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V-8 Engine Tech

Compression ratios on new alloy Edelbrock performer heads

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by regb2004 » Sun Jul 29, 2018 5:28 pm

Hi all. As I previously mentioned, my engine is coming out of the car (it's already out!). All looks good so far - no particular wear anywhere. The r/H exhaust manifold is blowing, but that's about it (although we have yet to take the bottom end off to check it). The car already has an Edelbrock carb fitted, and i have an Edelbrock inlet manifold ready to fit to match to it. The current heads are original H-Series 440. They look fine, but won't be unleaded compatible. As the heads are off, the suggestion is that we fit Edelbrock performance alloy heads from Martin Robeys, as there isn't much additional cost compared to having my own heads rebuilt. I'm no engineer, and have read on here about the alloy heads reducing the compression ratios. Should I be worried about this? Should I also replace the pistons if I'm going for the Performer heads? If so, that sounds like a step too far and I'll probably get my own heads refurb'ed while they are off. I'm not bothered about trying to improve mpg, but some improvement in performance would always be a good plan.... Thoughts??
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by Jorrocks » Fri Aug 03, 2018 9:59 pm

I think I am correct in saying that your iron heads ARE unleaded compatible but someone will no doubt be along shortly to confirm (The Yanks were using unleaded decades before us!). The guys helping me with my rebuild advise this stuff about one point of compression loss for ally heads is a bit misleading. The big thing with your heads is that that they are open chamber, so have no ‘squish’, what our transatlantic cousins call quench, which helps the fuel/air mixture burn more efficiently and helps prevent detonation, or ‘pinking’. The Eddy heads, being alloy, will dissipate heat faster than your iron heads and are, I think, a quench design so potentially more efficient than your iron heads. Can I suggest you have a chat with either Geoff Hauser (01933 20682500), or PM Duncan ‘Cannonball’ Watts of this parish. They will give you sound experience based advice!
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by johnw » Mon Aug 06, 2018 6:52 pm

There are 440 Source aluminium heads which are $499 per head at the moment. These might bump up the compression ratio slightly depending on your deck height and CC volumes. There are online calculators to play with, and different thicknesses of head gaskets to choose from to change the CR. Iron 906 heads CC volume 88cc, 440 Source 80cc, Performer RPM might be 88cc.

I was working on a car for someone and we went through this. We checked his valve seats there were no cracks. He then thought why not keep it original, plus save $$$. In the end I showed him how to lap in the valves, he got more involved, and kept it stock, including stock cam and followers. If you change the cam, you might want to go to roller cam and roller rockers to simplify/avoid problems with modern low zinc oils and break in.

Heads and manifold are easy to change with the engine in the car. There is a lot you can do with the engine out, like heat shielding on the bulkhead, general clean up, etc. Lots of options, including using what you have as you could swap the heads later if problems ever developed.
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by slotcarone » Tue Aug 07, 2018 4:38 am

While your engine is out you should definitely check the timing chain set to make sure it has been changed from the original plastic tooth design! :)
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by johnw » Tue Aug 07, 2018 8:36 am

slotcarone wrote:While your engine is out you should definitely check the timing chain set to make sure it has been changed from the original plastic tooth design! :)

Yes! Vital that one! That is all you really need to change, you can keep the heads.

Also the old silver heat shielding in the engine bas is probably asbestos with aluminium foil stuck over, depending on the year. Does anyone know what year Jensen switched to Asbestos?
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by regb2004 » Tue Aug 07, 2018 4:06 pm

Hi....Thanks for the helpful advice guys! Yes, the timing gear was original nylon - but surprisingly all intact... So we have got a metal one to replace it, and a set of new bulkhead insulation coming from Robeys too! What started as a job to fix an exhaust manifold blow has grown to a full engine rebuild with new heads, camshaft, polished crank, timing gear, water pump, oil pump, inlet manifolds, core plugs etc etc etc etc.....scary!
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by FFPL » Tue Aug 07, 2018 10:54 pm

regb2004 wrote:.....scary!


yes it won't stop
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by slotcarone » Tue Aug 07, 2018 11:50 pm

I am assuming you are changing the timing chain set? That is a must! :)
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by regb2004 » Wed Aug 08, 2018 8:46 am

Yes....complete timing chain and gear set. And all this is before we tackle the bodywork.....
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