20 replies to this topic

[madaboutcherry]

that's why the 12g940 is USELESS on a road going 998!

[minimender]

Wouldn't say useless, it just moves the power up the rev rage and allows more revs, all low down wheel spinning torque is lost and it uses more fuel, compression is higher though and I have only once pocketed the block to improve airflow round the valve. (due to cost)

 

Port velocity is a lot slower as well.

 

Just a matter of choice I suppose

[tb205gti]

Funny, I have a 12G940 head laying around that I was going to fit on my 998.

 

I started with a 12G202 where I have improved the chambers as per David's guidelines, and I just started working on the ports when I decided it would not flow as good as a stock 12G940.

 

but if the 998 _really_ benefits from the higher gasspeed the smaller ports on the 12G202 has, then I might reconsider that plan.. 

 

It is to be fitted with the following specs.

 

- 2x HS2 with MED filter with stub stacks

- Kent 276 cam

- Maniflow freeflow manifold

- 1.75" Maniflow exhaust with centre exit

- MegaJolt

- Compression: I recon about 10:1 static would be fine with the cam and the MegaJolt? I'm only running Shell VPower 99 with a lead additive. 

 

The car is only used on special occasions during the summer.

 

What do you recon, should i crack on and port the 12G202 head (I have done a lot of heads for more modern cars (with good results), Peugeot 106 Rallye 1.6 (126BHp, 150Nm), Peugeot 205 GTI 1.6 (125BHP 148NM), Peugeot 106 GTI (180BHp 180Nm etc.) but never a mini head.

 

..or should i just fit the 12G940 head to it?

[Turbo Phil]

Bigger ports flow more air, but at the expense of gas speed. Higher gas speed will improve cylinder filling & volumetric efficiency will be higher. The key is to get the port the right size, not to big, so gas sped is slow, & not to small as to strangle the engine at higher rpm's.
So the head needs to be worked to suit the valve sizes, lift & intended rpm.
The 940 ports are to large for a road 998 really, so if you do use one a general tidy of the ports is all that's needed. The 202 ports are quite conservative & these heads can be made to work well, they can also be picked up for next to nothing.

Phil.

[tb205gti]

Yes I understand, but how much will it move the powerband if fitted to a 998 (with the Kent cam, and std. 3 angled valves).

 

Are we talking 1500 rpm, 1000, 500 ?

[jaydee]

With a megajolt you can iron out a lot of the lumpiness of a kent 276 and you can make it run smooth from bottom end straight to the top (if using a 3.44 final drive)

Carbs do make a difference in lumpiness, i'd really reccomend twin HS2, the HS4 is no good for such cam, my small bore on a leccy dizzy strated to pull above 2500 rpm with a 276, although still usable from 1500 rpm, it gave all its best from 4500 to 6500, thats with an HIF44 carb. Now i've changed cams and using twin su's though.