I just want to know what people think about such concept. We all know that when boosting your engine, in most cases compression reduced to reduce chance of a detonation/pinging. But surely if you really wanted you can build high compression engine & still run reasonable boost on it (lets call 7psi reasonable in this case) & run it wih 11:1 compression.

The obvious advantage would be awesome drivablity of such car (power in low end) ofcourse it wont be able to compete for high end power with low compression & high boost engines. Let say you have

Forged Pistons
Appropriate head gasket
Tune for 98 octane, perhaps thats to dangerous as you will rely heavily on quality of petrol (what if its dodgy)
What about LPG which has awesome anti pinging properties
Dont forget we have cast iron blocks & forged con rods in magnas which can take some punishment.

Bye bye engine unless you wanna spend MASSIVE $$$ in stripping it down and blue-printing the whole thing. Honda have good record on forced induction on high compression but huge amounts of money and investment involved

Atmoshperic pressure is about 14.1 psi from memory, so +7psi = 150% of normal air flow.
So if you jam 150% of the normal air into an 11:1 compression ratio engine, you might think of it being about the same as a 16:1 compression ratio. Yeah, bye bye engine I would think :)
Unless it's a diesel.
Turbo diesels run about 20:1 compression and N/A diesels 25:1 - at least that's what I've read about my 3.1 Turbo Diesel Isuzu MU that I used to have.

Anyway, YES you can build an engine to take ridiculous compression ratios, but I'd doubt a standard or lightly modified Magna engine could
(I've heard of many drag cars running 40psi)

from my understanding isnt the main issue with the magna too much heat rather than too much pressure?

Quite easily but you would require alot of one off components to withstand the pressure. Remember F1 motors making 1000hp or something from a couple of litres.

pV=mRT

Pressure equals heat - fuel the primary source of cooling must be used appropriately.

Why not - compression ratio is the key element in an engines efficiency, its the main reason why diesels are more efficient even though the fuel they use has a lower energy content therefore more midrange torque with boost coming on it's stupid in a fwd.

Edit* common diesels have the pistons made to withstand those sort of pressures (main reason why diesel motors last so long) but you won't easily adapt them as the pistons are key in the diesel combustion process promoting swirl in the mixture for the relatively tiny direct injection time.

and what fuel are planning to use in this imaginary engine?

Well ai i said before 98octane but with such high CR you rely too much on the actual quality of fuel that your local servo stocks, how many times i filled up at unknown servo & can feel straigh away something is not right.

So for this imaginary engine i was thinking of LPG which would never ping, BUT then we have an issue of heat as LPG (in vapour form) does not serve as additional coolant.

Again this just a discussion on how to have both FI & the efficiency.....

Water / meth injection would help drop temps too

tzaboy is running water/meth injection on his charged ralliart to good effect.

I have LPG as the primary fuel in my supercharged 380, it runs 7.5 psi boost and switches to 98 by ecu control when load is above 50%.

Never pings.

Produces just under 200kw at the wheels on the dyno.

If you can fing a servo that has 100% propane for it's pump LPG then you are 110 octane.

I just want to know what people think about such concept. We all know that when boosting your engine, in most cases compression reduced to reduce chance of a detonation/pinging. But surely if you really wanted you can build high compression engine & still run reasonable boost on it (lets call 7psi reasonable in this case) & run it wih 11:1 compression.

The obvious advantage would be awesome drivablity of such car (power in low end) ofcourse it wont be able to compete for high end power with low compression & high boost engines. Let say you have

Forged Pistons
Appropriate head gasket
Tune for 98 octane, perhaps thats to dangerous as you will rely heavily on quality of petrol (what if its dodgy)
What about LPG which has awesome anti pinging properties
Dont forget we have cast iron blocks & forged con rods in magnas which can take some punishment.

i would sad only about 3-5 psi make and even then it would unsafe still
if u got the much of a high compression a cheap way to help lower it is a think copper head gasket

it all comes down to money. Nitromethanol drage cars use 7L v8's that run between 6.9 and 9.1 CR depending on ambient air temp and pressure. On that 6.9 CR or anywhere inbetween (dependant on ambient air properties) will run up to 80psi through their superchargers. at 15000 rpm they make between 5 and 7 thousand hoursepower bepending on which CR they chose to run the strip with.

there is a CRX del-sol running around still that was a 1.6L boosted to 2bar with a CR of 11.3 It was one of australiaas first under 1.8L street cars to crack 300fwkw.

