Best 2.5l long stroke heads?

[JKarow08]

Ahoy,

I'm about to rebuild the motor for my '73 911t, planning on doing a 2.5l long stroke build with 86mm 9.5:1 pistons/cylinders, dc30 cams, and weber carbs. The heads on my motor were corroded beyond repair so I am am going to buy a new set of heads for the build. What heads would be the best for a build like this? Would the combustion chamber need to be enlarged to match the 86mm bore if I used 2.4l heads? The heads will obviously be refurbished before use so I don't mind if some reasonable modifications/machining are necessary, but I'm curious what the best bang for the buck is as a starting point. Thanks in advance for the help!

[PeteKz]

What's your budget? If not a big consideration, the Xtreme custom heads are about the best available, and you can have them made to your specs.

[JKarow08]

Originally Posted by PeteKz

What's your budget? If not a big consideration, the Xtreme custom heads are about the best available, and you can have them made to your specs.

Unfortunately budget is definitely a consideration. I actually did a little digging today to see what the xtreme heads were going for and I could only find a 2nd hand listing, and even those were well out of my price range. They are definitely works of art though!

[stownsen914]

If you’re going with carbs, 2.2 T heads could be a good choice because that’s a carb engine so doesn’t have extra ports for injectors, etc. I think the 2.4T was MFI. 2.7 heads have EGR ports to plug.

It wouldn’t be the end of the world to plug injector or EGR ports though, in which case any 2.2-2.7 heads would be a fine choice. S heads from any of those will have larger intake and exhaust ports = better performance. Avoid 2.0 heads - smaller valves and poor combustion chamber shape.

Not a big deal to port though, once you’re sending them to a machine shop.
Smaller bore heads may need to be machined for the larger cylinders, also not a big deal.

Cgarr here on the forum has a good reputation for his machine work and could help with the above.

[Flat6pac]

I’d look for 2.7 S heads as they are already ported factory with in a mm of the early S heads and are readily available. Plugging the air injection is 10mm x 1mm thread.
Bruce

[JKarow08]

Originally Posted by Flat6pac

I’d look for 2.7 S heads as they are already ported factory with in a mm of the early S heads and are readily available. Plugging the air injection is 10mm x 1mm thread.
Bruce

Thanks! My research has been leading me that direction too, it looks like the 2.7s heads are available at reasonable prices and the performance increase from the porting looks to be significant. I read somewhere that the chamfer on the edge of the combustion chamber on 2.7 heads prevented sealing when used on 2.4 cylinders. Is that true? Would that be true with 86mm cylinders too? It looks like I'd want to have 2.4 heads chamfered to match the 86mm bores, but I haven't found if the 90mm chamfer on 2.7 heads is too much. Thanks again for all of the info everyone!

[MoreGAS]

Not every engine needs the most costly components available, especially when schemed for street use w fairly low compression and moderate to mild cams. Best to seek out heads that have not been messed with too much, nice seats still, also good idea on 2.7 heads w stock ports that can be massaged a little, bad porting can kill an engine's performance and seek out heads that have had the least amount cut off them and from memory, roughly they are a thickness of 3.328" when original(I can confirm by measuring a few if you PM). We've seen a few sets recently from reportedly good machinists where the heads are cracked in the valve guide Boss area from poor work. Very costly to fix this. If budget allows - twin plug them, and get compression to 10.6 to 1 range and to help that little 2.5 GO, and look bad ass with a twin plug distributor. For the cost of the Xtreme or similar heads you can buy heads, build them up, and do almost everything noted with your new pistons included.
Any 2.2,2.4, 2.7 head will work and easily get to the 9.5-10.6 range of compression you want. We weld any air injection port solid for strength.

Good luck
Kevin
GAS Motorsport

[240kar]

I’m currently piecing together parts to make a similar engine. I originally purchased a set of 911T mfi heads but found a great deal on a set of 2.7s heads as someone suggested earlier. I think these will do fine for what I’m wanting.

If you want to nerd out talking engine build stuff feel free to pm me.

[JKarow08]

Thanks for thr input everyone! I tried to respond this weekend but it said my post needed mod approval so I'm guessing it will get posted eventually and make some of this redundant. The more I've dug, it looks like the suggestion to use 2.7s heads if possible is the way to go, the increased porting should provide a nice boost in power and they are cheaper and easier to find than 2.4s heads. I read somewhere that the chamfer on the edge of the combustion chambers of the 2.7 heads prevent them from sealing properly on 2.4 (84mm) bores. Is that the case when used on 86mm bores too? If that's the case, it's probably cheaper/easier to have 2.4 heads chamfered to fit the 86mm bores than it is to weld and rechamfer 2.7 heads down? The various mfi/air injection ports seem easy enough to plug so that doesn't seem to be a big factor.

