New Race engine build

[rgh0]

The engine I swapped out a year ago has finally made it onto the engine stand for a rebuild. I always love pulling the sump off a race engine - nice steel caps, Carrillo rods and Datsun crank

Bottom end will be a bearing check and replacement if needed and replacement of the front timing chain cover which has a crack in it plus routine replacement of seals and check of other components

Need to decide whether I clean up the current head or swap the head for the spare McCoy conversion race head shown sitting on my bench . I will be rebuilding for using 98 Octane premium unleaded the same as the current race engine in the car versus Avgas that this engine was previously built for. i will use the head which gives me the needed 11 to 11.5 comp ratio the easiest, current head and pistons in this engine give me around 12.5 to 1

End result will be like the current engine in the Elan at around !80 hp at 7500 to 8500 rpm. Always nice to have a spare ready to swap in if needed. Hopefully I will not need it for a few years yet as I aim for 4+ years from a race engine and current engine has only been in the car for 1 1/2 years

cheers
Rohan

[Frogelan]

Rohan

It is always very wholesome to have your views.

Race engines are quite fragile...have you any bright ideas as to how their life can be extended ?

Andrew

[seniorchristo]

Rohan
What is head your thickness for 11 to 1 compression? I have access to a head with 4.59 thickness and I?m wondering if this is milled too far for street use with standard Lotus pistons. Are the optional head gaskets available suitable for use with 91 octane fuel?
Thanks!
Chris

[rgh0]

Rohan

It is always very wholesome to have your views.

Race engines are quite fragile...have you any bright ideas as to how their life can be extended ?

Andrew

Three general principles govern race engine life for a twin cam that I have found. Following these principles i have not had a race engine failure for over 20 years and I have just had to swap out engines for a refresh or rebuild to an upgraded specification about every 5 years.

1. Use the best quality components you can get and check them very very carefully to ensure no defects. In particular pay attention to the crank, rods and lubrication system and valve train components. Understand the failure modes of components and ensure you select and inspect to avoid these modes
2. Assembly the components very very carefully to get the right clearances and bolt torques
3. Be careful how you tune and run in the engine and how you use it in races and in particular limit the revs to around 8500 rpm maximum as beyond that the high vibration leads to a dramatically shorter life span.

cheers
Rohan

[rgh0]

Rohan
What is head your thickness for 11 to 1 compression? I have access to a head with 4.59 thickness and I?m wondering if this is milled too far for street use with standard Lotus pistons. Are the optional head gaskets available suitable for use with 91 octane fuel?
Thanks!
Chris

The head thickness is only part of the equation, its the starting point but so no simple answer as a number of other factors follow on.

e.g. In my 11 to 1 current race engine head thickness is 4.60 but more important is the actual chamber size and intruder size on the piston. The chambers are a little bigger than standard due to the bigger valves and the machining to un-shroud the inlet valve. The pistons have a 6 mm intruder to compensate for the bigger chambers and bigger cut outs in the piston to accommodate the higher lift cam and bigger valve.

I normally build based on a 1.2mm thick fibre composite head gasket that compresses to around 0.8 mm when assembled. I have built engines that have used 2 mm thick gaskets but my small local head gasket guy no longer has the material to make these. I know Cometic will make gaskets to whatever thickness you want but in general I would not use Cometic head gaskets on a twin cam as I know a number of people who have struggled to get them to seal. i think the twink head is just to flexible. I dont know if companies like Ajusa or Payen make fibre composite gaskets for a twin in different thicknesses but its worth enquiring

11 to 1 works on my race engine on Australia 98 octane ( R+M /2 method) premium unleaded in part due to the longer duration cam of seat to seat 300 degrees inlet and 285 exhaust. Potentially it could go a little higher and I may target the new build to around 11.3 to 1. On a road engine with standard 272 seat to east duration sprint cams you should limit to around 10.5 to 1 I believe for Australian 98 octane premium unleaded unless you have a full electronic ignition with knock sensors and retard based on that. A road engine with the maximum duration thats best on the road of around 285 degree seat to seat set of cams you may be able to push towards 11 to 1 with the standard ignition on Australian 98 octane fuel.

cheers
Rohan

[SJ Lambert]

Does the newly available, in Australia, Elf LMS (Le Mans) 102 RON fuel interest you for a high compression build Rohan?

[el-saturn]

rohan, hows it goin'? this is certainly within the topic and hopefully the added picture show's the 11:1 stated?!?! my bottom (that alone creates a grin) where you can see 2 pistons show the setup ....... AND you and a friend
(??? or someone from elannet) claimed i may have gasket problems due to the dome height. the head has the standard machining and they could even be 1/4" high (dia 83,65 accralite) - i still have cream btw and head freshly skimmed sandy PS my swiss mate looked at the last ajusa (blown) and said this failing could be the compression??!!

