3.0 SC/ 40 IDA Dyno Tuning

[atelier14]

I had my 3.0L engine again on the MAHA rolling road last week.

The throttle response and the overall performance is now quite satisfying. As you can see from the dyno chart (see only the red line!) the engine has a small 'dent' between 4,300 and 4,600 RPM, which we couldn't eliminate during the dyno session.

We came out at @248 HP at the flywheel at 6,700 RPM with power starting to drop of at 6,900 RPM. Torque figures were at @214 ft-lbs at 4,800 RPM.

The engine is an early 'big' port 3.0L SC with 95mm JE pistons with 10,5:1 (ROZ 100/102 gas is available here at German gas stations), Bosch HKZ single ignition timing set at @30 degrees total, DC60 cams (John Dougherty), 40IDAs (34mm venturis, F3 emulsion tubes, 150 main jets, 170 air correction jets, 60 idle jets, 50 pump jets). The exhaust system consists of Dansk 1 5/8" headers with Dansk dual pipe 'barrow type" exhaust.

We desperately want to eliminate the dent in the curve between 4,300 and 4,600 RPM - what alternative jetting do you think we should try? Any further ideas are appreciated...

Many thanks for your help.
Christian

[lindy 911]

46 Webers

[1QuickS]

Here are some suggestions in no particular order:

1) Try some F7 emulsion tubes, a little fatter on the lower RPM's than the F3s.

2) Check the fit of the auxiliary venturis to the main throttle bodies. I resurface the fuel interface surface on the aux venturi making it flat again; an imperfect seal here (it is metal-to-metal) will cause lost signal to the main circuit.

3) Check if you have the tall aux venturis that they don't rock in their mounts. If they rock (side-to-side) they will not seal properly. You can peen the edges of the rectangular end plates to help re-establish a snug fit and then flatten the fuel interface per the above.

4) Be sure the spring in the aux venturi provides a tight fuel interface

5) Check your exhaust gas for rich/lean at the RPMs in question to provide insight to the issue.

By the way, I run 34mm venturis in my 2.3 using 42mm throttle bores and am VERY happy with it, you might want to try 36mm venturis...

Paul Abbott
Performance Oriented

[jpnovak]

Paul,

Those are some fantastic tips for anyone setting up a pair of webers.

Thank you for sharing.

[1QuickS]

Jamie,

Hopefully the suggestions help;

"Sometimes the magic works and sometimes it doesn't." (from Little Big Man)

Paul Abbott

[lvporschepilot]

Those are great number for a Maha dyno. I love those types of dynos. INcredible amounts of information

[atelier14]

Many thanks so far, especially to Paul.

We have tried 36mm venturis with the 40IDAs but had some problems here with throttle response especially when running partial loads. In addition we only gained 3-4HP at maximum RPM.

We will of course consider your recommendations concerning the emulsion tubes and the aux venturis.

Best,
Christian

[Steve@Rennsport]

That 4500 RPM dip is mostly due to camshaft profile.

One can sometimes compensate a little with exhaust changes and advancing the cams a little bit, but I would recommend trying different exhaust components, first.

[Eagledriver]

I had a similar issue with my carbs. I found that by adding a 1 inch spacer between the heads and intake manifold the power came in about 500 RPM earlier. You need a fuel/air measurement to see if you are lean or rich at the lower RPM as well. I had a local machine shop make the spacers. I've been told by a well respected engine builder that for our 3 liter engines, a longer intake tract will help with low end torque and not lose the high end. That being said your cams also are not optimum for 4000-4500 RPM.

-Andy

[1QuickS]

Some more thoughts:

1) Optimize advance for each 1000 RPM and check that your distributor provides something like that.

2) Check fuel float levels on your engine when running and be sure fuel supply pressure is 3.5 psi at the carbs. Each float must be shimmed individually using the external fuel level gauge to check . If fuel levels are not matched then the main circuits will not be activated simultaneously for all cylinders which could provide a power dip.

[camgrinder]

The DC60 in a 3.0 litre is typically a 4500 rpm and up cam. Looks like the 40 Webers are holding you back above 7000 rpm but giving you a little below the intended rpm range. Are you logging the Air-fuel ratio?

