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CDMA
08-01-2000, 03:22 PM
In my deliberation's over what I want to build for a new engine I have been thinking about how high I can go with the compression ratio. As far as I know Mercruisers are 8 or 8.5 to 1. However it seems that at least in an automotive application you can run close if not above 9.5 to 1 on pump gas. I don't mind watching and only running 93 but it needs to be able to run well with no detonation. I would like a 383 similar to PaulO's but just too expensive. I have what seems to be a good 355 block that was remanufactured three years ago in the boat now so I am just thinking about pulling that out putting on a good set of iron heads, bumping up the compression, a mild cam and with any luck one of the Holley retrofit for the Quadrajet. Any thoughts?

Chris

PaulO
08-01-2000, 03:59 PM
Chris,
Great to hear of your progress. Make sure you get that total advance right on the timing. As you stated, that motor/drive/prop combo would run up to 5000 RPM on my boat. As far as the upgrades, I agree that 9.5 to 1 is the safe limit if you want to be a little less concerned about fuel. Depending on the piston type, you may be able to add a set of WorldProducts S/R torquer heads and boost both flow and comp. ratio. Head gasket thickness plays a key role also in determining not just comp. ratio but quench if you have a quench type piston. I will be happy to loan you my book on Marine High performance SBC motors. By the way, I think you owe me a ride!!!!!
PaulO

RickSE
08-01-2000, 04:44 PM
I have a 400 (auto not marine) with 11.5:1 compression that runs fine on 92 octane. The only problem is that I have to shut the engine off with the transmission in drive otherwise it will diesel, obviously this would never work in a marine application. One time I accidentally put 87 octane in the tank and it wouldn't stop running.

Most new Mercruiser's seem to run 9.0 to 9.4:1 compression. Newer automotive engines can run higher compression because of better fuel control with close loop fuel injection and better cylinder quenching with aluminum heads. I would think you should be able to run 9.5:1 on any fuel and maybe even up to 10:1 if you use 93 octane and a Mercruiser Thunderbolt V ignition with knock control.

PaulO is right though, one of the keys to running high compression is ignition timing. Too much timing advance and you will break starters, to little and the motor will ping.

Forrest
08-01-2000, 05:10 PM
Generally Chris, you will need to build your compression ratio around your cam selection. Long duration camshafts bleed off cylinder pressure while running, therefore require higher static compression ratios in order to have enough dynamic compression or compression while running. Stock type short-duration camshafts on the other hand, require lower static compression-ratios since these cams don't allow as much cylinder pressure bleed-off and the for the most part, have a dynamic compression ratio that is very close to the same engine's static compression ratio. Check out Crame Cams website (http://www.cranecams.com) and other camshaft manufactures websites. Somewhere in there they tell you all about that good stuff.

Another thing that dictates octane requirements for a given compression ratio is piston dwell-time at top dead center. Generally, longer rod engines not only have a better rod-angle ratio, but they also have longer piston dwell at TDC. This keeps the combustion chamber small for a longer time as the expanding gases push against the piston. The effect is to slow down the burning process which extracts more power from the fuel/air mix and reduces the chance of detonation. You can build a 383 stroker small-block using over-the-counter parts with 5.565" (400 sb rod), 5.7" (350 rod), or 6.0" rods. There are other combinations, but these are the most common. Forget building the engine with the 5.565" rod. Not only is the piston dwell at TDC short with this rod, but the rod-angle ratio is not very good. In this case, the rod angle is such that there is high degree of side loading on the piston - in other words, it wants to push the piston through the side of the cylinder block. With all of this in mind, why not build your 383 with 6.0" rods? With the six-inch rod you would have the best rod-angle ratio and maximum piston dwell time - but it comes at a price. Usually the longer rods cost more and the pistons to accommodate such a long rod are a little more expensive than most as well. As it turns out most 383 engines are built using the standard 5.7" rod out of a 350 Chevy engine. That combination is usually best compromise between rod length and economy..

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Forrest

rayjay
08-01-2000, 05:16 PM
RickS,

AL heads allow you to run higher compression, generally +.5, because of their ability to cool better. Any head, AL or FE, with a properly designed quench area, good chamber shape to promote a good burn, and proper design and finish of the ports and valves to give good swirl for better mixing of the mixture will allow you to push the envelope of how much compression an engine will stand. Rod length ratio for dwell at TDC, F/I mapping, ignition timing, cam profile and timing etc. will all also effect how much compression an engine can tolerate and still stay in one piece. But I will caution you that actually modifying or playing around with these things beyond what some else (like Chevy) has developed is an area best left for people who know what they are doing. The reason being is that they have probably learned from experience what makes an engine blow up and/or not blow up.

