Compression ratio

[205freak 7 1 Cars]

Hi, i ve built a xu10 engine but i think its missing some bhp, the engine spec is;

Bore 86mm

Stroke 88mm

Combustion Chamber volume 30cc

Piston volume 9cc

Head gasket 1.25mm

KentPT27 camshaft

 

It gave me over 11.5 compression ratio using the cr calculator but today i took the spark plugs to check compression it gave me in all cylinders 10,5bars thats 150 psi can this be right?? Whats the reason to loose torque and bhp above 3000rpms???

Any thoughts how to solve the loss??

Thanks

 

[petert 549]

How far down the bore is the piston at TDC? Did you add that space to your chamber volume? I left it as zero and got 11.7:1.

Is the cam timed correctly?

The cam may be too large for the compression you have. You really need to get the DCR above 8.5.

[welshpug 1,621]

did you hold the throttle open?

 

what intake and exhaust does it have?

[welshpug 1,621]

oh, i think Pete probably nailed it, static compression of 11.5 ish with a 300°+ cam wont work so well.

 

Im using mid 12's with 286° which does work!

[205freak 7 1 Cars]

Hi thanks for the replys!! PeterT you´re correct about piston at zero deck height, the cam was timed correctly, so what can i do, to make it work properly, skimmed the head a little bit more ou change the head gaskett for a thinner one???

 

[petert 549]

Advance the cam around to 102º to begin with. However, a 0.7mm Cometic head gasket would be best.

 

What lift @ TDC does it have?

 

What does the ignition map look like?

Edited August 18, 2018 by petert

[205freak 7 1 Cars]

PeterT i´ll try to confirm everything by the end of next week, when i have more time available!!!

[205freak 7 1 Cars]

Hi again..so after a few tweeks on mapping it improved a little but nevertheless i bought a 0.91 gaskett, after a few calculations will raise both static and dynamic cr making the engine more alive.. petert my question to you is..how the pistons will cope with a 12.5 compression ratio and do i need to worrie about piston/valve???

 

Thanks

[petert 549]

Are they stock pistons? I'd just run it anyway. All will be fine if the mixtures are sound. No, valve won't hit piston.

[205freak 7 1 Cars]

36 minutes ago, petert said:

Are they stock pistons? I'd just run it anyway. All will be fine if the mixtures are sound. No, valve won't hit piston.

They are stock pistons and i hope going over 12 on cr wont damage anything?!?!! Going to swap the gasketts in when it arrives i believe in 2 weeks time, after instalation i gonna tell you the diferences!!!

[petert 549]

What ecu are you using?

[205freak 7 1 Cars]

Petert using emerald k3!!

[DamirGTI 323]

I had 12.3 CR for around 4 years , was my daily 205 .. standard 1.9 DFZ pistons and a bit heavy head skim , standard management (except the recurved dizzy , and an adjustable FPR) . No problems with the pistons they where fine @ 12.3 .

 

D

[jamie_1992 66 1 Cars]

How have you got such a hi comp on stock pistons? Presume you have larger piston cut outs as well 

[welshpug 1,621]

dfz is a large head chamber engine so has a tiny dish (or maybe flat top even)  in the pistons, put these under a small chamber gti head and hey presto, nice snappy high comp 8v cheap.

 

8v pistons dont need cutouts due to the angle of the valves and how far they sit away from the piston at tdc

[205freak 7 1 Cars]

On 10/28/2018 at 6:05 PM, jamie_1992 said:

How have you got such a hi comp on stock pistons? Presume you have larger piston cut outs as well 

Jamie just using std pistons from the engine xu10j2, the head just had a light skimm to clean but it has already been skimmed in the past, the oe combustion chamber for this head is 36cc..and now its 30cc, so i do not know how much is has been taking off!!!

 

Petert using the thinner gaskett, the timing will be off again, would need timing again and another question how much crank advanced do need again?!??

Thanks