If this magical engine is a magna engine, then there wo9uld be a few things you would hav eto do to make it sweet. Some engines were jsut never designed to allow for such hardware changes. Some engines though have been built with enough margin for error in them that they support rediculous modification without self destructing. Ultimately i would rather drive a car with as little PSI and as much CR as it could handle to make the power i wanted. That way when im not pushing it 10/10ths id have a responisve, easy to peddle around, streeter.

You would get good results running E85, but charge cooling would be critical and you'd be better of dropping the CR. It would still require a lot of work to make safe.

rear mount turbo would be a better option, because it has more time to cool. This teamed with meth injection would provide low intake temps (~50-80c has been sort of average temps)

Unfortunately rear mounts are an abomination for a performance car.

Well ai i said before 98octane but with such high CR you rely too much on the actual quality of fuel that your local servo stocks, how many times i filled up at unknown servo & can feel straigh away something is not right.

So for this imaginary engine i was thinking of LPG which would never ping, BUT then we have an issue of heat as LPG (in vapour form) does not serve as additional coolant.

Again this just a discussion on how to have both FI & the efficiency.....

How about LPG (100% propane) and water/meth iniection?

while lpg doesnt act as a coolant on the physical parts of the engine, it does cool the intake charge very well, well only if you are using SLI/ LiLPG or to a lesser extent SVI.

regardless of where the turbo is, boost is boost, its a measurement of air pressure above ambient in your inlet tract. Rear mount turbos' are massivly inefficient. It all comes down to money to be spent, how far you want to go. Some engines just dont like being boosted stock. Some engines as i said dont seem to mind. It all comes down to how much tolerance was built into the design for operating conditoins that really has the biggest effect.

Unfortunately rear mounts are an abomination for a performance car.

I would be interested to know why you believe this?

hmmm, well my perspective would be based on drive ability, particularly in competition and you get increased lag on the exhaust side and on the intake side with a rear mount. In pure power terms you also lose a small amount of energy with the long ex run and with added drag on the in side of things. Howeverthe treand for such a placement seems to have come from SEMA and other american shows, where a large muscle car is fitted with twin turbo's and they don't want to stuff up the engine bay aesthetics. I know this view will not be a popular one, however in the area where performance and drivability is paramount, i.e. racing...try and find any car that has moved the turbo out of the engine bay. I recently had a go in a wealthy man's modified turbo porsche. It was fast i.e. wheelspin of in third fast...I would HATE to drive it with more lag than it had, especially on the road.

i agree in some respects,


however, consider the needs for a FWD magna in a road condition. It needs to have the power later in the power band to get going

also, a rear mount turbo is smaller to ensure that it spools quicker


but agreed, in a racing manner a normal turbo setup would probably be better

using a smaller turbo to spool quicker because you have placed it so far back in the exhaust does not make it more efficient. Power comes from efficiency and turbos require the hottest/fastest moving gas to get the job done as efficiently as possible. Yes there are times when it is the only choice to turbocharge because there is NO room i the engine bay but magnas have plenty

In regards to water methanol solution to reduce pinging threshold.......... Lets say you get 1.5 gallon tank = 5.5 litres. How long would that last, i know it depends on when u want it injected but still what would be reasonable estimate?

if you are going to use LPG then just do water injection, forget the pricey methanol.

LPG will act as the heatsink in that case, and water can just be the coolant part of the intake charge. Cheaper. As for how long 5.5 L would last i would assume you would only have it spraying above 3psi or so, so prolly a long time if your not thrashing it.

IF you are using liquid LPG injection there is a lot of cooling potential.

im lead to believe the cooling is only of the intake charge, there is no contact evapouration cooling taking place with LVI gas systems. It is similar to NOS in that it chemcially supercharges the intake air at the inlet

Well cooling of the air going into the cyl is all that matters regardless of where it occurs. It allows more power to be produced partly because the LPG takes less of the overall induction mass due to the manner in which it is introduced to the induction charge vs. a gas introduced earlier ala' the old impco lpg converter systems.

