Convert My 2.7 to 2.8 10.5:1
 

If I can, convert my 2.7 CIS to a 2.8 using Mahle Barrels and Cylinders (10.5:1)

I still am yet to get my heads machined, I guess 36mm

I have already invested in GE60's, webers, ssi's etc... My car wll not for racing at this point, it's a stock looking 74 911, but a bad ass street rod under the rear lid

1. Do I have to go to twin plug set up if i go with the 2.8 configuration (I assume if my compression is at 10.5:1 it will need twin plug and may take the mag case to the limit)

2. What possible modifications could I do to bring the comp down to around say 9.8:1 (like deck height) to make it a little more streetable?

Feedback apprciated

Matt

You might want to take a look at this related thread. You might be pleasantly surprised, but you should verify the parts you have:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/306508-dome-volume.html

Sherwood

matty74

I have a 75 Carrera with a 2.8L engine. It was done by the previous owner but I did rebuild it. I have 38MM intake and 46 Weber Carbs. I runs fine and I use it for street and DE sometimes. I am still working on the fine tuning. it runs a little rich and I'm thinking to go with a MSD ignition and get rid of the points. still researching that idea but I think its what it needs. Just make sure to add an external oil cooler (something other than the stock trombone), it will be much happier.

10.5:1 compression is overkill even with twin plug in a street engine.
9.5:1 compression in a 2.8 twin plug on street gas with DC60 cams, SSI and 40mm Weber or PMOs and you get 240 real hp. Just about the max for a mag case that you want to live.

2.7 40mm Webers, SSI, 10.0:1 comp., 906 cams 38mm port twin plug. 239 hp street gas,
http://forums.pelicanparts.com/uploa...1160195482.jpg
2.8 SS slide valve, twin plug, 9.5:1comp., RSR sprint cams, 40mm ports, 1 5/8 headers, 300+ hp street gas.
http://forums.pelicanparts.com/uploa...1160195759.jpg

Re: Convert My 2.7 to 2.8 10.5:1
 

Hi Matt:

1) IMHO, you certainly do need twin-ignition when using the small-chamber 2.4-2.7 heads with the Mahle 92mm RSR P/C's unless you run 105 octane race gas. Anything over 9.8:1 needs dual-ignition.

2) These P/C's come in at 11.1-11.3:1 with these heads so the most you can reduce the CR would be to 10.25:1 and that requires some judicious machine work on the head to achieve that. Remember now, reducing deck heights make an engine MORE prone to detonate; you'll find more info on this in a recent thread about this subject.

No real-world shortcuts here but if its done right, these things really perform very well. :)

Thanks for the info fella's, very helpful, you have helped making my decision much clearer.

I think at this stage I'm going to stay with 2.7 9.8:1 CR with JE's, dual plug is a bit beyond the budget, I think that an RSR spec build may be the build I'll do after this car.