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smckeown

Inlet Bore Sizing For Tuned 8v

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smckeown

I have researched and discussed cam and exhaust diameters with people regarding my new build, but the one thing left is TB bore size. I am preparing spacers to tune the inlet length but I'm starting to think that reducing from my current 45mm TBs down to 40 or 42mm might yield benefits also. The new build will maximise mid range torque with better top end than the last build. I have done some research and read the BHP targets per cyclinder based on bore diameter (but these are just guides), plus every post on here mentioning 42/45mm etc.

 

I have also looked at various graphs based on bore size and does seem that the smaller bores do indeed yield better mid range.

 

So has anyone got experience or knowledge regarding bore size for a tuned XU9JA ? I'm thinking 40mm would be the optimim (based on longmans not increasing the head inley boresize)

 

cheers

sean

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Sandy

Bear in mind that the TB bore flows about the same as a plain bore of 10% smaller diameter, so a 40mm port in fact matches 45mm TB. I've been lectured by vary "experts" online about TB bore sizes. But one of my friends that has spent literally hundreds of hours testing a vast range of arrangements on the Dyno, insists that TB bore size isn't anything like as critical as people say. What he believes is critical, is the overall transition of the tract section (tapering essentially) from trumpet down the valve throat and the overall length, which he optimises for each cam profile he designs; much like an exhaust primary in reverse, unsurprisingly!

The other point I hear argued is that TB's that are too big are difficult to map well, I've found there's some truth in this, I've tried 45's on a 1360 (had them already), which was difficult to set up and a bit boggy at low rpm, but on the 1.6 8v and 16v's I've mapped with 45's, they've been excellent by the end and certainly not lacking in mid range, quite the opposite.

Edited by sandy309

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Rippthrough
Bear in mind that the TB bore flows about the same as a plain bore of 10% smaller diameter, so a 40mm port in fact matches 45mm TB. I've been lectured by vary "experts" online about TB bore sizes. But one of my friends that has spent literally hundreds of hours testing a vast range of arrangements on the Dyno, insists that TB bore size isn't anything like as critical as people say. What he believes is critical, is the overall transition of the tract section (tapering essentially) from trumpet down the valve throat and the overall length, which he optimises for each cam profile he designs; much like an exhaust primary in reverse, unsurprisingly!

The other point I hear argued is that TB's that are too big are difficult to map well, I've found there's some truth in this, I've tried 45's on a 1360 (had them already), which was difficult to set up and a bit boggy at low rpm, but on the 1.6 8v and 16v's I've mapped with 45's, they've been excellent by the end and certainly not lacking in mid range, quite the opposite.

 

 

Spot on - I remember seeing a tb (Jenvey?) setup that had the ports, bodies and the trumpets all in one long exponential-type curve, just like a big trumpet, and talking to them about it, they reckoned on a 5% more torque over even a decent tapered setup, even more at high engine rpms.

 

Was one an R1 at one of the bike shows. had really short trumpets and a long (for a bike, they don't usually have anything bar a couple of inches of silicon) manifold too.

Edited by Rippthrough

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pdd144c

Ask Sid if Longmans are doing your build.

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smckeown
Ask Sid if Longmans are doing your build.

 

Yes it's sid I deal with on the head, i'll have a chat with him tomorrow (assuming they are back)

 

Interesting info there chaps, cheers.

 

I have noticed a vast difference between the trumpets jenvey supply and the ones KMS supply. The KMS ones have much more of an open bellmouth (if thats the right terminology) especially at the entry end.

 

Jenjey pipes:

progress6.jpg

 

KMS 90mm trumpets:

pieces_cut.jpg

 

Therefore the KMS ones would support this tapered theory more by the looks of it

Edited by smckeown

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Sandy

I strongly recommend you take Longman's advice if they're doing the head.

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Robsbc
I strongly recommend you take Longman's advice if they're doing the head.

 

Yes Sean listen to your tuner!

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smckeown

I always do; if they actually offer any advice that is :angry:

 

I've been through the build details with a fine toothed comb in advance with des-developments, I thought absolutely everything was discussed, but there's always something else eh!

 

This build with be completely optimised, everything is being considered as a package etc. But I won't spoil the surprise for now :lol: Keeping this go low key, just a very few trusted peeps have been involved in putting this complete package together.

 

Longman's are just supplying the head

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Owain1602

Have you or are you selling Dave Bakers head then?

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smckeown

I'll be selling plenty of items in feb, it will be quiet until then

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James_R

All I've found from different TB sizes, having swapped between 40mm and 45mm bodies on a 1.4 16v nova engine (well corsa engine in a nova) was that the transient throttle was easier to adjust on the 40's less on off, well as TB's go. With the output you;ll be looking at 40's would be better, but if the inlet mani is designed for 45's then the step on them would be worse than the trade off of using 45's anyways. A fullt tapered inlet would be nice, but does anyone do one for the XU 8v engine? also speaking of ideals, matching the inlet angle to the port angle in the head would be nice, but can't really be done on pug engine.

 

The KMS trupets are better in the bell mount design as they are spun rather than pressed, but I doubt there much really in it.

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smckeown

I spoke with longmans this morning and the outcome was certainly interesting. Their porting method obviously differs from dave's in that they do enlarge the inlet bore, whereas dave has stipulated it's already too large. This may be down to the different combination of mid range vs. top end they design to. Anyhoo, He couldn't give me the specifics only that they do enlarge the bore. So my inlet will be sent to them for porting and the head will be ready on the 19th/20th Jan for collection (excellent turnaround time eh!)

 

They also said that they have a common 'track day' engine build and they regularly use 45mm TBs. So I'll stick with my current ones, which is good because the spacers i'm making are all 45mm in diameter (phew).

 

The +1mm Group A head is rather common, so i'd assume someone already knows the inlet port size foir this head.

 

They also approved the cam choice, which is nice to hear.

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pdd144c

You will be impressed with their work Sean. Their advice is always top notch too.

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