I just want to know what people think about such concept. We all know that when boosting your engine, in most cases compression reduced to reduce chance of a detonation/pinging. But surely if you really wanted you can build high compression engine & still run reasonable boost on it (lets call 7psi reasonable in this case) & run it wih 11:1 compression.

The obvious advantage would be awesome drivablity of such car (power in low end) ofcourse it wont be able to compete for high end power with low compression & high boost engines. Let say you have

Forged Pistons
Appropriate head gasket
Tune for 98 octane, perhaps thats to dangerous as you will rely heavily on quality of petrol (what if its dodgy)
What about LPG which has awesome anti pinging properties
Dont forget we have cast iron blocks & forged con rods in magnas which can take some punishment.
its 100% fine, i run 10 psi on standard compression in the 3.5 which is high compression... loud1 has never had a problem, i know of a ba ford.. running turbos on the grey engines and also running fine of 12psi,

What people never seem to understand about turbos is they enjoy thermal expansion not so much gas flow. Think ridiculously hi pressure post combustion out past small and hot valves (the hotter they air the higher the sonic velocity therefore the faster the gas may exit) to a low pressure exhaust -> massive fluid acceleration.


This is why exhaust manifolds are getting shorter and we're now looking at integrated manifolds in cylinder heads with turbos hanging straight off.

I just want to know what people think about such concept. We all know that when boosting your engine, in most cases compression reduced to reduce chance of a detonation/pinging. But surely if you really wanted you can build high compression engine & still run reasonable boost on it (lets call 7psi reasonable in this case) & run it wih 11:1 compression.



Generally the problem with such a setup is you have to retard ignition timing so much to keep it from detonating that you end up negating any advantages from running a higher static CR/boost, or even going a step backwards in terms of power and efficiency.

Some newer motors are able to run boost with a relatively high CR, but they feature tech such as Direct Injection and advancements in head/combustion chamber design not present in a magna motor.

IMO you are not going to be able to run anything near 11:1 with 7psi in a magna motor on ANY kind of pump fuel. Perhaps some kind of exotic setup like running on straight methanol, yes.

I do think 10:1 CR + moderate boost would be doable/work well on a magna on pump fuel with E85 and water/meth injection..and this is what I would be interested in doing if I decide to chase more power/do a rebuild at some point.


Quite easily but you would require alot of one off components to withstand the pressure. Remember F1 motors making 1000hp or something from a couple of litres.



Try ~1000hp from 1.5L, running 50+psi of boost! But those motors ran 'jungle juice' fuel nothing like what's available at the pump, had very short life spans and were designed a tad differnetly to a magna motor:p

This is why exhaust manifolds are getting shorter and we're now looking at integrated manifolds in cylinder heads with turbos hanging straight off.

Isnt that what they did with the 3.0 SIDI AJ? I have seen pics of that engine and it has only 1 single outlet in the head for exhaust gas. That should make it a prime candidate for turbocharging...

Isnt that what they did with the 3.0 SIDI AJ? I have seen pics of that engine and it has only 1 single outlet in the head for exhaust gas. That should make it a prime candidate for turbocharging...

New Honda Civic (07+) shares the same principle, single pipe hanging off the engine head... i lol'd

If your going to the trouble of building a forged engine and turbo kit why restrict the whole setup with high compression,drop the compression a little along with a smallish turbo exhaust housing then run high boost and enjoy the best of both worlds.

If your going to the trouble of building a forged engine and turbo kit why restrict the whole setup with high compression,drop the compression a little along with a smallish turbo exhaust housing then run high boost and enjoy the best of both worlds.

Because who wants an undrivable mess with the turbo not coming on song until 4000rpm??

Because who wants an undrivable mess with the turbo not coming on song until 4000rpm??

Who you say? well EGO for one :)

I think those that know would agree a 3.5l with a GT35R .86 housing would be producing boost way before 4000rpm.

Who you say? well EGO for one :)

I think those that know would agree a 3.5l with a GT35R .86 housing would be producing boost way before 4000rpm.

werent we talking about compression though, not the size of a turbo?

werent we talking about compression though, not the size of a turbo?

Yes we were but i was pointing out that it makes more sense to run a well matched turbo for response while leaving headroom for further power down the track.

