Dougs951S

Before I even begin, *puts on flame suit*.

There, thats better. I've been throwing this idea around in my head for years, but recently a conversation with a friend prompted me to consider it again, and to my knowledge nobody has ever done it. I'd like to fab an ITB intake using 44mm butterflies from a Honda CBR1000, and integrate velocity stacks. They are extremely cheap, a set with the linkage can be had for 40 bucks all day long. Thats 60.8 cm^2 of valve area, and nearly 3 times the area of the stock 951 TB. A stock 951 intake hacked off before the plenum would let the stock fuel rail be used, and I would drill each runner and tap it for a boost/vac reference signal and delete the ISV. It wouldn't take much massaging to make the factory throttle cam and TPS work, so the main hurdle for me is figuring out exactly how long I want the runners so I know where to cut and designing the plenum. Can any of the great minds here give me their opinion on calculating ideal runner length and plenum volume? This will be going on a big turbo 2.5 liter engine with lots of breathing mods. Trying to achieve max VE at a relatively high engine speed.

Much thanks

Thom

44mm ITBs when stock inlet valves are 45mm?

Dougs951S

Thom, the idea to use CBR1k throttles was kind of just a "well, its available" thing as i was tossing the idea around in my head.. I thought about that fact too today, the hayabusa butterflies would be a better choice. ~100 for a set and they are 49mm.

nick_968

Not sure if you have seen the intake on this thread (link below) but it has the handy feature of being able to use different lengths of tubing between the plenum and the throttle bodies giving you the ability to adjust until you find the right power curve.....nice idea and potentially takes away some of the guess work or allows you to tune for different tracks. Not sure if that was the intention or if it was just for ease of installation but it makes sense.

https://rennlist.com/forums/944-turb...fo-thread.html

67King

It ain't that simple, and this isn't the place to learn what you need to know to do it. Oh and the turbo complicates it further.

Thom

The barrel throttle intake from JME uses a plenum of relatively limited volume compared to what we can see on other types of engines. Same as on the 944 GTR linked above.

Corleone used a 6L plenum on his 3.0 engine with remarkably short runners in order to maximise top end power. Such a large plenum should affect negatively throttle response, particularly when not used with ITBs, but in his case it must have been more or less compensated by a very high ignition timing profile coupled with a high CR as he was running on E85.

My short runner intake has about the same plenum volume as the engine and offers IMO a satisfying compromise between throttle response and top end breathing, considering the ouput of my compressor and a 1.1 backpressure/boost ratio.

If things like cam specs and runner size/shape of course take some place in the whole picture, we hardly ever get to start designing anything when we want to factor in all parameters right from the beginning, and I would say that plenum volume should be initially approached as a compromise between engine capacity, throttle response and compressor output.

Just my 0.02€ worth.

Cheburator

Why is that a problem?

According to your logic, my 16v heads should have never had 50mm throttle bodies fitted on them since total valve area is 78mm... Engine still made 438Rwhp/520RWNm out of 5.5ltrs on 99RON fuel and passed smog while remaining normally aspirated, so we must have done something right... My partner in crime runs a Lancia Delta rally car with 500+ bhp from its 2.0 ltr engine on GSXR 49mm throttle bodies...

Thom

Did I say it was a problem? What is "my logic", btw?

lee101315

Intake manifold design is extremely complicated, just slapping some motorcycle itbs on a 951 without calculating runner length and plenum volume is a lazy approach. I know of a local honda S2000 track car that spent ten hours on a dyno while the builder was modifying his plenum volume and shape for more mid range torque. By the way, there were about a dozen designs before they settled on one that worked, and it was good for an additional 10 ft lbs of torque.

Yes something will change with itbs.

In my opinion, a 951 engine with stock internals isn't meant to rev fast or up to the moon.

What is the exact result you are expecting from this modification?

Cheburator

I dunno, you tell me?

The way you phrased your response, it sounded like it would be a problem? I always find it a lot more useful when people actually explain things, rather than be witty to start with - just like you did in your second post, which is very useful...

