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1500 engine performance mods

Posted by oldag98 
oldag98 Avatar
Jeff and Brittany Brackenridge
Choctaw, Choctaw, Oklahoma, USA   usa
1975 MG Midget Conversion
1976 MG Midget 1500 "Yellow One"
1977 MG Midget 1500 "Dunkirk"
1979 MG Midget 1500 "Parts Car"

I started a similar thread on the MGE and got quite a bit of good feedback, so I thought I'd see what you guys have to say. I have 3 1500 Triumph engines in RB Midgets I'd like to rebuild and upgrade for greater performance and reliability, to take advantage of modern engine components, the years of tuning and development on this engine since the 70's, and the wealth of engine building experience that has amassed since the engines were introduced. Does anyone have a good engine building recipe I could replicate for my 1500's? (Not for racing, just to bring them into the 21st Century and to get my little Midget screaming down the road). Please comment on anything from main studs to cams to roller rockers to engine machining procedures, to compression ratios, to carbs, etc.....Please post any good links on this topic as well. Thank you!

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Mike Soltez
Irwin, Pennsylvania, USA   usa

I'll be the first.
I shaved the head to get around 10:1 compression ratio, ported and polished the head as Kas Kastner describes in the Comp. Prep manual, bigger valves from a mark 4 spit, a "fast road" cam from Kent, a 4,2,1 stainless header from Rimmer Bros emptying into a 1 3/4" exhaust, and best of all Dual weber 40DCOE. When I built the bottom end I machined the center main cap flat and used longer ARP bolts to hold a 1" piece of keystock against the bottom of it. one corner had to be ground away a bit to clear the oil pump. I'm guessing that Max torque comes on at 2100 RPM. The carbs growl and it really starts pulling. I think that the carbs will flow so much air that the engine would detonate into a million chunks before they would start to reach their capacity.

My to do list
.40 over pistons
Megajolt ignition
lighten the flywheel
lighten valve train and bottom end for mor Revs

I have hammered this car at autocrosses and track days and it seems to love it. Which means that it will explode on the way to the grocery store.

oldag98 Avatar
Jeff and Brittany Brackenridge
Choctaw, Choctaw, Oklahoma, USA   usa
1975 MG Midget Conversion
1976 MG Midget 1500 "Yellow One"
1977 MG Midget 1500 "Dunkirk"
1979 MG Midget 1500 "Parts Car"

Come on fellas, Mike can't be the only one to reply...??? What have you done to upgrade your 1500's?

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cokerart Avatar
Bob Coker
Atlanta, Georgia, USA   usa
1973 Triumph GT6 MkIII "Spit-6 Or Dr. Frankenstein"


I think it would probably be wise to rebuild the lump and have a good machine shop go through it.

Magnaflux, top and bottom ends.
Hot tank both.
3-angle grind on valve seats with hardened exhaust seats.
Bronze valve guides (Or bronze guide sleeves)
Micro-polished journals.
Align bore the crank and balance the rotating assembly for smoothness and long life.

Mike's suggestions are good if you are going racing, but you intimated that you are looking for a street car with greater reliability and a bit more poke. I'd hesitate to overbore a 1500 engine. .40 over piston bores would not leave you any room to expand if there were any wear issues down the road.
I'd look at a Pertronix or Mallory replacement distributor and ignition. This will eliminate points and create a little more reliability with the potential for better spark.

An internal combustion engine is really just a big air pump. The more air pumped in, the more air (exhaust), out. If you are thinking about better breathing, consider the process to be Rubic's cube: when you change one side of the breathing equation, you must change another. Big headers without a better breathing carb, are not efficient. A better carb without a higher lift cam means you'd be drowning. A 3" diameter exhaust pipe will eliminate back pressure. Back pressure is necessary for low-end torque, not so much at 6,000 RPM. Roller Rockers are great for cars that are always in the upper RPM ranges. The payoff is less rewarding for around town.

Keep in mind what you want to accomplish. Do you want a racing car or a street car? Compromises can be made, just be aware of what each change will affect as you create the car you want.

Once you are done with the engine, don't forget the recondition the Trans and rear end.

There's probably much more. The other guys will continue to fill in.
Good luck


Mark Jones Avatar
SW, Ontario, Canada   can
1970 Triumph Spitfire MkIII
1995 MG MGF "Barney"

Over the past 8 years I ahve been upgrading my 1500. It is now running with high compression pistons, UK-spec cam, UK-spec 4-2-1 exhaust manifold, and HS4 carbs. It is a completely different car than it was when I bought it. I really like the improved proformance.

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Mike Soltez
Irwin, Pennsylvania, USA   usa

My engine is very user friendly around town and on the highway. I have 185/60 13 tires which are a little lower so I put TR7 gears in the diff. I think that they are 3.67 to 1. Anyway even with the taller gears the engine isn't so high strung that it lugs at all. The Kent fast road grind has a lot of lift but not a lot of duration which keeps the power band from being pushed too high. The thing to remember is that the engine is many different systems that all have to work together. Bolting Dual DCOEs on a stock engine would would hurt driveability. I'd start with some head work a header and dual SUs.

oldag98 Avatar
Jeff and Brittany Brackenridge
Choctaw, Choctaw, Oklahoma, USA   usa
1975 MG Midget Conversion
1976 MG Midget 1500 "Yellow One"
1977 MG Midget 1500 "Dunkirk"
1979 MG Midget 1500 "Parts Car"

Has anyone done anything to deck the block to remove the recesses between the cylinders and run a diff head gasket, i.e. from a 1300? Do you need custom pistons for this?

