Increasing compression and cylinderhead gas flowing

[delcbr]

I have a BMW e36 3.0 m3,
Comp ratio is 10.8:1 across 6 cylinders I can get a 0.3mm thinner head gasket so equivalent of skimming the head 0.3mm is there any way of calculating the power increase ?

Also have a cylinder head off the car just now I am thinking of getting it gasflowed but don't know of anyone decent found one website cylinder head developments Andy Weston anyone heard if him or recommend someone but not cnc heads!
D

[GavinPearson]

You are best off buying a cylinder head from an expert like Peter Burgess or Pumaracing.

Compression ratio needs to be chosen in conjunction with all of the other mods you are doing, if you go too high you can cost power trying to retard the ignition to avoid detonation.

[PaulKemp]

Any cam changes will need to be calculated in more overlap equals less dynamic compression

Now this is a very ball park figure as I would need to know a few more measurements but 0.3 to 0.5 increase

I believe the BMW ports are pretty good as standard

[delcbr]

Any idea how much power that would be standard is 280bhp so increasing it say from 10.8 to 11.3 how much increase can I expect in power?
D

[PaulKemp]

Not a lot
Compression ratio on its own wont make a huge difference perhaps a bit more torque due to a more efficient burn, to get the most from engine mods get a plan, certainly if you have had to replace the gasket a thinner one will help.
Did you rolling road before so you can note the increase or is this just a pub boast? Most engines don't make what it says on the tin and blue printing usually gets what the engine was designed for.
Me I research a project to death before I turn a bolt
To many people talk about bhp
Not enough talk about torque and the torque curve

[PaulKemp]

Just a thought
How close do the valves get to the piston at TDC
You'll now be 0.3mm closer
What's your red line?
Old Skool Fords need about 0.9mm @ 8000rpm
I tend to run a mm as its a road car

[PeterBurgess]

Based on Gish et al 1958 research paper into effects of CR on bmep you may achieve around 290 instead of 280 all things being equal ie no detonation problems etc etc

Peter

[Kokkolanpoika]

PaulKemp said:

Just a thought
How close do the valves get to the piston at TDC
You'll now be 0.3mm closer
What's your red line?
Old Skool Fords need about 0.9mm @ 8000rpm
I tend to run a mm as its a road car

I have seen Old Skool Fords running 0.7-0.8mm quench at similar rpm or even more.

I increase my Rover V8 5.2litre street engine compression ratio 10.8->~11.7:1. We made 10hp more at rear wheels and 10Nm more. 340->350RWHP.. and 544Nm->552Nm (so engine power near 390-400hp)

[delcbr]

Yea it was 267bhp @the wheels and flywheel said 302bhp

[delcbr]

Just to add its a 3.0 engine but have the 3.2 head and throttle bodies ready to go on which is a common upgrade so it would be the 3.2 head that would be getting worked on the thinner head gasket is double the price of a standard one but I am keeping the standard cams for the time being.
D

Edited by delcbr on Tuesday 20th November 17:04

[PaulKemp]

You need to measure the combustion chamber volume, if it is different to the 3.0L then calculations will change.
To find comp ratio
Bore, piston bowl, flat or dome volume, chamber volume, gasket thickness, piston to deck height all need to be found.

[packman10_4]

It all has to work together , no point doing a quick head job on it BMW spent a vast amount to get that , improving that would take
many hour on a flow bench and a lot of experience.

After the head cams and the Vanos will need to be looked at .....

Best off Supercharging it you would gain 60-80 hp with a good set up without opening up the engine try getting that in N/A form.....

[Pumaracing]

PeterBurgess said:

Based on Gish et al 1958 research paper into effects of CR on bmep you may achieve around 290 instead of 280 all things being equal ie no detonation problems etc etc

Peter

You're a country mile out there chap I'm afraid. The theoretical thermal efficiency power increase going up from 11:1 to 12:1 CR is just over 2%. So for half a point increase in CR (if that were actually the case which it ain't) it would be just over 1% = 3 bhp

So three fifths of FA.

To answer the OP properly.