The dieseling your engine is doing is not good for the pistons or the margins of your valves. It could be caused by too hot a plug, a hot spot in the combustion chamber somewhere, or just the carb not closing completely as it's being held open by a solenoid designed to make the carb not close so fast for emissions purposes. You should get that corrected.

Forrest, SBC 6" rods are just made from Olds blanks and are becoming not much more costly than using the 5.7's. In fact KB Sil-vo-lite Hypereutectic pistons for a 6" rod 383 or 400 are easily available and, from my experience with them, a great piston unless you are going to run nitrous or a blower. rj

[This message has been edited by rayjay (edited 08-01-2000).]

[This message has been edited by rayjay (edited 08-01-2000).]

BERTRAM BOY
08-01-2000, 08:14 PM
Hey, Paul O.
What Hi-Performance Chevy Marine book are you speaking of? I think I need to get this book!!!
BERTRAM BOY

Jamesbon
08-01-2000, 10:06 PM
FYI, I had 9.75:1 in the last 355 cu. in SBC, ran fine on 93 octane. My 388 stroker has 10:1 and aluminum heads, I don't expect any surprises from that. I do plan to add octane booster to the 93 once I get her back in the boat.

CDMA
08-02-2000, 12:24 AM
Thanks for the info. My main limit is money but I have been looking around and contemplating the following

355 4 bolt main block ( already in boat)

Basic rebuild with 9.5 to 1 compression

Chevy Vortec heads ( $432 assembled and ready) with specific intake manifold

A cam set up that would bring peak power at about 5100 rpm

Probably keeping the stock Rochester

I figure that might give me the extra power over the stock 260 that I have now while still staying not that far over a grand.

Any thoughts regarding the Vortec heads or any guestimations of the power I could get out of this set up.

Chris

GeneD
08-02-2000, 08:05 AM
Chris,
My equation for chevy power is just about the same as yours, all dependant on money.
What I have done in the past is make use of things I have laying around the shop, or can get easily.
I haven't any experience with the Vortec heads, though I hear they are great.
Using a flat top chevy piston is something you can't go wrong with. A nice forged, flat top is just about standard on all the 350 Merc motors.
Using a cast iron semi-free flowing head design like a fuelly in either 2.02 or the 1.98 intake valve diameter is a good choice and relatively inexpensive. Make sure they have the holes in front to mount the power steering unit and the fuel separater.
Cam selection on these engines can utilyze the old not too much, and not too little profile equation. I think a cam with around 300 degrees duration and about .480 lift will do you fine. These cams are all over the place, and cheap.
For a head gasket, I would go with a standard Merc. After all, they are marine and you can't go wrong.
The quadrajet is not a great carb, but you DO have one, and a marine holley is not going to be cheap, unless someone on the board has one laying around - hey guys!!!
The other cheapo way to do this is to find yourself a 400 engine in the classifieds and build that up at your leisure using the same priciples I have just outlined.
Yet another way is to put a BB in there, I know where a relatively new one (seized) is that can be had for cheap. Manifolds and all, but no other accessories, (alt, PS, fuel stuff, ect.).
Sounds good don't it? But hey, building a motor is never cheap. Soft parts (rings, bearings, seals, gaskets) cost a bundle.
Find another motor and go from there. You don't want a lot of down time do you? Or are you already looking towards winter to do this?

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GeneD
007
Melbourne, Florida

CDMA
08-02-2000, 09:08 AM
Gene,

This is most definetly a winter thing. Right now I just need to use the boat a little. I would seriously consider a BB but for money and also time reasons it seems easier to do a nicer SB. I have a 260 now and really think I would be content in the 340-350 range which is something I think is possible to get. I too have heard good things about the vortec heads. What I figure is if you can get one of those new chevy vortec marine base engines that puts out 325 hp my engine with a mild cam as well as a little more compression should make 350. I know what I WANT to do and I know what I CAN do. I think a rebuild and a head change is all I can stomach this winter.

Also this may be ingnorant but what is the difference between forged and hypenumamatic pistons?

Chris

Riley
08-02-2000, 09:24 AM
Chris...take a few minutes and check out the 350 H.O. crate motor offered by Jim Pace (www.paceparts.com). This is a 4 bolt main 350 with Vortec heads, 9.1 to 1 comp., and rated at 330 hp at 5000 rpm. with an Edelbrock Perfromer RPM intake and a stouter cam the motor should be in the 350 hp range, no problem. I understand your budget constraints, but this engine lists for $1995! Not a bad deal when you check out the specs. I'm seriously looking at this crate engine for my winter project. Good luck with your boat....Chris Riley.

