Marine engine rebuild - what compression ratio is safe for regular fuel?

Hello, I am planning a 350 chevy build for my 25' trophy command bridge.
This is my first marine build, so any suggestions would be greatly appreciated.

I have been reading up a bit and have made a few choices, but would like your expert opinions.

Using a new set of chevy vortex heads and a keith black D-dish piston I have the option of two different size dish pistons.
The +12cc pistons would yield 9.3 to 1 compression
The +18cc pistons would yield 8.7 to 1 compression

Assuming I will be running regular fuel which would be a better choice?

I am looking for a vortec low profile intake manifold - any suggestions ?

possible candidates:
Edelbrock Vortec EPS performer (even comes powder coated black)
professional products Cyclone (is it really 1/4" shorter? [or just marketing hype])

Can I run a edelbrock 500cfm street 4-barrel? What would be a good marine carb in this size range?

Thanks for your help, Roy

Roy, take a look a this thread beginning at post #7.

roysbb said:

Hello, I am planning a 350 chevy build for my 25' trophy command bridge.
This is my first marine build, so any suggestions would be greatly appreciated.

I have been reading up a bit and have made a few choices, but would like your expert opinions.

Using a new set of chevy vortex heads and a keith black D-dish piston I have the option of two different size dish pistons.
Roy, the D dished piston profile would certainly work well if using a Pre-Vortec cylinder head.
The Vortec cylinder head quench surface most ressembles a LCQ style piston deck or quench surface as seen here.
The idea is to have the piston deck (quench surface) mirror the cylinder head quench surface best we can.

The +12cc pistons would yield 9.3 to 1 compression
The +18cc pistons would yield 8.7 to 1 compression
Assuming I will be running regular fuel which would be a better choice?

Short of over-heating and/or no oil pressure, Detonation is the Marine gasser's worst enemy.

With a good tight quench dimension, our static CR can come up a bit with no increased Detonation issues.
If you build a quench effect into this engine, the 9.3:1 would be a good choice.


Here are few static compression ratio calculators.

http://www.csgnetwork.com/compcalc.html

https://www.rbracing-rsr.com/compstaticcalc.html

http://www.summitracing.com/expertadviceandnews/calcsandtools/compression-calculator

A good Marine SBC quench dimension will be approx .038"..... and will be a result of the piston deck clearance and compressed head gasket thickness.
So you'll want to know these dimensions before making some of your selections.

Your first over bore = 4.030"
Your stroke = 3.480"

After you have decided upon your "knowns", plug in your numbers and play around a bit.

I am looking for a vortec low profile intake manifold - any suggestions ?
If planning to use aluminum, will this engine be Raw Water cooled or fitted with a Closed Cooling system?

possible candidates:
Edelbrock Vortec EPS performer (even comes powder coated black)
professional products Cyclone (is it really 1/4" shorter? [or just marketing hype])

Can I run a edelbrock 500cfm street 4-barrel?
The requirement for a 5.7L operating at 4K rpm is only 344 CFM.... at 4.5K rpm, this jumps to only 378 CFM.

The automotive carburetor fuel metering is not the same as for Marine......, nor are these designed to port over fuel into the venturi area.


What would be a good marine carb in this size range?
Many to chose from, and you'll get many suggestions from us here.
I like the Holley and the Rochestor Q-jet, and I do not like the Edelbrock.
Your call.

Click to expand...

When we build a quench effect into the SBC Marine engine, we can pull more torque from it via a better LPCP.
(LPCP = location of peak cylinder pressure)
The quench effect allows for more ignition spark lead, of which is what gives us this better LPCP.
Target LPCP is 12* to 14* ATDC.



.

Roy, I just now updated the other thread that I linked you to.

You are certainly on the right track by building a Qench Effect into your combustion chambers.
You won't regret it!

Roy, you asked about using regular gasoline, and I assume that you meant gasoline with a lower Octane rating.
I'm thinking that your question also included how the compression ratio will affect the use of a lower Octane fuel, or visa-versa!

The higher Octane rating gasoline doesn't offer any more power, but it will not flash as easily and will help with detonation.
With a good quench effect, your fuel economy will improve so that using a higher Octane rated fuel will not be as costly in the over-all scope of things.

