Compression Ratios

[jas-E 1]

Hey guys,

 

I was looking at getting forged pistons for my ally mi16 engine as i might be able to get a good deal on them and was reading through the catalogue which listed the mi16 pistons as the ones on a 405, a citreon BX and a 309 gti16v (which sounded about right).

 

The problem is that for the XU9J4 1905cm3 engine, the compression ratio listed is 9.7:1, the engine power output is 150PS (which is about 147bhp) and the bore diameter is given as 83mm.

 

I did a search and gathered from there that the mi16 ally block engine runs 10.4:1 compression ratio and know that it should be listed as producing 160bhp with a 83mm bore diameter.

 

Am I looking at the piston size for an iron block engine??do all the XU variants have the same bottom ends but different cylinder heads to allow for different compression ratios..?

 

The engines are listed under different dates to when they were released into the market atarting from about 1988 - 1992, but they all have the same dimensions for bore and same compression ratios.

 

cheers

Edited March 27, 2008 by jas-E

[James_R 3]

Could be for the cat engine, the later Mi's had lower Cr's and cat's.

 

Does it like the engine code aswell?

[Mikey G 3]

Iron block engines are 86mm.

[jas-E 1]

Could be for the cat engine, the later Mi's had lower Cr's and cat's.

 

Does it like the engine code aswell?

 

The cat engine came in after 1993 iirc. Theres an engine dated after 1993 listed as XU9JAZ which has 120PS and 9.2:1 compression ratio. I assumed thats the XU9 cat engine.

[jas-E 1]

Apologies for posting this in the wrong section.

 

Wasn't sure where it went.

[jackherer 543]

The cat engine came in after 1993 iirc. Theres an engine dated after 1993 listed as XU9JAZ which has 120PS and 9.2:1 compression ratio. I assumed thats the XU9 cat engine.

 

 

Thats the low comp 8v engine. The low comp Mi16 is XU9J4Z (high comp is XU9J4)

[jas-E 1]

Thats the low comp 8v engine. The low comp Mi16 is XU9J4Z (high comp is XU9J4)

 

So is it safe to go for the XU9J4 piston even though it says its got a compression ratio of 9.7:1..?

[petert 558]

Yes, the XU9J4 (10.4:1) and XU9J4Z (9.7:1) share the same block and liners. The XU9J4Z was fitted to cat equipped cars from '89 onwards. It has the engine code of DFW. The XU9J4 version has the code D6C.

[jas-E 1]

Yes, the XU9J4 (10.4:1) and XU9J4Z (9.7:1) share the same block and liners. The XU9J4Z was fitted to cat equipped cars from '89 onwards. It has the engine code of DFW. The XU9J4 version has the code D6C.

 

cheers for yuor reply mate..

 

Just out of curiosity if they had the same block and liners, is the differenc ein the compression ratio due to the cylinder head..?

[madaxgt 0]

Jas

 

Why would you want to use forged pistons in the mi when basically it will be a std engine? Cast pistons are ideal for near std and even modified engines, they have v.low expansion and very good life.

 

Forged pistons are only necessary in very high comp ratio high power na's (say 110hp/litre+) and forced induction motors to deal with increased cylinder pressures and temps etc. They are a ballache on std engines, you have to install them with like a 0.75mm piston to wall clearence depending on grade of forged Al used. When you use clearences that big they slap around like a b*tch and make a lot of noise and wear out very quick. Hence the issue with impreza sti piston slap at high mileages. (forged pistons as std.). (Other reasons this engines does that also.)

 

IF you were gonna go for forged pistons cos the deal was so good then why stick with 9.7:1 or even 10.4:1 if your thinking of uprating the engine. May as well get compression ratio increase while your at it for more torque almost everywhere. Its a complete waste otherwise imho. Its not like the mi has issues with its pistons anyway as i beleive theyre fairly strong and take a decent amount of abuse. If you were thinking light pressure t/c or s/c then the 9.7:1's forged pistons would be ideal for that concept.

 

Unless you are planning on being really clever and increasing your comp ratio in other ways to maintain good combustion chamber shape and secretly building a monster engine without telling anyone...........like me!

 

[petert 558]

\, is the differenc ein the compression ratio due to the cylinder head..?

 

No, it's the compression height of the piston. The top of a DFW pistons is approx 1mm lower. I've studied both pistons and they come from the same casting. ie the valve pocket depths relative to the pin height is the same.

 

Liner wear is more of a problem. Providing you have good liners, I'd skim the block and liners to obtain zero deck height and get 10.8:1.

[Powers 13]

Some really good points there.

Jas, are there any other uprated parts you can get discounted from this "catalogue" , if there are then why not go for this monster build!