[rgh0]

Hi Sandy
Good to see you have your engine running again Nothing like the sound of a healthy twink at 7000 rpm. I doubt the compression you are using is the source of your gasket failure problem.

I have used compression up to 13:1 on engines with relatively short duration cams at 285 degrees seat to seat duration without problems with failure of composite gaskets like the Ajusa. You do however need to ensure the fire ring does not go into the cylinder space as it can then crack and leak you have used a 87 mm bore on gasket on a 84mm bore cylinder and that should ensure you dont have that problem.

You do need to ensure the head and block are flat and well bolted down and the bolt torque is checked a couple of times during the first 10 hours of running by re-tightening each head bolt. if it does not move when checked I back the bolt off a very small amount ( maybe 5 degrees) and then retorque.

My race engine currently uses JE pistons like yours and an 11:1 compression ratio like yours and runs on 98 Octane (M+R/2 rating methods as used in Australia and I believe Europe) premium unleaded. I expect no problems with the head gasket.

The cams are a .460 lift 300 degree seat to seat inlet and .440 lift 285 degree seat to seat exhaust. This cam setup is a little to aggressive for normal road use especially with the 38 mm chokes. The setup is aimed at around 185 HP on the track and a 5000rpm to 8500 rpm power band

I believe you problem related to the head not being flat enough and the bolting having lost tension due to it not being sufficiently retorqued as the engine was run in.

cheers
Rohan

[el-saturn]

thanks rohan - it's leaking again! but i finally went to one of my motor men where we finally measured compression!! the gauge got stuck at 12 (not compression) - but way too much! i was just talking to schaffner and he's finishing a similiar engine BUT in order to get the 11: 1 compression the dome is 3,8mm high and not near the 6mil i think i've got - do you have the formula for calculating compression - right now i may have 13:1?? cheers sandy

isn't there a THICK GASKET available which solves my problem??

[el-saturn]

your bottom there is also breathtaking rohan, complimenti!! ! it's good my budget doesn't allow me to get that heavily involved, even though all MY bottom is forged (an original lotus 23 crank by farndon) and "covered" by a 701 block - valves (in; are 41,5 and i think 33,5 out??) lift is 10,9 and rel short duration - they are NOT identical at all! sandy

[oldchieft]

This might help

https://en.wikipedia.org/wiki/Compression_ratio

Jon the Chief

[el-saturn]

TX CHIEF -- i knew that both volumes would, displayed in simplist algebra (arithmatic) would reveal exactly this, many thanks sandy

[rgh0]

12 bar compression pressure or around 180 psi is not excessive but again it depends on the cams you are using. Longer duration cams give a low measured compression pressure at cranking speeds due to the later closing of the inlet valve.

You need to accurately calculate the the nominal compression ratio using the formula given in the reference next time the head is off by measuring the combustion chamber size in the head by filling it with liquid and measuring the liquid required with a syringe or pipette and similarly measure and calculate the intruder volume by filling with liquid with the piston down the bore.

Those measurements and that nominal compression ratio combined with the cam duration you are using will enable me to comment on whether you need to lower the compression or not and how best to do it.

cheers
Rohan

[el-saturn]

it isn't 12 bar - his gauge would (ef) indicate a ratio of 10:1 (ca) if the reading would've shown something between 9 and 10 on his instrument, so at way over 12 (where the needle stops) it could even be 14:1!!! plus, as i said, mine have a dome height of 5 to 6mm and schaffner "uses" 3,8mm for a 11:1 compression - i'm also just thinking about shaving off 2 (or so) mm with a mini mill (magnetical) as included (pistons centered at the very top and everything well covered to prevent shavings getting anywhere Sandy - .......... just maybe a crazy idea

[2cams70]

Yes that compression ratio looks more than 11:1 looking at the height of the dome. When you next take the head off again I suggest you measure it. An easy way to do this is to rotate the engine so you have one piston at exactly TDC and then consequently another piston at BDC. Seal around the edge of both pistons with some grease. Plonk the head back on temporarily with the gasket and fill those two cylinders with some light oil. Medicinal grade paraffin (not medicinal grade alcohol in your case Sandy!) works well. Measure accurately the volumes dispensed. Make sure to fill them both up to the same level through the spark plug hole. Once you have both volumes divide the larger figure by the lower figure and you have the compression ratio.

Whilst not accuracte enough for a race engine build because you are measuring across cylinders it's an easy method and good enough to give an indication of the actual ratio and therefore help determine the course of action that needs to be taken. The maths for calculation is also very simple with this method!

Forgot to add also that you'll need to back off all the camshaft caps so the valves are all closed of course whilst doing this. If the initial result shows the compression is too high considering the other specs. of the engine and the fuel being used the bottom end really needs to be stripped and the piston intruder heights corrected individually to achieve a satisfactory compression ratio.