Raising compression in 2.7 rs spec
 

Hi, is there a method to raise compression on an rs spec 2.7 motor.
I've got pmo's and e cams and it runs really nice, but a bump in cr seems like it would make some hp, but I don't want to replace pistons or twin plug...yet.
Any tricks or do I run 8.5:1 until I do a rebuild with new pistons?
Thanks,
Kevin

We used the J&E pistons 9.5 in our 2.7 with the Dougherty DC-40 cams set at 4.75mm and weber 40's Very nice motor. Don't know of anyway to raise it without some work.

With a LOT of massage work, you can get it to 9:1, but that's about it. You need different pistons to go above that.

Cut the heads by 1mm and add a 1mm spacer between the heads and cam towers to get cam chain geometry back.

While Kenik's suggestion will raise the compression ratio, it brings up another problem. The valve to piston clearance is marginal to start with for this configuration. Shaving the heads 1mm will make this even worse so you'd probably need to cut bigger valve pockets in the pistons (and so lower the compression ratio again). New pistons would be far cheaper and easier than any of mods to raise compression.

-Andy

Thanks for expert advice
 

Thanks all for your expert advice. I had a feeling the answer would start with new pistons.
I like the build on your 2.7 Craig, I'll pm you for more info.
Thanks again,
Kevin

This is what I did on a 2 liter S converted to 2.7 with E cams, RS P/C and 2.7 crank/rods:

Mill heads 1mm (0.040"), true cylinder heights, true cylinder/crankcase spigots.

Trial fit to measure valve-to-piston clearance, then machine piston crowns as needed for adequate clearance.

Trim chain boxes as needed to recenter cam within oil seal opening.

With the above machining, my stack height was reduced such that I had to have larger tensioner idler sprockets made to take up the excess chain slack.

Exact C.R.? Unknown. Factory RS pistons/cylinders are slightly optimistic @ 8.5:1, so I'm probably somewhere between that and 9:1.

Hope this helps,
Sherwood

have similar 2.7rs situation, want more power, but i don't want to open the engine completely -

currently running solex cams, weber 40's (34mm venturis), pmo tall manifolds on 2.7 heads, rs pistons, recurved distributior by steve@rennspot - cams "come on" about 2800 and pulls well to near 7000

[B]update [/B ]I visited John Dougherty Friday (02-24)

he did not recommend DC30's due my low 8.5CR rs p&c's

- ordered a set of E regrinds - $345 on a spare set of 143/144 CIS cams i have - he says this should increase the effective compression due to less overlap and bring my torque curve down about 600rpm

It shouldn't be "anemic" with that cam grind. Assuming all else is good, should be closer to 180 (at the flywheel) and with 15% loss, about 150 at the rear wheels.

Is the engine in good condition? e.g. good compression?
Cams timed correctly?

Sherwood

Regrind that cam to DC30/mod-solex.

Anything with carburetors is not RS spec.

You say the engine is anemic. Was it done by Motormeister or just the cams? They do not have a good reputation.

yes, compression is great (150 all cyls, cold engine - 60*F)

- genuine rs p&c's - 8.5 CR - i am not claiming it is "an oem RS spec engine"

this 2.7 engine was built to run on 91 street pump gas and for occasional vintage racing

cams set at 0.206 on the dial guage according to my old build notes (typ 901/02 spec according to BA's book - early 911S) - apparently too far advanced now that i study it

- degree check in the engine - they appeared to meet solex specs, also measured the lobe lift to be .0445

(the cams are nose-marked .143 & .144 - which are '76 CIS cams i've learned)

- engine is otherwise in fine condition; msd-6, pertronix, magnecors, etc; carbs tuned with a Halmeter - 02 sensors in exhaust

it pulls "good" all the way from 2800-7000; just doesn't have that strong kick-in-pants feel that even my tired '76 911 had

[B]update [/B ]

I visited John Dougherty Friday (02-24)

he did not recommend DC30's due my low 8.5CR rs p&c's

- ordered a set of E regrinds - $345 on a spare set of 143/144 CIS cams i have

- he says this should increase the effective compression due to less overlap and bring my torque curve down about 600rpm

the reason i don't want to further open the engine is that it is an otherwise fine and correctly updated 2.7, and i have two - 3.2's in the shed, and plan to do that conversion eventually

.

[QUOTE=911pcars;6543398]It shouldn't be "anemic" with that cam grind. Assuming all else is good, should be closer to 180 (at the flywheel) and with 15% loss, about 150 at the rear wheels.

I suggest a new set of pistons.

The best way we found to boost the compression and still use the RS spec piston was to modify the chamber.
The peanut chamber (pictured) is not a cheap modification but offers other benefits.
This chamber configuration reduces the propensity to detonate.
http://forums.pelicanparts.com/uploa...1328748485.jpg

Those numbers should be 141-142 or 143-144 otherwise they are a mismatched set.
Raising the compression ratio will typically bring the engine into the powerband sooner.
Have you done a compression test? Curious how low it pumps.

OH NO. Please search "Motormeister" on this site for some very unfortunate information. This sets some very important context.

Sounds like a good plan Larry. Lets see what you have and then decide.

Hard to say which camshaft they are by just the graph. I wonder why the torque dipped just before it started climbing? Do you have a big flat spot when you floor it below 3000 rpm?

930 turbo would have a 930105142 and 143 part number, while the 2.7's start with 911.

MM has been known to regrind the "desired" cam profile on cams that don't have the requisite minimum material. By grinding the base circle to a smaller diameter, they can theoretically create the desired lift and duration from whatever material exists. However, in so doing, that process compromises the resultant rocker arm/valve train geometry and can affect either performance or service life down the line.

Sherwood

thanks for related information share here. Keep posting.!

An E cam will increase compression due to less over lap at the cost of VE. basically move the power curve more to the low end at the expense of the top.

Chris

73 911 E

[B]update [/B ]

I visited John Dougherty Friday (02-24)

he did not recommend DC30's due low 8.5CR rs p&c's

- ordered a set of E regrinds - $345 on a spare set of 143/144 CIS cams i have

- he says this should increase the effective compression due to less overlap and bring my torque curve down about 600rpm

he also says it will run on 87 pump gas

for me - a 'way better solution for street & AX use



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