good compression ratio for supercharged cologne 2.8?

[carbon1993]

Hello,

I have an 1984 XR4i with a 3.1 cologne v6. I want to fit a sprintex/power engineering. This supercharger normally will fit on a stock 2.8 cologne.
The stock 2.8 has a CR of 9.2:1 and my engine has a CR of 9.9:1 which is pretty high for an old N/A engine.
I know that some turbo technics cars have a CR of 9.2:1 or 8.9:1 and i believe the tickford capri also 9.2:1

How much will this 0.7CR extra have an effect on the engine. Has anyone ever tried a high CR on a boosted car.
the car will run about 8psi max. and it will run a intercooler.

[Boosted LS1]

You should be fine. If you get any pinking then lower the boost or fit a simple water injection set up. A decent management system would be very useful.

[stevieturbo]

It's mostly all about fuel/spark control.

If it's good, no problem. If it's crude....need to be more careful.

[carbon1993]

I still don't know how i want tun run the fueling. The original kjet or EFi with an aftermarket ECU.
And also it has Volvo b23 or b230 pistons. So 9.9 won't be a problem for 8psi boost as long the spark timing and fueling is right.

[Boosted LS1]

I'd use an aftermarket ecu because it'll allow you greater flexibility. The compression you have isn't an issue in itself.

[blade7]

I fitted a Turbo Technics kit to my 2.8 injection Capri in the late 80's. The TT setup had an intercooler & basic ignition retard and ran around 6lbs boost standard. I ran 9lbs in cold conditions but it was right on the edge for detonation. If I'd have kept it I reckon new pistons/rings were on the cards...

[carbon1993]

It has diffrent pistons from a volvo b230 or b23 engine. They are reinforced but it still looks like cast pistons. If your capri 2.8 ran on 9.2:1CR on 9psi boost and was almost detonating i guess the 8psi i want to run won't work with 9.9:1CR.

But then what is safe number to run on standard ignition?

[stevieturbo]

carbon1993 said:

It has diffrent pistons from a volvo b230 or b23 engine. They are reinforced but it still looks like cast pistons. If your capri 2.8 ran on 9.2:1CR on 9psi boost and was almost detonating i guess the 8psi i want to run won't work with 9.9:1CR.

But then what is safe number to run on standard ignition?

Again it will come down tuning.

You cannot compare in that manner without knowing many many other details.

As long as your charge temps are not insane, and using a half decent fuel ( ie super unleaded ) then I would not see 10.0:1 CR being any problem at 8-9psi.

But if the fuel system is crude and using say a dizzy vs mapped ignition etc etc...then obviously there will be more compromises and things can get more difficult.

[carbon1993]

I don't know which fuel system i want to use. I might keep the K-jet or get an programable ECU which will only control the fueling probably semi sequintial. The fueling i use is something the firezone sells here in holland called 102 competition. Which has a higher octane rating than super 98.

what is smart and safe to do just skim a bit off the pistons to be safe?

[Boosted LS1]

I wouldn't interfere with the pistons, it'll weaken them and your engine will be out of balance. Concentrate on getting good spark management and fuelling.

You could skim the block and run the pistons closer to the head if you want to improve the combustion process but only do this if the pistons are more then .045" from the head at tdc. Personally I'd just sort out an ecu and run the engine to find out if it works well or what needs to be changed.

[stevieturbo]

carbon1993 said:

I don't know which fuel system i want to use. I might keep the K-jet or get an programable ECU which will only control the fueling probably semi sequintial. The fueling i use is something the firezone sells here in holland called 102 competition. Which has a higher octane rating than super 98.

what is smart and safe to do just skim a bit off the pistons to be safe?

If you're using 102 RON fuel ( lucky you ) I dont see any need to adjust your CR at all.

Again, it's all down to tuning.

[carbon1993]

these are the pistons,i had the whole engine balanced with the clutchcover and flywheel. The deckheight is 0.05mm(0.002inch)
If i skim the pistons till its flat that wouldn't hurt the pistons too much right? I am just affraid of detonating. And i don't won't spend too much money and time in getting the highest horsepower out of the engine. Just wan't to get it runnig reliably at the cheapest possible method without engine faillure. As a student i don't have 3K to spend on the engine.

Oh and i have zero experience building boosted engines.

i think the pistons are casted but they don't have very rough casting lines in the pistons.

Edited by carbon1993 on Sunday 1st January 23:23

[Boosted LS1]

They may be mahle forged? If they're cast you'll see the joint lines underneath the crown. If you skim the pistons you'll lose squish effect and that'll make the engine worse imo. What shape are the combustion chambers? I'd still leave it all alone.