Thom

The size of the circular (if you omit the recess for the injector) intake port on a stock 951 2V head is roughly 41mm.
Fitting larger intake valves means reducing air velocity between the inlet port and the valve, whence the interest to maximise air velocity at the inlet port by using runners that are wider than inlet ports.

I don't have a 4V head handy to measure, but if the overall area of the oval inlet port is larger than the resulting 19.63 cm² runner area of your 50mm ITBs, then you'd probably have made even more power with larger ITBs.

As a point of reference, inlet runners on the stock 951 intake are ~50mm where they reach the plenum, then taper to that value of 41mm.

Is that witty enough for you now, or do I have to explain to you why water is wet?

67King

+ infinity.

Folks, trust me, this is NOT simple. I have actually done this, for a 3.0L BMW. The results were incredible (+50hp naturally aspirated), but I probably had over 50 hours in modeling alone. And I have two engineering degrees, and used to do engine development for a living. I did it in Excel, and the spreadsheet was 24MB. That is a TON of data. Oh, and by the way, there isn't any inherent advantage to ITB's, but there are a LOT of disadvantages. The primary advantage is that it allows you to run a bunch more overlap without having to set the idle at a stupid high RPM point.

We used ITB's on the one I did, but it was because it was easier to modify an intake to make work than it was to come up with something that was all new. Here's what it looked like:


By the way, since the question of throttle size came up - what matters is port size. An approximate way to figure this out would be to take intake valve OD, subtract about 3mm for seat shape, then take a 10% reduction in area to account for taper. Now you CAN work your way back up to a 44mm butterfly, but to do so and maintain the same diameter, you'll have to make the runners longer.

Summary is size the diameter for your peak power RPM target. Then size the length for your peak VE RPM (close to, but not exactly peak torque) target. Make that VE RPM point say 15-20% or so below peak power RPM.

Cheburator

Hehe, someone got up on the wrong side of the bed...

50mm was actually properly sized eventhough the shape on a late GTS head is kind of funny and is a bit different from a 944 16v head...

I think my low tech engine made the most power of any 928 lump with standard displacement last time I checked... Btw, I think you are oversimplifying things with regards to flow, but now that you have told me off properly, I may as well keep schtum...

lee101315

There are a few people contributing to this board that are well versed in automotive engineering and have experience with ITBS.

Respectfully, Are you looking for a meaningful way to increase power? Or are you looking for the "cool" factor? You should jot down the objectives of your project and the characteristics youre looking to change first so that we can all help.

Just something to add, Lindsey racing built a prototype a few years ago, and Im not sure if they followed up with a production manifold or why they havent yet.

Cheburator

Look, I actually have an engine with ITBs, which happens to make almost an extra 160bhp on top of the standard 928 engine. Now I also have a 944 Turbo S and I see scope for improvement...

With regards to my 928 we were looking to increase power, as the base 928 engine, even in GTS form is pretty relaxed, making just 350Bhp out 5.4ltrs. Given that we changed a few things, it is very hard to pin-point how much power each modification made, as things are not cumulative (unless we are talking about the Fast and Furious Franchise). We fitted much higher compression pistons at 12:1, which were also lighter and of modern design, bigger cams, bigger inlet valves, ported the heads on a flow bench, simulated runner length, played around with injector positioning in the tract, added trully equal length headers with proper merge collectors, dual 70mm exhaust all the way, and spent probably 30-odd hours on a Mustang dyno, till we shreded a set of rear tyres. The motor is also drysumped...

With regards to the 944, we will go in a slightly diferent direction, but again we will use the motto "Some is good, but more is better"... My crystal ball tells me higher compression, lighter pistons, bigger roll bearing turbo, bigger intake valves, custom exhaust headers, ITBs and a proper modern ECU. If by then I am not divorced, I can also see a dry sump being fitted. Will be quite happy with 400 reliable crank horses - after all, the pecking order in the garage has to be maintained...