46 replies to this topic

[minidave54]

Hi guys, engine modification time again ready for the summer.

 

I have a 1995 Sprite SPi which i am in the process of converting to carb. The engine as standard only produces in the region of 53bhp and has a low compression ratio of 9.4:1

 

The engine is out of the car and the head has been removed. The identifying number on the piston is 21962. Looking this number up it quotes as standard bore, with a low compression piston with an 8cc dish.

 

Modifications will involve fitting a better head (Mini Spares MSE4) which has larger inlet valves and is flowed with a combustion chamber of 21.4cc.

Uprated camshaft (Mini Spares Evolution 001)

 

 

My question: the pistons do not come to the top of the bores, is it possible to increase the compression by skimming down the block.

 

cheers Dave

[Turbo Phil]

Yes you can deck the block, but surely it would be far easier to get a head with the correct chamber size in the first place ?

Phil.

[dotmatrix]

but if you allready have a compression of 9.4:1 isn't that good enough?

[ACDodd]

Leave it as is.

Ac

[minidave54]

Leave it as is.

Ac

Thanks for the reply, this had also crossed my mind. The reason I was looking to increase compression is to improve bhp and performance.

 

I will be putting the block to an engineering company to assess if it needs a hone or a re-bore, (a warm engine compression check gave readings of 185;185;200;190)

As I mentioned the pistons are 29162, is it still possible to get new piston rings for these.

 

Would you be able to say what sort of improvement I could expect to get with the modifications I have mentioned.

 

cheers Dave

[ACDodd]

I think the rings are the same at the 21253 piston R41570 from memory. Increasing the CR above what the modern fuel can tolerate actually reduces power.
My top of the line road spec uses 9.9:1 Cr and gets more than 100hp.

Ac

[minidave54]

I think the rings are the same at the 21253 piston R41570 from memory. Increasing the CR above what the modern fuel can tolerate actually reduces power.
My top of the line road spec uses 9.9:1 Cr and gets more than 100hp.

Ac

Thanks again for the information.

 

With keeping my compression as standard at 9.4:1 and with the upgrades i'm guessing I could be looking at between 70-80 bhp. does that sound about right?

 

If the bhp figures are about right, would there be any benefits in fitting a Duplex timing chain and a Cross pin diff?

 

Dave

[ACDodd]

Always fit a cross pin diff if you like to spin the wheels!

No need for a duplex just fit a high quality iwis simplex chain. If it's ok it shod do about 85hp on 1.3:1 rockers. It will need a stage 1 kit an HIF44 and a recurved dizzy to get the best from it.

Ac

[Earwax]

a duplex chain is for for peace of mind- one with  iwis chain

Edited by Earwax, 30 December 2019 - 10:51 PM.

[Cooperman]

Our lovely old A-Series engine has always responded well to high compression ratios.

 

However, as has been pointed out, the days of being able to buy 101 RON unleaded petrol at your local petrol station have long gone.

If you wish to run a relatively high CR you need do two things:

 

1.Always run 97 or 98 RON petrol and add a combined lead replacement and octane booster (typically 'ValveMaster Plus'). 

2. Have the distributor re-curved to prevent the ignition timing from being 'over-advanced' as the revs build up at full throttle.

 

For a normal road car this is really not necessary and I normally don't go over 10:1 for a road car. Even at 10:1 it is best to fit a dizzy with an adjusted advance curve. The Aldon 'Yellow' is a good choice for that. For really high compression ratios in full-on competition engines where CR's of over 11:1 are not unusual, a fully customised distributor is needed with a high dose of octane booster every time refuelling takes place.

 

Octane booster/lead replacement is not cheap and when added to the price of 98 RON petrol it makes high CR's an expensive luxury for normal road cars. In normal driving the difference between 9.5:1 and 10.5:1 will really not be noticed so long as the engine is nicely set-up and the 'right bits' are fitted.

 

As above, make sure the exhaust and induction systems are as good as they can be. Fit an HIF44 (or even twin HS4's), a good exhaust manifold and a 1.75" exhaust system. Match the components and then have a rolling road session.

[minidave54]

Thank's eveybody for there input. This is the spec and components of the engine i am thinking of building.

 

Head; (Mini Spares MSE4). flowed, valves 35.6mm x 29mm. Combustion chamber 21.4cc

 

Camshaft; (Mini Spares Evolution001)

Seat duration In 257 Ext 252.

Cam lift In 264 Ext 289.

Valve lift In 327 Ext 358

Timed @107 deg

 

Block; As suggested by AC Dodd I will leave as standard 1275, CR 9.4:1 and 8cc dish. It will just get a hone and new rings

 

Distributor; Lucas 59D Electronic

 

Coil; Lucas Sport GCL105

 

Carburettor; HIF44

 

Exhaust: Maniflow manifold and twin box 1.75" system.

 

I am hoping all that I have mentioned is compatible. I welcome all comments If anything looks out of place or can be improved

 

Dave 

[Cooperman]

That will give you a nice friendly car.

[minidave54]

Thanks Cooperman, I certainly hope so. With that spec would anybody have any idea what bhp I might be getting.

[Cooperman]

Don't be too bothered about the max bhp figure.

For a road engine it is good torque which is needed.

That specification should give really good torque from around 2800 rpm right up to around 6000 rpm.

If it was my engine I think I would have got to around 9.75 to 10:1 CR, but you'll be fine with 9.4:1.

Make sure the cam is accurately timed in with offset woodruff keys and that the manifolds, carb and head are all nicely matched with no 'steps' in the interfaces.

For really good acceleration I would be fitting a 3.4:1 final drive ratio which will give about 16.5 mph/1000 rpm in top when cruising, so 70 mph will be at just over 4000 rpm which is fine for the A-Series engine.

[minidave54]

I have heard back from the engineering company carrying out the cylinder block work. Unfortunately it will need more than a hone, so will be getting a re-bore. The thing is I have been offered a used but good set of AE 21251 +40 FTF pistons. I know this would take the engine up to 1310cc, missing out one re-bore option. I don't think this would made a huge difference to the spec already mentioned, just a slightly bigger engine.

What are peoples thoughts ? Are these a good and reliable pistons.