View Full Version : TR4/4A TR4 Optimal Compression - 91 Octane Pump Gas

01-28-2012, 11:50 AM
Hi All... building a street performance engine for my '53 +4 Morgan Roadster. Now have a TR4 (not 4A) cylinder head. <span style="font-weight: bold">For 91 Octane pump gas, what is the optimal compression ratio?</span> I read the Kas manuals, he suggests ll.7 to 1, but this may be dated (assuming higher Octane gas). This will be a 5k RPM engine, 89mms, 3/4 cam, headers &amp; SU's. Must be drivable in low RPM's...I owe beers to whoever has the answer..

01-28-2012, 11:53 AM
I'd suggest 9.5/1

01-28-2012, 12:05 PM
I wouldn't try to go any higher than 10:1 on pump gas; but it does depend to some extent on how much (and how well) you are modifying the combustion chamber. The factory chamber had a lot of sharp edges, which tend to promote detonation.

01-28-2012, 12:28 PM
Agree with the above 9.5 or 10 to one tops, I know a few guys who have gone higher street LBCs and regretted it, head leaks, detonation.

As Randall suggested, eliminating the shard edges in the combustion chamber can reduce tendency to detonation, I did this to my TR250 head with a dremel and carbide burr, pretty easy to do, the sharp edges come off fast, work slowly and carefully.

Simon TR4a
01-30-2012, 03:07 PM
To elaborate slightly on the answers above:
1) Kas is telling you how to build a race motor- you are building a street motor. His suggestion relates to race gas, probably 107 octane.
2) The big bore pistons will give you higher compression automatically as the swept volume is greater while the combustion chamber volume is unchanged.
3) A longer duration cam has the effect of lowering the effective compression ratio, the manufacturer of your cam or your engine builder may be able to tell you how much.
4) It would be worth unshrouding the valves to improve breathing, you will have plenty of room with that big bore!

In summary- I use 94 octane with 9.7 compression and a Kent cam with 290 degrees of duration and .310 lift (stock rocker ratio.)

I hope this is helpful.

02-07-2012, 11:10 AM
Thank you gentlemen...I am very glad I got this feedback before ruining a head. Networking via Doug Sallen (races Morgans) I have found someone in the Bay Area that really knows these heads and how to tune them. I'll suggest within 10:1 and the below suggested valve/porting/flowing work ....thanks

02-07-2012, 03:45 PM
I would look up Dave Rebello. He builds some of the best Nissan/Miata engines in the country, but I know he has done a TR4 engine that put out an honest 198bhp with 1 3/4 SUs. He's in the San Fransisco area. He does a lot of street work too. Same goes for Huffaker Engineering. There are some other high profile race shops who will be good at figuring these type of things out. Farther south is Greg Solow, he races Morgans.

02-07-2012, 09:22 PM
Definitely don't want to go over 10:1. I have 10:1 in my Tr250 and did a lot of head work and got rid of all the sharp edges with a lot of die grinding work. Ported out the inlets and used ceramic coatings on the head and top of the pistons. We can't get 94 octane in Ohio any more and my car ran great on it. Only a good quality 93 or 92 works well now after changing the timing a little. So if you want to run 91 octane then you will need to come under the 10:1 compression.
There is also some talk of taking sulphur out of gasoline like they did in diesel. If they do that we will all be running expensive additives to run our cars on so you may want to ask your head guy about that too.

Your compression ratio should be geared more towards what cam and RPMs you want to run. Especially in a street car lift over duration is much more prefered. Too much duration and you will have idle problems. When in dought take the lessor cam

02-08-2012, 12:22 PM
Ok so definitely will keep the compression down to 9.5 or 9.7 range. Assuming less milling on the surface as a result. Kas suggested grinding out some of the squish area, but assuming this is overkill with the lower ratios. I can take the sharp edges off with my dremel on the combustion side. (templates/jigs for consistency or is this overkill?) What are your thoughts on oversize intakes?

