E30 Zone

What's the best way to increase the compression on a 2.5 engine? New pistons or skimming the head? Or both?

Start off with the best parts, bluprinting. So pre face lift H/C pistons.

Skimming the head is a posibility, but you have to be careful as there is not much room to do this before the piston will hit. A dummy build will be needed to check the tolerances

Custom pistons are an expensive way of doing it.

A longer stroke crank, you can leave the chambers and pistons as standard, with a greater volume of air in the cylinder (swept volume) the compression ratio will increase.

A larger capacity with a standard head would definitely increase the compression but I want to keep my engine a 2.5. Early pistons and a skim looks like the way to go. If I need non standard pistons, where should I go? I'd like to achieve 10:1, maybe a little more, but I don't want to have to use super unleaded all the time.

885 heads are (comparatively) delicate and expensive; blocks are cheap and common.
Skim the block, not the head.

So, standard late pistons and skim the block? Or early higher compression pistons and skim the block?

Why do you want to stay 2.5? If you are going to the trouble of changing the pistons then you may as well go 2.8.
The capacity increase will be more benefit than the raised compression.

10:1 should be fine on normal pump unleaded.

The squish clearance is ~1.7mm so it is safe to either skim the block of use a basket 0.7mm thinner to tighten that gap and gain some compression.

Don't just go half way, fit both high comp 9.5:1 pistons and tighten the piston to head clearance.

At the moment I'm trying to gather parts for my build. That includes sourcing another motor that's local, everything seems to be miles away from me and collection only. I want to stay at 2.5 because I want it to rev and if I wanted more capacity I'd prefer to buy something that was already bigger from the factory. I think I mostly want to stay at 2.5 because no one else bothers much with 2.5, the 1st thing people do is go for a 2.7/2.8 conversion. I'd like to see what a 2.5 can do.

A 2.8 will rev just as well as a 2.5, the limit is not the stroke its the rockers, both engines will still happily got to 7000rpm. I don't know where this "2.8 motors won't rev" comes from but its bollocks.

To get the real benefit from the smaller stroke you would need to do some real work reducing the reciprocating mass and deal with the valve train issues that prevent these things running at high speed.
To do that your looking at custom rods and pistons and a new updated valve train, £2k at best.

A 2.8 is still a square engine so its not like its a long stroke motor.

I can appreciate wanting to go for something different however the arguments for more capacity far outweigh the benefits of the shorter stroke, the only reason to stay 2.5 is if you were limited to class regs.

Not to mention the port size of the m20 head is a bit too large to be effective on the 2.5 so a lot of expensive and risky/short lived head work would be needed.

By all means stick with the 2.5 but you will have it against you vs a large capacity motor, any gains will be less and it will be harder to break 210hp.
Fit the biggest version of a motor practical and then tune that.

I definitely understand the advantages of the 2.7/2.8, the success of the conversion is well documented and I was tempted by it. But for now I just want to stay 2.5. No turbo either.

i think what people mean when they say will rev better they mean make the peak power at a higher rpm which is a given. it would also be silkier feel with the shorter stroke.

strictly speaking the rate at which the engine revs depends on the engine torque and equivalent mass (i.e. rotational inertia and mass)

HairyScreech wrote:Don't just go half way, fit both high comp 9.5:1 pistons and tighten the piston to head clearance.

Does this mean the squish clearance is still ~1.7mm even with the early pistons?

By my calculations, a 0.7mm skim + early pistons should give 10.2:1

Is it safe to take off more than 0.7mm? I'm on LPG only and need all the compression I can get!

Be careful skimming the head too much. You need minimum .030 clearance between head and pistons, and .060 thou clearance between valves and pistons. I fitted a head which was around 124.4mm thick, I think stock thickness is 125.5. So My head had 1.1mm skimmed off it after several rebuilds. to get the right clearance I needed for piston to valves I needed a 2.5mm thick head gasket, up from 1.7mm stock.

Use the early pistons for sure, if you can find some.

So you just need different con rods or both piston and con rod to do the higher compression?

basketweave wrote:Be careful skimming the head too much. You need minimum .030 clearance between head and pistons, and .060 thou clearance between valves and pistons.
Use the early pistons for sure, if you can find some.

Bang on, closest i would tell someone else to run is 1mm from the head.
Thus dummy build the bottom end then check the clearance between the head and the piston.
Shooting for 1mm will be nice and tight but still safe. 1.5mm is a bit slack but still better than 1.7mm stock or 2.3mm on an undecked block, (which is pretty much a non squish chamber).

On a related note, what happens when we add a lumpy cam to the equation?
Thanks to a series of helpful diagrams someone posted, we know that a standard M20B25 cam lifts 1mm @ TDC, and a Schrick 288 which also uses OE valvetrain parts, lifts 3 mm.
Assuming Schrick are leaving a minimal clearance @ TDC, what scope is there to use e.g. a catcams 298* which lifts 2.8mm @ TDC with, say, a 0.7mm block skim?

Interesting question. I'd like to know too. Surely the pistons will need a pocket machined. Would the effect on compression be noticeable? I bought a set of 2.8 m52 rods and pistons, (came as a set), and the pistons are flat topped. Could the pistons be used? They're the same diameter as m20 ones.

The early pistons already have deeper pockets than the later ones; I'd imagine the combination of greater valve lift and a closer piston wouldn't be affected.
Machining the existing pockets 0.5mm deeper will increase cc volume by less than 0.7cc; dropping compression by about 0.1 point.
I also bought a set of M52B28 rods and pistons, did you notice that ARP rod bolts are $20 cheaper for these than for M20 rods
The M52B28 pistons may suit a 731 head.......

Cheaper bolts? I'm not ready to fit the rods yet as this is a (very) long term project but that's good to know. Can the original bolts not be re-used?

handpaper wrote:The early pistons already have deeper pockets than the later ones; I'd imagine the combination of greater valve lift and a closer piston wouldn't be affected.
Machining the existing pockets 0.5mm deeper will increase cc volume by less than 0.7cc; dropping compression by about 0.1 point.
I also bought a set of M52B28 rods and pistons, did you notice that ARP rod bolts are $20 cheaper for these than for M20 rods
The M52B28 pistons may suit a 731 head.......

This is something i have been looking at, the b28 pistons stock would need ~2.5mm taking off the top to clear the head, thus the crown would be a bit too thin in the center.
CR is also an issue, i can't remember right now what it came to but i think it was not great (irrecoverably high).
I have been a little quiet on the pistons front for a while but i think i may have a solution using the m52b28 piston.

I did a clearance test today (yay for Play-Doh) with early pistons and my piston-to-head clearance is ~1mm. I don't think the head has been skimmed, the telltale dimples are still there and about 0.5mm deep. How deep are they on a new head?
So there's very little scope for skimming the head or the block, on the other hand I don't need a vernier pulley now!
Clearance between the piston and closed valves is ~5mm on the exhaust side and a little more on the intake, which bodes well for the fitting of a naughty cam later.

If you want to check the valve clearance by the same method stick an old cam belt on and time it up before turning it over a couple of times.
That way the valves should leave an impression of there closest point. (some time around tdc during overlap.)