Mi16 Maximum Compression

[Rik 0]

As mentioned in previous posts i'm after a similar low down torque of the 1.9 8v but in a 1.9 16v.

 

Engine; Running 45mm bodies, Will be completely standard internals apart from a flowed head, matched ports, 3 angle valve seats (as if they mean loads)! anyhow.

 

I've heard here Changinf Mi16 Characteristics

 

that by fitting throttle bodies i can achieve a similar low down torque to the 8v.

 

This is NOT what i want to discusss..

 

I need to know without having to use stupid calculations, i do not want to chang ethe cam, and i do not want to play with the deck height etc.

 

SO WHAT IS THE MAXIMUM COMPRESSION I CAN RUN ON THE STANDARD Mi16 Alloy ENGINE?

I also need to know what i need to do to obtain it? E.g. Skim head x.xxx thou or use a thinner gasket like spesso etc.

 

I understand its a stab in the dark here, but i'm sure a technically minded/experienced person will know this.

 

Thanks

 

Rik

 

P.S Am i asking too much?

[petert 554]

Assuming you have 10.4:1,

 

1mm gasket = 10.8:1

0.7mm gasket = 11.1:1

 

I don't take metal off the head.

 

You wouldn't go much past 11:1 without forged pistons. I really have no idea what the actual limit would be.

Edited May 15, 2006 by petert

[veloce200 3]

Assuming you have 10.4:1,

 

1mm gasket = 10.8:1

0.7mm gasket = 11.1:1

 

I don't take metal off the head.

 

You wouldn't go much past 11:1 without forged pistons. I really have no idea what the actual limit would be.

 

 

surely it's too much for std cams - the dynamic compression ratio will be too high also the pumping losses on the downward stroke with a std cam would restrict revs a the top end? I would have said a better option is 40mm bodies and leave the compression alone and spend time tuning the air intake. Then again Peter has a lot more experience than me...

[Rippthrough 98]

The mi16 does have the bottom end of the 8v, or very close

[Rik 0]

Thr bodies are actually GSXR 750 - so about 42mm i believe.

 

I'm happy to discuss your thoughts Veloce200, how do would you know if the compression Peter suggested would be too much for standard cam?

 

Rik

[M3Evo 0]

Just out of interest, those Spesso gaskets for the XU engine are thicker than standard by a fair old margin I seem to recall!

[Rik 0]

I wanst saying i especially wanted a Spesso gasket, i was just using them as example.

 

I've seen peter mention another gasket manufacturer in another thread but cant find it now???

[James_R 3]

cosmetic

[veloce200 3]

Thr bodies are actually GSXR 750 - so about 42mm i believe.

 

I'm happy to discuss your thoughts Veloce200, how do would you know if the compression Peter suggested would be too much for standard cam?

 

Rik

 

 

i don't have pratical experience of the MI but from a theoretical POV the key to engine tuning is not static compression - it is dynamic compression and expansion stroke. eg a mild cam with 13:1 would be awful. A race cam with 10:1 would be awful. there is a balance to be struck. I would focus on giving yourself the longest induction length before and after throttle body and also a decent 4:2:1 exhaust with long primaries. as someone has already said though there is nothing intrinsically wrong with 16v bottom end - it just feels sluggish compared to the top end. this is why the MI runs shorter gears (as do most 16v's) effectively a 16v engine with a the same cam spec as an 8v will be much more peaky. better flow, less cam you need. eg kent cams race superbike profiles are not much hairier than the std 205 8v cam. this is because a bike engine is the absolute nemisis is all other areas so it doesn't need much cam.

[petert 554]

I've seen peter mention another gasket manufacturer in another thread but cant find it now???

 

I didn't think I'd ever posted the manufacturer on this forum. I have to keep some things up my sleeve! Ask me off line.

Edited May 18, 2006 by petert

[petert 554]

I had posted it before, in this thread: http://forum.205gtidrivers.com/index.php?showtopic=49664

 

They're copper gaskets however, and you really need to know what you're doing. You can only leave them in situ. for 2-3 years. I always machine at least the head before using them. Ideally you should o-ring the head, but I've found the thinner section of a liner squeezes into the copper nicely, creating an excellent mechanical seal. You definitley need to make sure your liner protrusion is correct.

