Engine rebuilding or swapping

[aknight123456]

Afternoon all,

I'm at a point where I am starting to consider my options. I've got a MK3 spitfire which has a MK4 engine which I feel is going to need some work soon as the oil pressure when hot is triggering the low pressure switch at idle. 

I've been doing some research and it seems that my engine model is the large journal engine post the change in camshaft in the MK4. I believe that means that the main differences between the MK3 and MK4 engines of my age is a larger heavier crank and a less sporty camshaft all meaning less power (boo) by the tune of about 14bhp...a fair amount in an already underpowered car.

I want to keep the small 4 cyl triumph engine as opposed to swapping in a gt6 lump etc but do you think it's worth sourcing a MK3 1300 or is it easy enough to convert my current engine back to a small journal MK3 esc engine as I've read they are the most fun and freer revving ones. I'm looking to have it mildly tuned whilst it's out of the car to bring it closer to 100bhp to give it a bit more of a sporty feel so it makes sense to tackle everything together. 

Cheers in advance

[RobPearce]

Converting a large journal engine to small journal is not practical - you'd need to replace the block, crank and con-rods, so you're into new engine territory.

If you can find a small bearing Herald 13/60 engine (GE prefix) then that can be fairly easily converted to Mk3 Spitfire spec - cam swap and head skim pretty much covers it.

If you're trying to get 100BHP out of a 1300, though, you're into serious tuning and you'll likely end up with something not very tractable, suitable for racing but not so great on the road. You don't need that. The car only weighs 700kg so 80BHP is plenty.

[aknight123456]

Thanks for your reply Rob. 

The 100bhp comment was based on what I'd read as a roughly realistic flywheel bhp but it's by no means a target. I just want a bit more than what I currently have without getting too serious on the tuning side!

In your opinion is the difference between the small and large journal 1300s that big? As alluded to I'm not going to race it but I'd like it to be sportier for the roads and maybe the occasional track for fun.

Thanks

[glang]

Are you sure you dont want to go to a 1500 engine? It would be easier to source, produce more torque so reduce gear changes and still offer the possibility of tuning for more power....

[thescrapman]

I have a Mk3 Spitfire and. Mk4 Spitfire, the Mk4 has had 1300 and 1500 engines in it, the Mk3 is still more fun.

Revs much more freely. Sounds great being thrashed.

I do have another engine to try in the Mk3, supposedly got a lumpy can and runs twin Webers. see what that is like. interestingly it is a large crank 1300. See if the free revving is compromised.

 

[RobPearce]

I don't think there's actually that much difference between small and large bearing 1300s. Most of the performance loss from Mk3 to Mk4 was down to the increased weight, followed by a cam change for emissions. The drop in quoted BHP was at least partly down to a change in how it was measured. The Mk3 engine is a little smoother revving than the later one but using a Mk3 profile cam on the large bearing engine gets some of that back.

If you're mostly doing road driving, glang's suggestion of a 1500 is reasonable. That's actually what I have in my Mk3 (the original engine was knackered and I had a good 1500 lying around).

[Clive]

Quoted from RobPearce-

Converting a large journal engine to small journal is not practical - you'd need to replace the block, crank and con-rods, so you're into new engine territory.

If you can find a small bearing Herald 13/60 engine (GE prefix) then that can be fairly easily converted to Mk3 Spitfire spec - cam swap and head skim pretty much covers it.

If you're trying to get 100BHP out of a 1300, though, you're into serious tuning and you'll likely end up with something not very tractable, suitable for racing but not so great on the road. You don't need that. The car only weighs 700kg so 80BHP is plenty.

I would disagree. My mk3 has a FR89 cam, modified head, and a pair of dellortos. It is VERY tractable from sensible revs. (and quicker than a GT6!)

 

[Nick Jones]

If you want a small-crank 1300 as a start point I have a Herald 13/60 "GE" lump here getting in the way.  It's dismantled and incomplete but includes the block, crank and rods.  Currently all at standard size, but it will require boring/grinding.  Located in South Somerset.....

As an aside, while a 1500 does have it's plus points and makes a good road engine, it really needs taller gearing than the Mk3 has to make the most of it's virtues.  IMO you need the 3.63:1 diff from a 1500 Spit as a minimum and ideally overdrive as well.

Nick