Variations on RV8 combustion chamber CCs

92rrrandall · Post by **92rrrandall** » Fri Jul 31, 2009 5:10 pm

When you CC the RV8 cyl heads, how much variation do you find in the combustion chamber sizes?? Just trying to determine if I should add this procedure to my usual head work. Primarily interested in the newer 28CC type heads.

Randall

topcatcustom · Post by **topcatcustom** » Sat Aug 01, 2009 8:36 am

I think the only 2 variations are 36cc for all heads up to around the 1993 mark, then 28cc from then on. The rest of the different compression ratio engines changed because of the pistons, though I suppose there may have been a few exceptions I think that is generally the right info!

TC

spend · Post by **spend** » Sat Aug 01, 2009 9:32 am

Id say its only really necessary if you have de-shrouded the valves or done some other sculpting in the heads?

Odd's of sucking in the same charge and adding the same fuel across all ports seem very low to me (particularly with RV8 EFi manifolds), which kind of makes it all a bit unbalanced anyway?

topcatcustom · Post by **topcatcustom** » Sat Aug 01, 2009 9:47 am

Oops sorry mis-read the Q!

ChrisJC · Post by **ChrisJC** » Sat Aug 01, 2009 10:09 am

I've only ever done it pretty crudely, but +/-1cc is about what I measured. How much is due to the measurement variation I don't know. Was on 36cc heads

Spend - why do you think the EFi manifolds are unbalanced?

Chris.

CastleMGBV8 · Post by **CastleMGBV8** » Sat Aug 01, 2009 10:17 am

Randal,

With any standard rover cylinder head you are going to get some manufacturing tolerance variations in the CC's of the chambers, possibly 1-2cc between chambers.

Note variations can also be caused by the installed height of the valves, to check this you can run a straight edge across the top of the valve stems, don't be surprised to see a 20-30thou variation.

If you are modifying the heads to a high spec, stage 3 etc, then the chambers should be equalised, if to a lesser spec then the variations would probably nor be noticeable in performance terms.

Any reputable head modifier should of course do this work.

Kevin.

sidecar · Post by **sidecar** » Sat Aug 01, 2009 10:44 am

I buretted all of my 36cc chambers, they were all less than 1cc in difference.

BTW every 8 thou skimmed off the heads is worth 1cc out of the chambers.

Pete

spend · Post by **spend** » Sat Aug 01, 2009 2:56 pm

ChrisJC wrote:Spend - why do you think the EFi manifolds are unbalanced?

Just look inside them and compare lengths / routes / volumes... I've never understood the common idiom that 1/2" difference in trumpet length magically makes them equal? The flow is completely impaired on the straightish and the snaking longer outer ports (they are not all the same) that cross over compared to the straight short inner ports..

ChrisJC · Post by **ChrisJC** » Sat Aug 01, 2009 3:17 pm

spend wrote:
ChrisJC wrote:Spend - why do you think the EFi manifolds are unbalanced?
Just look inside them and compare lengths / routes / volumes... I've never understood the common idiom that 1/2" difference in trumpet length magically makes them equal? The flow is completely impaired on the straightish and the snaking longer outer ports (they are not all the same) that cross over compared to the straight short inner ports..

I rather thought they are all about the same, hence the complicated routing and varying length trumpets.

Chris.

Wotland · Post by **Wotland** » Sat Aug 01, 2009 3:20 pm

The inside diameter of each trumpet is 3.81cm. The cross-sections of the runners in the cylinder heads are significantly smaller, but in the intake manifold the runners quickly transition up to round cross-sections of about 3.8cm diameter each. The average total length of all eight runners measured from intake valve to open plenum is about 38.75cm. (Runners 1 and 8 are approximately 41cm long. Runners 2 and 7 are approximately 39cm long. Runners 3 thru 6 are approximately 37.5cm long.)
http://www.britishv8.org/Articles/Rover-14CUX-EFI.htm

92rrrandall · Post by **92rrrandall** » Sat Aug 01, 2009 4:51 pm

Thanks for all you responses! The main reason I asked the question is to decide if it would be worth it to buy a precision burette, then take the time to go thru the process.

By telling me that there is ~1cc difference then that would be ~2% difference in chamber size. This is based on a 4.0L engine with 9.5 compression ratio.

So I agree that a very high performance head should have this. A stage one or two head would just be questionable. But if I could do it myself??

Looking at the "balance" of the intake runners: I did CC my intake runners and found the same results posted above. Even though the four inner runners look longer, they are actually just a little shorter overall than the four outside runners. I also CCed the intake runners on the cylinder head and found that all 8 are exactly 100CC.

To CC the intake runners I put the injectors in the holes and sealed the outlets using 3" wide packging tape. I used a Gerber baby formula bottle for the measing device. This bottle is clearly graduated in 10cc increments up to 270CC.

Randall

sidecar · Post by **sidecar** » Sat Aug 01, 2009 5:31 pm

92rrrandall wrote:Thanks for all you responses! The main reason I asked the question is to decide if it would be worth it to buy a precision burette, then take the time to go thru the process.

By telling me that there is ~1cc difference then that would be ~2% difference in chamber size. This is based on a 4.0L engine with 9.5 compression ratio.

So I agree that a very high performance head should have this. A stage one or two head would just be questionable. But if I could do it myself??

Looking at the "balance" of the intake runners: I did CC my intake runners and found the same results posted above. Even though the four inner runners look longer, they are actually just a little shorter overall than the four outside runners. I also CCed the intake runners on the cylinder head and found that all 8 are exactly 100CC.

To CC the intake runners I put the injectors in the holes and sealed the outlets using 3" wide packging tape. I used a Gerber baby formula bottle for the measing device. This bottle is clearly graduated in 10cc increments up to 270CC.

Randall

I have a burette and I keep dropping the bugger, various bits have broken off and I can not remember where I got it from so I fear that the next time it takes a knock that will be that!

As to whether its worth doing well in my opinion it can do no harm and then you know for sure but if you are not pushing the CR up high then it really won't matter. I also buretted each piston at TDC after using a DTI to get each one to TDC. You can then work out the CR of each and every cylinder. Again many would consider this a bit OTT.

You need a nice picce of perspex to seal the chambers and cylinders.

Oh its a bloody messy job to!

Pete