M20B28 Cam Advice

[Kedge]

Guys,

I'm in the process of getting all the parts together for an M20B28 build. I want to add in a performance cam but can't make my mind up which on to go with.

List so far;
Schrick - 272
Schrick - 284/272
Schrick - 288 (think this might have clearance issues though)
Cat Cams - 285

Since I'll have an extra 300cc I don't mind sacrificing some of the torque in a chase for some good top end power. I know Goosie was running a Cat Cam 285 but that was with future turbo plans and I understand that would affect what cam you would chose compared to a strictly NA build?!

Also will fitting any performanace cam require uprated springs or only when you start going to really high durations as I've read the following after some googling and wondered whether anyone could confirm this is true?

As I understand it, our M20 heads are sensitive to valve spring stiffness --- too stiff and the head will develop cracks around the seats. So I am wary of upgrading to stiffer springs.

Being that Schrick and DBilas both recommend stiffer springs only once you go w/ a 288 or higher degree cam, I won't bother w/ stiffer springs for a dual pattern 282/272 cam --- just new standard i springs, to account for 125k miles of aging on the current springs.

[Mikey_Boy]

Definitely you need to go for the lightest spring you can for your cam - helps in many differing ways...!

Standard cam runs about 242 degree duration if I recall correctly - depends on application, but anything above 280 degrees is going to be a bit lumpy on idle and may have MOT issues with HC emissions. See exhibit A below:



Also, there are a few folks here who would suggest that the cam is the last thing on the list to change - you will need to up your compression ratio to at least 10:1 to get a 270 cam to work effectively - standard 8.8:1 isn't going to do any good at all - quite the opposite in fact.

If you are doing all the right things on the 2.8 (higher CR, better exhaust manifold, BBTB and maybe a bit of head work) then a 270 cam is going to work very well.

A bit off the wall, but a very good cam comes from Newman:

http://www.newman-cams.com/pdf/bmw.pdf

270 degree cam gives very good results from what I have heard, plus they grind cams for several F1 teams so their quality is bang on...

Any 270 degree cam should get away with standard valve springs with no issues...

Good luck!

[Kedge]

Thanks for that, I'll check the Youtube link out once I'm home.

I hadn't considered the MOT emission but to fail that it's got to be pretty extreme surely?

The engine will be pretty well specced, BBTB, BTB3 manifold, full Scorpian exhaust, refurbed injectors. CR increases and headwork are also on the list but until I drop the engine off with DanThe and talk through things with him as he's building it for me. Also thinking about MAF in the future as well.

[reggid]

A 285 catcam (formerly 273)in 2.7/2.8 has less torque than a B25 before it comes on at 3500rpm or so. Just look at all the dyno plots on these forums I have seen about 3 or 4 with this particular cam and I also run a 298/285 in my 3L and have seen actual plots of the profile. An adjustable cam gear advanced 4 to 6 degrees would help as it runs a wider lobe separation and advancing it maintains a better dynamic compression. If you can cope with a very cammy engine that is flat off cam then a 285 catcam would work well for you (i.e. track work). IMO a 284/272 is a good compromise even tho split duration is not ideal for these engines which have a ”anormal”a intake-exhaust flow ratio. IMO a 288 is a bit grump with factory manifold and Motronic and I would want 10.5-11:1 CR. a non split 284 would probably might be worth trying it has a wider LSA than most schricks but with a cam gear setting the intake up at 108 centreline would work

[Bullet_Ride]

I'm running a schrick 284 with stock valve springs. My block was shaved 0.5mm and I needed to cut deeper valve reliefs into the pistons because I only had ~0.5mm of valve clearance on the intake side. The car idles fine, I haven't had it through emissions testing yet but I have it tuned so the AFRs are good. I don't think it should have much of a problem passing.

On a hot summer day with the car running rich because of a bad O2 sensor this is what I put down...



I still need to go back so I can play with my cam gear.

[Kedge]

All this reading about cams is making my head hurt, all very new to me

I've not found a 284/284 cam yet but I'm edging towards the 284/272 Schrick, that seem's to be very popular from reading the US forums. Is there any risk of cam/piston collisions with this cam?

By "normal" intake-exhaust flow ratio I assume that means the intake gases flow at the same rate as the exhaust gasses. What negatives would the split ration cam cause in this instance?

[Kedge]

What bottom end was that with Bullet_Ride?

