Engine and Transmission Tech

I asked this over on the MGE Swaps forum but only one respondent so I'll try here.

If I understand correctly with a 5 litre V8 I should be aiming for around an equal above trumpet to underside of plenum volume ie 5 litres. A stock Rover plenum measures around 30cm x 11cm so to achieve the 5 litres of air I need a plenum height of 16cm plus trumpet height ( volume = 30×11×16 = 5280 centimeter3 ). I plan on trimming 2cm off the plenum base like Glenn does but even so achieving this ideal is going to leave a fairly tall plenum unless I widen/lengthen a DIY plenum. So what volume do you have?
If I have a bonnet bulge I don't want it any taller than that on the C.

As I don't know how to add a "snip" here I've just copied the following from S-C Power in the UK which I found after visiting Pistonheads:

"As a basic rule the air Plenum volume should be at least equal to the cylinder volume of the engine i.e. 4-5 litres of air for a 4.0 litre engine and 5-6 litres of air for the 5.0 litre engine. TVR and Rover consistently used the plenum designed for the Rover 3.9 litre engine in all applications, this imposing a limitation of volumetric efficiency especially with larger engines, the 5.0 litre suffers most due to the undersized plenum volume."

I don't know how critical it is, but I figure it's likely Rover tested various configurations before ever going to production. Whilst writing this article, I measured the volume of a stock Rover 14cux plenum to be ~3250cc. My casual observation has always been that the GEMS plenums look a little larger (and "boxier"). I don't know the exact volume of my homemade plenum, but I expect it's in the same ballpark. Measured to the butterfly valve, not to the flange, right? (Runner volume on the other hand is slightly smaller.) It's sitting atop a 3.5L engine. One day I may move it to a 3.9/4.0L, but not to a stroker.

I am by no means an expert, but I am also planning a custom RV8 plenum.

Based on my internet research (so take this with many grains of salt) I think the key metric if you're modifying a gems plenum is total plenum volume exclusive of volume of trumpets, not volume above trumpets.

The website below skims a bit too quickly over the physics and math, but if I read it correctly, for a normally aspirated engine, as a general rule, the volume of air inside the plenum should match the total volume of intake runners, which in turn should match total displacement. Mismatches in volume generally lead to power loss:

[www.enginelabs.com]

I've also read that plenum volume is far more important than plenum shape, as long as you aren't restricting one intake runner more than others. Having all the plenum air above the trumpets presumably would be best, but that may be a marginal difference?

Curtis' figures above suggest that several stock RV8 plenums are too small. Perhaps far too small for the early 4.6. The Bosch/Thor manifold/plenum I have seems to be about double the volume of the gems, and the two plenum boxes are "above" the runners.



Edited 1 time(s). Last edit at 06/15/2017 08:17AM by RDMG.

My original idea was to build a plenum styled to look like an old-fashioned (14" round) steel air cleaner housing. It would have had a lot of volume! But, it also would have made servicing throttle body, sensors, wiring, and injectors less convenient. I also thought I'd hide my throttle body behind and under the rearward end of it because I wanted to keep my cowl induction scheme... but the packaging was just way too tight.

Without flow bench or dyno time, it'll be hard to optimize any design. Whether you make your plenum three liters or six, I expect you'll be way ahead of all those folks who simply bolt a throttle body to a manifold that was originally designed for a carburetor.

Thanks. If I'm to believe the blurb from SC-Power TVR & or Rover took the cheap way out and just used the existing EFI manifold and trumpet system. There are a number of UK companies offering 44mm wide trumpets over the stock 38mm but I can't find much about changing the plenum volume. SC-Power has a spacer that raises the height of the plenum thus "...by fitting or Spacer between the inlet manifold and trumpet base, resulting with an increase of 0.55 litres, a 15% increase of air volume that improves throttle response." but the issue that creates adds to the B's under bonnet clearance, The stock plenum has a volume of around 3600cc. I just assumed that the volume excluded the trumpets but if it does then 16cm will be about right. My guru can easily make a spacer and probably a new plenum base so we'll just be left with a bonnet clearance issue. And I hadn't included the TB spout. So if we place the TB in the intake pipe away from the plenum can I include that volume as a part of my strived for 5 litres or is it purely the volume of the plenum?

When I bought my engine it came without induction as I didn't know if I was going carb or EFI. I decided on the latter and bought a GEMS off the local LandRover wrecker - maybe I need to look into Thor to achieve sufficient volume? No I found a discussion in relation to a GT6 fitting; not a great idea.



Edited 3 time(s). Last edit at 06/16/2017 05:57AM by MGB567.

It looks like you and I are dealing with similar issues.

Excellent point regarding location of the TB distant from the aluminum plenum casting. My understanding is the "plenum volume" includes any airspace between the throttle body and the trumpet face. If I recall my trips to the junkyard correctly, GEMS engined land rovers had a length of plastic hose between the TB and the aluminum plenum, which would increase volume quite a bit.

I think the GEMS setup for the 4.6L engine may still have undersized intake runners, at least for truck applications. If it's correct to conclude that Rover engineered the 14CUX Hotwire castings and trumpet lengths for 3.9L truck applications, its reasonable to infer that the stock GEMS runner/trumpet lengths (same as the Hotwire I think) are less than ideal volume for 4.6L truck applications, and are closer to ideal length for a 4.6L application in a lightweight car than one might think at first. Volume still too small though. That probably explains the market for wider custom trumpets. As Curtis said, we're getting into dyno territory if we want to know for sure.

I will measure the volume of my Thor manifold one of these days, but I am certain it won't fit under the B bonnet. The intake runners on it are very long and thin, which makes a great torque profile for a 2-ton truck, not so great for a 1-ton B.

Let's look at this from a slightly different angle shall we? Sometimes older technology can offer some insights, and there has been an extreme amount of experimentation with runner length and plenum volume on carb equipped engines, with everything from "Xtreme" type single planes to "Red Ram" hemi long runner dual quads to large plenum tunnel rams with replaceable tops and tune-able volume. Since in all these cases the throttle plates are before the intakes, other than the location of the fuel metering they are, for all practical purposes interchangeable with the late Rover manifold and associated pieces.

So what have we learned from them? In terms of plenum volume by and large an increase in plenum volume is similar in effect to increasing carb size. Or by extension, increasing throttle body size, the purpose of which is to enhance the top end.

Do you need more top end?

Jim

Sure. Can you really use any more low end torque? ;)

Always.

Not without traction. :)