Comression ratio advice, 6 cylinder motor build Options

[team914]

Hi,
I am planning to build a 6 cylinder street motor for my 914/6 factory replica. My original 84mm barrels were worn, so I elected to machine the barrels to 84.5mm and have bought a set of 84.5mm Wossner pistons. With a 70.4mm crank, the compression ratio is stated at 10.11:1 with the pistons out of the box. I intend on using Weber carbs, E cams and an E distributor/123 Distributor. My machine shop engineer has recommended reducing the compression ratio from the 10.11:1 to a more suitable figure. The petrol quality in Ireland is poor, typically 95 octane, and we now have a compulsory addition of 10% Ethanol added (E10)!
Any advice greatly appreciated

[mepstein]

You want to measure the actual compression ratio and not just count on what’s stated in the tech sheet. Especially if you are making changers, you want a known baseline, not just theoretical.


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[gereed75]

The general wisdom says 9.5:1 is the max you can do on pump gas with a single plug.

Not familiar with the Wosners but it is fairly easy to remove material from the top of the pistons to adjust compression. Measure the volumes, do the calculations and then adjust.

[Retroracer]

Sounds like a nice build! I'm +1 on the "conventional wisdom" of <10:1 on single plug/high street petrol - the Dempsey book suggests as much - but I've never tried to build & run a high compression -6 motor daily, so what do I know?

HOWEVER: there are a few Porsche specialists on your time zone which might be worth a phone call for recommendations / guidance: Design 911 in Essex, and Tuthill Porsche in Oxfordshire.

Both have run older 911s in competition/ historic rallies all over the UK, so can't hurt to ask.

- Tony

PS. post pics of the build pls? (IMG:style_emoticons/default/smile.gif)

[nditiz1]

You want to measure the actual compression ratio and not just count on what’s stated in the tech sheet. Especially if you are making changers, you want a known baseline, not just theoretical.


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Agree with this. With my 3.2, you would think 9.5:1 with JE pistons being 9.5:1 ,but when measured CR is actually 9.4:1.

[rgalla9146]

Summit racing has an easy to use compression ratio formula on their site.
You'll have to measure deck height and provide other info that is probably
included with your pistons.
Wossner is a high quality brand. Good choice.
9.5 is a reasonable target. You'll have to use the best pump gas available
CR can be adjusted incrementally by raising the barrels with thicker than stock base
gaskets. This method is also reversable.
You've got a beautiful plan.
Good luck
Rory (IMG:style_emoticons/default/clover.gif)

[Krieger]

You do need to cc your heads.

[rgalla9146]

You do need to cc your heads.

OOPS. Yes, that too

[ogdougy]

I would also add that some piston manufactures will provide on their website or, if you ask, what values they use to calculate their advertised compression ratio.

It helps give a rough idea with minimal work. For example, I ordered a set of pistons from Arias for my 4age. Advertised with 12:1 comp ratio. But their calculations used a combustion chamber size larger than any 4age head had. So i knew i was getting more compression than advertised which is what i wanted.

[Krieger]

I would also add that some piston manufactures will provide on their website or, if you ask, what values they use to calculate their advertised compression ratio.

It helps give a rough idea with minimal work. For example, I ordered a set of pistons from Arias for my 4age. Advertised with 12:1 comp ratio. But their calculations used a combustion chamber size larger than any 4age head had. So i knew i was getting more compression than advertised which is what i wanted.

It's called a spec sheet. I would expect Wossner to supply one with your pistons. Dome volume is one of the specs you need. This is a complex engine to build correctly.

[r_towle]

For a Daily Driver, I would suggest 9.5:1 to ensure cooler running on worse fuel.
We don’t drive them daily
We don’t race the (really)

So build it to last do 15 years from now it still runs great.
I have 240k on my 84 3.2 powered 911. Easy to run

I am building a 3.2 to similar specs because 200 hp in a 914 is more than I need.

[targa72e]

As others have said you need to CC the heads, figure out deck height and cc piston dome volume. Once you know all these you can calculate actual compression ratio. Since you are changing pistons and can specify more valve clearance I would look at more modern cam shaft design like Mod Solex or Mod S.

john

[gereed75]

The "9.5 compression" JE's i got were more like 11 after I measured and calculated it all out. I suspect that my combustion chambers were significantly smaller than JE figured

Using the density of the piston alloy I calculated how much weight had to come off the domes to get the correct volume reduction. All six came out perfectly the same height to within a few thousandths thus the same volume.

Worked pretty slick. I would also recommend the mod solex grind - sacrifice basically no bottom end and get about the same top end punch as an S but at a 500 lower RPM

[porschetub]

The general wisdom says 9.5:1 is the max you can do on pump gas with a single plug.

Not familiar with the Wosners but it is fairly easy to remove material from the top of the pistons to adjust compression. Measure the volumes, do the calculations and then adjust.

(IMG:style_emoticons/default/agree.gif) considering the fuel he is running this has to be taken into account,his motor will still be fairly mild so no point in introducing more heat into an iron bore engine ,these motors are known to be rather under stressed and last well so no point in risking that for a few horsepower.
The cams will make the best single increase, and even better with an upgrade to better flowing 2.2 heads as Mr Dempsey suggests will be a good move,the larger valve sizes will benefit the better than stock cams which were very tame stock.
Cam choice will be important ,appears to be the Solex cams or 'E" cams but don't know so hopefully someone will chime in here.
End of the day this will be a rather strong short stroke and should make good power with well matched combo.
Cheers.