Trumpet bore

adamnreeves · Post by **adamnreeves** » Thu Mar 22, 2007 4:19 pm

Measured my trumpet bores the other day on my 4.6 RV8 build. There are 38mm. Looking at one of my generic performance books it suggests that 40mm is good for a maximum of 46HP per cylinder. My aim is 300BHP max for the engine in the initial state of tune, which is 37.5HP per cylinder. So from this I conclude that 38mm is just about fine.

Comments anyone?

Post by **katanaman** » Thu Mar 22, 2007 5:06 pm

Like I said to you before the standard setup is good for around 300bhp.

adamnreeves · Post by **adamnreeves** » Thu Mar 22, 2007 5:18 pm

Whoops,

Mind like a sieve at the moment. Okay good. cheers.

BTW, my foam shaped plenum chamber is coming on nicely!!
It is gonig to be 90mm in height which I believe is the standard height which satifiies the bore width * height ratio, whatever that is called. I note that the westfield plenum chamber does not comply with this!

katanaman wrote:Like I said to you before the standard setup is good for around 300bhp.

Post by **katanaman** » Thu Mar 22, 2007 5:21 pm

Nice one keep us posted.

Eliot · Post by **Eliot** » Fri Mar 23, 2007 12:35 pm

I'm going to be working on my trumpetless base on the weekend. Gavin kindly sent me a trumpet base i could use.

Plan is to make one for the std 38mm bores and then take it out to 44mm (TVR spec) and make another one.
I'm using 1" nylon this time, i was going to use tufnol as described before - but the plastics supplier didn't think it would come out very well. (and nylon is cheaper)

adamnreeves · Post by **adamnreeves** » Fri Mar 23, 2007 1:19 pm

Why trmphetless. I thought the longer the trumhet the more power at top end, lower increases low end torque at expense of power, etc. Also the standard trumphets are different lengths I presume this is something to do with firing order and reducing turbulence or something.

What you trying to achieve with this? Sorry that sounds harse, but genuinely interested in the advantages.

Eliot wrote:I'm going to be working on my trumpetless base on the weekend. Gavin kindly sent me a trumpet base i could use.

Plan is to make one for the std 38mm bores and then take it out to 44mm (TVR spec) and make another one.
I'm using 1" nylon this time, i was going to use tufnol as described before - but the plastics supplier didn't think it would come out very well. (and nylon is cheaper)

Post by **katanaman** » Fri Mar 23, 2007 1:57 pm

The way I understand it you have got it the wrong way round. The longer the lower it will rev, the shorter will rev higher. High revving engines generally have short trumpets And rover performance ones are short. Course it isn't as easy as that and its all about resonance tuned length.

kiwicar · Post by **kiwicar** » Fri Mar 23, 2007 2:12 pm

Yup Marki is right
Short trumpets higher peek power point, Longer trumpets more torque at lower revs.
It is not a simple thing to calculate, and the predicted results don't often match reality if you try and calculate it all out. But a general rule is as above.

You need to match the exhaust and cam though to the modification.
If you choose the exhaust for peek torque at low revs and inlet tract (short trumpets) for high power up the top end and a cam for midway between you will not end up with an engine with a good spread of power that hangs on at the top you will most likley end up with a dog!!! you may need to shorten the headers a bit and re time the cam a little earlier.
Best of luck
Mike

Eliot · Post by **Eliot** » Sat Mar 24, 2007 9:22 am

indeed; long narrow for torque (look at a thor manifold) - short wide for high power - tvr style.
Yes, the four inner trumpets should be longer - as the distance from the valve to the trumpet base is shorter.
What are the advantages ? - urr - i dont know. TVR blended just the ally base on their "taraka" upgrade which looked a bit nasty to me - so i thought this was a better version.

The blue one was done for a guy on here who's force feeding a 4.6 rover - so probably doesn't make much difference.

I'm hoping to get this one fitted to my mates griff 500 and do a comparison against the ACT trumpets he currently has fitted.

i may look at making the inner 4 longer(higher) also.

will also try the 71mm bored out plenum at the same time.

any changes (for better or worse) are only likey at the very top end anyway.

my other mate just got his seight running the other week, it would spin the rear wheels at tickover just by letting the clutch out !

More editing..
I can make the plastic bit thicker - hence make the inlet tract longer - but if you look inside the standard plenum top - the air flows in exaclty 1" above the base. If i made the plastic bit thicker, it would hit the side and block the inlet off.

pitsnow · Post by **pitsnow** » Sat Mar 24, 2007 9:48 am

Eliot, are you going to produce / sell this base modification?
If so, I would be interested.

ihatesissycars · Post by **ihatesissycars** » Sat Mar 24, 2007 9:53 am

Hi Elliot! Did you need an inlet manifold to put it on to do some testing with? I have a spare flapper one that only gonna go in the bin if not used.

Eliot · Post by **Eliot** » Sat Mar 24, 2007 10:18 am

pitsnow wrote:Eliot, are you going to produce / sell this base modification?
If so, I would be interested.

That's the vague plan - i'll see how long it takes to make. Std size one should be reasonable, but 44mm more pricey as it takes time to machine out the base.

Gav,
Dont need a base yet - but that may be on my radar in the future to see how much effort it is to port these things out (my late father was a gas-flow specialist of 30 years - ive now got all his tools..). He did flow out my mates seight one and he said it took longer than doing the heads :-O

ihatesissycars · Post by **ihatesissycars** » Sat Mar 24, 2007 10:23 am

Eliot,

Yes when i was alot younger reading millions of bike and car mag's your dads name was everywhere when something was modified, a legend!

I'll the manifold on the shelf then so if you ever want it let me know and i'll get it up to you (foc, all in the name of development!)