[Rik 0]

Peter! told you so lol

I'd like to know more on your views of what i should do with the engine and the compression, do you disagree with veloce?

 

Veloce - i understand a longer induction length gains more torque?

 

Whats the usual concensus on the Mi16 induction length?

 

Thanks for all so far guys

 

Rik

[veloce200 3]

Veloce - i understand a longer induction length gains more torque?

 

 

not necessarily more torque just lower in the rev range. more compression will give you more torque, but there is a limit to what you can run on pump fuel. are you going to run aftermarket management?

Edited May 18, 2006 by veloce200

[petert 554]

I think the effect of pumping losses from std. valve timing are very small compared to the physical limitations of cylinder pressure and knock on the cast piston. The exhaust ports are very efficient and I'd assume throttle position has more effect on the intake pumping losses than valve timing. The piston material and design is now quite dated compared to modern hyper-eutectic pistons. Taking the piston into the high 11's with commercial pump fuels is a recipe for disaster. As I said earleir, I have no idea where the actual limit is, as the cost (and result) in finding it is substantial. There are many modern engines of similar bore size and layout that run low to mid 11's on 98 octane fuel. However, they have knock sensors and better piston design.

 

Reducing squish height will have a greater effect than the increase it provides in compression ratio.

[petert 554]

You would also need to maintain a wide lobe separtion angle if using high 10 - 11 CR with the std. inlet cam. ie 114/114, ensuring the inlet closed as late as possible.

[Rik 0]

I'm not looking to run a high compression ratio, just one that will not blow my engine in 5 minutes but will give a power increase?

 

You've all gone a bit too technical for me here! wide lobe separtion angles being the main one! Is that changed by the cam timing?

 

Veloce - yes i'll be running aftermarket management, why is it you ask?

[Rik 0]

Anyone else got any comments on max compression?

 

or even induction length?

Rik

[James_R 3]

I'm just running in my new Mi, it's a 2045cc XU10 bottom end (UX9 Mi crank) and a skimmed head, so its in the 11+:1 ratio I'm running optimax or BP ulitmate and it's going fine, got anopther 800ish miles before I can abuse it enough to see how it is, I'm using 2,2 pulleys so the overlap is wider (I think) so shoudl be better set for the high CR.

[petert 554]

I'm using 2,2 pulleys so the overlap is wider (I think) so shoudl be better set for the high CR.

 

but which cam(s)?

 

You wouldn't run non-standard cams that wide. Two #2 pulleys gives 114/114 with the standard cams.

Edited May 23, 2006 by petert

[Rik 0]

Whats the best overlap to use then peter? i'm aiming for a high tens comp ratio.

 

Would one of your offset keys do the trick?

 

Rik

[petert 554]

If you're using std. cams just use two #2 pulleys. If you're using an inlet anything like my Stage I cam (226 deg. @ 0.050"), you'll need an offset key or a #3 pulley. That will then give 110/114.

[James_R 3]

I'm running std cams and 2,2 pulleys. I'm building up a UX9 Mi to replace this stand in. Was thinking of just using a thinner H/G on it to raise the CR enough, should a 0.7mm do the trick??

 

James

[Rik 0]

So onto my next question - how do i know if i have even 1 two pulley in the first place? Can verniers be used instead?

 

Rik

[B1ack_Mi16 67]

So onto my next question - how do i know if i have even 1 two pulley in the first place? Can verniers be used instead?

 

Rik

 

The number is stamed into it, so just take a look, it will be either (2), (3) or (4).

Verniers can ofcourse be used instead.

[Rik 0]

I'll try find another 2 then!

 

I presume most engine tuners/builders will know what i mean when i want to achieve 114/114 overlap then?

 

How is it calculated the overlap i mean?

And how is it adjusted when the engines built-up.

 

Rik