Type 4 mechanical compression ratio, Why is the factory compression so low? Options

[Beebo Kanelle]

OK, here's a fun one for the novice to ponder and you experts to answer:

Why is the factory mechanical CR so low?

and for reference, what is the highest mechanical CR that can be physically achieved? assume no octane limitations -

I'm just looking for the design limit for the highest CR that can be / has been in a Type 4 engine using available technology.

Thanks

[914_teener]

OK, here's a fun one for the novice to ponder and you experts to answer:

Why is the factory mechanical CR so low?

and for reference, what is the highest mechanical CR that can be physically achieved? assume no octane limitations -

I'm just looking for the design limit for the highest CR that can be / has been in a Type 4 engine using available technology.

Thanks

Might need some context to your questions;

ie. Octane is a fuel. So what is the engine? Internal combustion and what fuel?

Diesel uses compression for the spark plug and a A bomb uses tnt to start the fission reaction.

So for reference you need to add context to the question. When you say Octane I suppose you are referring to Octane rating. But what is the fuel?

[Mark Henry]

Kind of funny way of asking what is the max HP a Type 4 engine can make. (IMG:style_emoticons/default/biggrin.gif)

Often when these questions are asked, you have a younger guy, a novice who is only looking at the top number.

You can run at lower CR
The factory CR is about normal for the era.
Over about 9.5 you need to twin plug.
At to 12:1 your done for gas

Above has too many variables, but is ballpark for an aircooled, you can get a bit higher out of a watercooled

[colingreene]

Compression ratio is often set by quality of fuel. Head effiency and desired results.
At the factory compression these motors are very reliable as long as maintained properly.
cant really sell someone a car that's constantly exploding.

[Dave_Darling]

Assuming you aren't actually trying to burn anything, and are in a vacuum to boot, the maximum compression ratio is infinite. If you have zero volume in the chamber when the piston is at the top of its travel, you will have ((swept + 0) / 0):1 compression, which is infinite or undefined (depending on who you ask).

We need some space up at the top of travel for the air and fuel, obviously. We also need space for the slop in the rod bearings, wrist pins, etc., and for things to grow as they heat up. So there is a minimum amount of space needed, but how much depends on a lot of factors.

The stock 914 compression ratio is relatively low because it is set up to avoid pre-ignition, or detonation, or pinging. Much like all engines.

The engine has a pretty primitive design. The wedge-shaped combustion chambers are OK, but not great, so you can get detonation more easily than with a more advanced shape.. The cam is very mild so you don't have compression pressure "leak out" on overlap, which means you need a lower compression ratio.

The engine management system is primitive (D-jet, L-jet) or even more primitive (carbs) so you can't tune it closer to the edge and rely on the engine management to save you from detonation.

An air-cooled motor cools the way it wants to, and it is hard to get parts of it to run cooler than they want to. Water will absorb heat a lot better than air will, and you can run fine passages where you want them to go, and control the heat in various parts of the engine better. Having specific things get hot will lead to detonation more easily than keeping them cooler, though of course there are things you want to be hot for efficiency's sake.

The 914's compression ratio is not out of the normal range for the time the motor was designed, as Mark said. There were cars that ran higher compression, and some that ran lower.

Today, there are gasoline-powered cars that run over 14:1 compression, but those are pushing things! The highest I remember hearing about in a 914 motor is about 12.5:1, and that required race gas and had a big lumpy cam and cubic dollars sunk into the heads.

--DD

[r_towle]

13:1 at 200 hp
Race fuel only.

Could not move the car under 4500 rpms...
built tuned between 5500-7500 rpms.

stupid to drive anywhere aside from racing.

As Jake always says, its all in the combo.
He gets 200hp on pump fuel now....

The thing to remember is that higher CR creates heat...the worst enemy of an aircooled motor.

Rich

[Beebo Kanelle]

Thank you one and all.

I'm afraid I wasn't clear enough in my original inquiry -

I am modeling the Type 4 based upon other engines I have been involved with in the past.

I am not, however, familiar enough with the air-cooled engine to know if there are any hidden constraints that are unique.

