Differences in power outputs at power challenge day

[Ian Durrant]

Can it really be explained by domed pistons??

This seems to be the current theory, that engines still using the (no longer available) domed pistons, produced substantially more power than even otherwise modified engines (manifolds, holley etc)

However triumph claimed the change only gave the engine 1bhp more power despite the dome changing the "squish" and compression ratio.

I found a calculator on the web that does all the major maths for engines based on bore, stroke combustion chamber cc etc.

http://www.csgnetwork.com/compcalc.html

First we use the calculator to estimate the combustion chamber volumes. To do this I have assumed that the head gasket is .020" that its the same size inside diameter of the fire rings as the bore (its not but I can't see it would make that much difference).

I have also assumed the compression ratio of a car fitted with flat top pistons would be the 8.8:1 of the early cars (or was the combustion chamber modified as well on Mk2 type heads?)
We get a combustion chamber volume of 42.694 cc

Now I have really had to guess. I am assuming that skimming the head 0.020" would be (very roughly) equivalent to halving the thickness of the head gasket

This alone brings the CR back up to 9.04:1

Now most engines will also have been rebored so I have used the calculator to give CR's for a 0.010" , 0.020" and 0.040"overbore

Respectively this gives us 9.09:1, 9.13:1 and 9.23:1

So an engine on maximum rebore size and with 0.20" off the heads its pretty much at the same CR as one with domed pistons.

Now using this displacement calculator http://www.johnmaherracing.co.uk/enginecalc.htm
using my guesses for the standard engine without skimmed heads and without over bore we get a cc of 2995 which is pretty close to the actual 2997 so my guesses aren't widely off it would seem

However the 40 though rebore now has a cubic capacity of 3067cc which is only 2% bigger

so neither compression ratio nor other factors such as head skimming would appear to cater for the wide differences in standard stag outputs. Unless of course triumph so massively overquoted the mk1 engines output that the improved squish and CR characteristics of the mk2 domed piston merely brought the engine up to roughly what they had originally claimed.


My only purpose here was to try and work out if its worth the outlay for SOC TFL to get domed tops remanufactured and if they did whether it would be worth me buying them if they did. I am really not sure based on this.

I think its much more likely that some engines are more worn than others, some are better tuned than others, some could do with sessions on the rolling road to get the jetting and advance curve right to suit their individual chacteristics. Modern engines are better toleranced and the ecu changes the "tune" to suit the fuel using knock sensors etc. Maybe we would be better spending money on performance mods and then a rolling road session?

[SaphireSpud]

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and I thought I didn't get out enough!

I'll read this in the cold light of tomorrow morning but I cannot promise a sensible answer!

Regards, John.

[flying farmer]

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Here are some real world figures for compression ratios as measured on two of my own engines with mark two heads.

Chamber volume 35.5 cc- valves not far off flush with head face

Standard head gasket is 40 thou which gives a volume of 6 cc

Domed pistons protrude from the block but have no effective dish at tdc, however dished pistons have a bowl of 5.5 cc

On the first engine I built the heads were better than the above examples but it was about 12 years ago but i didnt write down the chamber volumes. What I do know is that the compression ratio worked out at 8.6 :1 so the chamber volume must have been near 38 cc. The same pistons with the above heads gives 9.1:1 and with the domed pistons you get 10.15:1. If you then add extra thick gaskets this puts an extra 3cc in the combustion chamber which alters the compressions to 8.6:1or 9.5:1. Of course if you have good heads, dished pistons and extra thick gaskets you could get as low as 8.2:1. All figures have been worked out using 20 thou oversize pistons giving a single cylinder capacity of 379cc or 3032cc total.

So it is possible to have two apparently identical engines with compression ratios varying from 8.2 to 10.1

One other fly in the ointment is that extra thick gaskets ruin the squish area of the combustion chamber by making the gap between piston and head bigger, and this reduces the swirl of the mixture just as ignition begins. This gives rise to a poorer burn than would be achieved with the same compression ratio but a tighter squish area.

Neil

[Stevekid1]

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I'll get me coat.

[Ian Durrant]

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and I'll get me anorak

I did warn you this thread might contain maths

Neil's post is interesting (to me at least). I hadn't thought about extra thick head gaskets ruining the squish characteristics but if the 20 odd thou difference can do this, then this alone could be a major contributor to why some engines performed badly. I know for a fact that the heads on my car have been skimmed at least 17 thou. 10 when i first rebuilt the engine and 7 when I rebuilt the heads a few years back.

The head gaskets supplied by a well discussed supplier at the time were terrible. after fitting the heads and torquing them down, I could get a 20 thou feeler gauge between the head and gasket. I assumed the heads had been machined badly and took them straight back to the engineering co. Here we checked the gaskets against a set of original (old stock) Payen gaskets he had and not only were they thicker than the payen's, the fire rings were substantially thicker than the gasket. Utterly useless. However these were not the "extra thick" gaskets but supposedly standard ones. Maybe slightly less "faulty" gaskets are in use in may cars and changing the design characteristics.

My car felt significantly more powerfull just after the heads were rebuilt but I had fitted tubular manifolds which I had assumed were the cause of the increase, but its possible, even likely, that decent quality gaskets and the head skim raising the CR were equally beneficial.

This raises another thought, are the HRS "head saver" shims shaped like the gasket or the combustion chamber? If the later the they should not affect the squish characteristics in the same way the extra thick gaskets do and maybe give better horsepower.

