Which Heads?

[martin_t]

Hi
Can someone advise on which stage (1,2 or 3) heads would be best, where to get them from and what peformance increase I could expect, if I were to get new ones for my Rover 3.5 V8 in a Cobra replica? The engine is standard bore with a Viper Hurricane cam, duplex timing chain, Holley 390 carb and bespoke Gardner Douglas exhaust system.

Thanks a lot

Martin

[kiwicar]

As the heads are the biggest restriction on power on this engine then you need the best heads you can afford, stage 3 should be a logical choice, but make sure they match your cam. If your cam has alot of extra duration but very little extra lift and your new heads produce all their extra flow above .5" lift with reduced flow up to that then it will not run well or produce more power. Try and get a flow chart form whoever is doing the porting so you can see how it fits your cam, a good porter should ask about what cam you are using, more to the point if they can't supply one don't buy! .
Mike

[sidecar]

On the Cobra forum you posted about the engine being smokey, as you are now enquiring about heads as you think that it's the valve seals that have gone?

When I fitted V8 development stage III heads to my 3.5 lump I did notice a drop in the performance at the bottom end of the rev range. In the end I got fed up with having to rev the nuts off the engine so built a 4.6 using the same heads. (Pulls top from 1300 RPM now!)

The problem is that everything has to be in balance, the valves in the stage III heads were possibly slightly on the large size for a 3.5 lump but are not too large for a 4.6 lump. Having large valves means that the gas speed at low RPM is slow, the ram effect is therefore reduced along with some other issues. (Fuel drop out ect).

Rover/Buick/GM picked the valve sizes to suit a 3.5 ROAD car so they won't be far off what's needed. (Just a bit small to makesure of good mid range) Over time the engine got bigger but the valves did not so effectively they got slightly too small for the bigger engines. (Good low down torque but not alot of revs). (I think the size went up once from the P6 to the SD1 lump but I'm not sure)
The shape of the valves changed a bit and the stems got wasted but this is just Rover pissing about really!

I'm not trying to put you off going to gas flowed heads but just don't go too mad on a small capacity engine.

In fact knowing what I now know, I would do it myself and save a shed load of money. 80% of the work that is needed is just behind the valve seat, there is a huge lump where the seat "blends" badly into the track. With the valves out you could clean this up with a dremmel and some tungsten burrs. You could then get the seats done as a three cut job.

(You need GOOD burrs, not bits of sandstone stuck on a stick!)

Finally you can get the heads skimmed to get the CR up to 10:1 whilst using comp gaskets. (You can ditch the dodgy outer head bolts that are fitted to your engine at the same time! )

Des Hammills book has some full scale drawings of the tracks in the heads, you can see what needs to be removed from them.

A total guess would be 25-35 BHP more, even with a home port for maybe 2 days work. (plus bragging rights down the Pub!)


All of the above is just my "humble" obviously!

Pete

[martin_t]

On the Cobra forum you posted about the engine being smokey, as you are now enquiring about heads as you think that it's the valve seals that have gone?

When I fitted V8 development stage III heads to my 3.5 lump I did notice a drop in the performance at the bottom end of the rev range. In the end I got fed up with having to rev the nuts off the engine so built a 4.6 using the same heads. (Pulls top from 1300 RPM now!)

The problem is that everything has to be in balance, the valves in the stage III heads were possibly slightly on the large size for a 3.5 lump but are not too large for a 4.6 lump. Having large valves means that the gas speed at low RPM is slow, the ram effect is therefore reduced along with some other issues. (Fuel drop out ect).

Rover/Buick/GM picked the valve sizes to suit a 3.5 ROAD car so they won't be far off what's needed. (Just a bit small for good mid range) Over time the engine got bigger but the valves did not so effectively they got slightly too small for the bigger engines. (Good low down torque but not alot of revs). (I think the size went up once from the P6 to the SD1 lump but I'm not sure)
The shape of the valves changed a bit and the stems got wasted but this is just Rover pissing about really!

I'm not trying to put you off going to gas flowed heads but just don't go too mad on a small capacity engine.

In fact knowing what I now know, I would do it myself and save a shed load of money. 80% of the work that is needed is just behind the valve seat, there is a huge lump where the seat "blends" badly into the track. With the valves out you could clean this up with a dremmel and some tungsten burrs. You could then get the seats done as a three cut job.

(You need GOOD burrs, not bits of sandstone stuck on a stick!)

Finally you can get the heads skimmed to get the CR up to 10:1 whilst using comp gaskets. (You can ditch the dodgy outer head bolts that are fitted to your engine at the same time! )

Des Hammills book has some full scale drawings of the tracks in the heads, you can see what needs to be removed from them.

A total guess would be 25-35 BHP more, even with a home port for maybe 2 days work. (plus bragging rights down the Pub!)


All of the above is just my "humble" obviously!

Pete

Hi Pete

I think I'm going to take your advice, finish the car and drive it for a while and see if the engine beds in and the smoke goes away. I was really just making enquiries for the future in case I did have to whip the heads off. Your advice on the power gain makes a lot of sense.
I am not to bothered about having a hugely powerful engine, as I really just want to drive the thing and enjoy it. I am sure the 3.5 will give me plenty to keep me occupied in a lightweight chassis, its all just very interesting, and the more I know about the heads etc will make things easier and more understandable if I do eventually have to change them or mod them.