I've considered going up to 10:5:1 compression and going twin plug, but I'm not sure if it's a good return on investment or not. Unfortunately I'm going to sell this car after the resto is complete so I'm trying to weigh what the best build combo is not only for performance, but also on resale. If I was keeping it, I'd go "balls to the wall" as that's how I drive, she'd be a 2.8l, 11:1, twin COP, efi ITB, peak power cammed screamer...but alas, it was not meant to be��

[JKarow08]

Hey guys, thanks for all of the input! I've been trying to respond to this for days but every time I tried to respond it said my response needed to be approved by a moderator, yet they haven't been posted days later. Then my account was locked, guessing my thread questioning if all the mods died had something to do with it�� Anyway...I'm back...I think��

Based on the feedback here and further digging, it definitely looks like 2.7s heads would be the best option for this build. One question I have relates to the combustion chamber chamfer though. I read somewhere that the chamfer on the edge of the 2.7 combustion chamber prevents the heads from sealing properly on 2.4 (84mm) cylinders. Is that true? If so, is that also the case with 86mm cylinders? If so, I'm guessing it's cheaper/easier to chamfer 2.4 heads larger than it is to weld and rechamfer 2.7 heads smaller.

I have considered going the twin plug route and higher compression, I'm just not sure if it makes sense financially for this build. Unfortunately I'm going to sell this car after it is completed, so I'm trying to figure out what engine build will have the best return on investment. If I was keeping it I'd be going "balls to the wall" because that's how I drive... I'd be building a 2.8l, 11:1, twin COP, efi ITB, peak power cammed screamer. But alas, it was not meant to be��

When looking for a set of core heads, are there any easily visible cues I should look for to indicate condition? Thanks again for all of the help!

[911mhutch]

And remember, if you don't want to port, 2.7 S heads, not 1974 2.7 Normal heads with the smaller port sizing.

[JKarow08]

I found a set of nice rebuilt 2.7s heads I'm considering purchasing and I just wanted to confirm...the sealing ring on 86mm cylinders will seal to 2.7 heads? There is a thread on here that states the chamfer on the 2.7 heads won't allow them to seal to 84mm 2.4 cylinders, haven't seen any definitive info on them sealing to 86mm cylinders though. Thanks

[JKarow08]

Originally Posted by JKarow08

I found a set of nice rebuilt 2.7s heads I'm considering purchasing and I just wanted to confirm...the sealing ring on 86mm cylinders will seal to 2.7 heads? There is a thread on here that states the chamfer on the 2.7 heads won't allow them to seal to 84mm 2.4 cylinders, haven't seen any definitive info on them sealing to 86mm cylinders though. Thanks

Nobody knows?

[JKarow08]

Originally Posted by JKarow08

I found a set of nice rebuilt 2.7s heads I'm considering purchasing and I just wanted to confirm...the sealing ring on 86mm cylinders will seal to 2.7 heads? There is a thread on here that states the chamfer on the 2.7 heads won't allow them to seal to 84mm 2.4 cylinders, haven't seen any definitive info on them sealing to 86mm cylinders though. Thanks

Somebody...anybody...help!!!

[JKarow08]

I wanted to circle back on this to clear this up for anyone doing a 2.5l build in the future. After consulting with the Porsche gods (John Walker, Neil Harvey, Henry Schmidt), 2.7 heads will not work for a 2.5l build with 86mm jugs. John noted that there is not enough inner sealing surface with 2.7 heads on 86mm jugs, and it looks like the numbers support that. The OD on the chamfer of 2.7 heads measures around 89mm (saw one at 89.2 and another at 88.8, Henry said he has seen them as low as 88mm), while the ID of the CE groove on AA 86mm cylinders is 91mm, leaving only 1mm of inner sealing surface.

More importantly, Henry noted that with the chamfer not matching the 86mm bore there is an exposed hard edge at the top of the cylinder, leading to combustion/detonation issues.

Henry did mention that early 2.7 heads (74 RS) had no chamfer and an 86mm combustion chamber so they would be perfect, but obviously rare. 5-74 dated heads did have the chamfer so they had already made the change by that point.

So it looks like 2.4s heads are the best option for a 2.5l build with 86mm bores, but they should have an 86mm chamfer machined onto them or you may encounter piston clearance issues. I would assume the exposed hard edge on unchamfered 2.4 heads may also lead to detonation issues on high compression builds.

Hope that helps somebody in the future!