16 replies to this topic

[Jonnym71]

Good evening, I’ve just recently bought a mini project that has a rust free shell. I have a Sprite 1275 A+ engine, which I believe is a low compression version. I also have the following:

A Stage 3/4/5 head , however it is defined these days; it has 37mm inlets, 31mm exhausts, beautifully worked ports and 18.5cc combustion chambers.

MG Metro, camshaft, 10.1cr Pistons and Rods
Minispares Evolution001 camshaft

Single HIF44 on a decent inlet manifold

Twin 1 1/4 HS2 carbs on an unknown manifold

Standard LCB manifold

RC40 twin box exhaust

I think the head+standard bottom end will give somewhere between 10.5: and 11:1 combustion ratio

I haven’t yet thought about ignition but might go with an Aldon yellow dizzy and for cam timing Inmight go with a lightweight duplex vernier.

With this in mind, what would be the best combination of parts and ballpark boho.

I’m also contemplating a rebore to 1293, having the bottom end lightened and and balanced as a later option as I would like to enter some hill climbs/sprints.

Thank you!

[Spider]

Hi and welcome to the forum.

 

Sounds like a fun project and a lucky find on the rust free shell.

 

In regards to your engine build, perhaps a bit more information here might be helpful, eg, what are you seeking as an outcome here / what is your intended use for it ?

 

I'll just add here, if you are contemplating changing pistons, I would advise reboring the block to suit the new pistons.

 

For inlet, I'd suggest running with a single HIF44.

 

Given the other specs of your proposed build, a standard but overhauled MG Metro distributor may not be a bad option here.

[Jonnym71]

Thanks. Initially fast road fun and then looking at sprinting/hillclimbing. I’m not looking for mega horsepower but something that’s torquey and reasonably quick to start and then develop over time.

My last mini, 30 years ago, had a Howley Racing ‘1340’ with a 286 cam and 45DCOE carbs, great down the lanes but awful around town!

[KTS]

sounds like a good project

 

something to bear in mind at this stage is that your choice of camshaft will be the primary factor in determining what the optimal compression ratio should be

 

Also, if you're planning on developing the motor it would be worth considering using a programmable ECU rather than a distributor; the up-front cost would be a little more, but much more flexible when it comes to setting up and/or modifying the ignition timing

 

..and don't forget to give some consideration to how you want to get that power on to the road !  choice of gearbox and final drive ratios can make all the difference..

[Jonnym71]

Thanks for the reply. Eventually it will have a straight cut gears/drops, an lsd or x-pin diff.

The car currently has 7x13 split rims but I have bought some 6x10 revolution rims and a 7.5” brake conversion kit.

I hadn’t thought of an ECU, but it certainly sounds like a good idea.

Long term goal is 100+ bhp, and will use the right components to achieve this.

[cal844]

If you plan on doing hillclimb/ sprints I'd be looking at the rules on the class you wish to run and build it appropriately.

Personally for reliability, ease of maintenance I'd go with a 65D distributor with vacuum advance and a HIF 44 getting them rebuilt and recurved by someone like AC Dodd.

Edited by cal844, 10 June 2025 - 09:08 PM.

[Shooter63]

I'm not sure if this will help, but it gives you an idea of what compression ratio you will end up with, with certain combinations. Personally I would sort the compression ratio out before doing anything else. The 10cc dish piston was the biggest I could find off the shelf apart from the 18cc dish job for turbo engines. Again personally I would machine an exta 2cc into the dish to make things more comfortable allowing a milder cam to be used, but your engine your choice.

Shooter

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[Jonnym71]

If you plan on doing hillclimb/ sprints I'd be looking at the rules on the class you wish to run and build it appropriately.

Personally for reliability, ease of maintenance I'd go with a 65D distributor with vacuum advance and a HIF 44 getting them rebuilt and recurved by someone like AC Dodd.

Thanks, I had a look at the MSA UK refs at lunch time. It’ll be modified road car class. Will need to look St the finer detail as I’ve currently got a bare rolling shell and will need to decide if I need to put things back (and add weight) or leave them off.

[Jonnym71]

I'm not sure if this will help, but it gives you an idea of what compression ratio you will end up with, with certain combinations. Personally I would sort the compression ratio out before doing anything else. The 10cc dish piston was the biggest I could find off the shelf apart from the 18cc dish job for turbo engines. Again personally I would machine an exta 2cc into the dish to make things more comfortable allowing a milder cam to be used, but your engine your choice.

Shooter

Thanks, that’s really helpful. I wonder going up another size will reduce the CR a bit .

[Shooter63]

I'm not sure if this will help, but it gives you an idea of what compression ratio you will end up with, with certain combinations. Personally I would sort the compression ratio out before doing anything else. The 10cc dish piston was the biggest I could find off the shelf apart from the 18cc dish job for turbo engines. Again personally I would machine an exta 2cc into the dish to make things more comfortable allowing a milder cam to be used, but your engine your choice.

Shooter

Thanks, that’s really helpful. I wonder going up another size will reduce the CR a bit .

How do you mean?

Shooter

[Jonnym71]

There’s extra capacity in the bore. Oh hang on, when the piston is tdc it won’t make any difference..

[timmy850]

If the bore is bigger, the “uncompressed” part of the calculation is bigger, and the “compressed” part is the same, which means the compression ratio will get higher

It may work out, rather than having to work the entire engine around your existing head, it might be more efficient to sell that one and start with a different head

[ACDodd]

Match the compression to the cam and rocker combination. Most people use too much static compression. Road cams with smooth idles up to 9.5 to 9.75:1. Cams with lope to the idle 10:1 max. 280 duration cams 10.5:1 max. All using E5 97+ octane fuel.

Ac

[Jonnym71]

Thanks Timmy and AC. I’m thinking to match the cam to the CR, I don’t mind a lumpy cam as I’m planning on building the car up for sprints/track days or hill climbs, but.. if a different head comes up I could go down that route, but it would be a shame as the head is really nice.

[Jonnym71]

Match the compression to the cam and rocker combination. Most people use too much static compression. Road cams with smooth idles up to 9.5 to 9.75:1. Cams with lope to the idle 10:1 max. 280 duration cams 10.5:1 max. All using E5 97+ octane fuel.

Ac

Thanks AC

Is there a spreadsheet/ready reckoner that would suggest what cam would suit a particular CR. I suspect mine is around the 11:1 CR with the current block/pistons (lower compression) and that is probably going to be race cam territory.

I don’t think I’d want to go down the decompression plate route.