BillK

Heys guys,
Wondering what you guys are getting away with as far as compression ratio goes on typical Marina fuel. Or possibly 93 octane from the gas station.

To make a long story short, one of my customers had an 11 to 1 383 "sitting around" and decided to put it in his 23' Wellcraft Nova. Promptly burnt two pistons and cannot figure out why I am trying to talk him into around 9.5 to 1 max. This is a cast iron head engine and I am not comfortable with any more than that on pump gas. I think he also has way too much prop on it, but that is another story.

What is working for you guys ?

Bill Koustenis
Advanced Automotive Machine
Waldorf Md

BajaRunner

9.5 max on iron heads and 93 octane

SB

iTrader: (1)

BillK -

If going in a normal V hull, pretend this engine is going in an RV. There is a ton of load, no multiple gear transmission, and therefore some time at WOT and lower rpm's until engine can reach proper higher rpms.

Also, max pump octane is going to be supposedly limited to 91 sometime soon.

It's a 383, therefore can be made to make great usable power. Pick the intended usage including peak hp rpm, and the rest will start to fall into place.

Remember, the camshaft has a lot to do with cylinder psi's also, not just power points. IVC vs compression can be very key.

SB

iTrader: (1)

Actually, it really isn't another story. Too much prop will also make more time at load and create more load. This can insuce detonation also, not to mention possibility of running WOT near/at torque peak instead of hp peak.

Sounds like you need to really talk to this customer and get this stuff squared away before it becomes miserable for the both of you. He's already ' lit the fire' by trying to run that other engine and melting it down, so you must put it out and keep it out.

BillK

SB,
I understand the prop deal and have built quite a few marine engines, so I understand the conditions for certain. This guy has it in his head that he can get away with more compression, but I am going to tell him its at his own risk. I am not building the engine, just doing some machine work and rebalancing the assembly once he decides on pistons.

We did a 400 small block for an almost identical 23' Nova about 10 years ago and it is still running strong ! Good point about the 91 octane deal, I will use that as ammo to calm him down some. I am sticking to my guns at 9 1/2 to 1.

When I said that the prop was "another story", I meant as far as this customer goes. He has a 23" on it now, not sure of the pitch, and has gone and bought a 4 blade composite 19" if I remember what he said

I just wanted to see if anyone was managing higher compression without any problems, looks like not.

Thanks,
Bill Koustenis
Advanced Automotive Machine
Waldorf Md

cuda

I have 10:1 with iron heads. I run 93 octane with no problems.

jmherbert

10.5:1 , 92 octane, no signs of detonation.

Dart heads, .038 quench, and 228@050, 512 lift cam.

Started w/about 95 octane and have worked back to 92 w/no differences.

SB

iTrader: (1)

BillK - sounds good. I don't know you or your knowledge,beliefs, experiences or etc,etc so I just spilled that out as if your where a normal, regular non-engine guy.

Seeing that you know what's up and you have a very viable concern since this is a customer that you know and are concerned about, I'll give you a little of my compression speal.

All things remaining the same, what does an extra point of compression give you ? Somewhere around 3%-5% more power ? On 400hp that is what - 12 to 20 more hp ?

Now some will say "Yeh but if we change cam to the more compression then we gain more power." Yes, this is true, but as we all know in the typical marine world with typical exhaust, typical rpm range, typical etc, etc,etc we are camshaft limited anyway and prob won't be able to use a cam that would complement such a high compression.

We could use a slow/non-aggressive camshaft that has a lot of seat duration vs .050" and .200", but that costs power anyway. This would offset or even be less power than having lower compression but a good modern day well designed camshaft.

So, my personal 02 with my customers is to keep them safe by using lower compression and making hp/torque in other ways.

You can't control a customer's habits and especially that one time they have no choice but to run 87 or 89 octane even though they where warned to use 93 or more. And as you mentioned a possibility of using too much prop and/or lugging engine around. Even if they are good about things, what happens with that one bad tank of gas ?

I don't mind being blamed/bad mouthed for something I did. No problem there. But having someone even the last bit upset/letdown, or etc towards me over something they did, uugghhh, that stinks bad !!!!

So, low safe compression and good cylinder heads, intake, carb and camshaft !

Would I run 10:1 on one of my motors. Yes and no. Yes, because I'm in my own control every minute of the day. No, because even though I have trailers and get fuel from land based gas stations, I do have to get fuel once in a while out on the water and in our area it's just 89 octane fuel at the docks. BTW: it's prob not close to 89 at the beginning of the season after sitting for 8 months or so.

I'll stop, sorry.