Camshaft effect on fuel consumption and idle?

[dwomby]

As I have posted in previous threads, I get poor fuel consumption and a Muscle Car sounding idle. My 408 had its engine replaced with one built in 1991 that I have the following info. for. I know it used .030" oversize pistons so it must now have slightly more than 360 Cu In.

1978 Monaco 360 engine.
Mopar Performance Cam 4452761.
Mopar Performance Intake Manifold P4529116.
Piston rings 4.030"
Pistons unknown
Springs 998?

I am attaching the specs I can find comparing the cam that is fitted and the stock 1978 Monaco 360 cam.

I think swapping my cam for a stock one is just about within my abilities but, before I try it, I expect a stock cam would reduce the idle noise and lumpiness but would it also help fuel consumption to any significant degree?

I also wonder if any other type of carburetor (it has a relatively new Summit Racing Carb SUM-M08600VS. 600cfm) would help?

David

[Thor]

Hello. I can't really get into recommending cams as it tends to be highly speculative, but a 408 with a well-built 360 should be fast enough for anybody. It may well be that the carburettor and distributor could be better adjusted for idle and low-load running (which is 99% of all running in a low-drag lightweight car like a Bristol). For every-day road use the cam should be either one of the hotter factory cams, or a very mild street cam. A set-up which might work well would be to use a truck or RV cam and a low-stall torque converter, but who knows unless someone tries it (?).

A Holley Sniper might a be good route towards improving things if you take some time and patience in setting it up carefully.

[dwomby]

Thanks. I think I need to recheck everything in the fuel/air system before I worry about swapping the cam.

David

[Green411]

I think Thor's right in what he says. My own view is that for a car like a bristol the recipe should be effortless torque and tall gearing to take advantage of that torque. Just how bad is your fuel consumption? Have you checked the basics, eg the ignition timing vs the handbook vs what mopar forums says suits the 360? I say that because my 383 ran very flat when I bought it but the timing was as per the main handbook. However I found an addendum that said 5* more advance for chassis numbers including mine. Mopar forums thought even that was too conservative and wedge heads liked to be in the high teens advance at idle with the vac advance disconected and plugged. And lo, it runs a lot better.
The overbore wont make any difference. Forums also say a good carb for normal driving is the Edelbrock AVS - v good fuel atomisation for a carb, giving good driveability, which is what's needed.
Good luck with it!

[dwomby]

Thanks. I'm going trhough everything simple at the top - timing, carb, vac leakes, plugs, compression, etc.. Hopefully I'll find something obvious.


I did set timing back in the spring and fitted limiters to the distributor. With vac plugged the car idled well in neutral at 900rpm with20BTDC timing and at about 750rpm in drive. The limiters are set to 14degrees. The mechanical advance starts at about 1100rpm and reaches 34 (the limiters hold it there, of course) at just before 3000rpm. The car drives well, pulls easily from a standing start, responds well to throttle including enough to kickdown. It also fires right up when restarting the hot engine which has been a problem before. It just seems to use too much fuel - c. 8mpg (Imperial) in country driving.

David

[Green411]

8mpg???? Blimey............When I drove my 411 home from the dealer a fag-packet sum based on the fuel guage suggested 10mpg, which I just couldn't live with, hence investigating Sniper efi as a possible option, and thus this series of posts. But subsequent, more accurate, sums show c.14mpg.
I honestly can't think what to suggest.....fuel leak? Did you get the carb new? If used, was it from someone who drag-raced a 440 or a Hemi and jetted accordingly? Are the plugs sooty? If not, then that might suggest a leak or evaporation?
Maybe you could fit a wide-band oxygen sensor, which would need a controller which could give you some info about air/fuel ratios etc?
In any event, good luck with it! I'm intrigued as to what the problem actually is, so hope to hear more.