Right,

Bought some forged pistons and rods (Burton rods + accralite pistons) for my zetec turbo build, but bein the dick that I am sometimes I've bught standard Zetec ones which raise compression to 11.4:1 !

Now I kno this is not suitable for turbo use, my question is..

Burtons can supply me with a 3.5mm ish head gasket, which with my dodgy maths I think will drop the c/r to approx 9.0:1. Is this still too high? Or will I be ok as the pistons an rods are good quality forged jobs? Also the gasket, is it ok usining a Cometic head gasket that think, or is it a bodge up job?

My heads battered with this now!!

 

If it works out to 9.0:1 thats fine mate and I would have no qualms about using a Cometic HG made for the job.

 

Cheers for your reply! Any idea what sorta boost I can run at that c/r? Some people sayin I'll ave to run low boost?

 

Fine for what?

What power are you aiming for?

Highest i'd go on a turbo car is 8.5:1 and even then, if going for say 400bhp on a 2.0 with 25-30psi IME is too high for comfort.

 

If I got a reliable 300 horses I would be happy! That's ideally what I'd be lookin for?

 

can't you speak to burtons and swap yours for turbo spec package as it's a genuine mistake as the gasket is going to be your weak link unless your after a lowish bhp figure

 

Wish I could! But I bought them second hand so that's not an option! Would the gasket be a weak link? Because there not cheap to start with? So if it's gonna be a weak link I'd rather give it a miss!??

 

IMO the thicker the gasket, the more likely it is to fail. That's not to say it will happen though.

how thick are the piston crown's? is it possible to have some material machined off to give them a dish?

Basically, the higher the comp the more limited you are to how much boost you can run before you have to retard the ignition sevearly, closer to detonation, more heat generated. Unless you have a high octane fuel (higher than V power!!).

Put it this way, a standard RS Turbo is 8.2:1.

 

A stock RST is not 8.2:1 it's 8.5:1 and there is no problem running at 9.0:1 with forged pistons/rods, good mapping and 99RON fuel, plenty of big power cars running that sort of CR, Stu Collins and myself for example, I ran 30psi circa 280bhp and Stu makes another 100bhp on top of what I did
IMO the chances of a Cometic multi layer steel gasket failing if the block/head have been decked/skimed and is torqued down correctly are very low, you would need some serious heat to do that.

For the OP's target power of 300bhp I think it will be 100% spot on, will be a nice responsive engine with the right turbo

Also just to add... My current ZT build is 8.9:1 running a bar, 95ron fuel and there is still plenty of boost and advance to dial in when my new manifold is finished

 

Was thinkin of usin standard focus rs manifold with the frs gt25, I'm usin frs inlet, but unsure on management yet! Jus spoke to someone this morn to see If anythin can be shaved off the pistons to allow a thinner gasket! But gotta send them to them to see. proper head wrecker really! Dont wanna spend decent money an get shitty results!!

What management are you on Karlos? Is your c/r 9.0.1?

Cheers for the comments peeps, any help or advice is always appreciated!

 

Just read 8.9.1!! My bad!

 

lol
Yes 8.9:1 thats using stock ST170 bottom end, stock black top head and the thicker FRS head gasket (2.5mm IIRC).
My limiting factor now is the rods and pistons, they are only good for about 350bhp... My CR is not an issue, the original Focus ST (not ST170) ran a CR of 9.0:1 and is turbo charged for example.
I'm on MS2 mate, self built and mapped.

 

as said above multi layer gasket will be fine , my car runs 7 layer gasket with 2.2 bar with no issues

john

 

You don't wanna go too low compression, its makes for a horrible off boost driving car.
iirc, fiesta rst's are 8.3:1.
Karlos what does your drive like off boost, bet its nice n responsive with minimal lag (depending on turbo)

 

Yup thats exactly what it's like Rog, it's actually almost hard to pull away in the wet without a little wheelspin and yeah very little lag with a 0.48 housing lol
After a quick google it seems the RST's were all 8.3:1 my mistake earlier when I said 8.5:1.

 

Ok, so 9.0.1 will be ok! Still gonna see it I can get a bit machined off the top an run a thinner gasket,an aim for 8.5,1 but if not at least I kno I can carry on with what I've got! How easy is MS to map yourself? Ive never done anythin like that before but I'm willin to ave a go an learn as I go, If its easy enough for a DIY'er!

 

It's not easy, there is an awful lot to learn mate... Thats not to put you off, just an honest answer.
But if you do want to go MS (and there are many reasons to do so) there are plenty of people on here that map it if you don't want to, sure there will be someone local to you.

 

depending on were he is were on the wirral which borders cheshire

 

I'm only in Runcorn! So the wirrals not too far! Could trailer it to you if I couldn't do anythin with it meself!

 

Id def go for the MS, its the best thing ive done to my car... I'd never mapped before and after having a go and several words of advice from Karlos & RobDOHC i've got my car driving really well and it goes well too on less than a bar of boost!

Karlos - You on a 48/55 t3 then? Does it run out of puff on the 2litre?

 

I'm on a .48/60 Rog, the .48 is what will become the restriction at high RPM, a .55 or .63 would see me bigger numbers but my IB5 is going to be struggling around 300bhp anyway so I'm not worried lol
I've not actually had a chance to see what I can squeeze out of it though cos of all this rain, no traction means no mapping... also rain hitting the det can tubing sounds a lot like det too! lol

 

would a t34 be a good option? I need to learn more about turbos and what all the numbers mean.

 

Yes a T34 .48 as used on a lot of Cossie's should see high 300's, RS Matt has just made 400bhp on his Cossie with one.... at 36psi peek!!