Edited by Boosted LS1 on Sunday 1st January 23:45

[stevieturbo]

And as already said before. It is all down to tuning. As long as this is done correctly and you have proper control over fuel/sparks, there will not be a problem.

Exactly what blower are you using ? And what usage is the car ?

You could possibly machine some of the piston crown...but you would need to know how thick it is in order to remove some, or you could slightly enlarge that dish etc.
But I dont see much point.

And you will need to spend money on an engine management system, although the budget options are pretty damn cheap these days.

Base Megasquirt or Spitronics are about the cheapest around.

[carbon1993]

Do i really need that squish effect? Normally that's used on flat cylinderheads.It only needs 1.4mm to get the dish out. If its machined 0.7mmm of it is already on 9.45:1CR. Normally the pistons are flat in this engine.
I bought the car for a daily driver but taxes changed i only driven 3000kilometers in 4 years now (because things broke all the time). I wan't to use the car a little more like 3000Km a year.

Can i calulate how much pressure it is going to deliver when i know the size of the pulley's?

The blower is a sprintex s102 sold by power engineering. It was a straight bolt on kit for the 2.8 back in the days (9.2:1CR). Here's some info: http://www.vondrachek.org/automotive/tech/screwed/...
http://www.xrstyle.co.uk/forums/showthread.php?412...
http://www.pistonheads.com/gassing/topic.asp?t=367...

this is the shapeof the cylinderhead.

[blade7]

carbon1993 said:

these are the pistons,

Oh and i have zero experience building boosted engines.

i think the pistons are casted but they don't have very rough casting lines in the pistons.

It looks like the bowl on those pistons could be machined wider while still retaining the squish. 10lbs boost on standard comp ratio without a standalone ECU is heading for tears IMO. My 2.8 could detonate at 8lbs on a warm day with a couple of mates in it. The TT setup had a vacuum advance/boost retard module fitted to the distributor, so although basic it did retarded the ignition on boost, and a mod to the K-jet fuel head.

Edited by blade7 on Monday 2nd January 13:20

Edited by blade7 on Monday 2nd January 13:21

[carbon1993]

already did some modifications on the kjet my self with a lot of help, it's alot of testing to get it right. If i am informed right a supercharger at the same PSI would be less likely to detonate as the temps are lower. So that's a advantage!

The dish is now 66.1mm*1.4mm. If i machine it to 80mm it would give a CR of 9.6 if i would flatten it completly it would give me a CR of 9.18:1
Maybe i should start looking into a aftermarket ECU.
.

[Little Pete]

I would leave the pistons as they are. I agree with Stevie in that the fuel you are able to use shouldn't cause you any issues at the CR you are running and the squish effect will help to overcome any detonation problems. I did some work on a 2.9 4x4 in the early nineties that had a supercharger on a standard engine. It had crude modifications to the vacuum system to retard ignition timing and extra fuelling was via the fast idle/warm up injector in the plenum chamber. I balanced and rebuilt the bottom end for the guy after some oil pressure issues but the pistons were fine! I was surprised but the system did seem to work. I think it ran 8/9 psi. The customer was looking at a very early Emerald ignition ecu as the next step but he wrote the car off before he got anywhere with that. Sorry but I can't remember what make the supercharger was.
Just another point. Have you measured the CR or are you going off someone else's information?

[blade7]

carbon1993 said:

already did some modifications on the kjet my self with a lot of help, it's alot of testing to get it right. If i am informed right a supercharger at the same PSI would be less likely to detonate as the temps are lower. So that's a advantage!

The dish is now 66.1mm*1.4mm. If i machine it to 80mm it would give a CR of 9.6 if i would flatten it completly it would give me a CR of 9.18:1
Maybe i should start looking into a aftermarket ECU.
.

I'd try running it at lower boost initially and see how you like it. Use a knock sensor and gradually increase the boost if you want more.

[stevieturbo]

If you were really stressed about it, you could machine the piston a little, but I would not go full flat top. Save a large ring land area above the top ring.

Or it looks like you could have the chambers in the head opened out a bit to reduce the CR ?

Also, do the pistons have holes behind the oil ring, or a large slot ? The latter being a much much weaker piston, and yours do appear to be basic cast pistons.

And yes an engine management system would be best. However it does depend on budget and it sill needs a competent person tuning it.

You could undoubtedly make it safe on the original systems, but there would also be more compromises with performance too and it would also take a lot more testing and tuning.

Modern systems are just very convenient.

I would always say buy a system where you can get good support for...ideally local support. So on that end something from a local supplier might be best.

Although the cheap Megasquirt stuff or this Spitronics...really can be done very very cheaply. Not something I've ever used myself, but both quite viable.

http://spitronics.com/products/index.php