injection manifold

[Badger]

Howdy, my Gemini is using a piazza turbo manifold with a fabricated plenum chamber with injection system from 1.8L astra/pulsar (delco computer). It works well, but the runners are really too short for n/a engine. I was wondering if anyone knew of another injection manifold which has longer runners and fits the gem head? I was thinking about rodeo manifold, but I don't really know much about these.

ps how much difference would accurate port matching make in my setup?

[MatMan]

It is impossible to tell how much difference port-matching will make, but it will always be an improvement. Every engine is different in that regard, and by that I don't mean a G161 opposed to an SR20DET, I mean that every G161 will be diffent to every other G161.

Why do you say the runners are to short? Do you want more torque down low? If yes, than longer runners may help, but tecniquly, shorter runners should be better for response and mid to top end power. Probably doesn't really help you but hey, FYI )

[Badger]

thanks MatMan, the guy I bought it from said that the next thing he was going to do was find a manifold with longer runners to get more torque. being a rally car, torque is king! I am not planning to actually do anything for some time, but I think that when I do, I will go to throttle bodies with long trumpets, giving good throttle response and torque. the reason that I asked about port matching was because no special effort was made to do this, but there is probably not too much of a mismatch.

I was after an idea of any cars which match the gemini intake pattern and the manifolds could be used, for future reference

[Ben Wight]

Hey Badger,

If you are going to go with quad throttle bodies and trumpets, then the manifold is going to be a lot shorter than your current one, (unless you build a custom one like a VK commodore six-cyl, etc).

I would look at it another way though, whatever sort of racing you are going to do, you are going to be revving the engine higher than normal, so a short runner manifold will increase engine torque at higher rpm, which I bet would propel you around the rally track quicker.

Ben Wight

[Badger]

1. with throttle bodies you can add intake trumpets to make the runners longer ( doesn't matter that they are before the throttle)
2. My engine is rev limited to 7000 rpm and has a cam to suit. I am not sure whether there is a serious problem with revving the gem engine higher ( conrods breaking etc) I think my bottom end is pretty standard, ported early ( big valve) head. I only bought the car a few weeks ago, and I don't know all about it yet...
3. in rallying you don't know what the next corner is like, so you have to be prepared to wash off speed quickly, and you often find yourself doing lower rpm than you expected - need lots of low down torque...
4. I only have std ( diesel - is this different ratios to petrol?) 5 speed, so ratios are not close enough for a narrow peak power screamer engine
5. torque curve is currently very flat, according to dyno at rear wheels, 3rd gear we have: 2500 - 118Nm, 3600 - 138Nm, 4800 - 136Nm, 6400 - 118Nm, peak torque at 3600, peak power at 6400 - 79kW. Now that I look at dyno sheet, it seems pretty good.
6. while more torque to high rpm means more peak power, this only makes you faster if you can keep the engine revs at this level all the time (see point 4). lots of people tune their cars this way, so they can bragg about how much peak power they are putting out, but it is faster to have a higher _average_ power over the used rev range - case in point is my 92 pulsar. Nissan tuned the engine to have a high peak power ( 82kW - 1.6 L engine) as this is good for bragging, but low average power and torque down low ( to give good fuel economy - also good for bragging). this means that if you want to accelerate quickly, you have to slip the clutch heaps to keep it in the high rev range and then change on the red line, even then you need to slip a bit on the changes...

[Ben Wight]

Yeah sorry, I forgot about the trumpets being part of the intake.

The reason I mentioned revvinh high was that in the rallies I have done where they have been organised, you are aware of what corners are coming up, and I recall always being in high rpm, at least over say 3000 all of the time. If I dropped below that, I downchanged. But I spse this would be very dependant on ones engine characteristics and driveline ratios.

Ben Wight

[Badger]

the whole point of rallying is that you don't know 100% what is around the corner. even with pacenotes (only ARC level) when you know how sharp each corner is, you don't know the exact layout of the corner, and you may get halfway in and find that it is really slippery, more than you expected. in lower levels ( what I do) the road book only has intersections and unexpected/dangerous corners listed, so you ahve to drive by the seat of your pants
that is why rally is harder and requires more skill and faster reactions than circuit racing. when I refer to high rpm, I mean above 5000. around 3500-4500 is where you want the torque (as you don't really want to downchange in the middle of a corner)

[Ben Wight]

I understand your point, we have different driving styles, thats all.

BUT, the RPM range you are talking about, say 3-5000 rpm is where the short runner manifolds and the like are designed to make their torque in factory applications. The very long runner intake manifolds such as those on current falcons, old falcons (XF efi), VK comm efi, etc, are all designed to improve torque at really low, caravan pulling, off idle rpm, like 1500-3000 rpm.

Of course, you build your engine the way you want to, I just feel that a manifold swap to a longer runner setup wont give you the results you say you are after. Then again, i read an article ages ago about trying different length ram tubes on an efi 2 litre alfa engine in a clubman kit car, the longest tubes they could find made the most torque, and I think from memory it was higher in the rpm range than the shorter tubes. So I guess it depends hugely on the entire engine combination like cam and compression as to where peak torque and the biggest spread of torque will be.

Good luck to you, let us know what results you get.

Ben WIght

[Badger]

if you look at most little efi engines (eg my pulsar), they have quite long runners too, compared to my gem. I think that is the point, not that I want really really long runners, but that the runners I have are very short, and I am thinking about the effect of medium length runners

I am quite happy with the setup now so I am not planning to change anything for some time, when I do, I will probably go to throttle bodies. I think the idea of longer runners is to increase the velocity of the air, so when the valve is open the air/fuel is stuffed into the cylinder by the air behind it "pushing" = more torque. this would obviously have more of an effect at low rpm where the air is going slower in the first place, but should still work at high rpm. it would certainly depend on the duration of your cam etc. there would also be some resonance effects, ie when the valve closes, the air "bounces off" the back of the valve, goes back to the plenum (as a travelling or sound wave), and then back to the valve. best torque will be when the runner is the right length for the valve to have just opened when the wave gets there. this will obviously happen only in certain rev ranges.

I recall that some car manufacturers (honda, toyota, audi, cosworth are now making engines with variable length intake runners (how? I don't know!) in order to have the best of both worlds, long at low rpm, getting shorter at high rpm, matching the resonances etc