[Jimbob]

Hi mate, my old 2.7 had the Dbilas 284/272 with standard springs. Was perfect during my ownership and think it also went well when the car was converted to a track slag.

[Kedge]

Cheers dude, didn't realise you'd cam'd that, you ever do a power run? I'll lookup dbilas as well, hadn't thought about them.

[Bullet_Ride]

What bottom end was that with Bullet_Ride?

sorry, I should have been more specific. My cam is a 284/272. Schrick doesn't sell a 284/284. The bottom end is an M52B28 crank, 130mm rods from the M20B27 and late model M20B25 pistons. With the block shaved 0.5mm I had this much valve clearance on the intake side...



Then I machined the intake reliefs deeper...

[Kedge]

Thanks for that info, need to findout how easy/expensive it is to do that work to the pistons.

Going on the Schrick spec sheets they one give 0.01mm clearnace on the 284/272 cam.

[Bullet_Ride]

Thanks for that info, need to findout how easy/expensive it is to do that work to the pistons.

Going on the Schrick spec sheets they one give 0.01mm clearnace on the 284/272 cam.

It's easy if you have the right tooling. I machined my pistons

Going on the Schrick spec sheets they one give 0.01mm clearnace on the 284/272 cam.

Where do you see 0.01mm on their spec sheet? They specify 0.25mm for the valve clearance, however that's for doing a valve adjustment. That is not the valve to piston clearance. I'd say 2mm minimum is what you want for VTP clearance.

[Jimbob]

Yeh it was 200bhp/198ftlb, I think the Dbilas cam was a bit cheaper than the Schrick.

[Kedge]

Going on the Schrick spec sheets they one give 0.01mm clearnace on the 284/272 cam.

Where do you see 0.01mm on their spec sheet? They specify 0.25mm for the valve clearance, however that's for doing a valve adjustment. That is not the valve to piston clearance. I'd say 2mm minimum is what you want for VTP clearance.

I'm going off the following from their catelogue, I've probably read it wrong.

M20 Valve lift at TDC = 2.3mm
272 Cam Valve lift at TDC = 1.6mm
284 Cam Valve lift at TDC = 2.2mm/1.7mm (Inlet/Exhaust)

Valve clearance on all their cams is meant to be 0.25mm

Nice bit of kit you've got there, wish I'd got into proper engineering from school.

Yeh it was 200bhp/198ftlb, I think the Dbilas cam was a bit cheaper than the Schrick.

That's good performance, most M20B28 dyno charts have been about that.

[steve_k]

I've got a cat cams 276* in my 2.6 with a Griffin head & at first it seemed a bit flat low down but after fitting the magnex 6 branch & a X-pipe at the end of the down pipe's (next to the gear box) & it seems to have given it a bit back down low, yet to have it dyno'd yet.

@jimbob, got a build thread?

[Jimbob]

No mate, nothing complicated about it though, 525e crank and conrods, early 325i block and pistons, took the block to a local machine shop and had them lower the block, standard 325i head. Ant's chip and BTB 6 branch

[Mikey_Boy]

Thanks for that, I'll check the Youtube link out once I'm home.

I hadn't considered the MOT emission but to fail that it's got to be pretty extreme surely?

The engine will be pretty well specced, BBTB, BTB3 manifold, full Scorpian exhaust, refurbed injectors. CR increases and headwork are also on the list but until I drop the engine off with DanThe and talk through things with him as he's building it for me. Also thinking about MAF in the future as well.

^^^
If DanThe is helping you out, honestly, you have no worries there...

Unburnt HC's is all about the valve overlap rather than the absolute cam lift - also, the M20 piston is robust but quite an old design so crevice HC's (between the crown and top ring) are also quite high - you are right, things do need to be quite extreme to fail an MOT, however, the M20 is quite an old 2V per cylinder engine - great in many ways (it's a tough unit for a start), but you can't change some fundamentals!

Your spec of engine sounds great - especially the BTB3 manifold - I had one on mine - super bit of kit...!

[Mikey_Boy]

Oh - by the way - do you have a MAF conversion as well? If not, then let me know - I have a BMC carbon airbox with a MAF attached for sale...

PM me if you are interested...

[Kedge]

Getting Dan to do it was an obvious choice, planning on building it myself from a spare engine once my car was on the road but a change of plan due to a few issues with my B25 means I want it doing much quicker than I'd manage.

What spec was your engine?

