9 replies to this topic

[Ginge620]

My 998 is currently at the machine shop. Having rebore +40 thou with flat top pistons, new cam bearings etc.

Its having a Kent 266 cam, stage 1 inlet + exhaust and im porting a 12G202 head, keeping the standard valves. Sticking with single HS4 may change to HIF44 later.

 I know the 202 head with drop compression ratio down and i will get it skimmed to compensate although i dont want to go to crazy.

What compression ratio does the standard 998cc A+ HC engine have?

What compression ratio should i aim for with the mods mentioned above? 9.x something i guess

Edited by Ginge620, 03 February 2015 - 11:52 PM.

[Ethel]

10.3:1 but that was with leaded petrol at the pumps. Even so it shouldn't be far off the mark with a longer duration cam.

[Cooperman]

Between 9.7:1 & 10.3:1 should work fine so long as the distributor advance curve is altered to suit (Aldon 'Yellow' distributor should work well).

[Ginge620]

Between 9.7:1 & 10.3:1 should work fine so long as the distributor advance curve is altered to suit (Aldon 'Yellow' distributor should work well).

Thanks, didnt know about the dissy, mines standard as far i know. So you're suggesting one of these http://www.minispare...px|Back to shop

Can you justify the aldon being essential or just as something to get the most out of the setup. Didn't realise the price of these things and well i have a perfectly good one already just a shame its not an aldon yellow type.

Also do you know if the Aldon ones are points or electronic ignition? I have electronic ignition on mine atm so i imagine i could just swop it over?

Edited by Ginge620, 04 February 2015 - 12:38 AM.

[Cooperman]

If you go for a higher compression of over 10:1 the advance curve needs to be slightly retarded from standard to prevent 'pinking' at full throttle through the gears. Either that or you may lose a bit of top end power due to the max. advance being not enough. You don't have to have a different curve, but having one makes the most of the engine's potential. They come with points or solid-state electrics.

[Ginge620]

If you go for a higher compression of over 10:1 the advance curve needs to be slightly retarded from standard to prevent 'pinking' at full throttle through the gears. Either that or you may lose a bit of top end power due to the max. advance being not enough. You don't have to have a different curve, but having one makes the most of the engine's potential. They come with points or solid-state electrics.

Ah right, ill keep that in mind. Thanks

[Shawbags]

looks like your going down the same route as me , it will be interesting to see how your engine performs when it's done , I'm still getting the bits together , is it your first build , Shawbags .

[Spider]

Fuel of course is the first thing that comes in to play for CR, so it depands on what's available to you and it's quality / consistancy of octane.

 

Here's a graph that is a guide - not gospel - only a guide

 

 

The Octane Numbers in that are in RON.

 

Intake Air Temp also plays a part, the hotter it is, the lower the CR needs to be.

 

And as others have touched on, how the Cam is timed in. For these numbers, it is FAR safer and more accurate to work to Dynamic Compression Ratios rather than Static numbers which don't take in to account the Closing Angle of the Intake Valve. It's only after that has closed that Compression starts.

Edited by Moke Spider, 04 February 2015 - 02:32 AM.

[carbon]

As pointed out by Moke Spider, before settling on a particular CR you need to decide what fuel you will be using.

 

Are you going for standard unleaded 95 octane, or premium 99 Octane such as Shell V-Power?

 

If you set the CR too high for the octane then you will need to retard ignition from optimum and this will lose you a lot of power.

[Ginge620]

looks like your going down the same route as me , it will be interesting to see how your engine performs when it's done , I'm still getting the bits together , is it your first build , Shawbags .

Yeah, first full engine rebuild, will be interesting how they turn out.

 

Fuel of course is the first thing that comes in to play for CR, so it depands on what's available to you and it's quality / consistancy of octane.

 

Here's a graph that is a guide - not gospel - only a guide

 

(snip)

 

The Octane Numbers in that are in RON.

 

Intake Air Temp also plays a part, the hotter it is, the lower the CR needs to be.

 

And as others have touched on, how the Cam is timed in. For these numbers, it is FAR safer and more accurate to work to Dynamic Compression Ratios rather than Static numbers which don't take in to account the Closing Angle of the Intake Valve. It's only after that has closed that Compression starts.

Yeah i run 99 from either Tesco or shell. Im not trying to go balls to the walls as such just something ideal.