I believe it is possible to increase the output to 100 - 110 hp / liter in a street driven, smooth idling engine of two valve design - but I know there are limits -

and it appears that, without going to exotic extremes that the reasonable limits would be 13:1.

Has anybody gone higher ?

Any and all info is greatly appreciated

Thanks for the input / advice / sage wisdom

[jmill]

IMHO twin plugging and race gas are exotic extremes.

[brant]

The materials used in a stock 914 engine do not like high compression.
Water cooling provides more controlled cooling than does steel cylinders on an air cooled motor

It's expensive to rebuild with modern components/nickies/etc
additionally the metallurgy of the block/heads can play into high theoretical numbers

[ConeDodger]

The materials used in a stock 914 engine do not like high compression.
Water cooling provides more controlled cooling than does steel cylinders on an air cooled motor

It's expensive to rebuild with modern components/nickies/etc
additionally the metallurgy of the block/heads can play into high theoretical numbers

(IMG:style_emoticons/default/agree.gif) engine sealing becomes an issue. Cylinder head temperatures become an issue. Oil cooling which is important to consider separately from CHT becomes an issue.
Your numbers, 100 HP/liter are doable, but I know only a couple builders who can make that live very long. At the raw number, 200-210HP, you might as well put a 3.2-6 in it as you've spent the money...

[Mark Henry]

The materials used in a stock 914 engine do not like high compression.
Water cooling provides more controlled cooling than does steel cylinders on an air cooled motor

It's expensive to rebuild with modern components/nickies/etc
additionally the metallurgy of the block/heads can play into high theoretical numbers

(IMG:style_emoticons/default/agree.gif) engine sealing becomes an issue. Cylinder head temperatures become an issue. Oil cooling which is important to consider separately from CHT becomes an issue.
Your numbers, 100 HP/liter are doable, but I know only a couple builders who can make that live very long. At the raw number, 200-210HP, you might as well put a 3.2-6 in it as you've spent the money...

(IMG:style_emoticons/default/agree.gif) To a point ...

100hp/liter is a crack pipe dream for a novice Type 4 builder. A Type 4 is an unforgiving bitch that will granade if you don't build it right.
Your pistons and cylinders alone will cost $3440, I'd say you are easily well over $12K in parts alone. I know look at my signature line for my '67 bug.

If the price doesn't scare you then look at my sig line again and what's going into my 914. Once you get to the cost of a super HP Type 4 in a 914, you may as well do a nice /6.

But then if HP is all you are looking for... do a V8.

[Mueller]

The cooling, or lack of is why such low compression.


If you look at almost all street engines the lowest compression motors for car and motorcycle are air cooled.

Water cooled have a point or more higher.

Direct injection water cooled motors have even higher compression due to the fuel cooling off the combustion chamber.

[Bulldog9]

Thank you one and all.

I'm afraid I wasn't clear enough in my original inquiry -

I am modeling the Type 4 based upon other engines I have been involved with in the past.

I am not, however, familiar enough with the air-cooled engine to know if there are any hidden constraints that are unique.

I believe it is possible to increase the output to 100 - 110 hp / liter in a street driven, smooth idling engine of two valve design - but I know there are limits -

and it appears that, without going to exotic extremes that the reasonable limits would be 13:1.

Has anybody gone higher ?

Any and all info is greatly appreciated

Thanks for the input / advice / sage wisdom

A few thoughts from a novice 1 time Type 4 builder.

100-110HP/liter is not doable in a N/A Type 4 without a SERIOUS cash infusion, unless perhaps it is an alcohol burning bracket racer you rebuild after every run. That's optimistic even for a modern N/A inline 4 street motor....

I have two liter+ motorcycles that make this kind of power, but at 8-10K and no torque. Not a problem to push 700lbs, big problem for a passenger car.

I'm sure it can be done, and maybe has, but after spending the last 5 years watching, reading, learning and doing, I havent seen it..... The Type 4 is a different animal.. Do some reading and educate yourself. The CR is not really low for that era. 13:1 CR reasonable? you need to put down the crack pipe freind (IMG:style_emoticons/default/drooley.gif)

[Mark Henry]

I'm sure it can be done, and maybe has, but after spending the last 5 years watching, reading, learning and doing, I havent seen it.....

