I build 10.5:1 /6 engines pretty regularly. 93 octane and single plug. That's about the limit without going twin-plug, but all of them on the road have been fine.


Why don't I see this in Type 1 or Type 4 engines?

Some rough math suggests a 2.0 @ 8.5:1 is an 85hp engine, while the same engine at 10.5:1 would give almost 120hp.



Just thinking out loud.....

It's all about the cam you use. Longer duration requires higher compression.
You want to be around 8:1 dynamic compression. Search the web for dynamic compression calculators.

If you just increase the compression without a longer duration cam, your HP improvement will be really small, like few HP only.

Case design/strength?

Running a 9:1 CR T4 now ... hasn't exploded yet

where ya planning to buy gas for it?

i paid $10/gal for leaded 110 at the track 2 weeks ago

short answer - yes it will break

if you are going to run 91 octane pump gas, stay below 9:1 - better yet just buy euro 2.0 p&c's and live with 8.5:1 & do the other things proven to make power like cam & throttle body

- i'm not sure your rough math is correct - ask another question -- ask the guys to put up their dyno results

i think it's more a question of what heads you use and the quench area design.

resistance to pre ignition and cooling the head along with a modern efi and timing would allow higher compression.

carbs and stand alone dizzy stock heads with street gas and 10.5 would prolly bang the motor apart and run pretty inefficiently.

valve seats and ring lands would not be happy.

No short answer Joe. Here's my short answer. 10:1 is totally do-able on pump gas. I built 11:1 BB Chevs and ran on pump gas. Several of my 6 cylinder BMW engines were 10:1 and higher. One of my BMW 2002 engines was 14:1

Alot has to do with increasing the amount of air/fuel you can get into the cylinders, the ability to light it up and the amount of air/exhaust you can remove. Then as you know it comes down to cooling. The bigger the explosion the more heat. Fuel can help cool the cylinders as well as oil and air with an air cooled engine.

But I'm not telling you anything you don't already know. I think you were just thinking out loud.

The 911 has a better combustion chamber and better heads than the Type IV motor does. I think it would be kind of challenging to run a 10.5:1 Type IV motor on any street gas. I'm sure it's been done, at least for a while, but I would bet it's a lot of work to set it up well.

--DD

If you real want the 12-1 comp. motor to run you need to change the cam so the engine runs on an Milliner cycle, Like some of the Mazda's. Basically the intake stays open until the crank hits bottom dead center. As the piston starts up the vale slams shut= giving you a running compression of 8.5-9.0 ish but with a much smaller quench area.

Best designed for the "pen head" intake exhaust systems(16 valves).

I had a 12-1 type1 1300 that broke 2 trannys on pump gas. That little motor only lived about 4k miles before it pulled all the exhaust valves thru there keepers. it was like a 2 stroke, rev until it quits pulling and shift!

But really would i do it again? no because i would probably have a ton of hours getting the subaru heads to fit a T4 or T1 or start form billet and make my own using the single over head cam 16 valve design.

jcl

What about doing a modern efi system with aquamist injection of 50/50 mix of water/methenal.. Still would need some big cams and nice flowing heads..

I'd love to see that.

In Florida, 93 octane is readily available. For my 10.5:1 /6 customers, that makes all the difference. They are told when they take delivery that 93 octane is mandatory. 91 octane won't do it.



My question isn't about cams. It's simply about reliability. I had assumed the 911 heads are better at getting rid of heat than T1 or T4 heads. So heat dissipation is the real issue? What's the failure mode?


I'm always keeping my eyes open for a fiberglass-style street buggy. If I find one, I may build up a 10.5:1 T1 engine and see how long it takes to make it go boom.

It would have Megasquirt and EDIS, so it would be fully programmable.

Should be fun.

I had a 10.5:1 engine in my street 914 for a couple years.
It was 103 x 66 (2200cc), had a 3/4 race cam (EMW J grind) and 44 IDFs.
The heads were built for top end power, having 48mm intake and 38mm exhaust valves and ports to match.
The engine made 180hp around 7000 rpm and was terrific on the track. I often ran it up to 7500 before shifting.

Lack of bottom end torque made it a poor choice for a street car though. Pulling into traffic from a stop sign required sidestepping the clutch at 4500 rpm.
I ran it on 93 octane except on the track, and sold it before ever needing to rebuild.

... you need to check your math!

How could a 2.0 T4 at 8.5:1 be only 85 HP, when the stock USA version GA 2.0 at 7.6:1 was 91/95 HP (SAE/DIN methods) & the Euro at 8.0:1 was 100 HP!!??

Check with Ron at FAT Performance in Orange....IIRC he does them at 10 & 10.5 with single plug & 145-185 HP, as does Jake Raby & probably Chris at Tangerine too! They change the intake & exhaust breathing, carb/FI delivery, etc. too, in order to get there of course.

So I don't see why you couldn't....

When I mentioned the case, I should have elaborated - I was referring to installing case saver/inserts, the larger dia studs and the even extra studs to keep the heads secure. I've seen lesser compression pull head studs on stock cases (at least on T-1s).

On the other end of the scale the 2007cc Gene Berg engine in my '67 bus has 7.3:1 CR, 120hp and is 22 years and 4 cars old.
Never been apart, has at least 80K miles on it. I should do the heads this year, then it will be good for another 80K.

a 2056 with 9 to 1CR with the right cam and valves/heads gives you 120hp. Its plenty for a street 914 with lots of low end torque.

Side note...
I just visited my wifes cousin in San Diego.
a Fellow aircooled brother who I wish I had met 25 years ago when we got married.

He most likely could have built that motor for you...
He did two or three per day for Bergman...they had a little assembly line thing going.

He does not work there anymore, but had some great stories and a garage full of not only a cool double cab ( that he wont sell...I asked) but tons of engines apart, in process, etc...
Full machine shop also.
All collecting dust (he is in his 60's)

rich

Yes, it can work.
I guess you need to work out the camshaft to support it, and the head sealing, but they can be made to work fine.

Large cylinders need to be re-ringed often (think 5-10k miles)
You could get around that problem with Nickies or Alusil cylinders.
That seems to be where things wear the most.
Obviously you need to work on the heads for the higher CR...but I am assuming you would do that anyways.

Side note.
What would you charge me to put in a 3.6?
PM me if you think you are the right guy.
I would ship it down.


Rich