8000 rpm/300 bhp 3.2 rebuild

[911hillclimber]

No idea!
The Pistons are described as 11.5:1
The heads are standard so imagine they used a stock head to call them that (?)

Whatever it actually is, I don't have the mojo to find out!

I want to get on with it.
Need to visit my local rolling road, BHP Motorsport about the ignition side.

[RobFrost]

What was the average your plasticine squashed down to against the cylinder heads, as a proportion of the stroke? That gives you a quick, crude estimate of compression ratio. Just for sense checking.

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[Cortina]

AAgggghhhh .............. Comp ratio is "key" - above suggestion tells you very little . Measure it , do the sums. CR is directly related to hp (see my post from ages ago) Too low and you are simply throwing away hp ... lots of it , too high and you are asking for potential problems. Yes , there is a "debating" range in the middle .... but if you don't actually know where you are. Burette (I think is the correct word) , piece of perspex with two small holes in it , bit of grease and a calculator. Yes - sums can get a bit tricky with shape of piston , where it ends up at top of stroke , etc etc ................. but not knowing ................ . Assume nothing. Lecture over (wait till mojo returns !)

[911hillclimber]

Since Gary's question today a few things come to mind:
1
I can't change anything to what I have, stock heads and valves, JE pistons, thus the CR is what it is.
If I measure (and get it right) then I have some greater knowledge to pass on to the tuner man to accommodate in his actions.
2
I'll look tomorrow, but I think JE gives the volume of the piston above the top ring, and it is easy to find out where the piston comes up to due to staining of the top of the barrel.

If I do measure the volume of the head chamber (no gasket on a 3.2) I could have a reasonably accurate set of figures to calculate the CR.I do not have a Perspex plate or a burrett, but I'll look at ebay.
3
If I cock the measuring up it could open a tin of red-herrings needlessly.
4
Time is not on my side. Everything has to be up and ready to race by end of March.

[PeterK]

I know a man who has a burette, stand & a sheet of perspex with a hole in it

[KS]

TBH I can't believe after all the other minutiae you've checked that you have no accurate idea of what the true CR is on your engine other than going on a product description. It's such a basic necessity on a non-stock engine build and I have never met a race engine builder who doesn't check chamber volume, deck height volume, etc. If it all measures up and the CR turns out to be bang on 11.5:1, then great, but you'll never know without checking. Oh, and if the engine isn't all done and the car ready to go by the end of March, do you know what? The world doesn't end - you won't lose a multi-million dollar contract with Red Bull, Gulf Oil or Petronas.

[PeterK]

I'm using 0.5mm cylinder gaskets as my heads had previously been machined 0.25mm. My pistons were spec'ed at 10.5 CR with my calculations coming out at 10.23

[RobFrost]

The 1.7mm plasticine. Was that the thinnest side of the side plasticine, with the pancake of plasticine tapered to be thicker near the centre of the cylinder?

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[hot66]

With Keith on this … you’re going to all this effort, but the very basics should be knowing your compression ratio as there are too many variables to make assumptions

I’m assuming you know the procedures but if will help

https://youtu.be/i9io_DknQ3U

[911hillclimber]

Sometimes it is had exposing your efforts.

Lets say I measure, I know the procedure, seen the diagrams, don't have the equipment. Thanks again for the offer Peter but imposed too much already, s will look at ebay after this.

I plan to use the 0.25 base copper gaskets that came with the gasket set, the same size that went on after I did the last build when I did not of this, just bolted it together and got 260 bhp.
Rob, the plasticine was placed in the 3 places shown a few pictures above, the sides being the most critical (I've read).
I did the test without the base gasket to allow a bit of a bonus to the actual built figure.

The two thicknesses on both sides of the engine were 1.73mm. Add the base gasket and it will be more, maybe 1.83mm. I've read that is ok.
The pellet I placed right on top of the crown was 4.6mm crushed.

Just to stop all the shouting I'm doing a crap job of this I'll do the CR test. The parts will arrive before I've gapped all the rings etc and have the lot on the engine less heads etc.