Take a look at this dyno graph
Another RSR Clone Project - Dyno Day

[atelier14]

Thanks Steve, Paul and John.

I know the 40IDAs might be to small for my displacement - but they are what I have at the moment. I am considering to move to MFI with 42mm butterflies and twin ignition next year and not going the 46mm Weber/ PMO route.

During the dyno session we found it strange that going to 36mm venturis made the dip at around 4,5k RPM even larger. As it is right know I do feel quite a lack in acceleration in this RPM range in the car.

Best
Christian

[Steve@Rennsport]

Originally Posted by atelier14

During the dyno session we found it strange that going to 36mm venturis made the dip at around 4,5k RPM even larger.

Totally predictable, especially with those cams

As it is right know I do feel quite a lack in acceleration in this RPM range in the car.

JMHO, but its sounds like you should be using DC40's not the 60's to get the powerband you expect.

[lindy 911]

Or 46 Webers.....

[lvporschepilot]

+1 with Steve. Cams with this kind of duration are going to yield numbers way up in the rev range and really nowhere else.

[camgrinder]

Your results are very good so far. If you convert your numbers and match the dyno graphs with Tom's 3.0 you will see the difference the 40's make over the 46 PMO's.
Drive it like a 911 S. Keep it above 5000 rpm until you get the MFI done.

Steve is right, for a 3500 to 7000 rpm band the DC40 is better.

[1QuickS]

Hi Christian,

You might be interested in my 42mm conversion for your 40s, more breathing without too large a throttle bore.

Paul Abbott
Performance Oriented

[atelier14]

Hi Paul,

I have had my 40IDAs extensively rebuilt very recently but certainly will consider your services for future needs.

Thanks,
Christian

[Jimmy18E]

Christian,

I had a very similar dip/"dent" in my curve, the shop who built my motor tried a number of exhaust combinations (different headers/muffler mods) and was able to straight it out a good bit.

Jimmy

[KTL]

I'd like to add a data point for reference, and maybe get some feedback as well.

My 3.0L single plug Electromotive crankfire 9.8:1 (Mahle) with 46 Webers and ..... uh ..... can't recall the cams at this time, Holcombe equal length headers w/no crossover, was jetted with 55 idles, 160 air correctors, 200 mains, F24 e-tubes, 42mm main venturis, 4.5 aux venturis and typical 50 pump jets.

On the dyno, mine was quite lean at ~ 15.0 and power flattened out at 6000 rpm, peaking at only 201 HP. Power production was disappointing and the bigger concern was the lean condition. Since I was quite high on the main jet scale, I changed out the venturis to 36mm since it appeared the 42's were much too large for my 3.0L. This decision was based on a friend's 3.4L currently using the larger 42mm chokes and smaller mains (180), reference manual (Bob Tomlinson's Weber Tech Manual) venturi size vs. cyl. capacity chart and same friend's former 3.0L used the 36mm venturis with 46 Webers.

My dyno guy (Dynojet) is a drag race & carb specialist and he also indicated that "smaller" carbs are advised in order to pull more fuel since I was already so high in main jet size, yet not getting the rich A/F I need. So with the smaller 36mm chokes, I also changed the e-tubes to F7. Reason being is because I understand the F24 to be for very large mains, which with the smaller chokes I shouldn't need to use?

My Halmeter AF30 doesn't seem to be too reliable because even with a new O2, after a few laps on the track the signal fell back off the chart in the lean zone. I'll get it back on the dyno to confirm where i'm at, but i'm just wondering how I could potentially still be lean at my current configuration (36 chokes, 200 mains, 160 A/C, F7 tubes, 55 idles) compared to what Christian is currently running. My next step would be to take a big swing at the air correctors by going down a few sizes.

Lastly, I found my engine to have a power dip/stumble at 4000-ish whereas it wasn't there before. I'll take the stumble if it means i'm running richer. I just don't want to melt my pistons by running too lean!

What are people's thoughts on the downsizing of the air correctors? Since these are a threefold effect compared to the main jet, smaller air correctors would certainly richen the mixture. But it seems my result would be sort of uncommon based on other people's typical jetting which is smaller mains and larger air correctors.

Thanks,
Kevin