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21 Turbo Compression.
- Thread starter andybond
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- compression
So in theory using a ph1/2 head at 8:0:1 and then having a set of pistons ( forged or otherwise ) skimmed would drop it again. Or even a sandwich plate ?You mean increase the compression?
I've never bothered confirming it but I believe it is in the head height, ie, they just skimmed it down lol. @Red21 may be able to confirm.
Trying the think around the "problem" that @la21t is having running out of octane points.
DaveL485
Staff member
Dropping compression reduces throttle response and reduces drivability. 8.0 to 8.5 is bang on, your 21 was 8.5 and look how that went even on a standard lump.
Lee was pushing the limits of what his setup could do, 27psi on a T3 based turbo on a hot day at the rev limit. He paid the price, and in the rebuild has addressed some of the issues including a bigger Turbo, and the addition of water/meth not to mention the stronger internals.
If he wants to increase octane we have options but thats not the best idea as you want a consistent map on readily available fuel that you dont have to mix up (ie, toluene mix), or one that costs £5 a litre.
I'm planning on >400hp @ 30psi boost or thereabouts and my CR is targeted for just under 8.0. I wouldn't want any lower, i've tried it and I didn't like it. If it det's before I get where I wanna go then i'll do the same as Lee has done, reasearch, ask advice, deliberate and take my chosen action. No doubt at some point i'll try a full power all-or-nothing, and then I will lob a load of toluene in my fuel, open up the NoS taps and pray
My point is , or isnt what Lee coulda , woulda , shoulda done. Its merely an exercise in discussion of what can be done to a car. Same concepts across most boosted cars
I am also going to argue the point of your comment to strengthen the internals wisdom. Whats the point in strengthening something that will eventually fail due to the fact that something else is on the limit ? You are in effect strengthening a front door when a burglar is using a window to break in.
I also fail to see the point in doing a balls out power run. What is it proving ? That your block will stay together just long enough to get one set of figures ? Consistent power for me .
This isnt having a go , its just asking questions.
I am also going to argue the point of your comment to strengthen the internals wisdom. Whats the point in strengthening something that will eventually fail due to the fact that something else is on the limit ? You are in effect strengthening a front door when a burglar is using a window to break in.
I also fail to see the point in doing a balls out power run. What is it proving ? That your block will stay together just long enough to get one set of figures ? Consistent power for me .
This isnt having a go , its just asking questions.
DaveL485
Staff member
Split a liner.
Replaced with stronger liners.
Bad analogy!
Plus the cause of the split liner has been addressed as far as reasonably possible (reduction/inhibition of det).
According to Dialogys the cylinder head, piston, head gasket part numbers all match on a phase 1/2 and phase 3 engine.
The only part numbers that don't match up between the phases are the blocks although I can't be certain what the reason for this is I expect it is due to the difference in the main bearing caps which are tapped for the crank case strengthener/phase 2 sump mountings.
Time to measure a phase 1 and phase 3 head thickness and CC the combustion chambers.
The only part numbers that don't match up between the phases are the blocks although I can't be certain what the reason for this is I expect it is due to the difference in the main bearing caps which are tapped for the crank case strengthener/phase 2 sump mountings.
Time to measure a phase 1 and phase 3 head thickness and CC the combustion chambers.
No doubting you , but how was the comp dropped then ? was it actually dropped ? ( or raised in the case of the newer cars )
Good question, all Renault documentation states that the compression ratio on the phase 3 cars was raised form 8:1 to 8.5:1
My query is the same as yours, how did they achieve this?
We know some reasons why they done this. The catalytic converter and other emissions equipment on the phase 3 cars restricted them, the book figures for a phase 3 are 162bhp versus the 175bhp form earlier cars.
Dialogys is not the best way to confirm the answer as the part numbers may not reveal the exact changes in the the parts spec. I'm sure Dave had the head off a phase 3 a few years back and it didn't reveal an obvious differences to the components. The mystery continues!
Dave's Strip Down of a Phase 3 Head I found the old thread but couldn't get any closer to the answer after reading it and looking carefully at the photos. @DaveL485 any ideas?
The answer? Looks as though some of the images no longer load up which is a shame.
My query is the same as yours, how did they achieve this?
We know some reasons why they done this. The catalytic converter and other emissions equipment on the phase 3 cars restricted them, the book figures for a phase 3 are 162bhp versus the 175bhp form earlier cars.
Dialogys is not the best way to confirm the answer as the part numbers may not reveal the exact changes in the the parts spec. I'm sure Dave had the head off a phase 3 a few years back and it didn't reveal an obvious differences to the components. The mystery continues!
Dave's Strip Down of a Phase 3 Head I found the old thread but couldn't get any closer to the answer after reading it and looking carefully at the photos. @DaveL485 any ideas?
The answer? Looks as though some of the images no longer load up which is a shame.