PaulO
08-02-2000, 11:05 AM
Lot's of good info! I would like to add 2 things:

1. Longer rods on a 383 stroker ( or probably any motor) are desireable however, the price you pay is that the wrist pin needs to be moved closer to the top of the piston and you sacrifice ring efficiency by putting them too close together and too near the heat of combustion. For this reason, most builders prefer the 5.7 rod.

2. If your engine is dieseling after shutdown, it is also doing this while it is running (even though you may not be able to detect it)meaning that you are getting the damage of detonation and losing all sorts of power.

PaulO

GeneD
08-02-2000, 02:30 PM
Also this may be ingnorant but what is the difference between forged and hypenumamatic pistons?

$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
I think you want to say "hypereutectic" pistons, or some such. The hyper pistons have a crystalline structure in them that is superior to standard forgings. Both are forged, however the hyper pistons are made of a better alloy and better forging techniques.
Standard forged pistons will do you just fine I would think. I have never broken a forged piston.
Ooops, that's wrong, I did break one in the Corvette last year. But that is the only time in my 20 some odd years of doing this type of thing.

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GeneD
007
Melbourne, Florida

[This message has been edited by GeneD (edited 08-02-2000).]

Forrest
08-02-2000, 02:46 PM
Here is the low-down on hyperutectic pistons vs. forged pistons (http://www.kb-silvolite.com/page11.htm). Hyperutectic pistons are actually castings rather than forged.

Here is another link that tells all about hyperutectic alloy, piston manufacturing, engine building these pistons, even nitrous (http://www.kb-silvolite.com/page02.htm)! Keep clicking on Next Page for more info.

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Forrest

[This message has been edited by Forrest (edited 08-02-2000).]

GEOO
08-02-2000, 09:11 PM
CDMA,
Cast piston are for stock, low load motors not good for Supercharging ie( Turbo, blower or Nitrous).
Hypereutectic Cast pistons are stonger, good for heavier loads, can handle very small amounts of supercharging.
Forged pistons are the strongest, best for heavy loads and all types of supercharging.
I ran Keith Black Hypereutectic's in my 383. In my new motor I used Ross Forged piston's.
This is a general guide; you can buy all different grades of pistons in each category.
The thickness of the top of the piston and the strength varies dramatically.
Keith Black make a great Hyper.. pistons, good value. Mercury's 383ci ? 377ci uses Hyper.. piston. All Mercury big block HP's use Forged pistons. GEOO

avk
08-02-2000, 11:34 PM
I have to agree with Forrest on what C/r to build the motor with. I build many race motors here in my shop however mostly for vintage race cars. An engine has to be considered as an entire beast that is C/R, fuel, RPM etc I am sorry if I get to messey with this subject but you need to get an idea what the whole package will be. Talk to the your cam guy to get a sense of the C/R he would like to see, get an idea where you would like the motor to run. I build some race motors that are turning out 125+ HP per liter of displacement but when we check them with a commpression gauge they only show 100 PSI because of the long cam that means the compression stroke is not from BTD to TDC. Sorry for all the words but your cam guy can tell you what his cams need.
Al;an

GeneD
08-03-2000, 07:15 AM
I stand (sitting really) corrected on my explaination of Hypereutectic -vs- Forged Pistons.
See, even us old timers can learn something new, everyday!
But you know, Hypereutectic even sounds expensive!

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GeneD
007
Melbourne, Florida

rayjay
08-03-2000, 06:51 PM
KB Hypereutectic pistons (along with some of the other good brands) are "fully machined" and are very low growth. This means they can be set very tight which gives an incredible long life. If you use them, have your machinist read the instructions as their clearances are so small that some machinists balk at using the clearances. Also make sure your machinist does very straight and true bores (from experience, not all do!!) as with the tight clearances any lack of trueness to the bore WILL cause problems. And, yes I know nothing is truely straight, but there are degrees of straightness.

What any cast piston can not do is deform such as under severe loads imposed by supercharging and/or nitrous, or severe detonation, they crack instead. The forging will deform without cracking, but must be set looser because they grow more. Arias has a "low" growth alloy, but we have never had good luck with them. Look in this months Motorcyclist at the piston from a Harley Fuel Dragster. The crown looks as if it has been dented by a ball peen hammer without holing or coming apart. A cast piston, not even a hypereutectic, would never have survived this. Even a forging will eventually hole. I have learned this the hard way! rj

[This message has been edited by rayjay (edited 08-04-2000).]