3.0 Rebuild

[Winterfarm]

i am in the process of rebuilding my ROW 3.0 CIS motor from an 81 SC. i came across some 40 ida weber carbs that have never been installed, complete w/linkages, original wrappings, etc., so couldn't resist getting them. but this leads me to the problem of whether to change out the pistons to maximize the cam to complement the carbs.

currently i have the 964 cams and SSI with M&K sport muffler (and the 9.8 ROW CIS pistons). i am not looking to build a full race motor, but one for both street and track. the only piston option i'm seeing (besides the ROW CIS pistons i already have) is the JE 9.5 piston. and then using an appropriate cam.

so my question is, am i crazy even to consider not changing the pistons/cams, or is the incremental gain not worth it?? how do the JE pistons perform and hold up compared to those porsche put in the motor originally. i would be interested to hear from those who have built similar motors of either or both types.

thanks!

-Winterfarm

[lindy 911]

From what I understand, the CIS pistons don't do well with carbs. They have a dome shape that is supposed to help swirl the intake charge and atomize the fuel better with the CIS system that is not necessary with the carbs.

For what it's worth, I'm building a 3.0 (78 SC) with 10.5 : 1 JE pistons, GE 60 cams, twin plug and 46mm carbs. The long block is complete and I'm waiting on some bits and pieces to finish it up. The JE pistons are affordable and I've used them in other applications for years with no trouble.

I'll let you know what the dyno numbers are when I get this thing tuned up.

Lindy

[Henry Schmidt]

Originally Posted by Winterfarm

i am in the process of rebuilding my ROW 3.0 CIS motor from an 81 SC. i came across some 40 ida weber carbs that have never been installed, complete w/linkages, original wrappings, etc., so couldn't resist getting them. but this leads me to the problem of whether to change out the pistons to maximize the cam to complement the carbs.

currently i have the 964 cams and SSI with M&K sport muffler (and the 9.8 ROW CIS pistons). i am not looking to build a full race motor, but one for both street and track. the only piston option i'm seeing (besides the ROW CIS pistons i already have) is the JE 9.5 piston. and then using an appropriate cam.

so my question is, am i crazy even to consider not changing the pistons/cams, or is the incremental gain not worth it?? how do the JE pistons perform and hold up compared to those porsche put in the motor originally. i would be interested to hear from those who have built similar motors of either or both types.

thanks!

-Winterfarm

With the configuration you have now, 9.8:1 euro pistons one option would be to have your pistons eye browed. The real problem with CIS pistons and Weber carbs is the lack of piston to valve clearance because of the squish chamber piston dome.
With valve pockets added, cams with a better overlap can be added. The 9.8:1 pistons with valve pockets will still have usable compression. Of course this is a cost saving option not the "best" option.

[euro911sc]

I would not do anything to those 9.8:1 pistons as they are hard to find... at least it took me a while to find mine. You may be able to get a few more $'s out of them for one building a ROW SC for PCA Stock class.

If you are going to use the webbers you should really look at replacing the pistons. the CIS pistons, as stated, really limit your options (present and future).

-Michael

[Winterfarm]

ok, so you all seem to be confirming that i'll be better off with replacing the pistons and cams. is there any hard data on actual increase in output i can expect? will probably go that route anyway.

but, if i'm buying pistons, would i be better off getting the 98mm piston and building it as a 3.2 ss? assuming it was the 9.5 JE piston, not the higher compression 10.5, how would that configuration work with the 40 ida carbs? what cam grind would make sense?

btw, lindy i've looked at your build thread and am interested to see how it turns out. though i don't have plans to go with higher compression and twin plug.

thanks again for the feedback.

-Winterfarm