Compression ratio advice

derm.1785 · 01-28-2022, 10:08 PM

Hi. I’m about to start rebuilding my 3.0SC, I’ve already got some 98mm cylinders that are going for a re plate and hone , I’ll get the cams reground to suit whatever o need but I’m looking at new 98mm pistons, 70.4mm stroke and 22mm pins. I like the price of Wossner but their 11.1 CR is too high “possibly, Carrillo do a 10.1 and JE have several but are a bit more money. I’m UK based so import costs are steep but petrol octane is high so I can get away with higher CR. I would like some solid advise on piston choice and CR for my needs. Thanks in advance.

reclino · reclino's Garage

Maybe ask Wossner is the piston crown is thick enough that you could mill off material to drop the compression ratio. People cut valve relief pockets all the time, so cutting a piston top is not much different.

OSC911 · 07:44 PM

Static compression ration means zero without knowing what camshaft you have and when the intake valve is closing.
See post below about similar question:

When people talk of compression ratio, they usually are talking about static compression but that is only half the story. The most important compression ratios metric is that dynamic compression ratio. You can have an engine that has 11.8:1 static compression that will run nicely on pump fuel with little or no pinging or detonation. Toyota Yamaha headed engines (engines with GE in the engine code e.g: 4AGE 20v) have been using these high compression engines for years. Obviously these are 4 and 5 valve engine which helps, but they have low dynamic compression. The camshafts are designed to close the inlet valve quite late - quite a ways up on the compression stroke past BDC. This allows for a lower dynamic compression - usually 8.0 to 8.5 is ok for pump fuel.

I know I should be comparing apples with apples but this is a factor that is often overlooked in most engines - not just a 911. I know that the piston/cylinder design on an air cooled 911 engine isn’t ideal with higher compression - hence twin plug heads. Static compression is fixed measurement when combining pistons, rod stroke, combustion chamber volume. Dynamic compression is a more accurate measurement when thinking about detonation and pinging. Calculate your dynamic compression also when determining fuel type and or twin plug head requirements and go from there with your decision.

mikedsilva · 02-01-2022, 12:06 AM

Quote:

Originally Posted by derm.1785

Hi. I’m about to start rebuilding my 3.0SC, I’ve already got some 98mm cylinders that are going for a re plate and hone , I’ll get the cams reground to suit whatever o need but I’m looking at new 98mm pistons, 70.4mm stroke and 22mm pins. I like the price of Wossner but their 11.1 CR is too high “possibly, Carrillo do a 10.1 and JE have several but are a bit more money. I’m UK based so import costs are steep but petrol octane is high so I can get away with higher CR. I would like some solid advise on piston choice and CR for my needs. Thanks in advance.

Probably need to know your fuel delivery system too...CIS, itbs, CARBS?
This will dictate how big a cam you can run, which will determine how much compression you can use.

If keeping CIS, you are limited to 964, webcam 20/21 or William Knights M1 cam.
With Williams cam, you will be able to run a little more compression but I would imagine you should err on the side of caution and go for 9.5:1 unless twin plugging.

But either way, let someone like William know your engine parameters and he can help you choose cams and compression that will work with your setup.

He has helped design a few engines for me now and every one of his predictions have been spot on.

derm.1785 · 02-01-2022, 03:16 AM

Quote:

Originally Posted by mikedsilva

Probably need to know your fuel delivery system too...CIS, itbs, CARBS?
This will dictate how big a cam you can run, which will determine how much compression you can use.

If keeping CIS, you are limited to 964, webcam 20/21 or William Knights M1 cam.
With Williams cam, you will be able to run a little more compression but I would imagine you should err on the side of caution and go for 9.5:1 unless twin plugging.

But either way, let someone like William know your engine parameters and he can help you choose cams and compression that will work with your setup.

He has helped design a few engines for me now and every one of his predictions have been spot on.

Thanks again for your input, I’m going carbs and from what info has been given so far it looks like I might be able to go 10.5:1. I Have a set of cams that can be reground to suit and as suggested I’ll possibly be able to dial in a lift that allows the exhaust to compensate for possible detonation, I have read a lot on here about different gas octane and how it affects these motors but I don’t have a reply from any actual owners as yet.

Mark Henry · 04:34 AM

Lot of engine info missing, are we suppose to guess?

What octane? what octane rating system?
Octane ratings in NA are done differently than the ROW, we use an average of RON and MON (AKI), posted on most gas station pumps as (R+M)/2.

derm.1785 · 02-01-2022, 06:56 AM

Quote:

Originally Posted by Mark Henry

Lot of engine info missing, are we suppose to guess?

What octane? what octane rating system?
Octane ratings in NA are done differently than the ROW, we use an average of RON and MON (AKI), posted on most gas station pumps as (R+M)/2.

Over here our standard is 95 RON, 97/98/99 is available at most pumps but the cost is about 75 cents per gallon more, this would put it in the region of $7.50 a gallon.

Mark Henry · 02-01-2022, 07:57 AM

Is that imperial gallon?
I could figure it out but unfortunately I don't have the time, sorry. Maybe someone else can help.
Takes a bit of research time and maths but the info is out there.

OSC911 · 02-01-2022, 05:47 PM

Could be metric gallons��

derm.1785 · 02-01-2022, 10:25 PM

Quote:

Originally Posted by OSC911

Could be metric gallons��

US gallon would work out to be around $6.20.

I’ve just spoken to a very reputable engine builder here in UK and he’s advised me to stick to 3.0 with mods than risk the increase to 3.2 and add the expensive twin spark. With a light car a good 3.0 would be a great engine to own so it looks like I’ll be going that way after all.

mikedsilva · 02-02-2022, 04:25 AM

Quote:

Originally Posted by derm.1785

US gallon would work out to be around $6.20.

I’ve just spoken to a very reputable engine builder here in UK and he’s advised me to stick to 3.0 with mods than risk the increase to 3.2 and add the expensive twin spark. With a light car a good 3.0 would be a great engine to own so it looks like I’ll be going that way after all.

Want to sell your 98mm cylinders? pm me if interested.

derm.1785 · 02-02-2022, 04:47 AM

Quote:

Originally Posted by mikedsilva

Want to sell your 98mm cylinders? pm me if interested.

I think shipping would be a tad high Mike, but I could find out how much if your serious.

prschmn · 02-02-2022, 06:15 AM

I use a lot of Wossners and have done a number of 3.2-3.4 setups.
I found I could cut .080 in. off the dome without problems.

derm.1785 · 02-02-2022, 08:17 AM

Quote:

Originally Posted by prschmn

I use a lot of Wossners and have done a number of 3.2-3.4 setups.
I found I could cut .080 in. off the dome without problems.

At 3.2 have you always twin sparked ? This has been my major dilemma in setting out my build.

Mark Henry · 04:48 AM

On Super 91 octane 9.5:1 CR is as far as I push it on single plug. My logic on running the 91 is availability and it's pure gas. The 93 and 94 here the octane is 'boosted' by adding ethanol which I don't want because I run webers, plus it's only available at select stations.
What I'm getting at is pick the gas that's most readily available in the UK (and maybe EU), pure gas if available and do the maths and CR for that octane. At a certain CR you have to pull the trigger and twin plug, even on high test. Of course that advice is for a street car as I drive my twin plug 3.0 almost daily during the summer.