best heads for 2.7rs build... 2.4s or 2.7s ?

[whackit]

hi all,

i'm rebuilding a 2.7 motor to rs ish spec. 8.5:1 euro pistons, e cam, pmo carbs, building a good street motor with broadband power. i have access to 2.7s cis heads (35/35 ports) or 2.4s mfi heads (36/35 ports) my tendency is to use the 2.7 heads but others are suggeting the 2.4 heads. any opinions? does it really matter? i know that the combustion chamber dome is narrower in the 2.7 heads. does this matter? i don't need to squeeze the motor for every ounce of power either so consider that too.

thanks

[Bill Verburg]

The 2.4S head is the same as was used on the 2.7RS, I would use that.

[pwd72s]

I read a post saying that the original 2.4S heads have become NLA...but "T" heads can be machined to "S" heads specs.

[whackit]

Originally Posted by Bill Verburg

The 2.4S head is the same as was used on the 2.7RS, I would use that.

Are you saying it was the actual 2.4 heads? The ones where the dome in the combustion chamber is 84mm diameter? Or is it just that they had the 36/35 ports?

[Bill Verburg]

Originally Posted by whackit

Are you saying it was the actual 2.4 heads? The ones where the dome in the combustion chamber is 84mm diameter? Or is it just that they had the 36/35 ports?

The actual head is the exact same part.

Casting, ports, valves, combustion chamber config etc are the same.

[whackit]

any second opinions before i stick these on my motor?

[Eagledriver]

I think you need to have the edges of the 2.4 heads chamfered for the bigger cylinders so your pistons don't hit the heads. Talk to a Porsche machine shop about this. The parts are the same but I think the final machining of the mating surfaces is different.

-Andy

[Jay Laifman]

Just curious, if one takes CIS heads and puts carbs on, how are the injection ports plugged?

[whackit]

PMO actually makes special manifolds that mate up with the cis ports. not sure how they work but richard parr is a pretty smart guy! If using MFI heads a simple plug is all that's needed.

[Bill Verburg]

Originally Posted by Eagledriver

I think you need to have the edges of the 2.4 heads chamfered for the bigger cylinders so your pistons don't hit the heads. Talk to a Porsche machine shop about this. The parts are the same but I think the final machining of the mating surfaces is different.

-Andy

nope, 3 -> 3.2 yes, 3.2 -> 3.4 yes, 2.4 -> 2.9 yes

[whackit]

Originally Posted by Bill Verburg

2.4 -> 2.9 yes

that's 2.4 -> 2.9 no. correct?

There are physical different part numbers for the heads. I'm not super concerned about clearances since i'm running euro 8.5.1 pistons. I was just concerned about the combustion chamber shape. The 90mm heads have that slight chamfer spoken of wheras the 2.4 heads do not. I'm guessing this has no effect but again. not sure.

[Bill Verburg]

Originally Posted by whackit

that's 2.4 -> 2.9 no. correct?

2.4 -> 2.9 w/ stock crank and 93mm 9.4:1 Mahle wants a head chamfer no head gasket

2.4 -> 2.7 RS w/ stock crank and 90mm 10.3:1 Mahle, CE- ring head gasket, no chamfer needed

[RoninLB]

just another data point on a street 2.7

I use stock 35/35 heads. It pulls strong in 5th from 4.5-5.5k. No 5th gear data above that.

It'll also run clean at 3k in 5th for highway driving.

Heads were tweaked by Competition Engineering.

[whackit]

Originally Posted by RoninLB

just another data point on a street 2.7

I use stock 35/35 heads. It pulls strong in 5th from 4.5-5.5k. No 5th gear data above that.

It'll also run clean at 3k in 5th for highway driving.

Heads were tweaked by Competition Engineering.

sounds like you are building close to the same motor as me. What cam are you using. and when you say stock heads I assume you mean stock S heads with the 35/35 ports

[Ole Petter]

Originally Posted by Bill Verburg

2.4 -> 2.9 w/ stock crank and 93mm 9.4:1 Mahle wants a head chamfer no head gasket

Please expain this, do you chamfer the head to fit without head gaskets? What CR will this bring with 9.4:1 Mahle pistons?

Ole

[jluetjen]

When I read this thread, I had this sudden rush of deja-vu all over again. The engine as spec'd should provide a nice wide power band, although at higher RPM it will be leaving something on the table. As near as I can tell, the E cams really don't have enough lift (given the valve size) for the 2.7. I detail why I believe that on this thread.

[whackit]

I was mostly dealing with the fittment of the two heads that I have available to me in this thread. The cam grind is a bit up in the air still. John Dougherty is working on that.

[RoninLB]

I measure power in EGTs with a 6 probe computer.

jluetjen's premise is right on.

[Bobboloo]

Since your using carbs and E ish cams I don't see any advantage in using the 2.4S heads. The carbs don't need the MFI port on the 2.4S heads and since your going with a less radical than S cam you'd be better off with the 35mm ports. They're a closer match to the cam your choosing.

Since I see no advantage to using the 2.4S heads, in your application, I'd sell them because they are worth a lot more than 2.7S heads in the used parts market. I'd take that money and use it elsewhere.

That's just my 2 cents.

[shbop]

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