16v block + 20v (BT) head... what CR?

[Shifty]

Hey kids

Interested to know if anyone knows approx CR of this combo?

Also if there is a difference between bigport and smallport pistons?

Also I take that with 16v pistons it is interference

[BLK_20v]

CR Ratio : Depends on your pistons, AFIK all 4a blocks are the same

Interference : Depends on your cams particularly the lift. B/Top engines are interference for this reason

[trdee]

deck height is the same between 4A blocks so no CR change.
bigport 4age pistons are 9.4:1
bigport gze pistons are 8:1
smallport 4age pistons 10.3:1
smallport gze pistons 8.9:1

interference; yes as the 16v pistons have flycuts for four valves, not five

[oldeskewltoy]

16V combustion chamber, stock uncut, should be 36.25cc to 36.5cc.

What is the volume of the blacktop head?

[Shifty]

16V combustion chamber, stock uncut, should be 36.25cc to 36.5cc.

What is the volume of the blacktop head?

37.8cc according to a quick Google search.

deck height is the same between 4A blocks so no CR change.
bigport 4age pistons are 9.4:1
bigport gze pistons are 8:1
smallport 4age pistons 10.3:1
smallport gze pistons 8.9:1

interference; yes as the 16v pistons have flycuts for four valves, not five

You know that pistons aren't the only factor in compression ratio, right...? Capacity is dictated by bore & stroke... CR is dictated by volume at BDC vs volume at TDC hence cydlinder head & head gasket CC impact CR.

[Shifty]

Further on that topic...

CR of blacktop engine is 11.0:1 with 37.8cc head & whatever the standard HG thickness is
CR of silvertop engine is 10.5:1 with 34.5/35cc head (two different sources) & whatever the standard HG thickness is

So, if you put blacktop "11:1" pistons in a silvertop engine (assuming everything cleared) my basic understanding (right or wrong!) means you would end up with a higher CR than 11:1 and similarly if you put silvertop 10.5:1 pistons in a blacktop engine you would end up lower than 10.5:1

...and according to another Google search the 16v 4AGE has a 39cc volume so for a given piston the 16v head will be the lowest compression. Transposing that means that a silvertop 20v head on a 16v block, all being equal, will increase static CR... and a blacktop head on a 16v block would increase CR even further.

[trdee]

37.8cc according to a quick Google search.



You know that pistons aren't the only factor in compression ratio, right...? Capacity is dictated by bore & stroke... CR is dictated by volume at BDC vs volume at TDC hence cydlinder head & head gasket CC impact CR.

lol... semantics

you asked if the compression ratio will change if using a 16v block with a blacktop head. i said no it wont if you use a different block. who knows what pistons you want to run???

then you asked what the difference is between the 16v pistons and i told you their compression ratios as they are released from the factory. this is the best comparison between them as, once again, the only difference is between the pistons as the deck height is the same for all 4A's and all 16v's have the same combustion chamber cc's, and the bore and stroke is the same... all the pretty variables are static except the pistons in this comparison

HTH, have fun building your 4A!

[Shifty]

furry muff

so can anyone comment further with my latest comments?

[Bazda]

My blacktop head measures 37.5cc. 16v small port reads relatively the same.

So if your running a 16v gze 8.9:1 block, then change to a 20v BT head and run the stock gasket the comp will be around the same of 8.9:1, as both the heads are similar in CC size.

Changing the combustion cc by 3cc each way (from the stock form 16v 8.9:1) would put it like this, @-3cc 9.4:1, @+3cc 8.5:1

[Shifty]

My blacktop head measures 37.5cc. 16v small port reads relatively the same.

So if your running a 16v gze 8.9:1 block, then change to a 20v BT head and run the stock gasket the comp will be around the same of 8.9:1, as both the heads are similar in CC size.

Changing the combustion cc by 3cc each way (from the stock form 16v 8.9:1) would put it like this, @-3cc 9.4:1, @+3cc 8.5:1

Ta mate, exactly what I needed to know.

Amazing how much difference 3cc can make...! 20cc is the difference between an engine that won't run, and one that has so much compression it will shit itself!

[Bazda]

Stock head gasket is worth around 6.5cc, TRD 0.8mm 4.33cc.

[SHEPPO3930]

FWIW - i measured a standard (head not shaved) silvertop head at 35.6cc.

[BLK_20v]

What is the best way to measure cc of the head, fill with water and measure with a syringe style plunger?

[The Witzl]

way i saw it done was with piece of clear perspex covering combustion chamber, with a small hole cut in perspex.
turn the head upsidedown, and put some vaseline on the perspex so that when you cover the combustion chamber, the vaso creates a kind of temporary seal with the gasket face.
Thru the hole, using a syringe, inject some coolant (cos its green) into the combustion chamber, taking note of how much you are injecting.
You will be able to see when you are done with the green coolant, cos you can see the air bubbles coming out.

Voila - you now know combustion chamber volume.

Ive only ever seen it done once, and it was done this way.

[SHEPPO3930]

I purchased a burette and used dark green oil. Had some 6mm thick Perspex with a 5mm hole in it. Sealed the Perspex to the head using a very thin smear of grease. Worked out well.