compression and Oil pressure

[motomadness1947]

Fuel Injected California 1980 tr7
What is the correct compression for the engine and what should the oil pressure be hot and running (not idle) Thank you so much.

Bruce

[Workshop Help]

Your question has variable answers. The important thing with the compression is all four jugs are within 10% or less with each other.

Given a new engine with our 8.0:1 compression pistons, one could expect about 150lbs/sq". However, time has marched on along with the miles and ingestion of abrasive dirt thru the filter and innumerable cold/dry starts which have eroded the rings and cylinder walls. The resultant effect on any engine is for the compression to reduce. After 50,000 miles since my overhaul, about 125 to 135 is a typical reading on my questionable accuracy compression gauge. But, the engine runs smoothly, burns an expected amount of oil for it's condition, makes no internal noises, and accelerates nicely.

As to oil pressure, the condition of the rod and main bearings is a large governing factor. Again, the above conditions dictate where the actual pressure reading will be when the engine is cold and when it is hot, as heated engine oil will thin down. When cold, my oil pressure gauge will show about 65lbs at idle. When hot, it reads amount 30lbs at idle. All is well giving me complete confidence in the engine propelling us down the long and winding road and back again ready for the next pavement adventure.

Oil pressure on our engine can also be governed by the condition of the bypass spring tension in the oil pump and relative condition of the rotary pump gears themselves. Over time and mileage, some wear can take place in the pump.

One of these days, you may wish to pull the oil pan to examine the condition of the main bearing and rod bearing shells. If the shell overlays are being wiped away, they should be replaced with new shells of the same size. It may not be necessary to pull the crankshaft unless there is damage to the journals.

Mildred Hargis

[motomadness1947]

Thanks Mildred good answer
I just recently purchased this car witch was suppose to have only 15,000 miles since rebuild. My compression on the best cyl is just 115 worst 90 and oil pressure hot is 35. sounds like time to pull this puppy and decide what to do. Thanks again

[FI Spyder]

That's pretty low compression. I would take out the spark plugs, squirt in some oil in the cylinders, as the engine is on a slant I would spray in some engine storage spray so it coats the cylinder walls and take another reading. If the compression pops up the problem is the rings/walls. If it doesn't appreciably change I would suspect the valves/clearance are the problem.



- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

[Hasbeen]

What sort of mileage has the thing been doing since the rebuild?

If it has been doing small mileage, with lay up each winter it is quite possible your piston rings are gummed up in their pistons, & not sealing properly.

My current car had not run for some time when I bought it, after occasional minor competition. It had very low compression on 2 cylinders.

I gave it about 50 kilometres on shell Rimula X highly detergent diesel oil, an oil change with a bottle of STP oil treatment added for a couple of hundred kilometres, & all cylinders were up to 150 PSI.

STP is very good at freeing up gummed rings, & tends to clean a lot of carbon out of engines. It is worth a try, as it costs little, & may save a lot of work.

While you're mucking with the thing, check your valve clearances. They can close up quickly on rebuilt engines, & our engines particularly don't like tight inlet clearances .

Good luck with it.

Hasbeen

[Workshop Help]

One more note of interest.

Please have confidence in the basic soundness of the TR7 slant four engine. It is better than you know. An engine overhaul can easily be done without pulling the engine from the body, with the exception of a crankshaft replacement. Should you opt for an overhaul, the single hardest chore will be removing the cylinder head. Most all folks here in the neighborhood have fought this battle-royale. In my case, no removal problems were encountered. You may be so lucky, or not.

With the head off and the engine mounts loosened, the block can be lifted enough to remove the oil pan to access the rod & main bearings, as well as removal of the pistons. In my case while the bearing shell veneers were pretty well wiped clean, the crank journals remained in serviceable condition. This meant replacement shells of stock dimensions were used to good effect. Also, the oil pump was replaced with a SAAB 99 pump which, in combination with the new Vandervell bearings, increased oil pressure at idle by some 15PSI.

New stock size pistons and an expensive comprehensive overhaul of the cylinder head resulted in a fresh powerplant.

Mildred Hargis

[motomadness1947]

can you replace the main crank bearing shells without pulling the engine.
I have some new pistons that came with the car but not installed.
I did take the oil pump apart and checked clearances and relief valve. they looked excellent.
I really would like to keep the slant four as so many have been junked and or replaced with v6 v8.
Thank you everyone for the help. I have printed the answers and will try the high detergent oil treatment. If that does not do it I'll take her apart. Not my first engine by a many count but it will be my first slant 4

Bruce

[Workshop Help]

1. Yes, most definitely. Your secret weapon to remove the top shell will be a Popsicle stick. I prefer the orange Popsicle, but grape is good too. Loosen all the main bearing caps to give the needed clearance.
2. A new piston set is always nice, but check, re-check, and triple check the bore(s) dimensions in the usual three places. Removing a new top ring from one of the pistons, place it in the top, center, and bottom of the worn in bore. Measure the ring gaps. Compare them with the acceptable gap in the workshop manual. Remeasure after the bore has been deglazed. Make your decision to either use what you've got. or sell them on ebay to finance a new oversize set.
3. OKAY for the oil pump.
4. Spend the money on the highest rated machine shop in your area for the head and connecting rod overhaul.

