Difference between JT engine and JU engine

[Lykkehansen]

Hi

 

I was wondering if anyone could tell me how they make different compression with same engine.

We have a 401 nailhead with engine number JU490 which indicates it is a engine made for export only ( 25 was made for export).

I am wondering because when we want to purchase pistons I get confused by the compression difference and are afraid that I will order the wrong ones.

JT engine has 10.25:1 and JU engine has 8.25:1.

Could anybody help me figuring this out?

 

Thanks

Anita

 

[joe_padavano]

Either the pistons or the combustion chambers in the heads (or both) are different to change the CR. Neither of those changes displacement - it's still a 401 cu in engine.

[Lykkehansen]

48 minutes ago, joe_padavano said:

Either the pistons or the combustion chambers in the heads (or both) are different to change the CR. Neither of those changes displacement - it's still a 401 cu in engine.

But when I order new pistons they are higher that the ones we have.

[joe_padavano]

15 minutes ago, Lykkehansen said:

But when I order new pistons they are higher that the ones we have.

So get the correct pistons for your engine's CR.  There are different pistons available. Your problem may be that the extra-low compression export engines were made in such low quantities that no one makes replacement pistons for that application.

[Rusty_OToole]

If the top of the piston is higher that indicates they are high compression pistons. The piston seller or manufacturer should be able to tell you what the compression ratio is.

 

8.25:1 compression should be fine even on regular (low octane) gas. 10.25 may be a little high even for premium gas. Depends where you live, and what octane gas you can get. Also how you intend to use the car. If you can get good high octane premium gas, and are half way sensible when it comes to driving, the high compression should be ok. Especially if you live in the mountains or at a high altitude.

[C Carl]

Yes , in general if you live and drive at or over 5000' or so , there would be an advantage to increasing from 8.25 to 10.25 and using only high octane gasoline. If the car will be used lower than 5000' and frequently at sea level , I strongly advise you to go no higher than 9.00 , and at that , use only the highest octane available. If you can find the 8.25 Pistons , by all means use them. There might be a chance you could machine some of the height off the 10.25 Pistons , but I don't think that is likely. Before considering that and buying the pistons, you MUST consult the manufacturer, who might not tell you whether you could safely reduce to 9.00 that way. The other solution is custom pistons from Arias or Ross. That way you could get the original 8.25 compression ratio. If you hear any detonation whatsoever when driving your new engine  , immediately reduce the throttle, slow down , and perhaps shift to a lower gear. Any audible  pre-ignition knocking , no matter how slight , is called SEVERE DETONATION. EXTREMELY HARD ON YOUR ENGINE. Detonation begins even before you can hear it.  - Carl