Is there a point where too much compression is bad for the engine ??

**spybot** · 02-26-2018, 11:59 AM

Is there a point where too much compression is bad for the engine ??
What is the ideal range to try and hit say 130 psi +/- 5psi ?
If you wanted a strong mid range motor what would the ideal psi to aim for ?
Cheers Andy

**phillnjack** · 02-26-2018, 03:20 PM

well 150 psi is about as far as you realy want to go with your V4 or you will need to start paying a fortune for fuel, higher octane than at the
local garage would be needed...
also with more than standard compression comes more strain on the rods.
it is a case of do you want a motor to last a lifetime or do you want just a fast motor that you rebuild once a year.
if you do decide to go up to say 160psi on your motor then you will be rebuilding sooner than you might think..
the v4 your building is going to be too much for the boat anyway, plus if you leave it around the 135 to 140 psi mark it will last a very long time.
From what I have noticed the high compression is mainly for the low and mid and to compensate for higher porting than standard, but like I say your
going to have plenty of power in excess on that boat.
If you decide to go all out for a full race motor then go for higher compression, but be prepared to buy plenty of spare parts.
As you know we dont get the amount of spares in the uk and when we do they cost 3 times what they do in the states.
.
.

**Mark75H** · 02-26-2018, 06:44 PM

Max compression depends greatly on factors like spark plug location, cylinder bore, turbulence and combustion chamber shape.

If you optimize all those, you are then limited by the knock resistance of your fuel and flame speed. As you increase compression, you usually have to decrease your spark timing advance/lead. When your optimum spark advance/lead is within a few degrees of TDC, you are at max for that combustion chamber and fuel.

My 2 9/16" bore motor has tremendous turbulence, a moderately compact combustion chamber and apparently an acceptible spark plug location. It cranks 275psi and I run 5° advance on 115 octane gas.

Now you will get a lot of BS from those with absolutely no experience close to mine.

**powerabout** · 02-26-2018, 07:12 PM

how did you raise the comp on a inline?
custom pistons?

**Mark75H** · 02-26-2018, 08:22 PM

Cut heads off and welded on new pucks - for alcohol! Compression was too high for gas (APBA Mod requires gas, but can be anything without oxygen) so I removed material from the top center of stock pistons to make a pocket combustion chamber close to the spark plug. Reduced compression to the level recommended by O. F. Christner - and it worked!

The motor was one of several originally owned by Charlie Wicks (WIX filters) that I got from Pete DeLackner

**FMP** · 02-27-2018, 09:05 AM

Any pics, this engine still around, HP?

**phillnjack** · 02-27-2018, 11:31 AM

I think a mix up is going on here , from what I gather the original poster is talking about a general every day motor on a recreational basis.
infact its a OMC crossflow V4... no special hardened rods or coated pistons or billet heads.

the motor will be run on OUR regular type of fuel what in the usa would be called around 93 to 94 octane at best.
W dont have the octane rating that is available in USA.
Our Tesco 99 is not what 99 is on the American octane scale. Avgas 100LL is not freely available neither, our airports DO NOT ALLOW anyone to just
go in and buy fuel and take it away. that is not something that happens over here. (you might get a tiny airport sell you adrop, but its not freely available
and, twice the price of car fuel (AVGAS is now taxed heavy over here ).
so we have what you call 92 to 93 octane on average, so this is where the compression problems would start to cause problems.

.

**FMP** · 02-27-2018, 11:43 AM

You and others have mentioned 150-160 for 93 fuel with ign and timing. Is that the answer?

**powerabout** · 02-27-2018, 08:07 PM

everyone has better fuel than the USA
Uk has 99RON and no ethanol I think ( thats in the 95 by legislation)
its MON or AKI in the USA which is roughly 8 points lower on the scale than RON
avgas is 100 MON

Looks like you can fill a can in the UK with avgas?
http://www.fenlandairfield.co.uk/fuel.html

**phillnjack** · 02-27-2018, 08:15 PM

well everyone who talks of higher compression also talks about the high octane fuel needed that we cant get hold of ay normal petrol pumps.
higher octane fuel as you already know , plays a massive part of the higher compression wars. most newer motors have lower compression due to the
lousy fuel popping up all over Europe and most countries around the world since the mid 1980's (some countries had crap fuel in the 1970's too)
They put ethanol in the fuel today trying to bump it up another couple of numbers, but that is destructive in other ways.