Mildred Hargis

[motomadness1947]

I hear you on the machine shop. The original owner told me that he had 30 thousands removed from the head surface, so if it needs work I will have to find someone who knows to shorten the valve stems.. Oh I so wanted to buy this car and drive it not rebuild everything. I just finished a three year project on a basket case Jaguar e-type 2+2. For better or worse a brit showed up at a local show and agreed to pay me a ridicules price so he could send it back to england. It was not for sale at the time so I never thought he would bite
Thanks again Mildred

[Hasbeen]

Don't EVER shorten the valve stems in these engines. It is a no no leading to problems with the cam follower buckets & shims for adjusting the valve clearances.

Shaving the head has no effect on the valve clearance, so I can't quite understand your reason for considering it. Perhaps you are thinking of the effect on a push rod engine.

Head shaving can lead to problems with the angled head studs, & head positioning, but not valves. It can also alter valve timing, as it shortens the required length of timing chain, but not really enough to be a problem. If your engine has had it's head shaved, & the head gasket is not giving problems, the necessary work on head stud clearance must have been done at the time, so don't worry about it.

Our local triumph guru reckons the compression ratio has to be increased considerably to get higher performance from these engines. This is considering the UK spec engine. The US engine had very low compression pistons fitted to aid meeting US emission requirements. If you do decide to rebuild the thing, it is definitely worth fitting UK spec pistons. The difference in mid range torque & acceleration from about 50 MPH alone makes it worth it.

Hasbeen

[motomadness1947]

Thanks Hasbeen, my mistake. I was informed (wrongly apparently) that the valves would hit the top of the pistons if you shaved the head without shortening the valves.
Also in reading correspondence from the original owner, he states that the car has used very little or non at all for several years after the rebuild. I will try the high detergent oil and stp and see how that works our. Have you ever tried Marvels Mystery Oil Instead of the STP oil treatment?

Thanks
Bruce

[Hasbeen]

You were correctly informed. The valves could hit the piston if too much was shaved off the head, but the way you fix that is by fitting a thick head gasket, or a shim between the head & the block, to restore the distance between the valve seat & the piston, or by larger cut outs from the piston to clear the valve.

If the thing has stood idle for quite a bit, you could also have surface rust in the bores, which will affect compression until it has been polished off.

One other problem I have found with engines that hibernated for too long, was valves not fully shutting, due to a mixture of rust & carbon on the valve stems. This of course gives a low compression test also. Running & STP in the oil had fixed this, when not too bad.

Shouldn't be a problem except in a well worked engine that then sits unused. A recently rebuilt engine should not have a buildup of carbon on the valve stems.

By the sound of it, you have some chance the thing will improve with a little work. Don't forget to change the oil, & the filter a couple of times. Any carbon or rust you polish out of the thing should not be left to circulate around the engine.

Hasbeen

[FI Spyder]

If you don't have an OEM manual you can download a pdf from jclay's website. I don't think it has FI specific section but the rest applies. The slant 4 in the TR7 was the only engine of it's kind that was imported into N/A (not counting Saab 99) so unless the mechanic has specific experience on it (unlikely as they are mostly all retired by now) he won't know about the unique peculiarities of it so you have to take such info on it with a grain of salt.

If the engine was overhauled why did the previous owner have a set of new pistons? If I were going to replace the pistons I would replace them with a set of the higher compression UK ones (which I have for first overhaul). As there is only 104,000 + miles on it, burns no oil and should last past 180,000 miles (barring a chain tensioner failure) that won't be for a while yet.


- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

[motomadness1947]

oops wrong button. Yes I have a good manual, yes, I am taking the insides out of the cat conv. as I believe the old one is clogged, I retorqued the head today (seemed like it liked it) I have put some Marvels Mystery Oil in to the spark plug holes to soan for a few days as I await mounting the exhaust. I bought some oil detergent today which I shall add to the oil and run for a few miles, then use the stp oil treatment. Having full faith in the kind folks on this forum, I expect she will be purring like a kitten and running like a cougar very shortly.
Now for the new set of pistons. The original owner said that he ordered the pistons as he noted very low compression on two cyls before pulling the old ones. He verified (he is an engineer) that the originals measured properly so he just replaced the rings, honed the cyls and reinstalled. That was in 1987 or 88 the car has only gone 15, 000 miles since then with most being in the early 1990,s egro I am hoping that our mixture of chemicals does work as prescribed.

Thanks
Bruce

PS if it fails I have a bottle of Jameson's that need activity prior to further work on the old girl

[motomadness1947]

Okay it is time for an update: I used a bottle of CR2 detergent additive with my castrol 20/50 for about 300 miles and my compression has gone from 90 100 115 and 130 to 135 on two cyls and 140 on two cyls. Yeah. I have replaced the filter and am now running Valvoline 20w50 racing oil as it has zinc in it. after a few miles with that I will check my oil pressure again.
I had a horrible knock in the "engine" as told to me by a local greek mechanic. (no he was working on cars not greeks) I decided it was the catalytic converter buggering things up so i had it cut in two and emptied. Oh yes no more knock and she runs much better now thank you.
Latest trial is that my little gem is acting hard to start from cold (it is a FI car) I suspect the cold fuel injector is not working properly. Have checked themotine sensor and it seems to be working properly.
So thank you to all that helped me here on the forum. I am having so much fun with the 7 that I bought an TR8 that was advertised on the Triumph wedge association website. She will be here next saturday. yeah