8 replies to this topic

[bluminicoop]

I have. A question I'm about to build a 1380 with a stage 5 race head, how do I calculate the compression ratio so I know what thicknes of head gasket to use, I'm worried if I just use a turbo head gasket things will go bang, is there a safe way to do this or can you just put the head on with any good gasket. I'd like to hear your views and also experiences you have had any advice wold be helpfull also I would like to know what carbs everyone recommends as in the past it would have been twin sus but things seem to have moved on and people using bike carbs are these good or better. Which ones do I need.
Thanks again in advance to all who leave replys on this wonderfully site.

[bmcecosse]

You do have to calculate the comp ration - piston dish volume/piston to block clearance and combustion chamber volume are all important. Do you know what they are? The gasket thickness doesn't vary significantly between types. Just use a good 'copper face' gasket.

[klivins]

well, to start with to my knowledge head gaskets are more or less the same thickness (except turbo?). You need to calculate the CR for your project engine, do a search here or get yourself a copy of 'yellow bible', the process is nicely described there.

[oltonlad]

you need to measure the cc in the chambers of the head, to do this get a syringe from somewhere like boots the chemist and put some white spirit in it, then fill each chamber up with the spirit.
the ammount that you use (as indicated on the syringe) will determine what cc the chambers are.

Edited by oltonlad, 08 April 2012 - 12:07 PM.

[racingbob]

on my 1380 race mini i used to use a copper AF 460

i believe it is, i also put smear of grease each side

had no problems with it

[Cooperman]

C.R. CALCULATIONS & MEASUREMENTS

1. Measure the average distance from piston top to block deck level with the pistons at TDC. Take the average for all 4 pistons.

2. Calculate the capacity of this volume (Call it ‘A’)
For 0.010” piston top to block deck this capacity will be:
Standard bore at 1275 cc = 0.99 cc
At +0.020” overbore = 1.01 cc
At +0.030” overbore = 1.02 cc
At +0.040" overbore = 1.025 cc
At +0.060” overbore = 1.04 cc

3. Measure the volume of the dish in the top of the piston using the 5-cc syringe with paraffin, meths, or even clean water. With Hepolite 20250, 20251 & 20253 pistons this is 8.3 cc.
Call this ‘B’

4. Set the head up to be absolutely level using a spirit level. Measure the combustion chamber volume in the head using the syringe and paraffin, meths, or clean water. Do this for a couple of chambers and take the average. Call this ‘C’

5. There is a volume in the head gasket area and we’ll make an assumption here that with the average gasket, say a BK450 or AF460, it’s 4 cc. Call this ‘D’

6. There is the ‘ring land volume’. That’s the volume down the side of the piston to the top of the top ring. Allow 0.75 cc for this. Call it ‘E’.

7. Now divide the actual swept volume of your engine by 4 to get the swept volume per cylinder. Call this the swept volume ‘SV’
For a standard 1275 cc = 318.75 cc
At +0.020” overbore, i.e. 1293 cc = 323.25 cc
At +0.040” i.e. 1310 cc = 327.5 cc
At +0.060” i.e. 1330 cc = 332.5 cc

8. Add all the unswept volumes together i.e. A+B+C+D+E.
Call this the Total Unswept Volume ‘UV’

9. The Comp Ratio (CR) = (SV+UV) divided by UV
So, if you have a standard bore 1275 cc engine with the SV = 318,75
And if the total of the other volumes, the total unswept vol. UV =, say, 36 cc,
The CR = (318.75 + 36) divided by 36
= 354.75 / 36 = 9.85 to 1

However, if that is not the CR you want, you have to work backwards to calculate what the combustion chamber volume in the head should be.
Say you have the figures we just used, but you want the CR to be 10.4 to 1.

The formula to work out what the UV should be is:

SV divided by (CR-1) = UV

So, in the case we are using:

UV = 318.75/(10.4 – 1) = 318.75 divided by 9.4. = 33.9 cc

Add A+B+D+E, as established earlier and subtract this from the new figure for UV, in this case subtract it from 33.9. The resulting figure is the actual volume you need each combustion chamber to be.
Using the syringe again, after setting the head up to be absolutely level, put EXACTLY this amount of fluid (paraffin, meths or clean water) into a combustion chamber.
The distance from the head face to the surface of the fluid is the amount to be skimmed from the head. To measure this put a steel ruler across the head above the chamber and measure down to the fluid surface with a digital vernier, subtracting the thickness of the ruler afterwards. In practice, this is the most difficult dimension to measure as the fluid has a ‘meniscus’ which sort-of ‘reaches up’ to the bottom of the vernier calliper. You have to make a judgement on this, but don’t be put off, it is not that hard.

Once the head is skimmed as necessary you just fit it and your CR is correct. Remember if you are increasing the CR you will need to check and adjust the timing.

[bmcecosse]

Head swimming now ...... Stage 5 race head - where from ? What's the spec??

[Turbo Phil]

As already said, the gaskets don't vary much in thickness so if you need to alter your compression ratio drastically, changing head gaskets will not make enough difference.
Why would using the Turbo head gasket make it go bang ?

[TheOldOne]

Sorry Made A Boo Boo
Will Start A New Post

Edited by TheOldOne, 09 April 2012 - 07:05 PM.