Camshaft overlap for FI?

Post Tue Jun 13, 2006 12:27 pm

Anyone know how much overlap on the various M20 camshafts? as im starting to think some of my boost may be getting blown throu the exhaust

Also how much will the compression ratio effect boost levels? IE:will a high compression engine make more boost than a low compression?

Post Tue Jun 13, 2006 1:39 pm

As a complete guess, I'd say the M20 has very little overlap as it has a plenum intake system, but it'd be interesting to know for sure.

Not sure what you mean about the compression making more boost though dude?

Think boost and CR are pretty much independant.

Post Tue Jun 13, 2006 2:31 pm

although im not that technically minded dont lower compression engines generally run higher boost levels

Post Tue Jun 13, 2006 2:51 pm

Agreed....

Low comp = more boost and safely

High comp = less boost but much perkier engine off boost

Gotta say my 8.2-1 C/R hasn't effected the off boost grunt at all, which is nice!

Mark.

Post Tue Jun 13, 2006 2:58 pm

higher CR = higher combustion temp

higher boost = higher combustion temp.

the two are related in this way

so highish cr+low boost = power
lower comp with more boost = more power but more lag and more fuel required

its a balancing act, and as is always the way, compromises are made on both sides to err on the safe side for the materials in the enigine internals.

the quoted CR is always a static measurement, so the boost in effect raises the given CR , so 8.8:1 can be 12.5:1 with boost due to the cylinder filling.

Post Tue Jun 13, 2006 3:25 pm

Thanks for info guys

What i meant on the C/R thing is my supercharger will make a set amount of boost across the rev range. so if i use the same supercharger and pulley setup on a low c/r and high c/r engine will the higher c/r engine see more boost?

Im only asking as my old (phase 1) engine the pistons came out the top of the bores wich makes me think it was high comp whearas my new engine the pistons sit a few mm down the bore.

Post Tue Jun 13, 2006 4:58 pm

What you have to remember is boost (pressure) is only a measure of flow against a restriction.

If you have change the VE of your engine your boost will also change eg if you fit a better flowing head/exhaust manifold/inlet manifold your engine will be able to process a greater volume of air through it, the restriction that the air coming from the turbo will be less and the pressure (boost) will drop.

So the question is, will the VE of your engine change by dropping the CR slightly????

If it does i doubt it will be a big enough change for you to notice it in your boost pressures.

Doug.

Post Tue Jun 13, 2006 5:00 pm

Where's that link Doug? the combined CR cal thang we were gassin about last night...... may help Len out here dude.

Post Tue Jun 13, 2006 5:17 pm

Not sure i have a link for it, just saved on the laptop at home I'm afraid.

Quite interesting, boost pressures on supercharged engines, because they are not self requlating/closed loop eg no wastegate/actuator, so they will change. Where as a turbo engine the only way you would notice the improved VE is by the increased shaft speed at given boost pressure.

Have you got those tips re conditioned yet lentec?

Post Tue Jun 13, 2006 5:24 pm

Overlap/centreline for a turbo cam on a higher boost M20 is 110-114, NA cams you buy are about 106-110 i think, stock one will do upto 400 hp then it doesnt flow enuff and will start to cause detonation. I have a custom turbo cam with 114 overlap, i dont know what overlap is on stock cam

Post Tue Jun 13, 2006 6:18 pm

RowdyBurns wrote: Have you got those tips re conditioned yet lentec?

No not yet doug.

Havent got anyware to leave the car in pecies at the mo.

So my low c/r will reduce my boost

Think im gonna strip my old block down and see whats in there

Cheers

Post Wed Jun 14, 2006 7:51 am

lentec wrote:
So my low c/r will reduce my boost

No, what I'm saying is that the small CR change you have made will make negligable change on you boost pressure you are seeing.

However you will be able to RUN more boost if you so desired on the low CR engine without detonation beacuse of the lower peak combustion pressures achievable.

So stick with the low comp engine and get those rotors re tipped!

Post Wed Jun 14, 2006 10:30 am

RowdyBurns wrote: So stick with the low comp engine and get those rotors re tipped!

Ive found someone who can re coat them at a good price but cant find someone brave enough to dismantle it!
I think the rotors have to be pressed out of the bearings at the same time as they are a helicoil design.
Will be harder to put back together.

Anyone feel brave?

Post Wed Jun 14, 2006 10:52 am

Might be worth asking on the Merc forums as to where they get there's refurbed Lennie.

Or buy a BIGGER ONE!!

Post Wed Jun 14, 2006 9:51 pm

fozzymonster wrote:
Or buy a BIGGER ONE!!

It's finished but it was fairly cheap

http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... %3AIT&rd=1

Post Thu Jun 15, 2006 8:22 am

Dam that was cheep! shame i missed that.
Found a website last night that shows how to dismantle the rotor pack.
So as soon as time available i will have a go myself. all i need is a hydrolic press and a plate with 2 holes in?!?

Mine are nowhere as bad as theese tho
http://cgi.ebay.com/ebaymotors/EATON-M9 ... otohosting