Jake says he's done it, closest I've come on a slightly milder 2.6L build is 70hp/liter, but you're totally right on outrageously expensive and not a novice build.

Personally for the novice I'd start with a 2056cc build and if you are truly skilled a 2276cc would be the max. Both engines are great in a 914.
BTW just a FYI a performance /6 build is way harder than a /4. BTDT

[Beebo Kanelle]

Thank you all for all your input. I knew that the collective intellect could be relied upon for valuable insight and experience

To answer a few comments/observations made, I am trying trying to maximize the thermodynamic efficiency of the design, not just max HP numbers...

Whatever the numbers end up being, it has to be drivable, have sufficient torque throughout the range and without going to endless amounts revs - I always try to minimize frictional losses.

this is not to be a track / race engine; just a daily driver

So to sum up, the enemy that needs to be addressed is heat management, and combustion chamber limitations, correct?

and yes, this sort of stuff always costs... its just a fun project - for an old guy.

I always welcome any and all comments / observations / experiences so keep 'em coming.

and to R Towle, do you have the camshaft specifications from your 13:1, 200 hp engine? I am specifically interested in your valve timing events

thanks again.

[stugray]

So to sum up, the enemy that needs to be addressed is heat management, and combustion chamber limitations, correct?

There has been some discussion about a water cooled head, air cooled cylinders but I dont think anyone has ever done it.

I seem to recall pictures of it done on a type I engine long ago (what WASNT done to a type I in the 80s-90s ? )

If you want 100hp/liter in a 4cyl NA boxer, just get a FA20 from a BRZ (which happens to be 86mmX86mm with 12.5/1 CR)

[ConeDodger]

The materials used in a stock 914 engine do not like high compression.
Water cooling provides more controlled cooling than does steel cylinders on an air cooled motor

It's expensive to rebuild with modern components/nickies/etc
additionally the metallurgy of the block/heads can play into high theoretical numbers

(IMG:style_emoticons/default/agree.gif) engine sealing becomes an issue. Cylinder head temperatures become an issue. Oil cooling which is important to consider separately from CHT becomes an issue.
Your numbers, 100 HP/liter are doable, but I know only a couple builders who can make that live very long. At the raw number, 200-210HP, you might as well put a 3.2-6 in it as you've spent the money...

(IMG:style_emoticons/default/agree.gif) To a point ...

100hp/liter is a crack pipe dream for a novice Type 4 builder. A Type 4 is an unforgiving bitch that will granade if you don't build it right.
Your pistons and cylinders alone will cost $3440, I'd say you are easily well over $12K in parts alone. I know look at my signature line for my '67 bug.

If the price doesn't scare you then look at my sig line again and what's going into my 914. Once you get to the cost of a super HP Type 4 in a 914, you may as well do a nice /6.

But then if HP is all you are looking for... do a V8.

100HP/liter cost me $14000. For parts alone. If you have to pay for the labor? (IMG:style_emoticons/default/sad.gif)
I am very happy with my 3.2 six. 220HP with manners...

[Mark Henry]

So to sum up, the enemy that needs to be addressed is heat management, and combustion chamber limitations, correct?

I seem to recall pictures of it done on a type I engine long ago (what WASNT done to a type I in the 80s-90s ? )

Yes, in a dragster.
Only raced a few times, never amounted to much, was deemed illegal after just a few races. IIRC it's been a long time, it was a feature in HotVW's

[McMark]

If you're looking to improve thermodynamic efficiency, why are we talking about compression? Why are we talking about horsepower? Neither of things have any direct connection to head cooling.

You're being very cagey about what you're actually trying to do. It makes it hard to help.

[Mark Henry]

If you're looking to improve thermodynamic efficiency, why are we talking about compression? Why are we talking about horsepower? Neither of things have any direct connection to head cooling.

You're being very cagey about what you're actually trying to do. It makes it hard to help.

(IMG:style_emoticons/default/agree.gif)
And what is your budget?

We've seen our share of new members posting they want a 200hp type four, then after countless posts we find out their entire budget is $2k and a case of beer. (IMG:style_emoticons/default/blink.gif)
(IMG:style_emoticons/default/beer3.gif)