Big Valve cylinderheads-or not.

[CastleMGBV8]

Dave,

The Extreme top should give enough additional flow for a 3.5 without having to change the bonnet.

Kevin.

[tetlow]

Kevin

I have just looked into this. You are completely right!!!

A MGC bonnet is £300+

I have removed the air filter and found that the air after entering through the 2" high filter has to squeeze through a 1/2" gap before turning down into the carb. This must be a real restriction.
I will get an X-stream top. I can even get one in Red. I think I will remove the insulation from the bonnet above the filter too to give a bit more room.

Thanks for your help.

Dave.

[sidecar]

Those filter tops that are also a filter seem to make sense but I'm sure that I read somewhere that they cockup the airflow into the carb and cause turbulence.

Could you lower the engine on its mounts?

[tetlow]

Afraid not. I have had to jack the engine up slightly to clear the Power Steering rack.

I recon I will give the top filter a try and see what happens.

[sidecar]

Afraid not. I have had to jack the engine up slightly to clear the Power Steering rack.

I recon I will give the top filter a try and see what happens.

I'm sure that it will be OK, it's not as if we are talking F1 engines!

By the way you could also go for a head skim to get the CR up to around 10:1 even with comp head gaskets. That will get you a few more BHP and helps prop up the bottom end when a longer duration cam is fitted


What carb are you running and what ignition system have you got? (And what timing has it been set to?)

[davemgb]

Dave,

Have you checked the casting numbers against the reference list ?

When the 4.6 heads came out with bigger ports everyone confidently predicted that no one would bother doing stage 1 or 2 heads for 3.5 engines. Given they come up on ebay fairly regularly they might be a good choice.

Dave

[tetlow]

I have the Webber 500 carb with a Mallory dizzy with the Petronix electronic ignition and high volt coil. I have a std cam, with a JWR Dual Plane manifold.

I have 9.75 pistons and have had the heads skimmed so I can use comp gaskets with no loss of ratio. At the same time I had the valves re-cut with the 3 angle seat.

I had a special air filter top slotted so it would fit snuggly ontop of the filter but have just measured the area under the plate for the air to pass through to the carb. )thuis is the min restriction in the filter unit)It is only 94cm2. This does seem a bit smaller that I would have expected when you consider the area of my filter being 440cm2.

I am hoping to increase the flow through the heads by porting so I imagine I should first make sure the air can get to the top of the carb.

I have noticed that the K&N X-Stream top filter should only be used with a K&N round filter and the lowest they make are 3" high versions. That would mean the top of the filter would be about 1" above the bonnet.

Cheers

Dave

[mgbv8]

So fit an RV8 bonnet Dave...Honeybourne mouldings and Smith and Deakin do them in fibreglass. You will just have to put up with a big power bulge in the bonnet mate.
PS
I've just finished a V8 Roadster conversion with a mate using Clive Wheatley engine mounts.

He has the weber and low rider 2" filter and there is just about 1" clearance above the filter to the bonnet. But your PAS may be a problem??

[CastleMGBV8]

I've got the Honeybourme Mouldingd RV8 Bonnet, nicely made and very good value at approx £80.00

I think they do a C bonnet for about the same money.

For a carb set up the RV8 bulge isn't wide enough for a higher 14" round filter, an oval one might work depending on the dimensions and whether or not you can get a drop base in the oval shape filters.

Kevin.

[tetlow]

Hi Perry

Do you think your mate has a flow restriction problem that would hold the engine back?

I guess the best thing is to do the head changes and to test it with original filter setup then just try it with the filter cover off!

Dave