It only requires the bore and stroke to be known to calculate the effect on CR of a reduction in gasket thickness or block height. On your engine bore is 86mm and stroke 85.8mm if I'm correct. Assuming a gasket bore of 87mm a 0.3mm decrease in thickness would only take CR from 10.80 to 11.16. So actually only just over 1/3 of a point increase.

Power increase for a 1/3 of a point increase in CR starting from 11:1 would be barely 0.7% or 2 bhp. Not even worth fretting about.

Correct answer goes to Kokkolanpoika with a measured circa 2.5% increase in power from a 1 point increase in CR which is about spot on the money from the thermal efficiency calculations. Well played that Finn

Second place for Paul Kemp for the also correct answer "not a lot".

Edited by Pumaracing on Saturday 24th November 04:44

[PeterBurgess]

Dave

The threader asked a question I replied with some research info. What is the problem with that? You seem to take great delight in trying to knock people down, why not use that amazing brain of yours to build people up and help folk rather than play the 'Big I am'? As my wife's Granny said, if you cannot say something nice about someone then dont say anything at all! Life is hard enough without knockers. At my unit we look after kids who have been excluded from school and we also look after unemployed folk to help them gain some self confidence, it is good to see people stand taller when they are given encouragement, kindness and care. I will never be a wealthy person materially but it feels good to put something back into society and you cannot put a price on that!

You seem to be having a burst of posting yet you have not reposted to a thread I posted about wheel horsepoawer or replied to any of the questions I posed to you on the effects of flywheel lightening. You are very challenging but seem to shy away when questioned back.

Peter

[Pumaracing]

For some reason I'm recalling a wild life documentary I once saw about lions in the Serengeti relaxing after lunch. This little cub was desperate to get mummy's attention and make her join in the games he wanted to play. He kept jumping on her and biting her face but all she wanted to do was lie in the sun digesting wildebeest. Then eventually this big paw lashed out and swatted him gently and he got the message. I have no idea why that popped into my head - it'll no doubt come to me.

[delcbr]

Thanks for the input. So one persons saying next to nothing and the others saying around 10bhp.
I think 0.3mm is quite alot and accross 6 cylinders would make a difference but what do I know.
In the old days when you skimmed a head for more power how much would you skim off the head to make it more powerful?

[PeterBurgess]

Typical amounts on a few older heads and blocks we regularly work on;
MGB 1 to 1.25 mm to take from high 8s to high 9s
1275 Midget 1 mm to take from high 8s to mid 9s
T-Type cant slim enough off but have to protect white metal bearings from too much CR
Healey 3000 and MGC 3000 approx 0.5 mm
Rover 2200 flat head (no chambers) 10mm off block to get ridiculous CR, caused rumble ignition but flew!
VWs about 1-2mm off heads as v low CR standard.
small bore A series 1-1.5 mm restricted by oil feed hole breaking into chamber unless do loads of special work.
1500 spitfire/MG midget 1 mm
TR6 1 mm
Export TR 6 cylinder 4 mm as so low cr to start with.
TR4 3 mm at times.

Does this help?

Peter

[carlt5]

Pumaracing said:

For some reason I'm recalling a wild life documentary I once saw about lions in the Serengeti relaxing after lunch. This little cub was desperate to get mummy's attention and make her join in the games he wanted to play. He kept jumping on her and biting her face but all she wanted to do was lie in the sun digesting wildebeest. Then eventually this big paw lashed out and swatted him gently and he got the message. I have no idea why that popped into my head - it'll no doubt come to me.

You should obviously stop jumping on people and biting their faces then ?

[davepoth]

delcbr said:

Thanks for the input. So one persons saying next to nothing and the others saying around 10bhp.
I think 0.3mm is quite alot and accross 6 cylinders would make a difference but what do I know.
In the old days when you skimmed a head for more power how much would you skim off the head to make it more powerful?

Skimming the head alone, as said, makes little difference. A car engine generally has a four stroke cycle, which can be glibly referred to as

"Suck, Squeeze, Bang, Blow".

BMW will have been at great pains to get the sucking, squeezing, banging and blowing in fine balance, and by increasing squeeze without the other three you won't get the full benefit. Think about whether you'll have enough suck, bang and blow to match.

[eliot]

I'm in the "not a lot" dept on this one.