Compression and pump gas

[El Prosecutor]

Is there a general rule on how high of compression you can run on pump gas?

[OverKill]

I only use 100LL/AV Gas. I am just under 11 to 1, the highest I think I could go is 12.5 to 13 to 1 thats about it. IMO I woud'nt go any higher than just under 11 to 1 with 91 octane. There might be some others here with some different opinions.

[396_WAYS_TO_SPIT]

Hey James, I think you missed his question. He is asking about "pump" gas;)
I have seen peeps run 10.5-1 and burn stuff up. I wouldnt run over 9.5-1 on pump gas. If your at 10 and higher mix it with a little avgas and be safe. Its better to be safe than sorry. I know someone will argue with me but trust me, 10.5-1 isnt really a pump gas friendly compression. If you had a knock sensor and a ecu to retard the timing them you would be safe.

[Taylorman]

To may factors that can infulence that question. Static and dynamic compression can be different. Some have run up to 12-1 on pump gas but that is pro built engines. Lots of variable. Also alum vs iron heads can make a difference.

[OverKill]

Hey James, I think you missed his question. He is asking about "pump" gas;)
I have seen peeps run 10.5-1 and burn stuff up. I wouldnt run over 9.5-1 on pump gas. If your at 10 and higher mix it with a little avgas and be safe. Its better to be safe than sorry. I know someone will argue with me but trust me, 10.5-1 isnt really a pump gas friendly compression. If you had a knock sensor and a ecu to retard the timing them you would be safe.
Dee Dee Dee he did say LOW didn't he LOL.
So how was Mermorial Day a Suguaro?? You gonna be on the water soon??

[pw_Tony]

I've seen up to 12.5:1 compression. All the high compression motors are usually high polished shallow chamber, and detailed pistons with no dome, or with a dish. My car has 10.4:1 compression with chambers I polished and pistons I detail, I can run 87(have to be easy on her though) and when I have 91 in there I can hot rod all day, just takes a little grinding:D

[pw_Tony]

But can't jet boats run more compression than a car due to less load?

[ck7684]

Less load?? Arent boats ALWAYS under load?? Especially jets?

[Sleeper CP]

To may factors that can infulence that question. Static and dynamic compression can be different. Some have run up to 12-1 on pump gas but that is pro built engines. Lots of variable. Also alum vs iron heads can make a difference.
All true :if you run alum heads and keep the temp down you can get away with higher compression 11:1 or even higher with dump gas so long as you watch the timing. Two other factors you want to consider are: how do you drive the boat and cam specs? I've seen people that load up their boats with a bunch of people and toys then drive at ski speed for 10 miles at 35 mph just working the shit out of the poor engine. They never think about getting " on top" and getting on the cam alittle bit and running it, this takes a hell of alot of load off of the engine, less load less heat and alot easier on parts and will save you gas.
The other factor is cam specs. If you are starting fresh on your engine build talk to your cam company and they can design the cam to bleed off some cylinder pressure to help keep down the detonation if you're thinking of exceeding 11:1 or so.
Sleeper CP

[HBjet]

All Iron 496ci - 10.25:1 compression - 91 octane all day long - 700+ HP - 5 years strong!
When done right, there are no problems!
HBjet

[Taylorman]

But can't jet boats run more compression than a car due to less load?
Less load, i think you got it backwards. Boats are always under a load.

[HBjet]

Hows it running in the new boat?
Well, the new boat is about the same weight as the last one, but the hull design is better - so without any setup it was already quicker. The biggest difference I notice though was how I could stay on plane at lower RPM due to the hull change. The old boat if I was under 3k rpm I was coming off plane. Now in this boat I can run at 2600 rpm. Makes for some nice cruising, but I tend to be all over the place with the throttle... haha!
I have the same pump and motor from the old boat, but I do not have my place diverter so that is the main difference, and I just got a new shoe made up for this boat, but I have not played with it like I did with the old boat getting it to where I felt it ran best. Still a lot to do... But for where it is at now I am happy.
I think a blue bottle will be in order later in the future... best way to stay competitive with all these big HP guys at the river.
HBjet

[FuelInMyVeins82]

Damn tammi must be keeping you busy if you need a whole bottle :D

[HBjet]

Damn tammi must be keeping you busy if you need a whole bottle :D
I'll only need a few seconds to hold you over, the rest I will just purge into the air with my blue led lit purge valve as a victory celebration. :D

[FuelInMyVeins82]

lol. I think your flashing blue light is going to be more like a cop chasing a speeder. Maybe i should put a mirror on the rogers so I can watch it fade into the distance.

[HBjet]

lol. I think your flashing blue light is going to be more like a cop chasing a speeder. Maybe i should put a mirror on the rogers so I can watch it fade into the distance.
haha...
I won't chase ya while you look for CS to bring that boat to victory lane. :D I'll keep that light on for ya! :)

[fleetimus]

I have a 468, 13-1. I run 50/50 92 oct and 110 fuel. My timing is at 38*, I would rather buy the fuel than buy an engine!!

