XRTypeRS

Ive just done a compression test on my RST cabs engine and found that all 4 pots are giving a compression reading of 180-190 PSI . I believe the spec of my engine to be :-

1900 (tall block US type) CVH, 88 mm stroke bored out.
2 litre Zetec pistons
standard head and cam (head was skimmed)
Vernier to account for cam timing error introduced by tall block

Although im not 100% sure of this as I got it before I was as clued up as I am now (pistons are defo flat top though).

I found this post from Karl which said :-

"A std ERST engine will give between 150 and 160psi. The same engine with a performance long duration cam will give around 140 to 150psi.

A low comp ERST engine of say 7.3:1 could be as low as 120psi with a big cam.

These are just indicative Psi readings and not to be taken as gospel as I do not want people to be guessing their C.R based on a compression result!!!

These figures are what MY engines give."

So this doesnt sound good for me . Ive been running 8PSI boost for 4 years OK but hoped to raise it a bit (have briefly run it at 1 bar but spark kept blowing out - could that be related??). Not sure what to do now, engines done about 14K and runs sweetly but so I dont want to rebuild it but will she run into det easily if the CR is as high as it looks ??. What is the max boost a standard S2 can run on standard compression?

Any oppinions?

Thanks,

Stu

RichardPON

Am I missing something here?

Surely if all four pistons are giving the same compression readings (within tolerances) then there's no problem? (to a point obviously)..............

XRTypeRS

Rich, thats true and that is the main purpose of a compression test but Im pretty sure my engine is in good condition, what Im trying to get an idea of is the CR as I dont actually have a clue what mine is and given I dont trust the people who built it any further than I can throw it I wouldnt be surprised if it was running a CR more suited to N/A than a tuurbo application - most people who build ZVHs using the zetec pistons skim them to get a decent CR (or use a spacer) but Im worried I just have some full height zetec pistons slapped ioin there with no spacer and a skimmed head!! thus maybe too high a CR??

I know compression test isnt a v good indication but Im just trying to get an idea as I dont want to take the head off when its running fine.

Cheers

Stt

GARETH T

there is difference between a compression test and compression ratio, and they dont go hand in hand

XRTypeRS

Exactly Gareth and Im aware of the limitations but surely it can give a very rough indication of CR?? (given Im running a standard head, cam and turbo etc etc)

GARETH T

to be honest i thought the same a long time ago, but there is simply too many variables!

the onyl way is to take the top off

The Sludge

tru these peeps

http://www.escortrsturbo.co.uk/community/

XRTypeRS

Noo!!

So the test wont really tell me anything??

CraigN

i wouldnt run a bar on std zetec pistons cast jobbers

XRTypeRS

Agreed its risky mate but lots of people have done it on here and they usually can handle it so long as the fuel is there. I wouldnt have them if I was building an engine now but at the time I just didnt know what was what

philipeddowes

I see what you mean Stu, bit of a dilema mate

If it helps, not sure it does, mine (1.6cvh) has had 1mm of the top if the pistons and has 150psi on all 4, runs as much boost as my stg 3 T3 can produce LOL.

XRTypeRS

Nice one Phil, do you have any idea what boost your running then??

That must be scarey in a cabby!!, when I turned mine up it made 241 lbs ot torque - way too much for the super bendy cabby chassis IMO!!

Jim Galbally

compression test does not in any way shape or form tell you what CR you have... cant even give a rough indication

philipeddowes

Currently peaks about 29.5 and holds abouve 28 all the way to limiter LOL.

Re-inforce under the sills and sort the suspension and its okay though never had it flat out LOL did hit 148mph once but me balls ran out before me revs did I think my torque is about 260ft/lbs and all four wheels still seem to be on the floor so it ain't that bent LOL

philipeddowes

Pls explain Jim. I realise there are loads of variables and any estimate can only be an estimate but surely having a cr of 180 tells you that there are boost limitations.
Thought thats why we drop the cr, to allow us to up the boost, well thats why I lowered mine.

Jim Galbally

compression ratio and engine's compression are two seperate things not directly linked

philipeddowes

Well thats that explained then LOL

EIL132

As Jim said, it gives an indication of how well the rings/valves/gasket,etc are sealing, not compression ratio.

XRTypeRS

Jim I understand that valve timing etc can have a big effect on static compression but as my car is virtually standard I thought I could gain some kind of relative estimate(given Karl states that a standard RST will produce 150-160psi) - my extra 10+ psi must be coming from somewhere . Oh well will just have to get my home made det cans on and go testing!

Phil, what strengthening do you do??, sounds interesting?, did u fit a strut brace?, Ive heard they can make a big effect on a cab but Ive also heard inurance companies hate them and they can cause the shell to split around the front turretts!!. What suspension are you running?, Ive taken mine back to standard shocks and standard S2 springs (a bit harder and lower than standard cabby ones) + polybushes as she scuttleshook so badly on normal lowering springs - its still damn uncomfomfortable now - I might go 100% standard again yet and limit myself to straight line blasts!!

XRTypeRS

Bloody hell!! - thats some boost!!

philipeddowes

Pealed an intercooler open like a tin of beans the other week though and boost leaks seem to have become an issue

JesseT

What engine management and intercooler are you running?

EIL132

How long did you turn the engine over for? Was it warm or stone cold?

philipeddowes

If you are refering to my intercooler, the engine certainly was very warm

Mark V8

iTrader: (2)

You can get a fairly good idea of clearance volume by setting the engine to TDC and carefully measuring some light oil (3 in 1?) into the chamber via the spark plug hole until it is level with the bottom of the threads, granted you will need a torch and syring to measure the oil but this method worked OK on my bike and takes into account gasket thickness/piston crown shapes etc. You WILL have to blow the oil out afterwards with an airline to save any engine damage.

Mark

EDIT...... make sure it's TDC compression stroke!

XRTypeRS

Jesse, Im on standard Bosch KE and a GRS EVO 3 cooler - by the way thank you for your PM ages ago about the tech edge lambda kit, I intend to reply to you soon as I have some questions but I have not so for as Ive been very busy building the kit!! - its all done now and seems to work, just need to put it on the car - and get a decent laptop PC to drive it!!.

EIL, engine was an full operating temp, just got in from driving home 30 mins before (fan on) then warmed it up again for another 5 mins on idle before the test. Throttle was wide open when testing.

Cheers,

Stu

XRTypeRS

Mark so I could work out the volume of the head gasket etc with this and then simply work out the rest of the volume using simple maths?, i.e. cyl dia x stroke? type thing?.

Mark V8

iTrader: (2)

Piece of piss mate

Known engine CC divided by 4 gives you swept volume.
Measured clearance volume can be measured with oil.

Swept volume + clearance volume divided by clearance volume is your CR

Mark

XRTypeRS

Nice one!!! - think I may give that a whirl!! cheers mate!

Mark V8

iTrader: (2)

One thing to bare in mind
When an engine is stripped to do a CR test the ring gaps are sealed with grease to prevent leakage, as long as you don't spend half an hour trying to get some oil into your combustion chamber you will be fine. If you struggle to get the oil in it may be worth trying somthing thicker, this does carry the risk of air bubbles though!

Mark

JesseT

I'll say the compression pressure will not limit you going to 1 bar boost pressure. Having a decent IC really helps too. However, you might need a way to retard your ignition advance slightly more when on boost.

zvhturbo

My compression test gave me 117psi all round.

Mine is a low compression built to run 25psi boost.

If thats some sort of indication mate.

Matt.

XRTypeRS

117psi??!! thats well low!!