Build 3'7 - compression ratio question for single spark.
 

Hi all,

I've read as much as I could but did not find a definitive answer. Any real world experience on running single plug with the following spec? What is good compression ratio?

I'm building a 3.7 from a 3.2 with gt3 crank and 98mm mahles. Pistons are custom JE designed for 11.0:1 compression ratio and I will be using only 98ron gas.

My cam specs are 304/304 in/ex, 110 degree lobe separation, 12.4mm max lift and 3.0mm lift in TDC.

Thanks in advance for sharing your views.

The important number you're looking for when calculating safe compression on street gas is Dynamic compression.
General rule of thumb for acceptable dynamic compression ratio to run safely on pump gas is 8.5:1 with aluminum cylinder heads.
To calculate that you will need to know standard engine specs and intake valve degrees at closing. If you're using a formula, they generally want IVC@ .050" of lift.

GT3 crankshaft and 98mm pistons would be 3.4 liter according to my calculator. Now 102mm pistons would get you 3.7 liter.
I've read that you could go up to 10.0 CR with single plug and 92 octane pump gas. (?) I just twin plug every engine. :)

"I've always read" is a horrible strategy for designing an engine.
Error on the safe side or do the correct calculations.
Static compression is incredibly misleading especially when you're dealing with air-cooled, hemi-head engines driven by error prone week end racers.
Henry is right. The answer starts with dynamic compression, add to that, projected head temps, dome configuration, ignition variables and the list goes on.
You want to really screw things up, hang a blower on it.

…or was the calculation possibly done on 4L crank? With 80.4 stroke, that takes the 3.46 (98 on 76.4) to 3.64.

Hi Joe,

That is correct. I'll be using an 80.4 4L spec crank so the displacement is around 3.64.

Steve Wong got back to me and explained 98 RON fuel is equal to about 93-94 AKI octane in the US so with my cam he says I could get away with 10.5:1.

I'll calculate dynamic compression as well when I have all my measurements done. Thanks Henry!

Please post dynamic compression. … I ran some numbers with one of the on line calculations a while ago and was surprised at how much it can drop depending on cam ( like a static 12.7 with big cam, dynamic cr ~8.7.) Though yours is kinda wide at 110. What do you need to measure do first pass?

Anyone happen to dynamic CR to race fuel? 100 up to X, 110 up to Y, etc.

Few other 4L crank questions /comments:
- what ignition are u running? Not sure if dizzy gear fits on 4L crank.
- do u need to run 4L intermediate shaft & gears?
- clearancance’ing of cyl required to clear rod bolts?

… almost all above void if custom stroke SC crank.

Can you recommend an online tool to use?

It’s indeed a custom SC crank. The only thing i need to do is change my flywheel bearing since it’s g50 spec but I run a 915.

I think I used the Wallace racing one. …been a bit since I did it so not quite sure. I’d throw your numbers in 2 or 3 of them and see if ya get same answer. When you google dynamic compression calculator many come up.

This one is easy and effective .
remember this is only a general guide .
Further math is often helpful.

https://www.gofastmath.com/Compression-Ratio-Calculators/Static-And-Dynamic-Compression-Ratio-Calculator

Why go single plug with such an effort? Just use some ECU and COP or cheapest way would be twin Bosch 3x2 coilpacks.

I am also surprised you want to stay with a single plug, since this engine was clearly not built on a tight budget. What kind of engine management are you planning to run? Bosch COPs are cheap, you can (almost) buy 12 new ones for the resale price of your distributor...

If it's still not assembled, I'd seriously think about adding that second spark plug hole.

Henry has posted data on how much cooler twin plug engines run, and the additional power they make. I'd say it's a worthwhile modification so you don't risk detonating your nice new high compression pistons.... :o

Hi Lukas,

Currently I stay with the original Motronic DME but Steve Wong is a building a chip for my set up.

Do you happen to have a link to the thread of Henry you are referring to? I found many things twin plug in the search but not really what I was looking for.
EDIT: Think I found it, reading with great interest!

And yes... I am contemplating more and more on going for it after reading up here. Initially I wanted to stay 'visually' stock for inspection reasons (local DMV doesn't allow proper modifications) but I guess a 12 rotor will not draw too much attention and allow me to run the compression the JE pistons were intended for (11.0:1).
Discussed with Steve that the bleed of my cam should allow to run single plug on 98 fuel (europe) on 10.5 compression but that means not getting the potential out of the engine indeed.

Steve offers a solution to adjust the original DME for twin plug but I would prefer to stay with an RSR style single 12P rotor look instead of opting for 964 double one. Could I run the setup Steve has one his website with this distributor from Patrick Motorsport? So using the BOSCH 0227 100 203 250 module instead of MSD CDI?

https://patrickmotorsports.com/collections/twin-plug-distributors/products/ign911tpdist32
https://www.911chips.com/TPtype993.gif

https://www.911chips.com/TPtype993.gif

In addition, how much modification is needed to run the cables to bottom plugs? I've read you need to change shields on a 3.0SC but have not looked into what needs to be done on a 3.2.

Eager to learn, thanks!