Engine and Transmission Tech

Hi all
can any one tell me if the heads from a EFI 3500 rover engine will fit a EFI 3900 engine,
I believe mine are cracked and I have a spare 3500 engine,

thanks in advance Johno

Johno,

Basically the same casting, the valve sizes are also the same but the SD1 valves may or may not have waisted stems, if the waisted stem valves in the 3.9 heads are good you could just swap them over and lap them in,

If you want a bit more power then have the valve seats cut to actual seat size and then open up the area below the valve seat to match that size and then blend into the bowl

This is worth as much as 30 + BHP with no loss lower down with a standard cam.

Kevin..

Thanks Kevin
you have no idea how long it has taken me to get that info,

Have you any pictures on the valve seat mod, it sounds like a good idea, can I do it my self or does it need to be done by a machinist ?

thanks again in advance.

John,

If your handy with tools it is not very difficult.

You would need a valve seat cutter to reduce the seat size, the type that sits in the valve guides. The throat area below the seat you can increase the diameter by approx 3mm. the throat area is restrictive as standard and smaller than the existing seat size. This could possibly be done with a Dremel or similar power tool or with a porting kit from Summit or similar.

You can also reduce the amount that the valve guide protrudes into the roff of the bowl which also aids flow.

Have the heads decked 25 thou to increase compression to approx 9.8/1 which should be fie with pump gas

Leave the port runners as standard and don't be tempted to polish them, it will cost you power! The slightly rough surface of the ports helps to keep the fuel in suspension which improves combustion.

I would guess that this work is worth 35BHP on a 3.9 engine, add a fast road cam and you are probably + another 20 BHP say 245 BHP in total with good drivability and very free revving but whatch your rev limit with standard valve gear, 5800 RPM is a safe limit
and probably 300RPM above the peak power so no point revving higher.

Kevin.

Kevin.

Thanks again
I will keep you informed of my progress

Thank you from me too for the info above. I want to push it a bit further...I have a professionally ported pair of PRE Sd1 small valve heads that I would lke to fit to a 3.5, street cam and fuel injection on 9.35:1 comp. I like the wasted stem valve idea and was wondering if the standard seats can be retained and cut away more (they have already been cut for the current small valves) to accomodate the bigger valves?
Thank you,
Johan

Johan,

If you are talking about Rover P5 or 6 heads then I'm not certain as these heads had smaller valves than the SD1 and later heads.

Also there is conflicting information re whether the early heads are suitable for lead free fuel.

With the SD1 or later heads the largest valve sizes that can be used with recut existing seats is 1.63" Inlet and 1.4" Exhaust which are commonly used in up to stage 3 heads with appropriate mods to the port runners in addition to the throat and bowl area mods mentioned in my earlier post.

Kevin.