Head swap

[Paul V8]

Hello, i have a 3.5 rover in a TR7, at the moment i have a cross bolted block with 9.75 pistons with SD1 heads with the 36cc chambers and comp gaskets, i have a pair of ported 4.6 heads with 28cc chambers to fit, should i use the steel shim gasket or the comp type will it all fit together and would i notice a change in performance with a different compression, what would the compression be? Thanks.

[CastleMGBV8]

Tin gasket 12-1 Comp gasket 11.25-1

if the engine was bored 20 over then the compression will be increased by .15 tin and .12 comp
Too tired to write down all the maths but hope this helps.

Kevin.

[CastleMGBV8]

Correction, The figures I posted last night are too high, I shouldn't do maths after midnight!

To calculate compression ratios you use the following values.

1, the swept volume of a cylinder. Standard bore 3.5 = 441cc

2. the total of the combustion chamber 28cc + gasket tin 3cc or comp 6cc
+ deck height say 20thou = 3cc + volume of piston bowl approx 12cc.

So with tin gaskets you have 28cc + 3cc +3cc +12cc =46cc. + swept volume 441cc. = 486cc then divide by 46cc = Approx. 10.56/1

With composite gaskets compression Approx. 10.00/1

If engine bored 20thou/.5mm add approx .15 for tin and .12 for composite gaskets.

You could go with the higher figure but will need to use either Shell V power which is ok for up to 11.00/1 or possibly BP Ultimate, Shell V Power definitely makes mine run smoother.

Without knowing all the exact internal dimensions of you engine it is difficult to be precise with the figures but the above should be close enough.

Kevin.




Kevin

[CastleMGBV8]

Another thought, you say that the 4.6 heads have been ported do you know who did them? One of the most important areas is the valve throats just behind the heads of the valves these should be opened up to match the seat size and together with bulleted guides will yield approx +30BHP have a look at Peter Burgess Econotune heads on his web site.

Depending on the rest of the engine spec if the heads have been done to above spec you would get approx + 30BHP the increased compression would probably give another 5BHP the rest depends on the cam and what you are using for induction and exhaust.

Kevin.

[russell_ram]

Trouble is that the calc is highly dependant on the deck height volume - particularly with the tin gasket option. Without knowing what that is it's a bit difficult to say what you will get with your proposed set-ups with any degree of certainty.

My spread sheet calcs your current comp ratio to be 9.02cc assuming zero deck height (ie you're probably lower than this now).

Then, with 36cc heads, 11.40cc with tins and 10.39cc with comps - again both assuming zero deck height ie these are probably max CR numbers.

Adding in Kevins 3cc (.020inch) deck volume assumption I'm close to his numbers (witnin rounding errors). The number you end up with is likely to end up somewhere between the two.

If you have a small piston/deck clearance then I would suggest that 11.4 might be a bit too high even with Optimax etc. 10.5-10.75 is reasonable for a street Rover imo.

Russ

[kiwicar]

just a quick question, what cam are you planning on using with this set up? just I assume you are planning on a non standard cam . If so then when you close the inlet valve has a big effect on the CR you can get away with, I ran a 10:1ish ( 10.5:1 pistons (high comp P6 pistons from what I remember) and a comp gasket with SD1 heads, 10thou off the heads to straighten them up, on an r87 cam (about 215deg at .05 timing I think)in a rangy on ordinary unleaded OK on about 32deg spark timing. If you are using a fairly "big" cam you will want as much CR as you can get.
Mike.