On the manifold side heard you need to be careful...heard it's easy to "find" the water jackets. Will port to both manifolds and take out "obvious" irregularities...again this is just a street engine (5k and I won't keep it there)..just intend to impress my daughters boyfriends and keep the Porsche's honest.

As for the cam, I have the specs re a moderate street cam from Elgin (Santa Rosa CA).
44 44 72.5 72.5 296 .38
Worried about the duration...I want to idle within 1k and do not want stutter getting to the sweet spot. Happy to chamfer the liners for the intake lift but don't want rough running engine. Other ideas greatly appreciated.

Simon TR4a
02-08-2012, 08:36 PM
I have had two different cylinder heads on my own TR4a, the first time I had standard size valves of EN46 from Triumphtune, with a good 3 angle valve job, mild porting and port matched to the manifolds. This head cracked when the gasket failed.
The second head I had built with oversize valves from the same source and again prepared as before. I could not swear to there being any gain from the bigger valves, so likely not worth it for a street motor.
I would point out that to avoid pre-ignition on engines with raised compression it is important to get rid of the spent gases efficiently so a mild cleaning up of the exhaust ports and the slight shrouding of the valve will help.
If you have the "log" style inlet manifold it is worth changing to the later style.
The inside of my inlet manifold has been enlarged as well, easy to do with an alloy manifold, this might be worthwhile- I have an unmodified inlet manifold but have not tried it out to see if there is any difference.
I am impressed by the figure of 198bhp- that is a monster! I am sure any of the three places mentioned can give you good advice- I have spoken to Greg Solow and also communicated with him on some of the race forums- a most knowledgeable guy.

Some final thoughts- 1)it is usually best to buy lifters from the same source as the cam.
2)If you take a lot off the head you may need shorter pushrods to preserve correct valve geometry- I got mine from British Frame and Engine in Temple, Arizona, strong tubular ones.
3)I put my stock air filters back on in place of the K&amp;N filters I used on the track and found this smoothed out the power curve a little- I suspect a slight reduction in airflow had this effect.

Let us know how things work out!

02-08-2012, 08:48 PM
I have a later style inlet manifold and no TR4a to install it on, so if you want one let me know (message me through the forum), it is supposed to breathe a little better.


02-08-2012, 09:15 PM
Porting and port matching is critical if your doing extra large valves and special valve grinds. You want to get everything in and out with out obstructions. Good headers too are important. I too was very nervous when porting my heads. I had never done that before but followed the race prepared manual to a T. It took me about a week porting the intake and exhaust ports. I did about 2 ports a night after work. I have a good compressor and used carbide bits with a die grinder. A dremel my take a month.

Out of the work I did on my TR250 engine I know I got at lease 150 hp and a very nice idle at 1k. I've even had it idling at 750 rpm. There is a local x Triumph dealership mechanic that I some times take my car to when i get into trouble. In fact he degreed the cam for me which is another important component of a highly tuned performance engine. He also did the final timing and remove the vacuum advance and says there is still much more in me engine. So work with your local reputable experts they should be your best source of information

02-10-2012, 12:51 PM
Thanks for you input...very helpful. General theme seems to be "patience"! Will pull the engine at end of this month (after ordering parts/aligning machine shop for cam). I'm sure this journey will lead to more questions (last time I rebuilt this engine was with my father when I was 12). 38 years later (and dad smiling down from above)...find myself feeling like an anxious kid!!

02-10-2012, 01:24 PM
I'll take a stab.

Quench is good. I wouldn't modify the quench pads.

Big valves will work with 89mm liners. Not sure how they'll work at the lifts you're looking at; that's something that needs to be checked on a flowbench.

Work on your exhaust system. I suggest buying a program like Pipemax. You can figure out your primary tube and intake manifold lengths for where your engine is going to peak. Attention to harmonic tuning is one of the attributes that separates a so-so engine from GOOD engine.

Pipemax will also help you determine how much lift you'll need and port CSA (cross sectional area). A lot of cams might have similar specs, but lobe acceleration rates vary.

It's all about flow quality not quantity. Be wary of gasket matching as most people just open up the face and make it too large.

When you're done, spend time on a dyno with a legitimate tuner.