Calculating My Cr...where Am I Going Wrong!

[Chris H 10]

Sorry for another thread about compression ratios but i've got myself all confused working out my CR

 

I have a xu5 engine which seems to be low on compression 140psi on 3 cyls and 125 on one but that has a valve that's not quite sealing.

 

So before i strip the head, i've measured the cc's to an average of 27cc

 

it is a std 1600 bore of 83mm and stroke of 73mm

 

The engine currently had a 1.8mm head gasket fitted but i also have a 1.4mm that i could use as well as another 1.8 one.

 

I've been using 9cc as the piston volume is this correct for a std 1.6 piston (mine has small cut outs for valve clearance so will measure them accurately as soon as i have the pistons out)

 

84.2mm as the head gasket diameter

 

0.36mm as the deck height.

 

When i put this in to an omline calculator it gives me a cr or around 9.8:1 which is what it should be with a std head, not one with a much smaller volume.

 

What have i missed or miss calculated.

 

I'm trying to work out if my compression test results are low because there is a fault, or because the engine spec is wrong.

 

The engine all seems in good cond so far other than the one valve, but when trying to map it it only made 77bhp at the wheels! Rest of engine spec is piper bp300 cam, piper vernier, ported head with std valves by peter burgess, double valve springs, 45mm tb's

[welshpug 1,661]

standard gasket is 1.4mm I believe, so using a 1.8 mm may give those kind of numbers possibly? not sure without doing the maths

Edited November 2, 2015 by welshpug

[Biggles 65]

Hi Chris,

 

Using your figures above, I calculate a CR of 9.2 when using the 1.8mm gasket. Accounting for land volume will knock another 0.1 off this figure.

Based on the published timings for a BP300 (assuming I'm reading the catalogue correctly) (inlet closes 78deg ABDC) this gives a dynamic CR of 6.0 - which would explain your lack of performance - unless a couple of glasses of Shiraz have affected my ability to count. (I shall revisit this tomorrow as 6:1 is such a ridiculous number something has to be wrong.)

[petert 588]

It's never going to make hp with only 140psi cranking pressure and the faulty 125psi. A cam that big needs 11:1. Regardless of the CR calcs, throw those gaskets away and buy a 0.7mm Cometic gasket. 9cc is correct for the dish.

[petert 588]

I get 10.4:1 with a 0.7mm gasket. I'd also consider smaller cam.

[welshpug 1,661]

or get some flat top xu9 pistons if there's enough clearance, a similar cam and inlet in a 1.9 makes around 175bhp.

[Biggles 65]

0.7mm gasket = 10.4 for me too - so I can still count !

 

Sounds like you need more CR or less cam Chris.

[Chris H 10]

will have a check through my stock of XU cams, think i have a PTS max torque, a newmans phase 2 3, and 4 but not sure if any are in good condition.

 

if i were to get a new cam, what would be your suggestions? When i bought the engine i'm having issues with, i was advertised at having a 285 cam which was more what i was looking for, only on stripping did i find it was a 300 with over 12mm lift.

 

What i don't understand, is if the head has had a big skim and chambers of 27cc, why is the CR less than a std engine when calculating using a 1.4mm gasket?

 

Petert, do you have any more info on the gasket you recommend, i cant see anything on the cometic website for a wet liner XU.

 

Thanks for you input guys, all very useful.

[petert 588]

I was just using the numbers you provided and the formula CR= (Vs+Vc)/Vc

 

where

Vs = swept volume

Vc = 27cc + 9cc + gasket volume + deck height volume

 

If you do a search for "Cometic" and "Sweetbadger" in the XU9J4 section you'll find the part number. Sorry, I can't recall.

 

Something around 275-285 deg would be better for 10.4:1.

 

Your other and possibly better option, is to install a 1.9L crank and rods.

Edited November 3, 2015 by petert

[TAG 25 1 Cars]

I think some of your measurements may be wrong. Using your figures, but replacing chamber volume with 32cc (should be standard head), I get a static CR of 8.79 (with a 1.4mm HG). Figures I have for an XU5 is CR = 9.8. I can't find any figures for deck height, where did you get that value from?

 

Too small a CR for that cam would certainly explain low power figures. What are you revving it to?

Edited November 3, 2015 by TAG

[Chris H 10]

I got the deck height measurement from a post of Petert's but just noticeed he actually stated 0.38mm

 

Maybe i need to measure it myself.

 

I must have something wrong as i cant see how i can have almost 2mm skimmed off the head and have a lower than std compression ratio.

[petert 588]

Those pistons normally have zero deck height, thus you'd have:

 

Vc=27+9+head gasket

 

10.4 with 1.4mm gasket

10.9 with 0.7mm

Edited November 3, 2015 by petert

[Chris H 10]

I've done some more measurements and have a piston dish of 5cc and a piston to deck height of 0.36mm

 

i now get a CR of 10.8:1 with a 1.25 headgasket so getting a reasonable CR now.

 

Is the piper BP300 worth continuing with or should i look at something a bit milder.

 

If so what are the recommendations, PTS 7048, newman PEUI/280/470, kent PT37, piper p16bp285 or catcams 4900266 all look at possible options.

 

I have a pts cam already and possibly a newmans one...need to check the profile on that one though.

 

I am going to get the valve seats done and fit the longman valves i already have so would like to decide on what cam to use so i can get the head all shimmed up too.

[TAG 25 1 Cars]

Hi Chris, you haven't actually stated what you will be using this engine for?

I presume its a 1600 rally engine? Gravel? Tarmac?

What are you looking for? Something that revvs its tits off? Or a more flexible engine with a wider power band?

[petert 588]

Is the piper BP300 worth continuing with or should i look at something a bit milder.

 

If so what are the recommendations, PTS 7048, newman PEUI/280/470, kent PT37, piper p16bp285 or catcams 4900266 all look at possible options.

 

I have a pts cam already and possibly a newmans one...need to check the profile on that one though.

 

The only way you'll know is to gather all the duration @ 1mm figures for the cams above and compare them.

[Chris H 10]

I'm planning to use the engine for tarmac rallying, would like a reasonably good output from the engine but definitely want reliability rather than max power/revs. I'm aiming for something around the 140bhp mark which i dont think is too unreasonable.

 

The head has been ported quite nicely but i'm just stripping it now and very surprised to see it had std valve springs, they can't have been suitable for the piper bp300 that was fitted.

 

Thankfully i have 2 sets of double valve springs i can fit once the machining is done.

 

The engine will be running TB's and a mappable ecu. The bottom end is lighten and balanced, shot peened rods, PTS baffle plate, 1900 block stiffener and 1900 sump pan. Magnex 4 branch manifold.

 

I'll get the all cam data in front and see how they compare.

[welshpug 1,661]

DCC's 1.9 made 175 on the 300 degree ph4, cant remember exactly what his compression ratio was but its over 11 easily, it has std valves with double springs.

 

might not apply to an 8v, but I'd be quite wary of used longman valves, many a full house engine has met its demise from dropped valve heads..

[Chris H 10]

The valves, double valve springs and cam followers i'll be using are all brand new. Bought them cheap off a mate who went 16v instead!

 

I was surprised that the inlet valves are 10g and exh 13g lighter than std when i weighed them earlier