Some of you may not think this stuff important, but I would have to disagree. I am not strict on originality, but even if i were, original power would be at the top of my list.

However I am still not convinced that the domed pistons are capable of explaining the suprisingly high output of an otherwise standard stag (according to the test results).

[flying farmer]

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I had a set of head gaskets like that but only found out when I filled the cooling system

Most of the difference in outputs on the rolling road are due to engine setup.

At the TR register international a few years ago when they started doing rolling road runs, I asked the operator what a good standard spec 150 bhp engine should produce and he reckoned 125bhp. Best on the day was a totally standard TR5 with original exhaust system that made 122bhp. Most of the modified ones made less, one particularly sad case being a recent rebuildof a 125 spec with a supposed upgrade to 150 spec that made 85bhp. A lot of these cars had performance exhausts and manifolds and a large number of other go faster bits.

My stag engined TR5 made 159bhp @ 5250 rpm on the same day, this being the 8.6:1 compression engine refered to yesterday.

Most common problem on the TR's was over rich mixture, not an unusual problem with the lucas injection system but I have seen a number of stags on the rolling road chucking black smoke. By the time your car is producing black smoke power will be well down and the engine will be gradually wearing itself away with a combination of bore wash and oil dilution

Add to the mix an 8 cylinder engine that needs a damn good coil to supply enough sparks and cam timing that is probably a tooth out due to a combination of skimmed heads and stretched timing chains and you have a recipe for disapointment

It is probably safe to assume that the HRS built stag has properly timed in cams and a distributor that is working as it should

[Ian Durrant]

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Yes I was lucky to spot it really. It could easily have been back in the car and only found on filling the cooling system too! I can't remember what made me check, I think something just didn't feel right.

I had a think about the skimmed heads affecting the valve timing issue. If I am right I think skimmed heads would, if you follow the ROM, result in slightly retarded valve timing. From memory and, I admit a quick google, retarding the valve timing a little tends to increase top end power at the expense of lower end torque. Since the stag lump is hardly short on torque, I would have thought that this would be beneficial from a rolling road point of view. Of course attempting to correct the slight error could easily result in one tooth of advance which would tend to mash the peak power figure.

I'll check mine because I seem to remember that after fitting the chains and turning the engine over, the valve timing marks were slightly out and valve timing retarded. (Can you see the timing marks through the oil filler or do you need to remove a cam cover? Mines garaged for the winter so I can't check now).

[V Mad]

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If you add up all the possible variations of c/ratio, cam timing, ignition timing and fuel mixture, plus any other effects due to30+ years of wearits not surprising that there is so much variation in power.

As well as changing c/ratio I noticed that on early engines the spark advance curve was quite different too (much more adavnce than on later cars IIRC). So if you have had later pistons fitted, should you change the advance curve too?

And how many of us have even checked the spark advance curve? To do it properly you need to have highly visible lines markedon thetiming plate and a strobe light (plus rev counteror helper to look).

[gerty]

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Well I do try to get out as much as possible ! ! !but here are a few thoughts on the subject.

Twelve months ago after having driven a number of Stags for sale, most of which were truly gutless, I alighted on a Mk2 TV8 manual that had had a huge rebuild a few years back( but very few miles since)including a major but unsubstantiated engine rebuild, which was very quick for a Stag (or at least compared with the others I had driven), I bought this.

Having used itfor a while and confirming all the usual thing such as the actual oil pressure(Millers Semi synthetic 20W50), water temp,compressions, static and full ignitionadvance etc, I realised it was a really good engine.

I then fittedthe Phoenix tubular exhaust manifolds- these gave an amazing improvement in low end and mid range torqueand a noticable increase in power.

Soon after this I fitted an Edelbrock 1404 carburettor on a Tony Fox Canadian inlet manifold adapter, the results after trying several jet/rod settings was a substantial increase in power and an amazing improvement in the drivability, actually quite staggering.

Having sorted the approximatemixture settingsby roaddriving I then had it set upon a respected rolling road using Shell Vmax 99 Octane fuel, after checking the mixture at idle and under various loads the first power run gavearound 125 bhp@ 3500rpm and around 132 bhpbetween 4500& 5500rpm - probably what I was hoping for.All this this wasachieved at a maximum total igntionadvance of 26 degrees (static of 12 degreesplus centrifugal 2x 7 degrees, which is the standard for most UKMk2 distributors).

Adjusting the maximum ignition advance under full load to 30 degrees,the power output peakedat 158 bhp@ 5500 rpm, ie about +19% power increase. This advance of 30 degees is roughly what I would expect, my TR7-V8255 bhp 3.9 litre RV8 runs at 32 degrees asdoes a TR5 producing 165 bhp.

My point is that I am not too sure that the published Triumph ignition setting are at all correct for best power, Isuspectthere is a lot of power to be gainedby checkingthe best ignition advance on a powerrun ( the down side is that the distributor may needthe amount of advance altered, as well as possibly a change ofsprings ).

Before I get posts saying " no way for 158 bhp", yes I agree, but I am fairly sure it is well north of 140 bhp,I hope to get it onto Enginuity's rolling road to compare.These are all computed flywheel bhp figures after calculated losses had been added back in.



It was quite clear on this engine it is very sensitive toignition advance.



Regards, David



Ps I do not know the compression ratios (compressions hot are around 175 psi), but I feel it is probably around 9.1/9.3:1