Any rotrex mx5s lurking in here?

Postby **NitroDann** » Sun Jun 05, 2016 1:46 pm

You need to speak to a cam guy

No one is running 310* cams, that's advertised duration which is baloney.

I reckon you'd want 265@0.050 or close to.

Tell a serious cam guy you have the money and want to pay him for his expertise, you'll get a good result.

3gress · Postby **3gress** » Sun Jun 05, 2016 2:01 pm

If I had a well built normally aspirated engine (and I do!) and wanted to supercharge it, I would be decreasing compression ratio and/or increasing cam duration.[/quote]

So in effect i could still use my 0.075"/1.905mm shaved /worked head and replace the 256/9mm lift cams with something of a larger duration to bleed off compression.
Sure is cheaper that way.

Postby **NitroDann** » Sun Jun 05, 2016 2:27 pm

you aren't bleeding off compression.

You guys should buy some books, this stuffs hard to explain online.

StillIC · Postby **StillIC** » Sun Jun 05, 2016 3:13 pm

Compression ratio isn't really a compression ratio. The real compression ratio (called effective compression ratio) is a function of combustion chamber volume, and where the piston was when the inlet valve closed. This is always less than the quoted (or geometric) compression ratio. Therefore, detonation or knock can be controlled by closing the inlet cam later, thus reducing the effective compression ratio. This also has other effects on the engine, but in no way bleeds boost.

madjak · Postby **madjak** » Sun Jun 05, 2016 3:16 pm

NitroDann wrote:You need to speak to a cam guy

No one is running 310* cams, that's advertised duration which is baloney.

I reckon you'd want 265@0.050 or close to.

Tell a serious cam guy you have the money and want to pay him for his expertise, you'll get a good result.

My thoughts are to just switch the intake cam to 264 - 274 degrees. I'll have to do some calcs and see how much overlap there is with my existing big exhaust cam.

rowdytoot · Postby **rowdytoot** » Sun Jun 05, 2016 4:10 pm

ja9 · Postby **ja9** » Sun Jun 05, 2016 6:25 pm

Latest Kitcar magazine (April) shows how to fit a BMW Cooper S supercharger onto an Mx5 engine.

3gress · Postby **3gress** » Sun Jun 05, 2016 8:50 pm

NitroDann wrote:you aren't bleeding off compression.

You guys should buy some books, this stuffs hard to explain online.

Didn't mean literally bleeding off compression. My original unedited post rambled on about static vs. dynamic compression, cc volume, valve clearance issues and such.
Wondering whether to start again on a new head with larger cc volume/lower compression or use increased duration to counter detonation issues.
Cams would be cheaper than a full built worked head.
Was hoping someone found it easy to explain, would be happy with a book recommendation.

Postby **NitroDann** » Sun Jun 05, 2016 9:14 pm

3gress wrote:
NitroDann wrote: use increased duration to counter detonation issues.

Would only be true at low rpm. The opposite would be true at high rpm.

StillIC · Postby **StillIC** » Wed Jun 08, 2016 12:15 am

madjak wrote: My head is decked about as far as you can go (2mm) and I'm running wiseco 10.5:1 pistons. To get more than I'm running you'd either need some custom pistons with shaped tops or weld up the bowl in the head. Maybe with less cam you could deck the head a bit more.

Custom pistons? Not needed. Am I the only one who knows of Wossner pistons, as no-one I know of, or read about on this forum, uses them? They produce 12:1 pistons for B6 and BP engines. That is, on a standard head and block they give 12:1; more with a decked block or shaved head.
e.g.
http://www.wossnerpistons.com/products/product-details/id/WOS-9078D100/name/wossner-piston-mazda-miata-1-8ltr-16v-84-mm-bore
I have them in my B6. Plenty of room in the combustion chamber (so the valves aren't shrouded) and good CR from the piston.