1If the cr is 10.00 (say) I have miscalculated or have a cr of 10. What can I do about that? Machine the head down by some amount and then risk other issues, mainly the 1.7mm thickness, and if that is too tight, then what?
If the CR is 12.00 then I have mis calculated or have a CR of 12. What can I do? add a stack of base gaskets to jack it all up. (This can make the cam seal under eccentric load as the side engine chain cases would also need to be packed up) and there is a good risk of oil leaks. I could have the piston crowns machined to some measure I suppose to drop the CR or run the engine at 12:1 and get some more power. I fellow class competitor runs 15:1 and 1 bar turbo boost so I'm playing at the lower end of the scale.

I'm not going to be machining anything, but take encouragement from Peter's comments about his measurements and his calc's correlating to the piston supplier's data, as good as the same.
I have the 0.5mm base gaskets to fall back on if the cr is high, whatever 'high' means.

I was in a good mood to go into the 2 degs of the garage for the morning/day hoping to make some progress, but not so sure now.

Still, onwards and somewhere for this Non Race Engine Builder. Andy Lightweight summed me up well a few months ago, I am a compulsive tinkerer, so that puts my skills in perspective.

[911hillclimber]

If you are reading this Peter, how did you get the volume of the domed piston?
The paper with my JE's call up several nunbers.
Closest I think is 'Dome volume' which is given as a huge 46.46 cu in.

I can't imagine that is the volume of the piston head that goes into the head chamber.

In my mind it is more like 4.46 cu in.

Burette on order.

Dome height is 0.53", no other info on the JE web site.

[RobFrost]

Ok did a quick calc using your dims and photos and it gives 10.69:1 which should be reassuring. Here's the calculation.

Gap is the thickness of plasticine you measured, tan is the ratio to multiply that by if the plasticine was at a slope, to get the distance through it, if you were to measure parallel to the piston stroke. Gasket is 0.125 the compressed copper thickness.

Proportion is the proportion of the area of the top of the piston which is at that thickness. I've assumed stroke of 74.4mm which I think is standard for a 3.2. Cutout depth (and extent of the piston top covered) I've estimated based on a visual comparison of the cutouts with your 5mm plasticine.

I'll edit this post to link the spreadsheet in a moment so feel free to adjust the numbers yourself and see how it affects compression.



Here's the spreadsheet:
https://adstorm-my.sharepoint.com/:x:/g ... g?e=9pMIom

Also, I've coloured the editable cells green now.

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[Nine One One]

Graham,
I do not think anybody on here is shouting at you, or saying you are doing a crap build (your words). They are all trying to help with their vast knowledge of either having done this themselves, or from information gained from reputable engine builders. All things that have come up when they have done similar.
Keith has built numerous hot rod and drag racing engines, where the slightest miscalculation would just lead to total destruction - they are all trying to make sure you build the engine correctly.

You have moved on from your last engine build, changing various parts from standard to get more more, with that comes the need to listen to what they are telling you.

Do you really want to go to all this expense and effort to find you end up with less power than you originally started off with?

You originally asked for advise, which to go with this engine, what to use, and what was best to get what you might need. Now you have followed that advise, listen to what they are trying to tell you is the best way to check its all going together correctly.

It is only advise which only you can choose to ignore or follow and see the results of everyones efforts to help you with this build?

[sladey]

I’m also a tinkerer Graham, and as such we have to accept that there are plenty of others on here who know way more than me.

If you were just throwing it together then I suspect you’d be ok with what you’ve already done (but wtf do I know)

However you’re going for 300bhp and 8000 rpm - every last little bit is going to be important. So people are going to jump in and whilst sometimes it can come across as a bit critical I would say it’s worth taking their advice.

Even if it gets a bit raw at times I welcome you sharing the experience on here - I’m really enjoying the thread and hope you get there.

[KS]

That must be 46.46cc not 46.46cu in.

For example, for 10.5:1 3.2 JE pistons, AA Performance and others show a dome volume of 41.7cc...

We're not shouting, just wanting you to get the best out of your motor so we can all cheer you on throughout the season.