Throttle body sizing

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Throttle body sizing

Postby rjwooll » Sun Jul 30, 2017 1:00 pm

My engine (Lotus 912 2.2 with DTA S40) originally used Dellorto DHLA 45s with 37mm chokes. It now has 48mm twin Jenveys and for a while I've been thinking of moving to a smaller size like 40mm. I don't use top end power much if at all (the car is an automatic!) and my main requirement is to improve driveability on light throttle, as well as anything to help mid-range torque. I've already modified the maps to introduce a new 1% throttle position (between 0 and 2%) which has helped a bit. With a large TB such as mine, even a tiny opening of the throttle will allow disproportionately more air into the engine that it would with a smaller body, so I believe a 40mm body would help further. Additionally, as the chokes in the Dellortos were 37mm and they effectively dictated flow (rather than the 45mm barrel size), I shouldn't suffer in performance.

Do you think I'm right? (I already know I'm an oddball!)

Thanks, Richard

Richard
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Re: Throttle body sizing

Postby steveslowboy » Sun Jul 30, 2017 3:26 pm

Would it be worth considering making a plenum chamber with suitably-sized runners and feeding off a large single throttle body - I would think this will calm things down a bit and be smoother lower down.

You could also then implement MAP correction as well as TPS?
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Re: Throttle body sizing

Postby Rob Stevens » Mon Jul 31, 2017 3:16 am

My experience is that plenums just dont work well on a high specific output engine (NA) unless the engine has some sort of cam control.
I suspect the 912 engine makes 160 - 170 bhp and needs a fairly wild cam.
48 mm TB's does sound very large but an expensive trial to bring them to 40 mm but I do think this would help. I'm sure the 48's would sell just fine as I assume they are Weber bolt pattern.

From my experience of my Rover K road car, it made 175 bhp on a plenham and 180 ish on 42mm TB, but that is only half the storey, it drives beautifully on the TB but I hated it on the plenum, the inlet pulses get muddled up somehow on very light throttle and believe me I tried for years to get it to work. So I really don't recommend that.

As Steve suggest moving to TPS + MAP corrections (on the 48's) may help and all you have to do is a little vacuum plumbing, wiring and buy a sensor. But of course would need a re map or if you did a lot of logging (before changing the map) you could probably work out what the fuel numbers need to look like.

If you don't have one now get a wide band lambda.

Also I assume there are no engine issues like air leaks, TB's are balanced and the throttle linkage and cable are as smooth as a smooth thing.
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Re: Throttle body sizing

Postby rjwooll » Mon Jul 31, 2017 8:40 am

Thanks guys, very useful comments. I'm not sure that using MAP on the 48s would do it as the real problem is that the throttle is not sufficiently controllable at very light loads. Compounding this, I have fuel cut off on overrun set so I often find that on a light throttle (steady 40-50mph for example) I'm moving between 0% and 1% throttle, and the car is noticeably 'hunting' as a result. Yes, even 1% is too much sometimes, although it's better than when I was moving between 0% and 2% before I modified the map.

OK I could disable fuel cutoff but this gave me decent economy gains and I do like doing things as efficiently as possible. 40mm TBs would give me better and more controllable throttle at low openings, as the butterfly would need to be open wider to provide equivalent flow to the 48s.

Richard
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Re: Throttle body sizing

Postby Rob Stevens » Mon Jul 31, 2017 9:15 am

I kind of think that if you are getting noticeable mpg improvements with the fuel cut off active it must be massively rich? As I say do you have a wide band?
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Re: Throttle body sizing

Postby rjwooll » Mon Jul 31, 2017 10:48 am

I don't currently although I'd like to get one. Its rather frustrating they didn't fit one or even ask if they should when the system was installed.
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Re: Throttle body sizing

Postby katana » Mon Jul 31, 2017 12:49 pm

You could fit and eccentric throttle pull to give reduced sensitivity just off idle and / or increase the fuel cut low limit to 2% throttle, but tend to agree that you shouldn't be able to cruise on effectively a closed throttle.
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Re: Throttle body sizing

Postby rjwooll » Tue Aug 01, 2017 4:37 am

katana wrote:You could fit and eccentric throttle pull to give reduced sensitivity just off idle and / or increase the fuel cut low limit to 2% throttle, but tend to agree that you shouldn't be able to cruise on effectively a closed throttle.

I did think of a throttle cam (my old 944 had one) but I'd still have the binary transition between 0 and 1% throttle. Also I did try setting the cut off to 2% but it made the car far more jerky as it was transitioning between relatively higher fuelling and zero.
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Re: Throttle body sizing

Postby Rob Stevens » Tue Aug 01, 2017 7:01 am

It isn't a binary transition, there is full interpolation between the 2 sites. However you have made it binary by activating the fuel cut.
Please get a wide band before you go any further :)
Car probably doesn't have a lambda boss, so this is not the work of a moment.
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Re: Throttle body sizing

Postby rjwooll » Wed Aug 02, 2017 7:21 am

Rob Stevens wrote:It isn't a binary transition, there is full interpolation between the 2 sites. However you have made it binary by activating the fuel cut.
Please get a wide band before you go any further :)
Car probably doesn't have a lambda boss, so this is not the work of a moment.

Interesting - I didn't know there was full interpolation applied to the maps when running. I had asked the question before (on this forum I think) but had no answer. In which case, I may disable cut-off on overrun and replace it by zeroing out fuelling in the 0% column of the fuel map down to my cutoff limit - in which case the system should interpolate between 1% and 0% making it less binary.

However this is still a side effect of the size of my throttle bodies.

Yes, I do have a lambda boss on the exhaust that was used when it was set up on the rolling road. It is however in the underbody section just behind the junction of the two downpipes, and points slightly downwards probably due to clearance issues with the tunnel/propshaft, and so I'm concerned about underbody clearance. Can you get L shaped lambda sensors to overcome this problem?

Many thanks Rob

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