16 replies to this topic

[Willpill20]

Hi , I am wanting to turbocharge my 998 a plus engine. I want to use speical components fuel injection kit . I was gonna use the gt17 turbo and the kc500 cam . Do I need to do any mods to my crankshaft ? I have got flat top pistons , my pistons are plus 60 tho too , do I need a decommission plate ? what head should I use , I was thinking a 12g295 that has never been skimmed down . A lot of people use the 12g940 head . What about using special components 7 port head with twin throttle body’s ?
Let me know what is best .
Thanks

[Ethel]

Crankshaft mods would depend on power and, more so, rpm you're aiming for, but generally nothing special is required for forced induction just good standard spec. I'm sure you could make either head work, but the 295 could be less problematic - gaskets & valve clearance. 7 port head would require a lot of custom plumbing, but would allow easier multipoint fuel injection. Dunno if it's been done on a small bore.

[miniturbo01]

Hi willpill20, I am currently building a turbo lump 1275, high specs lined oversize pistons omega, wedged crank, dry decked etc. I'm just about to put the same cam in mine so will be interesting, lots of info from Turbo Phill on you-tube, he recommends the 295 great head with big chambers to get your cr down, ditch the flat top pistons you need a big dish, to much compression and you will get detonation and wreck your engine, don't worry about porting your head as is forced induction, crankshaft should be fine just get crack tested, and you could get it balanced that would help and engine. good luck with your build let me know how you get on, enjoy the booost............

[Willpill20]

So if I just had the 295 head that hasn’t been skimmed would be ok with flat top pistons for a turbo ? I will still use the special components fuel injection kit but for the standard 5 port head . Is the turbo and camshaft that I’m after ok for this ?

[MiniMadRacer]

The turbo guys here will know a lot more, but you need to calc and measure your CR properly, or you can destroy a turbo engine in a very short space of time...dizzy will need re curving too, and I am pretty convinced trial and error with heads CAMS and timing is going to cost a lot of money

[Ethel]

The flat top pistons original to the A+ are good, again it depends on what you're after. Without doing any sums, you should get a useable CR for a sensible road engine, say 8.5 to 1 as a rough guide.

 

I'm not clued up on the specs of a lot of cams, but for forced induction generally shorter duration & wider lobe centres are the way to go.

 

Engine management helps, particularly accurate & tuneable ignition timing - a turbo adds an extra dimension that a dizzy can't really measure.

[Willpill20]

Am I able to make the dish on the pistons I have now ? I can’t get rid of these as my dad bought them for me and he will go nuts if I sold them . If not then I’m gonna have to go for a supercharger instead.
Thanks

[sonscar]

If I had the budget for a seven port head it would be my only choice.I have no experience of these heads or turbo minis but starting with an injector per cylinder seems like the right way to start.Of course others are much more knowledgeable and other paths better trod.Have ( expensive) fun.Steve..

[Ethel]

Am I able to make the dish on the pistons I have now ? I can’t get rid of these as my dad bought them for me and he will go nuts if I sold them . If not then I’m gonna have to go for a supercharger instead.
Thanks

Doubt you'll need a dish, more metal on the crown is a good thing for forced induction. Do some sums & approximate the head volume you require.

 

Plenty of specs available:

 

CAM4180 is 25.5cc standard

12G295 is 28.3

 

998 "E" engine had a 10.3:1 CR with CAM4180 & flat top pistons

 

that looks like roughly 10% without any calcs.

[Willpill20]

Ok seem good so far then , only problem... ...how do you calculate cr ?

[Willpill20]

Also is that the cc of a standard unskimmed 12g295 head ?

[Ethel]

Yes standard head, you could get a few more cc's with some head grinding.

 

Add the swept volume (bore x stroke to the combustion chamber volume (in everything: head, gasket, piston dish, ring land) and divide it by the same combustion chamber volume. 

 

To estimate you can work backwards with the standard 998 ...

 

(249.5 + x)/x = 10.3

249.5/ x + 1 = 10.3

 

249.5/ x = 9.3

 

249.5/9.3 = x

 

x = 26.88

 

26.88-25.5 = 1.38cc to add 28.3  

 

1.38 sounds small but  it will be consistent whatever the reason - I've found myself quoting 24.5 for a CAM4180, 25.5 is what google said

[Ethel]

260.75 + 29.63 / 29.63 = 9.8    (Knew there was a reason we tend to keep turbo overbores modest)

 

260.75 + x / x = 8.5

 

 

x= 34.76

 

Sounds like you could be looking for 6 or 7 cc's, probably half of that in the gasket etc. Grinding 3cc's out of the chamber doesn't sound impossible, see what Turbo Phil & Co have to say.

[Willpill20]

Ok I’m a bit confused with all this cr ratio and stuff but if think it’s all good then I will go with it , I will talk to turbo Phil and see what he thinks. so if understand it properly, with my plus 60 flat top pistons and the 12g295 head , I need to grind off 3cc from the chambers ? Correct me if I’m wrong

[Ethel]

I'm just guessing at what's likely to be possible.