Yes we were but i was pointing out that it makes more sense to run a well matched turbo for response while leaving headroom for further power down the track.

i have to agree here, EVERYONE wants more power at some point. So really, why hamstring yourself to start with BUT if you are that worried about response/driveability, when you set your goals for your engine build i personally think you should be aiming for the higest compression ratio your engine can handle with as little amount of boost you need to get the power you want. Not the other way around of building a boost complient engine and then just winding in the boost until you get to your goal. Turbo choice comes into it but only as far as finding one that makes the flow you need without reaching high into its efficiency range. To keep a turbo in its efficency range, keeps air temps down, which wont hurt so much with more CR.

For thsoe not up to spec, an easy way to look at it is. CR is the amount of compression of 1 atmostphere inside the cylinder/combustion chamber on the compression stroke. So a 9:1 CR means there is 9 atmospheres compressed into the space of 1 at TDC of the compression stroke. Factor in that at 14.7psi (there abouts) you have added 1 atmostphere of boosted air before the compression stroke. So when talking pressures you are doubling the CR of the engine for every 14.7psi you add in boost NOT increasing the CR from 9 atmsotpheres to 10 atmospheres : 1 as a lot of people think. Without taking heat into the equation there is the same cylinder pressures at 8:1 CR with 1 bar (14.7psi) of boost as there is with a CR of 10:1 @ .6bar (8psi).

Now that makes sense. Good point -lynel- so at CR 11:1 that means at 4PSI the pressure in the cylinder would be the same as 9:1 at 14PSI? Im not saying that performance would be the same since you actually forcing about 30% of air vs 14PSI hence you wont be able to inject as much fuel as with 14 PSI. But i would love to see the power curve of a car running 4PSI with 11:1 CR.......

I should rephrase the start of the explanation. You arent compressing 9 atmospheres into 1 @ 9:1 compression, its more that you are compressing 1 atmosphere 9 times.

the injection of fuel is not relevant to the equation as such, although it changes and things become more complicated when you start discussing static CR's and what not, since fuel is essentially a liquid (although atomised during injection) it does increase compression dependant on how much in injected, as its harder to compress a liquid then it is a gas (hence why engines can throw a rod from hydro-lock).

To get the same cylinder pressure from 9:1 @ 1 bar without boosting the car you would need a compression ratio of 18:1. As you said with 11:1 compression you could use as little as 4psi (bad idea due to turbo effiency and exhuast housing design ruining the 2 positives of a high CR turbo engine). Now add to this using a random AFR of 13:1 a 3L v6 will have cylinders of ~500cc so at 1bar each cylinder will be effectively 1000cc. At 13 parts air/fuel, that means 130cc of fuel will need to be injected. The same engine at 11:1@4psi will be effective 670cc/ cylinder and require only 90cc of fuel even though both engines have the same/similar cylinder pressures, the 9:1@1bar will release more energy per combustion stroke due to the fact more air and fuel are present. Its a bit to get your head around, hence why engines builders are cheap. Now comparing difference engines with different CR's turbo VS non turbo is a black art, as results are hard to compare but ultimately it comes down to how much cylinder pressure can be supported because combustion chamber CC is the difference between high and low compression ratio engines with the same displacement.

Whether it be dished pistions/ thicker or thinner headgasket/ cc'd combustion chambers, shaved head or block. all you are doing is changing the amount of space the CR of the engine is compressing into. Lowering this CC will raise the static CR of your engine and reduce volume, while increasing the CC of the combustion chamber increases the volume of air present in the cylinder but reduces the static CR.

ALl values are given as absolute values, pretty much impossible to reach in a car due to inefficient design/ flow dynamics/ ultimately thermal effficiency. Its an idea of the science behind the math.

Now that makes sense. Good point -lynel- so at CR 11:1 that means at 4PSI the pressure in the cylinder would be the same as 9:1 at 14PSI? Im not saying that performance would be the same since you actually forcing about 30% of air vs 14PSI hence you wont be able to inject as much fuel as with 14 PSI. But i would love to see the power curve of a car running 4PSI with 11:1 CR.......

It's not quite that simple - we are talking about static compression ratio figures as a rough baseline, but in the real world dynamic cylinder pressure is going to be influenced by many factors, the main one being valve timing/overlap at any particular RPM you are looking at (IMO if you are going to the trouble of changing CR and running boost, you would probably be looking at changing from the factory camshaft profile too).

But yes when done right relatively high static CR/low boost setups can work quite well, some hondas I know of running around come to mind.