Compression Ratio

TVRleigh · Post by **TVRleigh** » Wed Jul 18, 2007 11:47 am

Hi,

I have complety flat pistons in my 3.9 / 4ltr TVR400SE 1989.
I think they are cosworth pistons.
I think it used to be 10.5 / 1 compression ratio with old style heads.
would this be correct.

Also what would the compression ratio be now I have the new style heads on the engine.

Also the car used to have a strange head gasket with copper inserts, it now has a compersit gasket.

I think the recon heads have a 10 thou skim as they have 010 written on them.

Also I had to put a 30 thou shim to get the right pre-load.

I sorry for being so vague, I wish I mesured the piston gap when the engine was apart.

Just trying to work out aprox compression ratio. without taking the engine apart again.

Thanks in advance
Leigh

Post by **katanaman** » Wed Jul 18, 2007 1:31 pm

You would have to let us know the thickness of the original gasket or there is no way we can work out what your compression is now.

If your gasket was the same as a tin gasket then your compression now will be around the same, slightly higher because of the 10 thou skim.

If it was the same as a comp gasket then your now looking at roughly 11.4 ish.

Course your original heads could have been altered as well to get the 10.5:1. Too many variables involved but not enough measurements I am afraid.

TVRleigh · Post by **TVRleigh** » Wed Jul 18, 2007 1:36 pm

From what I know the copper ringed gaskets was sightly thinger than the compersit ones.

Does 10.5 / 1 sould right tho with a flat piston.

Post by **katanaman** » Wed Jul 18, 2007 1:46 pm

No way to tell as the pistons could have been further down the bore than standard. Flat top pistons in a 3.5 give you 10.5:1 but this should be higher in a 3.9 because of the larger volume. If the heads were standard 37cc and the gasket was thicker or pistons further down then it could be right but its a lot of if's and buts. If the original gasket was thicker than a comp then your compression will be higher probably by a good bit but I don't know what your doing with the engine to know if its dangerously high or not.

TVRleigh · Post by **TVRleigh** » Wed Jul 18, 2007 2:16 pm

Well I not doing any thing special,
Just put stage 2 gas flowed big valve heads on.
I've been told it should have a Kent 214 cam, hopfully this is ok, as don't want to change it yet.

I had the option of getting the orignal heads referb or exchanged.
The orignal head had been over ported, as was told that its best to have the right amount and have them gas flowed, than just take as much away as posible, (they said if you using force induction then yes take as much as posible)
So I went with exchange heads, with big valves and gas flowed.

I'm close to finishing putting every thing back together. I've done a lot of detailing also hence taking so long.

Ali brackets, Header tank and overflow, and samco and braided hoses. Stainless caped bolts etc.

And a lot of polishing.

TVRleigh · Post by **TVRleigh** » Wed Jul 18, 2007 3:55 pm

I've been told the TVR copper ringed gasket is about 55thou
and they have kent 214 Cam's

so changes are.
Compersit gasket from TVR one 15 thou (55 - 40)
and new style heads with 010 thou skim, and big valves.
Orignal compresion ratio was 10.5/1

Does this help matters.

and should I worry about any of this
as I'm hoping to try and start the car saturday.
Need to sort fuel leaks 1st

Thanks again
Leigh

Post by **katanaman** » Wed Jul 18, 2007 6:31 pm

At a guess I would say your compression is up slightly which could cause you problems so take it easy until you find out for sure. Course you will need super unleaded or whatever they call it these days and probably octane booster. I don't think you have changed anything enough for anything to hit as in valves and pistons and turning by hand at this point wont tell you anything as your lifters will be down a bit. Obviously turn it by hand first anyway just to be sure.

Basically your pushing it for pump fuel but a few have managed and you might get away with it. Easy does it and keep an ear open for detonation.

TVRleigh · Post by **TVRleigh** » Thu Jul 19, 2007 11:18 am

Thank you for you help.

I have got some octane booster, plus had to run it on super before the mods anyway.

It turns over by hand so don't think any think is hitting.

is there anyway to find out for sure, or is the only way by taking the heads off again.

or get it RR'd and see how much I have to adjust the timing.

x.l.r.8 · Post by **x.l.r.8** » Thu Jul 19, 2007 2:13 pm

The 214 is a fine street cam, don't expect it to push you back in the seat but on roads you thought you knew how fast you could go you will start noticing a difference. The other beauty with this profile you won't hit the pistons, however if you go up a cam profile your going to either need to cut out the pistons (depending on crown depth) or go pocketed from another company.

TVRleigh · Post by **TVRleigh** » Thu Aug 02, 2007 3:19 pm

Thanks for all your advice,

Its all back together and working fine, after a big problem with the clutch.

I'm still keeping it under 3000 RPM. for 1st 450 miles, and have done a few 4000 RPM in the last 50.

All seems fine. will be doing a 2-300 miles this weekend, so after that will rev it to 6k. its not pinked yet but as said only had it to 4k. but am running SUL.