22 replies to this topic

[kyle9832]

I am wondering on the positives and negatives of there two?, basically I am looking at a vmaxxscart head with 35.7 inlet valves and 31 mm exhaust, what I wanted to know was weather I should go for a larger combustion chamber 23cc and flat top pistons or smaller combustion chamber 18cc and dished pistons? if anyone could give me pros and cons of each that would be great:) and with either one should I go larger than 23 or lower that 18? I am planning a very fast road build and this will be paired with 276 cam, wedged crank, roller rockers, twin hs2, maniflow manifold and new exhausts

 

[kyle9832]

any ideas?

[Craig89]

I would personally go with 23cc and flat tops. Reason being that to get 18cc means machining the face of the head making the walls of the water ways smaller and it limits it in terms of future skims

[Cooperman]

It is generally considered that dished pistons are best as they create a better overall combustion shape for optimum squish.

Edited by Cooperman, 04 November 2014 - 08:37 PM.

[kyle9832]

so which should I go for? haha? could you please elaborate on your answers.

[carbon]

Is this for a 1275? The CR with 23cc chambers and flat top pistons would be well over 12:1

 

Unless you build the engine with the pistons well down the bores I think your CR with flat top pistons is going to be too high for a road engine.

[kyle9832]

its a 998

[kyle9832]

and what cc would you recommend then?

[carbon]

With those valve sizes in a 998 you will need to be sculpting the block...

 

You'll need to take this into account when working out the chamber size that gives you the desired CR.

[Cooperman]

You will need to do the compression ratio calculations.

For a 998 it is good to use flat-top pistons. The head you are proposing has far too large valve diameters for best performance.

If using a 12G940 head the optimum valve sizes for a 998 are 33.5/29.5 mm with appropriate gas flowing and skimming for correct C.R. 

Are you boring it out to +0.060" which gives 1060 cc.

By the way, you won't need roller rockers with a 998 using a 940 head. There will be little enough bottom end as it is without losing more from 1.5:1 roller rockers.

You are going to really need to rev that so make sure it will sustain very high revs and fit a low final drive ratio and ideally a close ratio straight cut gearbox.

There will be little torque below about 3300 rpm and peak power will be around 6200 rpm.

The problem on the road will be keeping it on the cam in general driving.

By the way, there is no such thing as a very fast road 998, just a not quite so slow one  .I built a 998 rally historic rally car with a 286 cam, lots of other mods and the right transmission. It gave all of 72 bhp at 6500 rpm and I had to fit a 4.1:1 FDR with SCD CR 'box to make it work at all. On the road it was doing 70 mph at 5000 rpm in top. So although it was OK in its class on really twisty stuff, on the road it was awful and nasty to drive. In the end it was beaten by a Hillman Imp, so we fitted a 970 'S' engine, but it was still all revs & no go!

[jaydee]

Use dished pistons and calculate your CR based on the cc of the dish the HG volume and then you'll know the chamber size.

 

In example

 

Taking a swept volume of 318,75cc for a 1275 (1275/4=318,75)

The static CR is (swept volume + upswept volume) / upswept volume

The Cr you're looking for is 10,5

So upswept volume = swpt volume / CR -1

upswept volume = 318,75/ 9,5

Upswept = 33,55

 

NOW

 

The upswept will be chamber + piston dish + head gasket + piston to deck volume

Say the HG volume is 3cc, piston to deck 1cc, piston dish 7cc, it follows that the chamber needs to be of 22 cc

 

NOTE: take this as an example, you need to do this calculation with the real data.

 

 

Ah and you guys type too fast, just popped out a post saying its a 998 engine, ok but the maths are the same

Edited by jaydee, 04 November 2014 - 10:16 PM.

[Mrpeanut]

You need to look at your compression ratio. My old (low compression 1275 rover cooper engine) had an 18cc head. When I strapped that head to my rebuilt (high compression) 1330 it went bang on the drive back home after being fitted (and filled with 95 ron fuel - it was run in on the rollers with 99).

Its now running about an 23cc head and hasn't gone bang yet!

[Cooperman]

He has a 998

[kyle9832]

Z

Edited by kyle9832, 04 November 2014 - 10:53 PM.

[kyle9832]

Can someone please help me on how to do the compression ratio calculation please! That last post has completly confused me, even better if you could just sum up what i need for flat top pistons, 0.60