RV8 ITB Jenvey Inlet Manifolds - Downdraft vs Crossover

MPoxon · Post by **MPoxon** » Wed Feb 22, 2017 12:06 pm

I am after some advice from those in the know.

I am looking at rebuilding my TVR500 engine to a higher spec. Current thinking is bore out to 96mm, crossbolt, balance forged rods pistons etc to result in forged 5.2 bottom end.

Top end either H324 from an ease of maintenance point of view or possibly M248. Both will have a modern ECU to tame the low down manners.

I have spoken to a couple of engine builders and both have been helpful with the spec but one thing I cannot seem to get an answer on is what ITB system is best crossover or downdraft?

From my limited understanding crossover will provide more torque as the inlet track is longer whereas downdraft will provide better throttle response and potentially more BHP?

What are people's thoughts and experiences on the matter?

One thing to consider is the low bonnet on the TVR may influence this choice as you can fit a much bigger airbox on the crossover.

Crossover

Downdraft

Any advice would be greatly appreciated.

DaveEFI · Post by **DaveEFI** » Wed Feb 22, 2017 12:40 pm

I'd love to see some figures showing the gains by using throttle bodies over the standard plenum/trumpet arrangement on an RV8.

SpringV8 · Post by **SpringV8** » Sat Feb 25, 2017 4:11 pm

The crossovers will give a straight intake while downdraft give a ~45° angle between the head and the throttle.
We had that actually on a high reving 4 cylinder. The owner made a new intake manifold with a bend of ~40° between head and throttles ( to mount a bigger airbox ) - eveything else was the same - the result was 19hp less on the dyno.

Ian Anderson · Post by **Ian Anderson** » Sat Feb 25, 2017 5:51 pm

I have heard shat some of the cross overs have some restriction where the inlets cross over.

So you may have a larger throttle body but then the restriction is not that point.

Downdrafts do not have this problem. But then you have bonnet clearance issues.but may also be able to use that to hold a reversion plate to hold the back flow in the right place to be used on the next stroke.

Ian

MPoxon · Post by **MPoxon** » Sat Feb 25, 2017 8:19 pm

Thanks both

I will check out the JE cross over and see if there is a restriction at the crossover... good shout I did not think of that potential issue.

I don't think there is much in it regarding the bend required to go up or across. The inlet from the head appears to be roughly at a 45 degree angle....

SuperV8 · Post by **SuperV8** » Mon Feb 27, 2017 4:51 pm

You don't mention what heads you plan to use?
Having the best inlet system is just a waste of money without heads that can flow enough.

I believe the benefit of throttle bodies is to help tame radical cams. There's no reason why a plenum can't flow enough air for high hp but cross cylinder contamination from big cams hurts lower rpm.

Tom.

kokkolanpoika · Post by **kokkolanpoika** » Mon Feb 27, 2017 9:27 pm

In my mind if use throttle bodies, head´s will see better port shape, than really is. I will say it make a small difference to swap plenum to throttle bodies if use STD non ported head´s.. If use fully ported heads´s, you might get 40hp+.. I will tell this with my dyno experience.. My 5.2l make 40hp more from downdraft throttle bodies than plenum..

I think mgbloke has got similar experience when he tested throttle bodies vs carb.. Near 40hp more with 4.8l engine with stage 3 or merlin heads etc..

But if rover manifold/plenum is good enought to make +400hp, why they never use it? Plenum or trumpet base wont resist air flow.. I think inlet manifold port will..

MPoxon · Post by **MPoxon** » Mon Feb 27, 2017 10:02 pm

Heads I will be using TVR500 heads which are already fairly heavily worked with 43 mm (1.7 in) intake and 37 mm (1.5 in) exhaust valves.

Main motivation for throttle bodies is to tame the H324 or M248 cam in combination with an ECU.

I understand the cost vs the BHP is minimal but BHP is not the main goal as I am after the throttle response and induction noise. I have the ACT Twin plenum at the moment not a bad setup. I just want to make sure I do not end up with less BHP than the plenum!

I am interested in your 5.2 setup Timo, sounds similar to what I am after. May I ask how you got to 5.2? 96mm bore and 90mm stroke?

What do you use to filter your ITBs? I have heard the trumpet socks rob BHP but the ITG filters are better?

kokkolanpoika · Post by **kokkolanpoika** » Wed Mar 01, 2017 12:31 pm

Yes, it will use 96mm pistons and 90mm forger crank..

I will use pipercross air filter element, just like ITG.. Our local Jenvey dealer says that socks can eat +20-30hp for 400hp engine..

And if use ITG type filter, minium distance for bellmouth to filter is 40mm.. You can go less, but it might eat power..