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Thread: Stock Delorean throttle bodies - why so big?

  1. #1
    Back to the Omnipresent! Spittybug's Avatar
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    Stock Delorean throttle bodies - why so big?

    I've been EFI'd for several years now and would never go back. When I built my setup I got rid of the stock intake manifold and throttle body and replaced them with a Peugeot one with an old Camaro throttle body mounted to it. Without much thought I made sure that I had a restrictor plate in place to ensure that the larger throttle opening got sized down to be equal with the stock one. Well, the car has never liked to be tuned in the 'conventional' manner that Megasquirt EFI likes to use; manifold vacuum levels. I have used the throttle position sensor instead with great success. (That's alpha-n versus speed density for those of you with a similar setup). I assumed it was me doing something wrong and just went with what worked well.

    While interpreting some of the log files I created while re-tuning things after some post Harvey work I did (new fuel pump, new O2 sensor, etc..) I discovered that my manifold vacuum disappears very quickly - like 15% throttle and boom I go to near atmospheric in the manifold! There are no leaks or other issues at play here, just the amount of air coming in.

    So rather than rely on JZ's throttle size, I pulled up an online calculator and plugged in our engine parameters. It called for a throttle size of 53mm (2185 sqmm). Our stock throttle body is DUAL 47 mms (3470 sqmm). That is a very large opening for a 2.85 liter engine. IIRC there was no restrictor plate of any sort downsteam of the throttle body in the stock setup, right? I can only assume that the huge intake manifold and its convoluted air path through the horns, into the upper chambers and then crossing over to the opposite side intakes must have something to do with the need for such a large bore. Or maybe it doesn't matter because of the use of the air flow meter for fueling rather than manifold vacuum to meter fuel. Maybe some of you old school engine guys can opine? This may well explain why some people have had difficulty getting their cars to run well with EFI when using the stock manifold and throttle body.

    In any case the cure is now visible to me; a much more restrictive plate behind my throttle body will help maintain the vacuum levels in the engine properly and allow me to tune with the preferred method.
    Owen
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  2. #2
    EFI'd dn010's Avatar
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    I would think it doesn't matter because the air is already being metered but I could be wrong. This is why I always thought it was nonsense when I read about the 'new' plastic throttle body spacers "improving performance" by being less restrictive and letting more air pass. I'm running the B280F, maybe I can see how large the throttle of that is tonight for comparison sake.
    Last edited by dn010; 02-12-2019 at 11:46 AM.
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  3. #3
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    A lot of factors come into play. Are you using a throttle body with fuel injection or are you injecting at each intake valve? What is the velocity of the air you want? The throttle body in the stock set-up was sized for an already mixed air-fuel mixture. What is the temperature (density) of the air you are controlling? If you are going to make changes you must re-engineer the whole system to work, not just a few pieces of it. You have to size the throttle body and the mass air flow sensor to the amount of air you intend to control to maintain the proper amount of control.
    David Teitelbaum

  4. #4
    EFI'd dn010's Avatar
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    That's right, I forgot the fact that the B280Fs use MAF sensors so I guess the measurement won't matter. I think I'm the only one running MAF based EFI and it was a PITA trying to find a sensor that would be a good replacement for the B280Fs hot wire bosch maf.
    Last edited by dn010; 02-12-2019 at 12:47 PM.
    -----Dan B.

  5. #5
    Back to the Omnipresent! Spittybug's Avatar
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    Quote Originally Posted by David T View Post
    A lot of factors come into play. Are you using a throttle body with fuel injection or are you injecting at each intake valve? What is the velocity of the air you want? The throttle body in the stock set-up was sized for an already mixed air-fuel mixture. What is the temperature (density) of the air you are controlling? If you are going to make changes you must re-engineer the whole system to work, not just a few pieces of it. You have to size the throttle body and the mass air flow sensor to the amount of air you intend to control to maintain the proper amount of control.
    David, what I think you meant to say to not sound condescending was: "The Delorean was engineered for the whole system to work, not just a few pieces of it. They had to size the throttle body and the mass air flow sensor to the amount of air they intended to control to maintain the proper amount of control."

    I'm sure you didn't intend to sound condescending about how my EFI setup, which doesn't use a MAF sensor and automatically adjusts for incoming air temperature, needs to be setup. It is running very well (as good as stock) using throttle position to control fueling. The fact remains that the engine bore, stroke and max power RPM level determine the necessary air flow when using modern day fuel injection when basing fueling on engine load not throttle position.

    The stock setup indeed mixed the air with fuel prior to going through the throttle body. The Delorean engineering that you refer to takes into account such items as resistance of the metering plate, the long manifold lengths and the fact that the engine vacuum levels were indirectly used in controlling the fuel injection quantity (via the fueling plate, not directly off the manifold). Your lesson on engine design is best left for another day, thanks.
    Owen
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  6. #6
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    I don't mean my posts to sound condescending but sometimes someone can take it that way I guess. When I post, I post not only to the OP but to the general audience. I am not familiar with each person's Delorean especially if they have modified it. Not everyone is familiar with what it takes to install an EFI set up and all of the considerations it takes to get it to work well. Sometimes I restate the obvious. It is obvious to those familiar with the topic, not everyone reading these post is as familiar with the topic as the OP may be.
    David Teitelbaum

  7. #7
    '82 T3 FABombjoy's Avatar
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    The decel valves probably reduce the effective TB size a little.

    What issues have people had tuning with the stock intake?
    Luke S :: 10270 :: 82 Grey 5-Speed :: Single Watercooled T3 .60/.48 :: Borla Exhaust :: MSD Ignition :: MS3X Fully SFI Odd-fire EFI :: DevilsOwn Methanol Injection

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  8. #8
    EFI Squirted DARCOM's Avatar
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    I just finished my new setup with bigger throttle bodys. I'm liking them. They may not be practical for my car but i love the throttle response.
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  9. #9
    Back to the Omnipresent! Spittybug's Avatar
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    Don't you guys with stock intakes and big throttles have problems with vacuum drop off very quickly when you open the throttle? I had a restrictor plate in place to choke down to the equivalent of ~54.3 mm throttle body (stock being dual 47 mm) and I was having the issue that even a 12% throttle opening was making me go to just about atmospheric. Very hard to get fueling/timing right that was using speed density. I've been using alpha-n which listens to the throttle position and it has worked very well. Yesterday I made an incrementally smaller plate using an online calculator and our engine parameters as a guide. It is barely smaller; 53 mm total area, but that was enough for me to get a MUCH better MAP reading at more throttle. I did a quick run in the garage last night, RPM up to our max power range of 5500, and recorded the MAP. Stayed below 60kPa. That was with steady throttle opening, so punching it will of course rise higher.

    This will allow me to give speed density tuning another shot. It is better at cruising ranges than alpha-n and since more people use it and are familiar with it I won't be the exception any longer. I don't know if my Peugeot manifold or the adapter on top somehow affect my setup materially differently than the stock setup, but I know other Delorean guys with stock intakes that have struggled with getting the tip in to not bog, especially with an automatic. I suspect too much air is allowing vacuum to totally disappear and making tuning very hard.
    Owen
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  10. #10
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    Quote Originally Posted by DARCOM View Post
    I just finished my new setup with bigger throttle bodys. I'm liking them. They may not be practical for my car but i love the throttle response.

    That looks fantastic, what throttle bodies are you using?

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