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383 upgrade questions

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Throttlestop

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I am looking at building out an extra 350 I have to a 383. This will upgrade the current 350 I have mated to Volvo dp drive. This is going in a 28' cruiser so I am concerned about detonation.

Here is what I have penciled out so far with two different pistons :

.030 bore
Vortec heads 64cc
3.75 crank
5.7 rods
Edelbrock EPS Performer intake
Edelbrock 1409 carb

Kieth Black SKB121
+28 cc
.017 deck clearance
.026 Felpro 1142026 head gasket
8.76:1 compression ratio

Speed Pro H890CP30
+23 cc
.025 deck clearance
.015 Felpro 1094
9.17:1 compression ratio

I am hedgeing towards the 28cc piston to have compression ratio better suited to low octane fuel pushing a heavy boat. What are others opinions?

I haven't picked out a camshaft yet, though I am planning on going flat tappet. Does anyone have any recommendations?

I also have semi closed cooling on my current engine.
 
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Throttlestop said:
I am looking at building out an extra 350 I have to a 383. This will upgrade the current 350 I have mated to Volvo dp drive. This is going in a 28' cruiser so I am concerned about detonation.
Your concern is very valid, and is exactly why you will want to steer clear of the GM Full Dished pistons for this build.
attachment.php


The GM style F/D piston cannot possibly create a Q/E (quench effect) within the combustion chamber.
A good Q/E build will greatly reduce the liklihood of Detonation!

Take a few minutes and read this thread beginning at post #7.
http://www.marineengine.com/boat-forum/showthread.php?394668-V6-to-V8-engine-swap



Here is what I have penciled out so far with two different pistons :

.030 bore
Vortec heads 64cc
3.75 crank
5.7 rods Great choice!
With the 3.750" stroke, the wrist pin location is already raised. Longer rods call for an even further raised wrist pin and shorter piston skirt!
I would stay with the 5.7" connecting rods.

Edelbrock EPS Performer intake
Edelbrock 1409 carb

Kieth Black SKB121
+28 cc
Look closely at the description.
https://www.summitracing.com/parts/uem-skb121-std

They list this as a "D" shape profile, and call out double valve reliefs, but they are showing a LCQ style profile.
For the Vortec chambers, you will do much better with a LCQ style piston profile that will better "mirror" the quench surface.
We want a piston deck that can take advantage of the cylinder head's quench surface...... especially for the SBC 6.3L Marine build.

.017 deck clearance
.026 Felpro 1142026 head gasket
.017" + .026" = .043" quench dimension which should work well.
8.76:1 compression ratio

Speed Pro H890CP30
+23 cc
https://www.summitracing.com/parts/slp-h890cp30
No description of the piston deck, so we must assume that the image is correct.
If so, this reflects the LCQ style profile which will work well with the Vortec chamber quench surface.

.025 deck clearance
.015 Felpro 1094
.025" deck clearance?
You may not know the exact piston deck height until the lower end is assembled.
Then you can select the compressed head gasket thickness.
You may need to adjust these numbers in order to achieve the correct quench dimension.


9.17:1 compression ratio
With a good tight quench dimension, 9.17:1 should not be a problem.

I am hedgeing towards the 28cc piston to have compression ratio better suited to low octane fuel pushing a heavy boat. What are others opinions?
Use one of the online Static Compression Ratio calculators, and plug in your values.
https://www.google.com/search?q=eng...0ahUKEwivyZLg4YDTAhUQ52MKHYxDCx4Q_AUIBA#spf=1

Your "knowns" are:
4.030" bore
3.750" stroke
Cylinder Head Chamber volume 64cc

play with the other values until your SC/R ratio hits your goal.


I haven't picked out a camshaft yet, though I am planning on going flat tappet. Does anyone have any recommendations?
Once you have selected your rotating assembly components, I would call a good camshaft manufacture and speak with an expert tech who is familiar with the SBC Marine Cruiser builds.
Don't bother with speaking to a Hot Rod tech....... you want someone with true Marine experience!

I also have semi closed cooling on my current engine.
That is a real plus and will allow the use of the aluminum automotive intake manifold!
Click to expand...
 
Oh..... I forgot to mention that your 5.7L Duo Prop drive will be the 1.95:1 reduction (assuming that you have an AQ series drive).
This ratio is correct for a 5.7L SBC.

However, the 1.95:1 gear set is not quite as stout as is the 1.78:1 gear set.

A Q/E built 6.3L SBC will offer much more torque, possibly more than the 1.95:1 gear set is comfortable with.
You'll do much better with the 1.78:1 DP lower unit behind your new 6.3L SBC.


.
 
Thanks for the info Rick.

Both KB121 and Speedpro H890CP30 pistons mirror a vortec head. I think summit racing just doesn't distinguish between a LCQ and D shape piston. Here is the actual picture of KB121 http://www.carshopinc.com/product_thumb2.php?img=images/KBP/KBPKB121.030.jpg&w=1000&h=750 .

I measured the piston heights before the current piston where removed and I used their specs to estimate what the new piston deck clearance would be. This was just to see what pistons would put me in the ballpark. There isn't much available when you are looking at getting good quench and lower compression in a 383, most of the piston selection is geared towards much higher compression.

My main question does the a .41 compression ratio make that much of a difference in power, will it just be safer to go with KB121 piston and a 8.76:1 ratio?

Back on the camshaft - any recommendations on a manufacturer?
 
Rick,

First, thanks for the all the advice.

I believe both the pistons do have the matching peaks from the vortec heads, summit racing just lumps them together as D style pistons. Here is a picture of the KB piston. http://www.carshopinc.com/product_thumb2.php?img=images/KBP/KBPKB121.030.jpg&w=1000&h=750 .

I did mock up some other pistons with the lower end to see where the deck height was at. I used that info to ballpark the deck heights above based on the piston specs. I was mainly trying to just see what my piston choices were going to be. Most of the selection of 383 pistons people are trying to raise the compression. Those where the only two pistons that I could find that fell within the right range of quench and compression ratio. I will of course have the machinist verify my measurements when the final piston is chosen and that effect the final head gasket used.

My main question is with this type of build what is the benefit and/or drawbacks between the piston choices. There is less than .5 a difference in the compression ratio. Personally I would rather error on lower compression and less chance of detonation, than the benefits of a little higher compression.

Thanks again.
 
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