Tu5 Head Onto Tu3S Block?

[Kitch 4]

Hi all,

 

Just wondering whether to consider this as an option at this point.

 

Basically I have a TU5 blacktop Saxo VTR engine running bike carbs, PH3 and a 4-2-1. Makes 110bhp and will do it all day long.

 

However, it's in a Hillman Imp, still running the Imp transaxle. The torque of the TU5 could probably kill the transaxle, and it's an iron block, so I'm carrying a slight weight penalty at the rear end. That said, I'd like to keep the TU5 head as I have the carbs/cam etc already fitted, and it's all plumbed in nicely.

Was wondering if the TU5 head would fit a TU3S block I have? It would save me some weight, and reduce the torque slightly (certainly move it up the rev range a bit) while *hopefully* not losing too much in the way of power. I'm thinking the valves will clear the pistons, because they're smaller than the TU24 valves, which use the same bore as the TU3. Not sure about compression ratios though, and it's been suggested this could be an issue.

Anyone been there, done it?

Cheers

[welshpug 1,661]

TU3J2 and 5J2 heads and cams are the same, no idea which the "blacktop" head is though, Sandy could probably tell you what you'd get roughly compression wise with the injection head over a carb piston.

[Kitch 4]

Cheers. I knew the J2 heads were the same, as I fitted a 106 XSi head to a Saxo VTR block once. Difference there is the blocks are cast iron, not alloy, and the pistons are domed, which I assumed was to raise compression, but I suppose equally it might lower it depending on the ratio to the combustion chamber.

 

Basically the Saxo blacktop lump is the earlier TU5J2 engine, non-roller rocker etc. The TU3S head would physically fit, but I'm unsure what sort of results I might get.

[RossD 44]

The compression ratio will drop right down with a TU5 head on a TU3S block. At the least you would need to use the pistons from a TU3J2 (The iron block 1.4, used in the early 106 XSi's) to bring the compression ration up to a useable level.

 

The reason is that the TU5 and TU3J2 engines have larger valves and combustion chambers in the head than the 'small valve' TU3S engine.

[trogboy 17]

Like Ross said - CR will be way down if I recall correctly around 8.5:1 with that combo. So domed pistons, machine down the head to remove lots of volume from the chamber, or stick a turbo on it!

[And 15 1 Cars]

It's some thing like 10cc differents in the combustion chambers a 1mm skim is needed just to get back somewhere near std compression so a domed piston is required and or a thinner head gasket, ether way you need to take some of the extra combustion chamber volume away as you will loose a lot of cylinder volume going down from the 15??cc To 1360cc a rebore out to 76mm Pistons would help tho and give a 1398cc.

[Kitch 4]

Thanks chaps, reckon I'll just leave it as TU5 spec then.

[fourbanger 2]

So are the bigger valves worth it, assume big you get CR back to something sensible?

[And 15 1 Cars]

If you can get the compression back up then yes I feel it is I did it, the other option you do have is get the larger valves fitted into the TU3S head but depends on machine shops it can be costly the TU5 is by no means a cheaper option tho if you go down the route of Pistons to get the compression back up though..

[fourbanger 2]

​OK. this is the route I'm heading down I think. Money doesn't come into it or I'd just buy something else for what I'm going to spend!

​Do you know what the best heads to put on are? Looking on eBay now.

​And which pistons to pair them with?

[And 15 1 Cars]

I used the later TU5 vtr head the 5 stud in let it has the better spaced inlet ports but that then gives you other issues on the inlet side!!, I then had the warsner Grp A Pistons and TU5 forged rods as them Pistons require the longer rod but then the issue is the head combustion chambers need a lot of reworking, maybe best to try and find a 106 1400 xsi engine or ax gti is the same they have the big valve head same as the TU5 but have a domed Pistons and so no compression issues