RV8 single or multi throttles

[eff eff]

This may be a numpty question but here goes. Is the benefit of a twin or triple throttle plemum over a single on an RV8; to do with the total air flowed by the throttles, or the way the air is delivered to the plenum?

[HarryW]

Not wishing to sound silly I think its a bit of both. AFAIK for most applications a bigger single is all you probably need, 65mm out to 72mm, unless you have lots of internal upgrades.

[eliot]

Indeed. Tripples seem a bit OTT, but they do look nice. Have a look through my archive for other ideas;

http://www.mez.co.uk/ms12.html

Just added one by PH'er DNB, a nice twin plenum idea.

[trackcar]

72mm is good for 270bhp before it starts to lose out on flow to higher area set-ups. At reall hgih airflows though they lose out massively .. on two 320hp engines on 72mm we got +20hp with our 8 throttle base.

[Chassis 33]

It's all rather academic TBH if you're still using the standard flapper or hotwire AFMs. At the end of the day you still have to breathe through that restriction no matter what size your throttles are.

Regards
Iain

[eff eff]

Chassis 33 said:

It's all rather academic TBH if you're still using the standard flapper or hotwire AFMs. At the end of the day you still have to breathe through that restriction no matter what size your throttles are.

Regards
Iain

Indeed, that is what prompted the question, so does it follow that in any combination of throttles there is no point in exceeding the diameter of the airflow meter?

[trackcar]

There's no point keeping the AFM if you're after proper outright hp. If you do keep the AFM there's little point doing anything with the plenum either. Once you ditch the AFM things get a whole lot rosier Most people seem to be reaslising this these days hence a growing number of PHers RV8 cars running alternative ECUs.

[eff eff]

I can see that the ECU doesn't cut it, but I still can't get my head around the air flow. The AFM has a pretty large bore and only two cylinders are filling at once, so is the restriction really the AFM?

[trackcar]

the std AFM is restrictive from 150hp worth of airflow onwards. ie if your engine is more than 150hp (as most RV8 engines are) then the AFM is restricting it's hp potential. On a 5 litre with a cam and 72mm plenum, say around 300hp then it's typically sapping 15-20hp off what the engine could achieve with everything else unrestrictive.

[GreenV8S]

eff eff said:

I can see that the ECU doesn't cut it, but I still can't get my head around the air flow. The AFM has a pretty large bore and only two cylinders are filling at once, so is the restriction really the AFM?

The AFM bore is surprisingly small. You also need to consider that it's not just a matter of power being limited by a single restriction - each restriction in the system introduces a small pressure drop which saps power a little. The AFM doesn't have to be the most restrictive point in the system in order to be costing you power.

Edited by GreenV8S on Saturday 25th August 00:37

[Chimjunkie]

My 5.0 puts out 290bhp 320 ftlb. It has ported inlet manifolds, stealth cam, V8D stage3 heads. At present it is running the std throttle opening. What would be the benefit of just opening up the throttle body to 72mm? and then going back to Mark Adams again..

[rev-erend]

Chimjunkie said:

My 5.0 puts out 290bhp 320 ftlb. It has ported inlet manifolds, stealth cam, V8D stage3 heads. At present it is running the std throttle opening. What would be the benefit of just opening up the throttle body to 72mm? and then going back to Mark Adams again..

So - what is the throttle opening at present ? (68mm ? )

[HarryW]

rev-erend said:

Chimjunkie said:

My 5.0 puts out 290bhp 320 ftlb. It has ported inlet manifolds, stealth cam, V8D stage3 heads. At present it is running the std throttle opening. What would be the benefit of just opening up the throttle body to 72mm? and then going back to Mark Adams again..

So - what is the throttle opening at present ? (68mm ? )

Thought standard was 65mm.............

[Chimjunkie]

Yep. 65mm.

[Steve_D]

eff eff said:

I can see that the ECU doesn't cut it, but I still can't get my head around the air flow. The AFM has a pretty large bore and only two cylinders are filling at once, so is the restriction really the AFM?

At about 4000rpm it has to pull in 5 cubic metres of air per minute.
That's about the same as a kitchen extractor fan but with a vent over twice the diameter.

Steve