bad_monkey

I've been thinking about a supercharger on my 2.7 NA for a while now, but as I don't want to switch out the pistons etc, I was considering running lower boost and water/methanol.

But I've had a slightly different idea now. I'd like to add a deck spacer between the head and the block to bump the 10.9:1 compression down to 8:1.

I know - there's a lot of people breaking out the right now...

But, according to my calculations the spacer would only need to be 2.62mm ~= 1/10".

So, I'd get a plate milled up that matches the gasket and use a regular head gasket on either side of the sandwich - but here's the question....

Would the standard head studs cope with 2.5mm less thread (is there any "extra" normally?) - and if not, how hard is it to get the studs out of the block to replace them with some longer ones?

I figure the exhaust manifold, belts etc would handle 2.5mm movement without too much hassle.

The rest of the solution would be injectors, fuel pump, rising rate regulator, and eventually a MicroSquirt ecu to be a bit more accurate with the mapping.

Any sensible comments are welcome - and if you want to bring the math to argue with my calculations, please do - but I reckon I've got it right.

xsboost90

why bump the compression down? I know a guy w/ a 968 running full compression and a SC no problem- just need to tune it properly. Of couse low boost is a must w/o some more mods.

Ecos

I've been researching super/turbo charging my 944s and had the same idea. After reading a few books on the subject from what I gather it is a bad idea. In the book Maximum Boost by Corky Bell he talks about a "squish zone" above the piston that helps prevent detonation and by using a spacer it can actually cause more problems with detonation.

I'm far from an expert on these things, thats why I'm reading info from experts so I can't go into the technical details but supposedly its a bad thing.

bad_monkey

Ecos: I also read about about the effect of spacers on squish too - but the bore of the 944 cylinder is so wide anyhow, and the spacer is only 2.5mm. Also the higher RPM, the greater turbulence (the positive effect of squish) and with the supercharger, boost will increase with RPM - squish should become less important as the boost increases. Careful selection of pulley wheels should help.

xsboost: In this case, I'm looking for more power - the 2.7 head doesn't breathe as well as the 968 head - so I think a higher boost/lower compression solution is going to provide more peak power as the cylinder filling will be improved as well as the increase amount of air/fuel inout... but, theory aside, I also want to try it to find out :-)

Rogue_Ant

monkey,
You should be able to get a custom thickness Cometic HG... just specify that you are looking for x thickness.


-Rogue

bad_monkey

Thanks Rogue! I'll follow that up...

Now, the question still remains as to whether the existing head studs can handle the extra spacing...

krystar

but again...the question is...why decrease static compression at all? why not just run your intended boost level with same compression you have already? as long as you keep the detonation under control with either an air-air intercooler or a chemical intercooler like water/meth as you were thinking about it already. find out what the maximum rpm that your blower is still in its thermal efficiency range and don't exceed that. once u reach that rpm, it's going to be a power ceiling anyway unless you start increasing static compression.

spoolin51

+1

bad_monkey

krystar: the 2.7 is euro spec, with 10.9:1 compression - the higher compression means that I'll be able to run around 5psi max on (even 98) pump gas, given a good setup and without going to a standalone ecu. Someone correct me if I'm wrong here...

14.5psi boost is approximately 100% more air/fuel, which given that not all other factors are equal could maybe give 80% more power (optimistic?) - more boost is effectively a higher multiplicative factor of the displacement you have. I know it's not exactly as linear as that - higher CR as well as boost and lots of expensive tuning can give you a better number - but this is a street car and not a drag beast.

So, I'm hoping to run 15psi initially, at 8:1 - giving an effective CR of 11.5:1 at maximum boost - see; http://www.autocomponenti.com/booste...osted_tech.htm

That's about as high as I'm happy to go initially...

Then, I can add an FMIC, water/methanol, and look at running even more boost by changing pulleys, or I can add a cam, and MicroSquirt first... perhaps with a knock sensor and an active bypass valve to keep things in check.

Basically, this is going to be a staged project and I've done a fair bit of background research, talked to Blown 944 and others about their setups - bottom line is that unless you have E85 (more resistant to detonation) or you lower your compression ratio, you are left with the low boost option. This is pretty well established - the 951 and pretty much every other forced induction road car have lower CR from the factory to accomodate more boost while retaining driveability.

