Bore and stroke size, rod angle, piston speed etc. If you change any of these, they have a knock on effect (no pun intended ) of their ability to be resistant to det or not. You cannot compare different engines to their ability to run high compression - it absolutely has to be like for like - just because it is an in-line four of the same capacity, doesn't mean that it will respond in the same way to a compression hike. A good example of this is the Evo engine, which due to it's configuration, can take a much higher compression as standard (8.5:1 compared to the Cossies 8.0:1), as well as modified.

 

So what is the lowest comp that has been openly tried on a cosworth, i know 7.2:1 is what's normaly considered as low. Lowest i have been in was a 6.8:1 and didnt feel any different to my old 7.2:1 engine.

 

Exactly, it is a common misnoma that compression has a huge effect on driveability . However, the compression only needs to be as low as it has to be to allow the boost and ignition figures you would like to see. And I believe the engine you are talking about could take some more compression with no changes in the ignition / boost, which may give it a smidge more power .

 

Not the engine you think im on about Mike but was a ava build, dont know what comp Euan is running these day but sure it will be low and his car is responsive as any like for like and takes big boost with ease. Will be good to compare others i have been in to my new LOW comp engine soon

 

Have done and will be doing more testing on comp ratios on my car with true back to back testing after each stage so could be interesting

 

Any results so far ? You going up or down in cop ratio ?

 

only time will tell

 

a heard a rumour it was going up

 

wouldnt listen to rumours

 

come from a reliable source too .............

 

Told you Euan 9.5:1 is too high

 

no its not

 

Well i hope it is or im waaaaay off with my comp

So what other things will comp effect ? Will it have effect on boost threshold ? What about surge or is that more to do with the head ?

 

escrstJust noticed you are building zetc hc engine, who's the builder? I ask as i'm in the same process. Sounds more sense to me if you are not after high hp to run hc engine

cheers Richard

 

Good thread

 

so is euans new engine higher or lower

 

its higher than it used to be, but still low

 

Yeah, it used to start with a 5, now it starts with a 6 .

 

SHHHH Mike dont give all the secrets away

 

its a bit higher than it used to be, the difference in off boost driving is so noticable.......





























































NOT!

 

707bhp on Optimax from mine . Can I ask what MikeR considers HC in a YB engine producing near 800bhp, over 7.5:1 or maybe even 8:1 . Genuine question after Knowing what Mark now builds.

 

Rod hasnt Mark already stated that he can build them high comp but has to be a ground up build, so he can change rod lenght ect to get it to work.

 

Rod lenght realy has nothing to do with it, Its knowing how to make it work and the realy big engines can support a much higher comp than you think if you are stuck in the past

Mark

 

How about someone doing a technical write up about high compression engine building?

 

Rod,
I know you can do it - if you're prepared to accept an engine that doesn't make full boost until 5000rpm. However, driveability is what I want, rather than outright numbers, so for me achieving a a high power figure with a huge turbo is complete and utter waste of time for a track car .

My engine makes 0.7 bar at 3000rpm and 2.2 bar by 3500rpm, which it holds all the the way to the limiter and due to this, I personally wouldn't want to run any higher than the 7.3:1 that I am running, as I want good ignition values to keep engine heat at bay.

A top speed run lasts 35s? A lap of the Ring lasts 8mins.... .

Obviously you build an engine to its desired remit. To say it is stuck in the past is completely missing the point .

 

Mike.

you could run more boost than that surely for more mid-range torque? Or is there a reason? And no im not saying 2.2 bar is low, but i am a boost junkie

 

450lb ft used regularly and aggressively is already highlighting any weaknesses with the transmission, so why would I want to put this under even more strain, along with overspeeding the turbo and increasing the ACTS?

 

Static compression ratio is calculated from swept volume and combustion chamber volume etc, etc... We come up with a value of 7.5:1 or 9.5:1, or whatever we want.
Camshaft timing - specifically when the intake valve closes is much more important.
Dynamic compression ratio is the important factor. Japanese engines ( or most modern designs) have variable intake camshaft timing - where the intake camshaft is advanced or retared wrt the crankshaft. This changes the dynamic compression ratio. Much higher static CR's are used when variable or late intake closing camshaft timing is used.
On a Cossie the camshaft position is not variable, and is not a late closing profile, therefore CR's of 7.5:1 and lower are used for high boost applications.
Honda engines with variable camshaft timing (phasing) and an efficient combustion chamber
can use a much higher compression ratio before detonation takes place.
One cannot make a blanket statement regarding compression ratio in a turbocharged engine. There are many more variables than just static compression ratio alone.

Just a thought from a Canadian ERST fan.

Cheers

 

I have 425lb/ft of torque at 3000rpm and my engine is not a low CR one.

And without all Mike's Mountune inlet and other stuff. Thanks Harvey Gibbs !

 

Whats the difference in pistons for high and low comp, can someone post a picture please?

 

high comp one is more flat and low comp looks like a breakfast bowl charlie!

 

bmw f1 car! 1000 bhp from a 1.5! high comp turbo! if you got the internals and the octain then its better. i kno its an F1 car but thats where it all starts

 

what

they was NOT high comp, 7s and 6s.

and they ran on mega race fuel, on pump fuel high comp is a whole different ballgame...

 

Normal compression flat top pistons.


Higher compression domed/raised pistons.


Lower compression forged pistons.

 

Well Mike mine made 3.2bar of boost at 4400rpm on 8.4:1 on a GT42.

 

This post can end up endless. i carnt belive how fast they can make diesels now.

 

Lol. Diesel is for tractors, nothing else. Driving pleasure starts where diesel end..... at 4000rpm. Just try to find a 2.0L diesel engine 4cyl with more than 500bhp...

 

yes they did, dick head!

 

any need for that ?? how old are you anyway?

 

The F1 Turbos used 8.0-8.5:1 in CR and used ~85% toluene and 15% heptane fuel = poison!


I run 10.88:1 in CR on my WRC escort rallycar engine. Anyone want to argue if it works proper?