You have to take into account the price of high octane fuel here, unless actually racing its not a viable thing to go looking for.
We can buy it from a select few places, but the price is frightening, bad enough when using it on race bikes, but to buy it for a recreational boat is
just not worth the price.

for example.
I just looked up a place here that advertises about how cheap they are for the fuel. Yes they are cheap compared to most in the uk.
motor sport 109 that has a octane of 101 is £99 for 19 litres.
thats equates $27 per US gallon. so not exactly cheap for just 101 octane.
if you want to go mad and buy the 120 octane then that is $41.44 US per us gallon.

**powerabout** · 02-27-2018, 08:19 PM

avgas in the UK is around GBP1+ /ltr

**phillnjack** · 02-27-2018, 08:32 PM

Originally Posted by powerabout

everyone has better fuel than the USA
Uk has 99RON and no ethanol I think ( thats in the 95 by legislation)
its MON or AKI in the USA which is roughly 8 points lower on the scale than RON
avgas is 100 MON

Looks like you can fill a can in the UK with avgas?
http://www.fenlandairfield.co.uk/fuel.html

sorry mate your way off track.. the 99 is only at some Tesco garages and is equal to 94/95 USA octane.
our 95 is around 88 in the usa.. and 99.9% of our fuel is 10% ethanol, some is 15%...

and no you cant just go into an airfield and buy your fuel. you need to be a member of the CCA and have a license to fill up your plane.
you cannot just go walk in with a can and buy it.. you have obviously no idea of uk laws and airports. .HAZ/CHEM.

I used to have a special permit to allow me airside at any uk airport, but not any more. even then I would not be allowed to just go in and buy
fuel and then take it home.

We even have laws in the uk about carrying petrol in a can in a car that most people dont even know about, its 2 gallons max in a proper fuel carrying can.
if you got caught with a 6 gallon boat tank in the boot of your car full of petrol they could take your driving licence off you and give a massive fine.
you could even go to prison for it as your carrying flammable goods without a special licence (haz chem)..

UK fuel is nowhere near as good as some of the fuel available in Europe. not the eastern block, thats always been crap.

our top fuel at 99.99% of forecourts is 95 ron .. only Tesco millennium is the 99ron stuff.

**phillnjack** · 02-27-2018, 08:39 PM

Originally Posted by powerabout

avgas in the UK is around GBP1+ /ltr

I have no idea where your getting you information from as its completely wrong. its over £2 and has been for the last 12 months.
plus you cant just go and buy it.. All airports in the uk have very strict security policy. you cant just do what you like over here.
in the last 10 years all security has tightened right up.... we pay £130 per litre for our car petrol at the pumps. avgas was £2.20 at begging of 2017
and its been creeping up steady in price all the time. its now taxed heavy..
a lot of the prices you might find on the internet are prices from 2012. not updated since all fuels went up here from 2016.
cheap diesel here is £1.25 per litre today at esso ..

**powerabout** · 02-27-2018, 08:46 PM

£1.80 here
http://www.wycombeairpark.co.uk/wyco...acilities/fuel

£1.39 here +20%vat
http://www.nottinghamairport.co.uk/aircraft-fuels.html

avgas prices are published world wide, easy to find out
try and order a 205ltr drum from a supplier and see what happens?

**BUZZIN' DOZEN** · 02-27-2018, 09:11 PM

What is never discussed on a "lake,river motor" is the load on your crank, I used to run very tight heads with the good fuel, and when the motors got torn down the thrust side of the rod journals were definitely abused, since then I won't run anything but stock compression on the river.