SOHC carburation part 2

[James Bowen]

Hello Guy,

I know you¢ž¢ll probably remember but‚¦.

I currently have a 1498cc SOHC engine in a Fiat X1/9. It is running a cylinder head that has standard 36mm inlet and 33mm exhaust valves. The seats have been 3 angle cut, and the throats opened out accordingly, though there is still a little margin for further opening, as the valves aren¢ž¢t seated right out to their edges. The ports are smoothed and straightened, as per GC directions on this site.

The Camshaft is a 286 duration with a LCA of 106 deg and 10.75mm lift minus clearance.

The engine runs a compression ratio of 10.8 : 1

The exhaust has 4 -1 headers 32‚ long primaries that are 1 3/8 int dia, and 2‚ out dia secondary pipe with a George Polley AX810 silencer.

I have twin Weber 40 DCNF¢ž¢s, which are producing reasonable power, and a decent torque spread right from 4,000 rpm to 7,000 rpm.

But I haven¢ž¢t heard of them managing to produce more than 135 ¢‚¬Å“ 140hp in any SOHC engine. I want to improve the situation with the current engine, but‚¦‚¦I also want to …œFuture proof‚. I have plans to eventually build another engine, that will utilise a big valve head. (40mm inlets 35mm exhausts) and slightly larger primary bore exhaust (again as per GC advice on this site) The cam and compression ratio will stay the same.

Therefore I am interested in replacing my current set up of twin DCNF 40 weber carburettors (see thread viewtopic.php?t=647&postdays=0&postorde ... on&start=0) with some Dellorto DHLA 40¢ž¢s.

The idea is to get better cylinder filling by having a straighter and longer inlet tract. Thereby improving torque, and high rpm cylinder filling for more power. I want to improve power on the existing set up, but with half a mind to being able to use it on the next engine incarnation.

Is this feasible?

If it is‚¦‚¦‚¦ What jetting am I going to need? The reason for asking, is that I already have a set of servicable DHLA 40¢ž¢s but I think the jetting is more for a 2 litre.

For info. Currently fitted to the DCNF¢ž¢s are 34mm chokes
135 mains
170 air correctors
F24 emulsion tubes
50 idle jets
All as per extensive testing, and advice from GC‚¦ many thanks.

The DHLA¢ž¢s have (for interest) currently fitted;
32mm chokes
140 mains
160 air correctors
7772.5 emultion tubes
50 idle jets
7850.3 idle airs

I¢ž¢d appreciate what you have to say on the matter Guy, and anyone else with experience to share. The car is used for Sprints and trackdays. But also some road use to get to the events.

Very many thanks, James

PS attached pic of my car doing the Brighton Speed trials this year. Recorded a 15.97 with a fluffed 3rd to 4th gear change. I coasted over the line doing 73mph rather than the previous runs 88mph!! Cost me the Handicap class 2nd place spot I reckon, but there¢ž¢s always next year.

[Guy Croft]

MODEL POST!

Clear, concise, straight to the point in well laid-out way and a nice photo too! I do appreciate the way you have thoughtfully summarised the current setup - as you know it helps me no end.

You wrote:

Currently fitted Weber 40 DCNFs
34mm chokes
135 mains
170 air correctors
F24 emulsion tubes
50 idle jets
All as per extensive testing, and advice from GC‚¦ many thanks.

My recommendations for Dellorto 40 DHLAs:
8383.32 (32mm) chokes - GO TO 34MM
7484.140 mains - GO TO 135 TO START WITH
7484.160 air correctors GO TO 190 TO START WITH
7772.5 emulsion tubes - MIGHT BE OK, IF TROUBLE PICKING UP IN MID RANGE GO TO RICHER .6
7644.50 idle jets OK
7850.3 idle airs OK

You know the jetting regime by now, I cannot offer clsoer recommendations on mains and air correctors, sorry. Good RR proveout reqd!

Pump jet wants to be 7851.40. Go over the carbs carefully and make sure all galleries are clean, esp pump jet bypass in lower housing. Make sure the throttles open easily and close properly. Always use an OE Dellorto overhaul kit with new needles and pump diaphrams etc. You can get an exploded view of the carbs from the sole supplier Eurocarb in Reading.

Certainly will be more powerful on sidedrafts, the 90 degree turn with downdrafts does nothing for airflow. As for being better because the inlet is longer, moot point. Typically, for same engine with same cam timing and header configuration the lower end torque will be higher with longer inlet manifold but lower at the top end.

GC

[James Bowen]

Excellent. As always many thanks Guy.

With regard to the future development of the engine and improved head flow, I imagine these carbs will do, but will I be better off looking at 45 dcoe's with say 36mm chokes?

I'll keep you posted of developments.

Regards, James

[Guy Croft]

James, hi

I not sure that I would, but only because I have never done it on that motor. I figure the change to DCOE at 40 size will make a big difference alone. There might be a bit of a hole in the bottom end torque curve on 45s..

GC

[James Bowen]

OK understood. Many thanks,

Regards, James