 

I ran a t34.48 on my zetec turbo befor i sawped for a gt30. But after mapping i canged for a .63 exhaust housing and was the best thing i ever done. The .48 runs out of puff really low down imo

 

sorry to hijack the thread but i'm building a zetec turbo and thought that a comp ratio of 9.0:1 might be a little too high and leave no margin for error if the fueling were to run lean.

i'm looking at rod/piston packages from the states listed on ebay and the lowest c/r they seem to offer is 9.0:1, tbh i'm very tempted by the price.

i'll be running a 2 litre (silvertop) zetec with a t34.48 on megasquirt or microsquirt and am looking for 300 bhp.

i'm really just looking for more re assurance that the comp ratio will be ok with my spec.

thanks for any help

 

This thread is absolutely hilarious. Never have I read such utter bad advice and total incorrect information!

The focus RSmk1 which is a 2.0 16v zetec is 8.0:1 C.R.

A ERST is 8.3:1, A FRST is 8.2:1.

As for going higher C.R..................NO!

 

You see no bennifts of high compression engines Karl??

 

I was'nt just chuckling to myself about compression ratios, but also the advice about using thicker head gaskets and other daft comments!

I find it highly amusing that people who offer advice do so based on only their own limited experience which is often just one engine or at very best a few!! Thats simply not enough experience to go advising whats best!!

Regarding compression ratio's it's about using a C.R that is the best compromise for each application. That means suitability against fuel type, turbo type and of course power or more specifically specific power output per litre.

It is simply not the case that the higher you can run the better. If you all genuinely think that is the case then before I comment again please answer the following questions:

1. Why do you think Ford chose 8.0:1 for the focus RS mk 1 for a power output of 215bhp?

2. Are you all aware that the Focus ST is 9.0:1 for 225bhp but the same engine in the Focus RS is 8.5:1 for 300bhp and thats a 2.5litre 5 cylinder engine! (i.e. the ST is under 100bhp per litre! hence gets away with 9.0:1)

3. Are you aware Mitsubishi evo engines range from around 9.0:1 down to 8.2:1C.R as the power outputs rise on their engines? (Their engines are right on the det limit as std)

4. Why did Ford choose 8.3:1 for the CVH turbo engine when the N/A engine was only 9.5:1. Why not leave it at 9.5:1 for 130bhp?

5. Do you actually realise a lower compression engine generates MORE torque low down (i.e. off boost) than a higher compression engine?

Proper answers only please I'm trying to make people learn for themselves rather then me tell you directly!

 

The thing is Karl, we can only offer advice from our own experiences and knowledge limited or otherwise... This is of course how a forum works, we learn from each other.
None of us have the knowledge that you do so you are always going to laugh at some of the things we say because you know better!

Regards your questions, your obviously suggesting that if you want over 100bhp per litre a lower CR than 9.0:1 is needed....
But the CVH turbo is only 81bhp per litre so why is the CR so low?
I did not know that a lower CR will generate more torque low down, why is this?

Of course not all tuners agree that a lower CR is needed, take Stu's CVH for example, Jamie has managed to produce over 200bhp per litre with a CR of around 9.0:1

 

Karlos,

What do you think the benefits of higher compression than the manufacturers OE compression ratio are on a turbo engine?

And genuinely I want to know why do you think the OE manufacturer didnt make it higher to start with?

 

I suppose a factor of this would have to be emissions related? Or does that not come into it?

 

I guess rally cars run higher CR to get round turbo been restricted. And other requirements

 

I have always thought that manufacturers ran lower than necessary compression ratios because they have to make the car be able to run safely on cheap low octane fuel. Or because the ecu doesn't have a knock sensor.

Is there still gains to be had from lowering the compression ratio on modern engines (evo, skyline ECT ) I would have thought that combustion chamber and cylinder head design advancements (I assume there has been advancements since the 80s ) would have mean that a higher Cr could be used safely?

Please correct me if im wrong as these are just my thoughts on the subject.

 

I didnt say there were any benefits Karl, I just pointed out that engines can be ran at very high BHP/Litre outputs with a higher than stock CR... Like Stu's CVH for example.

IMO Stock CR is probably spot on for the average person wanting more power, there seems to be enough headroom for good gains and without sacrificing power off boost by lowering CR.... but this is where I'm confused, you said earlier that a lower CR will produce more torque? Could you explain this please mate as I always thought the opposite.
A higher CR will produce more power (bhp) and that is calculated by bhp = torque * rpm/5252 so surely it must be producing more torque?!

"A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air/fuel mixture due to its higher thermal efficeincy. High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression–causing better mixing and evaporation of the fuel droplets.Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree."

But all this aside the OP's question was is a CR of 9.0:1 ok for his ZT build with a target BHP of 300... I still think this is ok... I could of course be wrong.
What are your thoughts?

 

My zetec turbo runs 7:4:1 comp ratio

 

Karlos i know what you mean, cars with a lower cr tend to feel flat off boost, whereas stock or raised cr feel alot more responsive...
Also what Jed says is a valid point, combustion chamber design must make a diff to the safety of an engine not just cr.
Modern advancements in ecu programming aswell help us to run higher cr's safely... Whereas manufacturers engineer in a huge safety buffer for lower octane fuels, high intake temps etc etc...

 

Turbo size has a huge influence on the C.R an engine can get away with, anyone care to elaborate on why? (And Im talking the same power on two different turbos here, one big, one small!)

 

This thread is becoming very interesting......

 

Come on Karl explain so can get my head around it all

 

I have'nt got the time to explain in detail, its actually a very complex issue. I'm trying to get a feel for the average understanding , people have of compression ratios but I think the problem is people dont fully understand it is a dynamic condition that varies vastly with other engine components. So for example I could build two zetec engines both 300bhp, one would det itself to death at 9.0:1 the other wouldnt. Anyone want to guess why?