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Red21
Well-Known Member
The compression ratio on the J engine is varied by the piston crown either a dished, flat or a raised crown. Now I have multiple sources of official information and they do conflict about the CR between the 752 & 756 (even for NA versions its different in listings) with the update note when the J7R756 first appeared stating a 8:1 CR, BUT the revised engine manual has it as a 8.2:1CR but then again the final print of the repair manual which collates all the end of production information list it as a 8.5:1 for the 756 & 8:1 for the 752.
The variation in actual Psi between the two when doing a “End of stroke” compression test will not be a lot as CR is a calculated ratio of cylinder fill at BDC &TDC and I would imagine that the confusion is down to rounding up/down the decimal fractions i.e. Actual calculated CR is 8.26-8.35:1 that are caused by variation in production tolerance of components.
What I will try is to gather the cc volumes of the head, cylinder & piston crown on the items I have and run it through my CR calculator ( NPPB )and see what it is.
The variation in actual Psi between the two when doing a “End of stroke” compression test will not be a lot as CR is a calculated ratio of cylinder fill at BDC &TDC and I would imagine that the confusion is down to rounding up/down the decimal fractions i.e. Actual calculated CR is 8.26-8.35:1 that are caused by variation in production tolerance of components.
What I will try is to gather the cc volumes of the head, cylinder & piston crown on the items I have and run it through my CR calculator ( NPPB )and see what it is.
DaveL485
Staff member
I have never seen a turbo lump with anything other than a dished crown, however I do remember someone, I think it was Kieran Prentice, saying there was a slight difference in some pistons he used..two of one sort and two of the other. Long, long time ago mind.
The restrictions to the PH3 (Catalyst, inlet pipe diameter) but equal torque with less hp do tie up with the higher compression though.
If I ever take the head off my current engine, which is untouched from the factory, i'll have to measure it. If anyone does the same with an earlier engine a comparison would be good. Lewis' link to the J-Plate is defo an original lump as it had the non-updated HG on it, and it was changed cos of the oil leak at the back of the head.
Red21
Well-Known Member
The crown type & dimension is how they raise the CR from turbo to NA, and as you say all turbos have the dished variaty of about 0.75mm below crown face with the highest NA engine with a 10:1CR having raise crown of 4.3mm. The old carb'd J5R had a similar CR to the turbo variant with a piston dish of 1.3mm but the chamber within the head was of a different design.
As far I am aware the only difference between pistons on the Ph1 OE fitment & later engines and replacement items for all J turbo'd engines is the ring size.
I have measured the piston bowl cc on a set of genuine replacement pistons and it was appox 2.6cc, I will now dig out a known Ph1 piston tomorrow & measure that.
Red21
Well-Known Member
Done a few more capacity checks & calculations with two brands of used head gaskets, a Payen & a Victor Reinz, plus a known Ph1 J7R 752 factory fitted piston from a November 1988 vehicle.
The only difficulty I had was with the cylinder head cc as the ones I got have been slightly modified so the combustion chamber capacity may be non standard size so I have taken an average of the two which was close to the book quoted capacity of 57.15 cc.
These are the numbers I have used to calculate the CR
Stroke 82mm
Bore 88mm
Head gasket bore 89mm,
Compressed thickness for Reinz 1.6mm
Compressed thickness for Payen 1.8mm
Combustion chamber in head 57.15cc
New & old Piston bowl 2.6cc
The Reinz gasket CR works out at 8.15:1 and the Payen at 8:1.
What we need to do is to get a known Ph3 original & standard head to measure the chamber volume to confirm the cc of these heads but would imagine that it will be the same.
The only difficulty I had was with the cylinder head cc as the ones I got have been slightly modified so the combustion chamber capacity may be non standard size so I have taken an average of the two which was close to the book quoted capacity of 57.15 cc.
These are the numbers I have used to calculate the CR
Stroke 82mm
Bore 88mm
Head gasket bore 89mm,
Compressed thickness for Reinz 1.6mm
Compressed thickness for Payen 1.8mm
Combustion chamber in head 57.15cc
New & old Piston bowl 2.6cc
The Reinz gasket CR works out at 8.15:1 and the Payen at 8:1.
What we need to do is to get a known Ph3 original & standard head to measure the chamber volume to confirm the cc of these heads but would imagine that it will be the same.
DaveL485
Staff member
Excellent info Rich.
This is pointing towards there being no difference between the two, but driving the engines, you would swear there was. Even if we prove its the same mechanically i'll still struggle to believe it lol
Red21
Well-Known Member
The variance in output Hp but same torque can be done via mapping & as the 756 has a cat and coil packs I think it may be all down the mapping controls & the CR it just a typo that has been copied upon copy just like the mystory island in the Pacific that appeared on maps for 100yrs or so until they went to find it.
