User friendly 2.7L

Mike PCar · 10:39 AM

I’m in the beginning stage of building a 911 2.7L engine and have narrowed down my options. Either CP 9.5:1 Pistons and a DC-30 Cam or Mahle 8.5:1 RS Pistons and an E Cam while using Weber 40's. I have read that the higher compression can cause “sensitivity to detonation or knock sensing” issues that is above my pay grade. I am wanting an engine that is powerful for spirited street driving, but do not want to be constantly tuning the engine. Any advice is appreciated, thank you in advance!

targa72e · 06-24-2025, 07:33 PM

I would recommend the higher compression with 9.5 to 1 but with Mod S cams. I built several early engines (one 2.7 with RS pistons and Mod S cams) and dynoed all of them in the same car. You can see the results in the thread below. Mod S cams were better than E cams everywhere.

https://forums.pelicanparts.com/porsche-911-technical-forum/1151593-dyno-summary-testing-early-2-4l-2-5l.html

john

Mike PCar · 06-26-2025, 05:03 AM

John, thank you for the plethora of information in those links! Do you find that the Higher compression engines result in more attention to the engine to keep running correctly compared to the 8.5 built engines?

rwest · rwest's Garage

I did the 8:5.1 Mahle RS P&Cs and SC cams for my 1977 2.7 build.

I did this on the advice of Steve Weiner who sadly passed away several years ago, but was highly respected as a Porsche mechanic/shop owner.

When I first talked with him I told him I wanted a good street engine and didn’t want to have to rev the piss out of it to get performance.

I’m running EFI on it. Although I still have a ways to go on my tune, it pulls hard from idle to redline and I’m happy with my choices.

targa72e · 06-26-2025, 10:26 AM

The factory published compression rations are usually optimistic. So your 8.5 to 1 is closer to 8:0 to 1. Increasing the compression will give you some more power and make the engine feel more responsive. Compression increase has diminishing returns but the gains from going from 8 to 9.5 are pretty good. See chart.

With 9.5 to 1 you should be able to run single plug on 91 octane .

john

Mike PCar · 06-26-2025, 05:52 PM

Thanks Rutager. I'll look into the SC cams. Sounds like Steve gave good advice. Good luck with the tuning.

Mike PCar · 06-26-2025, 06:04 PM

John, I have read that about the regular Mahle 8.5. I was told that is why the Mahle RS 8.5 is more consistent in being 8.5. Are you referring to both? I have Carillo 9.5 Pistons on order, but it seems that the more I hear about higher compression engines being needy to keep running correctly, the more I shy away from them. I can work on the engine to a point, but I don't have extensive knowledge of them and don't want to feel like I'm constantly fiddling with it to keep it running right. A few local experts indicated that the 8.5RS is more of a user friendly engine that will run for years to come (if not abused). I'd rather have the extra 20-30 hp with the 9.5, but looking for confirmation from users that have experience with the 9.5. I also have to take in consideration of advice given by owners who say they are not anymore needy than lower compression engines, but who also take their engine apart often while racing. I need to keep everything I've read and advice given in perspective from the author who gave it!

silverlock · silverlock's Garage

Don’t over index on compression ratio here. A tremendous amount of this is over exaggerated myth.

Stock US 3.2 Carrera’s were 9.5:1, Euro 10:1, late US SC’s 9.3:1, Euro SC’s 9.5:1, late 60’s 2.2 S’s 9.8:1 and early 70’s S’s were 8.5:1. Many of us (myself included) run a _static_ 10:1 in a single plug application on street gas (91/93) without issue.

Importantly: your dynamic compression ratio is what matters wrt detonation risk, and in practice is going to be well below 8:1 with any moderately spicy cam due to how overlap reduces effective CR.

A very rough estimate of a 2.7L with DC-40 Mod S cams and 9.5:1 SCR pistons would put you closer to ~ 6.3:1 DCR in practice. That is plenty of headroom.

Henry Schmidt · Henry Schmidt's Garage

Mod "S" cams, 9.5:1 compression, 36 mm ports with 40mm Webers and early heat exchangers will produce around 220 hp.
Easy to drive with lots of wow when you hammer the loud pedal.