonder if I need to look for another piston with more dish. I have looked at these pistons

from here:http://www.ebay.com/itm/331716267488?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

When I saw his "computed compression ratio" I thought it looked high and contacted Skip White. Turns out it is wrong, and actually is right at 9.0:1 with a stock 9.025 block, .040 head gasket and a .010 deck height. Would consider this piston, although forged, and cut the block to 9.020, use a .030 head gasket and a quench of .035. Thoughts?

I have spoken with Dennis, as well as Phil with MichiganMotors, about the quench dimension.
The general consensus is that .038" is ideal for a 5.7L (3.480" stroke) Marine SBC Engine.
I would bump this to approximately .042" for the 3.750" stroke 6.3L Marine SBC Engine.

If you keep it tight, your SC/R will be ok!
If you were to loosen it even more, then detonation becomes more of a possibility....... almost as with the dreaded GM full dished piston.


.

I would bump this to approximately .042" for the 3.750" stroke 6.3L Marine SBC Engine.

Click to expand...

Ayuh,.... .035" sounds kinda tight to Me too,....

almost as with the dreaded GM full dished piston.

Click to expand...

The GM HT-383 would be a Great crate motor for boats, if not for that very reason,.....
Over the counter, from most any dealer,...

I was concerned that .035 was a little tight, no problem as I'll go with either a .038 or .039 head gasket for a quench of .042. Final choice will be after I mock up the block with the piston, rods and crank and have my final deck height figured out, 042 will be my target.

bondo said:

The GM HT-383 would be a Great crate motor for boats, if not for that very reason,.....over the counter, from most any dealer,...

Click to expand...

Interesting as I thought the very thing and gave it serious consideration except for the pistons. Can you imagine what that engine could be with a good piston?

After speaking with my machine shop, thinking about the fuel I get at some of the very remote marinas that I travel to in BC waters, I decided on a little lower compression.

The following is what I believe will do me a good job.

Scat ProStock 9000 cast crank

Stock GM PM 5.7" long rods

Stock 062 casting Vortec 64cc heads

Wiseco 2618 forged piston 24cc dish http://www.ebay.com/itm/331716267488?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

Deck the block to 9.020

Use a .038 -.040 head gasket

Deck clearance of .005

SCR of 9.1:1

Quench of .042-.043

• the SBC Marine Engine has no "coast time" nor any "no load" time as does the car/truck engine.
• the Marine (be it SBC or other) will always be under load.
• torque is created by many factors..... most important being a good LPCP (LPCP = Location of Peak Cylinder Pressure)
• LPCP should be at/near 12 to 14 degrees ATDC.
• ignition advance has the last word in LPCP.
• when ignition advance is held back..... so is the LPCP, and it becomes "lazy".
• short of Over-Heating or Lack of Lubrication....... "Detonation" is the Gasoline Marine Engine's worst enemy.
• a tight quench area allows for a higher SC/R and more Ignition Advance with less concern for Detonation.
• you are building a lower RPM Marine Engine, which means that there is no need to increase the Quench dimension as we would with a higher RPM Auto Engine.
• you should be able to allow more Ignition Advacne at/near 3.2K rpm. (instead of lets say a conservative 28* @ 3.2K rpm...... a more generous 31* @ 3.2K rpm

The GM full dished piston cannot possibly offer a quench, and should never be used in the SBC engine!
And by the way...... the GM 377 or 383 crate engines are both sent out the door with the Full Dished pistons!


.

RicardoMarine said:

• the SBC Marine Engine has no "coast time" nor any "no load" time as does the car/truck engine.
• the Marine (be it SBC or other) will always be under load.
• torque is created by many factors..... most important being a good LPCP (LPCP = Location of Peak Cylinder Pressure)
• LPCP should be at/near 12 to 14 degrees ATDC.
• ignition advance has the last word in LPCP.
• when ignition advance is held back..... so is the LPCP, and it becomes "lazy".
• short of Over-Heating or Lack of Lubrication....... "Detonation" is the Gasoline Marine Engine's worst enemy.
• a tight quench area allows for a higher SC/R and more Ignition Advance with less concern for Detonation.
• you are building a lower RPM Marine Engine, which means that there is no need to increase the Quench dimension as we would with a higher RPM Auto Engine.
• you should be able to allow more Ignition Advacne at/near 3.2K rpm. (instead of lets say a conservative 28* @ 3.2K rpm...... a more generous 31* @ 3.2K rpm

Click to expand...

It would seem with my 9.1:1 SCR I will be able to run "a more generous 31* @ 3.2K RPM" with no problem, or am I missing something?

Would changing rocker arm ratio from 1.5 to 1.6 be advantageous with using a stock marine roller cam designed for a 350 in a 383? With a 1.5 rocker intake valve lift is .430 where a 1.6 rocker would be .460. Exhaust with a 1.5 rocker would be .450, while a 1.6 would be .480 lift. I realize valve clearances will need to be checked and will be regardless of what ratio I use. The heads have been rebuilt with screw in studs and taking into account the use of a 1.6 ratio roller rocker arm. Roller rockers will be used regardless of which ratio I decide on.