Thanks for all the advise

Martin

[sidecar]

On the Cobra forum you posted about the engine being smokey, as you are now enquiring about heads as you think that it's the valve seals that have gone?

When I fitted V8 development stage III heads to my 3.5 lump I did notice a drop in the performance at the bottom end of the rev range. In the end I got fed up with having to rev the nuts off the engine so built a 4.6 using the same heads. (Pulls top from 1300 RPM now!)

The problem is that everything has to be in balance, the valves in the stage III heads were possibly slightly on the large size for a 3.5 lump but are not too large for a 4.6 lump. Having large valves means that the gas speed at low RPM is slow, the ram effect is therefore reduced along with some other issues. (Fuel drop out ect).

Rover/Buick/GM picked the valve sizes to suit a 3.5 ROAD car so they won't be far off what's needed. (Just a bit small for good mid range) Over time the engine got bigger but the valves did not so effectively they got slightly too small for the bigger engines. (Good low down torque but not alot of revs). (I think the size went up once from the P6 to the SD1 lump but I'm not sure)
The shape of the valves changed a bit and the stems got wasted but this is just Rover pissing about really!

I'm not trying to put you off going to gas flowed heads but just don't go too mad on a small capacity engine.

In fact knowing what I now know, I would do it myself and save a shed load of money. 80% of the work that is needed is just behind the valve seat, there is a huge lump where the seat "blends" badly into the track. With the valves out you could clean this up with a dremmel and some tungsten burrs. You could then get the seats done as a three cut job.

(You need GOOD burrs, not bits of sandstone stuck on a stick!)

Finally you can get the heads skimmed to get the CR up to 10:1 whilst using comp gaskets. (You can ditch the dodgy outer head bolts that are fitted to your engine at the same time! )

Des Hammills book has some full scale drawings of the tracks in the heads, you can see what needs to be removed from them.

A total guess would be 25-35 BHP more, even with a home port for maybe 2 days work. (plus bragging rights down the Pub!)


All of the above is just my "humble" obviously!

Pete

Hi Pete

I think I'm going to take your advice, finish the car and drive it for a while and see if the engine beds in and the smoke goes away. I was really just making enquiries for the future in case I did have to whip the heads off. Your advice on the power gain makes a lot of sense.
I am not to bothered about having a hugely powerful engine, as I really just want to drive the thing and enjoy it. I am sure the 3.5 will give me plenty to keep me occupied in a lightweight chassis, its all just very interesting, and the more I know about the heads etc will make things easier and more understandable if I do eventually have to change them or mod them.

Thanks for all the advise

Martin

Hi Martin,

No problem!

Alot of people are happy with a 3.5 in a cob and to be honest I think that you need to make a hell of a lot more BHP to make a noticeable difference to the cars performance. Chedz has a 3.5 in his cob and yes mine does accelerate better than his but he's not "blown into the weeds". (certainly not until over 100 mph) Again Noel has a 383 SBC and his car is faster than mine, but he does not leave me until 130 MPH.

My point is that for 0-90 real world speeds the 3.5 is fine.

You could always buy some old SD1 heads and work on them once your car is on the road, you could then swap them out over a weekend.

(Note to any Plod...All the figures are on a private testing ground! either that or I made all of the figures up)

Good luck with the build!

Pete

[CastleMGBV8]

good advice.

If you want a nice flexible 3.5 engine don't over cam it or go to mad on the heads.

I'd look for a pair of late 4.0/4.6 heads off e-bay which have slightly better ports and waisted stem valves so just need claning up behind the valve head to provide a decent power increase with no loss of bottom end .

If you need a new cam anyway go for something like a real steel hurricane and that combo should yield approx 200BHP at around 5500RPM
but do fit new lifters or you will wreck the cam and you must run the cam in by running the engine at 2-2500RPM for 20mins after initial start up and don't let it idle.

If you are getting oil smoke have you checked your crankcase ventilation is working properly, if not excessive crankcase pressure will force oil down the inlet guides especially if they are a little worn.

Kevin.

[martin_t]

good advice.

If you want a nice flexible 3.5 engine don't over cam it or go to mad on the heads.

I'd look for a pair of late 4.0/4.6 heads off e-bay which have slightly better ports and waisted stem valves so just need claning up behind the valve head to provide a decent power increase with no loss of bottom end .

If you need a new cam anyway go for something like a real steel hurricane and that combo should yield approx 200BHP at around 5500RPM
but do fit new lifters or you will wreck the cam and you must run the cam in by running the engine at 2-2500RPM for 20mins after initial start up and don't let it idle.

If you are getting oil smoke have you checked your crankcase ventilation is working properly, if not excessive crankcase pressure will force oil down the inlet guides especially if they are a little worn.

Kevin.

Hi Kevin

Thanks for the reply. I have a Viper Hurricane cam in all ready. Bought one from RealSteel when I was building the engine up. I had the Heads cleaned up and new springs put in but nothing else done to them although they did check the valves.
As said previously I will run the engine for a while and see if the smoking dissapears.

Thanks

Martin