[pw_Tony]

Less load, i think you got it backwards. Boats are always under a load.
Ok just a question not trying to be a smart ass:D ... but when I'm driving my boat I can pretty much jump to 5200 rpm in a few seconds, as when I'm in a car it takes a lot llonger and obviously goes through the gears. So when a boat can jump in RPM so fast wouldnt that mean less load? I know in a car most of the detonation happens at low RPM when it's floored, but I just never had a boat detonate on me that's why I'm asking

[396_WAYS_TO_SPIT]

All Iron 496ci - 10.25:1 compression - 91 octane all day long - 700+ HP - 5 years strong!
When done right, there are no problems!
HBjet
Look at the key words that Randy wrote!!!!! He has a motor that a very well know engine builder built. In fact Dave is one nasty guy when it comes to pushing the limits on pump swell;) Its not just a matter of a few parts here and there. When I spoke to Dave last year at firebird, he got very technical during our conversation. It takes kick azz parts and alot of knowledge to push those kind of limits;)

[396_WAYS_TO_SPIT]

Ask "BigErn" here on the boards about what happens when you run pump gas on a 11-1 motor:eek:

[lucky]

10.25 is all i would run along with a proper cam (ie overlap ) and anything to promote a cooler aircharge entering the cyclinders - also watch timing - there is alot of tricks -

[MudPumper]

There is no right or easy answer to this question. It's all relative to the set up you are running. Too many factors to consider.

[396_WAYS_TO_SPIT]

Screw it! Run 87 octane in a 11-1 compression motor all you want;) Ill continue to pump avgas with some vp110 and keep my 13-1 comp motor happy;)
I dont put a price on the ammount of fun that I have in my toys;) Whats an extra $2.00 per gallon of fuel to have more hp:idea:

[77charger]

keeping engine runnin very cool,alm headvs irons,cams,type of piston dome vs flat top,combustion chamber smooth vs rough,timing.
like mentioned above to many variables come into play.
can have a motor ruunin at 150 degrees,alm heads,flat top pistons,right timing and cam pushing almost 11-1 and be at limit on pump gas or a iron headed high pop up piston to make 11-1 with a thermostat set at 180 degrees with alot of timing that must have race gas.
IMO if going race gas why stop at 11-1 just go 13.5-1 or more:D

[396_WAYS_TO_SPIT]

Dee Dee Dee he did say LOW didn't he LOL.
So how was Mermorial Day a Suguaro?? You gonna be on the water soon??
Saguaro was off da hook brotha;) I killed a 5th of grey goose and was feeling like a million buicks:D Bubba on the other hand wasnt too happy. Blew a headgasket and melted a spark plug. Too much compression with a blower:eek:

[Ken F]

In answer to your question, a fairly stock engine with iron heads, about 9.0-9.5 would be on the limit.
With Aluminum heads you could push it one compression point higher.
Hope this helps,
Ken F

[sunkisst]

Dunno what your plan is,I just slowed my boat down with help of Duane HTP,8.5:1compression, 461 hydraulic flat tappet, Gullwing,81.6 MPH ,PUMP GAS,THATS RIGHT,81.6,GPS.Very happy with the recomendations.

[FuelInMyVeins82]

haha...
I won't chase ya while you look for CS to bring that boat to victory lane. :D I'll keep that light on for ya! :)
Lol. But as I recall it did see victory lane. :devil: don't worry even the river needs a pace car :D

[HBjet]

Lol. But as I recall it did see victory lane. :devil: don't worry even the river needs a pace car :D
With the training wheels on (rolling start) you sure did... I was a length behind ya!:D

[pattymelt18]

Geeze randy and jordan gettn out some nice friendly fire nice.

[FuelInMyVeins82]

With the training wheels on (rolling start) you sure did... I was a length behind ya!:D
lol we can do it from a dead stop if you want. :D

[Blown 472]

With the training wheels on (rolling start) you sure did... I was a length behind ya!:D
Boy you talk pretty big for a guy that doesn't take his boat to the drags.:D

[HBjet]

Boy you talk pretty big for a guy that doesn't take his boat to the drags.:D
Send me a cashiers check and I will go. Until then I am happy with where my priorities are at. :)
HB

[Blown 472]

Send me a cashiers check and I will go. Until then I am happy with where my priorities are at. :)
HB
What address should I send it to?

[HBjet]

What address should I send it to?
Check your PM's

[OverKill]

Saguaro was off da hook brotha;) I killed a 5th of grey goose and was feeling like a million buicks:D Bubba on the other hand wasnt too happy. Blew a headgasket and melted a spark plug. Too much compression with a blower:eek:
Don't tell me he was running 10 to 1 with an 8-71 :mad: :mad: shouldn't he be at maximum 8 to 1. Well anyway's what was his application?? I hear the Blower Shop makes a bolt on super charger that is like a 100 shot of NOS. You ever heard of this??

[396_WAYS_TO_SPIT]

Don't tell me he was running 10 to 1 with an 8-71 :mad: :mad: shouldn't he be at maximum 8 to 1. Well anyway's what was his application?? I hear the Blower Shop makes a bolt on super charger that is like a 100 shot of NOS. You ever heard of this??
He slapped a blowershop 250 on his 502. Come to find out it was like 9.5-1 to 9.8-1 comp :eek:

[Riverdog1]

Is there a general rule on how high of compression you can run on pump gas?
10.25:1 should be the max and only if your valve train was built bullet proof. And then on 91 Octane. To me, you would be defeating the purpose of going 12:1 by compensating with timing and such. That's to create serious heat and HP to go fast. Aluminum heads can take up some of the heat but you're still asking for trouble eventually. 110 leaded (believe it or not) is pretty close to 91 at the pump now days and you only need maybe a 10% - 15% mix.