PM on its way to

[reggid]

dbilas do a 284/284 cam but don't know anyone who has used one and it is not the same as schrick 284/272 with a different exhaust lobe (the lobe centreline are wider).

dbilas also do a 282/270 cam very similar to schrick 284/272

Schrick might do a 284/284 if you asked them

[reggid]

What bottom end was that with Bullet_Ride?

sorry, I should have been more specific. My cam is a 284/272. Schrick doesn't sell a 284/284. The bottom end is an M52B28 crank, 130mm rods from the M20B27 and late model M20B25 pistons. With the block shaved 0.5mm I had this much valve clearance on the intake side...



Then I machined the intake reliefs deeper...

awesome pics mate, people should remember that using a B28 crank and eta rod with 0.5mm skim is the same stacked height as a stock engine so always check your P to V with a performance camshaft

[Kedge]

Think that's worth me adding to the wiki at somepoint!

[basketweave]

I am running a 276 dbilas cam in my M20B28, and metric mechanic beehive valve springs, which I tested and have the same tension as the stock dual valve springs, just in one light spring.

I chose this cam for its good dynamic compression qualities, and high valve lift (11.5mm).

IMO it suits a short rod motor, as it has agressive ramp angles which suits the short piston dwell time at TDC . The door opens and closes fast, (but opens wide) and the piston is moving through TDC alot faster than the normal 135mm rod motor.

[Gunni]

OP : I don´t think you have mentioned what the purpose of your car is yet.
This is probably the most crucial information that would help in determining what specs you should be targetting.

[Kedge]

Thought I had, sorry. Fast road and very occasional track use.

I've made the decision to go for a 272 Schrick. Got hold of one at a very good price and following conversations with Dan I've chosen to go down the route of getting the CR to a good level without spending buckets of money on custom pistons due to cams with the high lifts.

[Gunni]

Sounds like a good plan. I´d reccomend a aftermarket ecu to bump up the idle speed to get rid of hunting idle and the tunability for emissions tuning if you are having problems passing.

[HairyScreech]

What bottom end was that with Bullet_Ride?

sorry, I should have been more specific. My cam is a 284/272. Schrick doesn't sell a 284/284. The bottom end is an M52B28 crank, 130mm rods from the M20B27 and late model M20B25 pistons. With the block shaved 0.5mm I had this much valve clearance on the intake side...



Then I machined the intake reliefs deeper...

awesome pics mate, people should remember that using a B28 crank and eta rod with 0.5mm skim is the same stacked height as a stock engine so always check your P to V with a performance camshaft

Very wise words there, PtoV clearance is something that can easily be overlooked when swapping stock parts around.

Kedge; Only just seen your PM sorry, the 272 shrick would have been my thinking for what you have asked as well.
Sticking massive cam shafts in these engine is really a band aid for air flow issues in the head and comes with as many downsides as benefits.
If you want more power then get multi angle seats and bigger valves in and stick with the 272 for road.

[Kedge]

Kedge; Only just seen your PM sorry, the 272 shrick would have been my thinking for what you have asked as well.
Sticking massive cam shafts in these engine is really a band aid for air flow issues in the head and comes with as many downsides as benefits.
If you want more power then get multi angle seats and bigger valves in and stick with the 272 for road.

No worries dude, good to hear I'm on the right lines with the 272. As for the head, that's just going to get the ports tickled a bit for now for simplicity and keeping costs undercontrol.

Later down the line I might look at serious head work.

[HairyScreech]

There are 3 spots that can be tweeked by pretty much anyone on the 885 and not hurt things.
These are the top of the port either side of the valve guide, some removal here helps the nasty pinch where the port transitions from port to bowl.
The top of the bowl where the casting cores end and leave a step into the bowl.
The sides of the port where casting core shift has created steps.

work these areas and you cant do any harm (within reason) you wont increase the port CSA by much more than 1mm but will actually increase the average velocity in the port by stopping the turbulence that can be heard on the bench.

Making the port more efficient gets more air in in the same time, where as fitting a bigger cam is just the same poor flow for longer.

Cam wise when you look at the profiles and duration used in multi valve engines (more flow per cylinder) then they are pretty mild compared to the m20, a 272* cam for a lot of 16v engines is considered a rally cam and anything over 285 is nearly a race cam with all the bad manners that come with it.
The m20 has a hot cam as standard for a stock engine, its just coupled with a poor induction system.