If you do not build a Q/E into this engine, you'll likely have no choice but to use the higher Octane rated fuel.
Your fuel economy will not be as good either.


.

Man this forum keeps logging me out and I lose all my response I have tried to enter! Twice!
How can I avoid this?

Hi Ricardo, I was hoping to hear from you, I have been reading all the threads you pointed me towards and quite a few others. I'll try and borrow the "Marine Performance" book from the library.

My goal is to build economy and reliability. I am looking at getting new vortec heads directly from chevy.
I want to be able to run the cheap fuel, as I never know where I will fill up, and what will be available at the fuel dock. Alaska/British Columbia all the way down to Mexico.

Here are the pistons I am considering - they look like they match the vortec heads - what do you think?
http://www.summitracing.com/parts/uem-kb193-060/overview/

I am raw water cooled, should I use a cast iron intake manifold rather than aluminum? I found an Edelbrock aluminum Vortec EPS that is powder coated on the outside.

My boat is a 1988 25' x 10' wide Bayliner Trophy Command Bridge with a Volvo DuoProp (290 I believe). She cruises at 25+ mph at 3500rpm.
I have no complaints with the current configuration, but she is starting to use a little oil - no leaks so it must be going out the exhaust.
It is also time to go through the outdrive and re-seal it.

As far as a carb and intake, what would YOU recommend? Anything I can do to squeeze a few more miles out of a tank .....

Thanks, Roy

[h=2][/h]

I've also had trouble lately.
I like the idea of typing the post in notepad or a word processor..… I will occasionally do that myself.

Look for the "auto save" to become active. If you get bumped off, you can sometimes use the auto save feature and it will bring back what you thought you had lost.


Roy, I clicked on the link to the Keith Black piston that you wanted me to look at, but you would need more info in order to determine the dish volume.
The Vortec chambers (I believe) are either 64 or 65 cc.

I would suggest that you play around with one of the "compression ratio calculators", and then vary the dish volume until you come up with a static compression ratio that fits your needs.
Your machine shop will know what to do with the piston deck height and the compressed head gasket thickness.

.

Roy, just for fun, I fiddled around with a CR calculator just now.

I used the following numbers for a 5.7L first over-bore giving us a quench of .040".

************************

Same, but with a 16 cc piston dish volume.

Bore: (diameter)	4.030"
Stroke:	3.480"
Cylinder Head Volume:	64 cc
Effective Dome Volume: [SUP]Use (-) for Dome and (+) for Dish.[/SUP]	+16 cc
Deck Clearance:	.016"
Compressed Gasket Thickness:	.024"
Number of Cylinders:	8
Compression Ratio :	9.24 : 1
Total Displacement (in.[SUP]3[/SUP]) :	355.12
Total Displacement cc's :	5821.64

****************************************
Same, but with a 14 cc piston dish volume.

Bore: (diameter)	4.030"
Stroke:	3.480"
Cylinder Head Volume:	64 cc
Effective Dome Volume: [SUP]Use (-) for Dome and (+) for Dish.[/SUP]	+14 cc
Deck Clearance:	.016"
Compressed Gasket Thickness:	.024"
Number of Cylinders:	8
Compression Ratio :	9.44 : 1
Total Displacement (in.[SUP]3[/SUP]) :	355.12
Total Displacement cc's :	5821.64

***********************************************

Same, but with a 12 cc piston dish volume.

Bore: (diameter)	4.030"
Stroke:	3.480"
Cylinder Head Volume:	64 cc
Effective Dome Volume: [SUP]Use (-) for Dome and (+) for Dish.[/SUP]	+12 cc
Deck Clearance:	.016"
Compressed Gasket Thickness:	.024"
Number of Cylinders:	8
Compression Ratio :	9.64 : 1
Total Displacement (in.[SUP]3[/SUP]) :	355.12
Total Displacement cc's :	5821.64

Here's an article by Allen W. Cline who suggests that 14* ATDC is optimum for LPCP.
Other engineers agree upon 12* to 14* ATDC.


Here's another one of my drawings.