The idea here is to find a cheaper alternative to rebuilding the bottom end where E85 is not available. Blown 944 has shown that the NA bottom end is strong enough to handle about 300hp stock - so why change the pistons if you don't have to? (and can't afford to!)....

EDIT: I've been over to the Cometic site and asked about a gasket with the extra spacing required - the gasket would be about 0.14" by my reckoning. I'll post what they reply.

bad_monkey

Well, I asked about making a thicker gasket - 0.14" to be exact - and I got a negative response.

I've now asked the question in reverse - what would the thickest sensible gasket be...

Again, I'll post the reply.

Maybe back to the drawing board though...

Van

Monkey, you can also get custom copper headgaskets of different thicknesses...

But, wouldn't the easy thing just be to pick up a set of turbo pistons? They're dished out to get 8:1 with a standard headgasket.

RC924

What particular blower do you have in mind?

................Or........................or.......................or... ......
Or develop a PLAN first.
Or consider a suitable engine management system.
Or just hope........... RRFPR + stock ECU ?

Seriously, if you can`t afford pistons you can`t afford a SUCCESSFUL blower install.

Sincerely hope you can persuade them to produce less than their required minimum of 100 units.

Jeremy Himsel

Not exactly. While I'm not trying to discourage you, more RPM = more heat which increases the need for the extra cooling across the CC that is generated by the quench. While you can offset the effects of opening the quench area a few different ways, it shouldn't be overlooked as an option. You also need to consider the heat and expansion rates of the 2.7L pistons at higher boost levels. Over the years there have been a few guys who have tore up perfectly good 2.7 & 3.0L motors by just adding FI. There is also a long history of guys who ran shorter rods, bigger HG, milled pistons, etc to reduce compression and have struggled with knock and making the appropriate power.

Superchargers do run a bit cooler so you'll have some room there but as said by a few others, Cometic can make the gasket you need but your block will have to be very flat.

bad_monkey

@van:

Turbo pistons are 100mm bore, 2.7l is 104mm.

@rc924:

Eaton m90 - I reckon it will manage up to the power level I'm looking at... see http://www.eaton.com/ecm/groups/publ.../ct_128485.gif and Blown944's posts for more details.

And I do have a plan - it's very well formed and accounts for contingencies based on the fact that this is a learning experience for myself. If I knew exactly what I was doing it would be a lot less fun! Incidentally you are currently located in the middle of the research phase of the plan The build stages might go something like this;

1. Blower, injectors, fuel pump, RRFPR, thicker gasket and stock ECU. Wideband (LM2) also. Low boost - 7psi

2. MicroSquirt and ignition/fuel fine mapping, knock sensor. Medium boost - 10 psi

3. Water/Methanol, FMIC or water/air IC, High boost - to maximum efficiency of blower (15psi?).

Well - if you have money to throw away before thinking your way through a problem you might go ahead and believe that... Please bear in mind these would be custom 104mm bore dished pistons - and would cost as much/more than the rest of the components put together. Blown944 has proven that the stock bottom end can handle the power, and a gasket/spacer was a potential solution (not a new one) to the problem. The true cost of a successful installation is measured in patient hours of research rather than $$$.

Besides, if everyone did things the same way... there would be no blower installations in the first place? (everyone would "sell the NA and buy a turbo")

The Cometic engineer seemed happy to entertain the idea of making a custom gasket (see the "custom gaskets" section of their site). Just not the thickness I had in mind initially...

@Jeremy:

Thanks for the input - that's food for thought. I've heard about people having trouble both ways - lowering static compression or retaining the stock/close to stock. It's very much about the CC shape and the swirl characteristics that come from that - and a whole load of other factors, including heat etc. Piston expansion was not one I had considered fully. I suppose that could be a strong argument to moving to forged 4032 pistons at some stage at least.

At present my reaction to the data I've been getting is that I ought to start by planning for less boost and then increase the boost over development, monitoring temps etc.

That should help avoid tearing up another block in the name of progress

Incidentally, as we have 98RON pump gas here in NZ - I'm looking for some guidance as to what maximum effective CR I can sanely consider? (see http://www.rbracing-rsr.com/compression.htm)

As the page says - there are many variables to consider....

Rogue_Ant

Not trying to sharp-shoot, but a roots style supercharger has generally much lower adiabatic efficiency then centrifugal compressors.


-Rogue