40's v. 46's

[juanbenae]

i will be tearing down an early US 3l this winter to build a more race worthy plant for a spec racer. will go all the way down to rod bearing without splitting the case unless condition dictates i do so once apart. it being a spec class i have to use factory CIS cams, pistons, intake with carbs are allowed on the 3l motor. i will go with the higher compression euro version pistons as the class rules allow and redo the heads with some minor porting work.

my question is regarding induction and driving style. my current set up is a never opened 90k mile large port 3l with weber 40's. i tend to not look for speed in the corners by down shifting to lower gear for high rev's that i see in other's competitors video from the same race weekends. i tend to carry speed in a higher gear to keep the chassis somewhat settled and feed throttle as the apex nears. right or wrong this approach is ingrained and im too old to change now... i also believe that is why ive been lucky to this point completing my 3rd race season and many track days on an original motor. many of the guys i see dropping down to get the high revs tend to be running 46's in most cases. from reading wayne's engine rebuilt book & BA's performance handbook i am wondering if the low end benefit of the current 40's more suits my driving style than the bigger 46's might? from what ive read the 46's are going to provide the biggest bump in HP at the high rev end of the power band while not serving the low end my approach dictates. as i stated i am wondering with the driving style ive developed that even post rebuild i might be better served by keeping the 40's i have now to work better in the low end grunt i ask for. i recognize the trade off for this may cost me some at the higher revs but im pretty set on my current approach to to driving the car. this style seems to be a lil less demanding on equipment as well.

looking from some input from the brain trust engine builders here. last couple tidbits- stock gears are required per class rules so i can not utilize gear sets to accommodate my approach. must run pump gas at 91 octane. i am currently 1-2 seconds off the fastest cars in the class that are using electronic injection systems, 3.2 with chipped stock induction & the cars with 46's. granted these are to the best of knowledge much fresher motors than mine already running the euro pistons i will install.

thanks for any insight.

[Steve@Rennsport]

You want 46mm carbs for that engine; simply use the appropriate size venturis to tune the powerband to your preferences.

[TT Oversteer]

I ran 46mm PMO's on my 78SC 3.0 with stock US compression pistons. I did use 66 solex grind cams later but it also ran well with the stock SC grind. It was a very strong running engine, much stronger than a very similar 3.0 that I drove with 40mm PMO carbs. My 3.0 pulled hard to redline and didn't give up anything in the lower revs.