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William Bonbright
Rincon, Puerto Rico, Puerto Rico   pri
1979 Triumph Spitfire "Verde"
1979 Triumph Spitfire 1500 "Verde"


These little cars were known for one thing, and one thing only...handling. The rear end will not hold more than 150 hp, the transmissions were decent but the O/D were plagued with problems. The engine has three bearings which make for a potentially unbalanced crank shaft. Paul's site dedicated to RELIABLE performance enhancements is


I would check out his site. I have modeled my rebuild after his suggestions and found them to very usefull.

What I decided to do is enhance the better qualities of the Spit...coil over shocks, independant rear suspension, a 1" sway bar, oversized brakes in the front, re-built rear brakes, and switched the rear to a 3.67 (from the stock 3.89)...I do not have O/D. I have not finished the build up...yet, but am eager to get her done.

As for the engine performace I am replacing the stromberg with a Makuni side draft carb, swapped the intake manifold to a 1300, ported and ceramic coated a pacesetter header (4-2-1) which leads to an inline muff using 1 3/4" pipe. I am going to change out the cam for a hotter one (1/2 race) and have replaced the electronics to Mallorey.

I am waiting until either the engine or the tranny goes until I rebuild it .4 over, while milling the head to get about 9.5:1 compression. Then I will replace the tranny with a T-9 ford 5 speed. After the complete engine/tranny rebuild I would expect somewhere in the 110hp range. Not bad considering it has about 47hp stock.

well hope the info helps. I'm going to follow the topic, as I have an interest in it as well.


emadak Avatar
Ed 123Ignitionusa
Ohio, USA   usa
1969 MG MGC

The 123 ignition(distributor) is a modification that employs spark balancing, like used in formula race cars. It will smooth out an engine and pull locked up hp to the fore. In addition there are no springs or weights to fool with. All spark advance is handled electronically. With your choice of advance curves or a tunable unit.

Member Services:
US Supplier of 123Ignitions "NEW" Fully programable Distributors perfect timing every time, Easy as 123, Alternator Upgrade Kits, Kyb rear shock conversion kits, Spal Fans, ZDDP Plus, Stainless Heat Shields for MGC
tktrain Avatar
Tom Kohlmeyer
Evansville, IN, USA   usa

Anyone know anything about the downdraft Weber carb and aluminum intake kit being offered by englishparts.com? The '78 spit I just bought has this and a 4 tube header on it. I don't have a clue what else is in there yet but I intend to pull it down and find out before I run it hard.

britsnspits Avatar
Michael Stoliker
Bethlehem, PA, USA   usa
1976 Triumph Spitfire 1500 "The Phoenix"
1978 Triumph Spitfire 1500 "Lucky"

The weber downdraft is basically the same carburetor that was used on early German Capris and Ford Pintos (1970s). I think Ford later made a knock-off of the carb that was used on 4 cylinder mustangs and later US made Mercury Capris (1980s). The carb is a two stage design, so it's essentially a one barrel with a secondary that comes on at full throttle. I think these carbs were also stock on 70's era Fiat 124 sedans and roadsters, but I may be wrong.

I always found these carbs to be finicky and difficult to get good performance from on the Fords and Mercs, but Spitfire owners seem to think they are more reliable than the stock Zenith Strombergs (one man's floor is another man's ceiling, guess).

I'd rather have twin SU's or a single weber side draft (dual weber sidedrafts seem like overkill and a tuning nightmare to me).

Mark Jones Avatar
SW, Ontario, Canada   can
1970 Triumph Spitfire MkIII
1995 MG MGF "Barney"

There was an article in Classic Motorsport comparing different SUs to different Webers on the same MGB B-series engine. The results were that the downdraft Weber gave essentially the same performance as HS4s.

tktrain Avatar
Tom Kohlmeyer
Evansville, IN, USA   usa

My best friend has a Fiat with a DCV on it that responded VERY well to it. He has a 2 liter OHC compared with the spit's little push rod 1500. I'm just curious where this little motor might go if tuned and built properly. I've seen talk of 100HP from some out there, but I have to admit that the 3 main bearing crank arrangement scares me pretty badly. I have a lot of experience in building engines, I even put together a current class record holding engine at Bonneville. A 439 inch small block that put out 739HP on pump gas. It set the record on 60% nitromethane, naturally aspirated.

Point is, I know my way around the inside of an engine pretty well and I'm not afraid to try things, but the 3 main bearing block is scary. Any and all insight is greatly appreciated.

spitfire50 Avatar
Paul Mugford
Rochester, N.H., USA   usa

From what I've read the trick with the 1500 is don't try to spin it too fast. Get your power out of it under 6000RPM and don't expect to rev it to that speed and live a long life. Yes you can make a 1500 survive revs, but it costs a lot more to do than the small crank 1296. The best thing about the 1500 is the torque.
Good luck,

oldag98 Avatar
Jeff and Brittany Brackenridge
Choctaw, Choctaw, Oklahoma, USA   usa
1975 MG Midget Conversion
1976 MG Midget 1500 "Yellow One"
1977 MG Midget 1500 "Dunkirk"
1979 MG Midget 1500 "Parts Car"

I suggest you guys go read the same thread over on the MGExperience board. I am looking for some specific engine modifications, specific cam recommendations, specific machining insight from the Spit crowd who have been tuning these engines longer and harder than we MG owners. The 1500 is a stroked 1300 and like all stroker engines, they make torque down low, but do not like higher RPMS. The 3 main floppy 1500 crank can live well at higher RPMS than a stock 1500, if rotating and recipricating assemblies are well ballanced and good ARP main studs are used. The Webers are 1 step up from the stock ZS carb, but suffer from a lack of tuning experience and info on this side of the pond. The dual HS setup appears to be the best way to go, either the HS2 set-up from a Euro 1300 Spit or the HS4 set from a Euro 1500 Spit. They are still relatively cheap, but are usually in need of new throttle shafts and waxstat to fixed jet modification. A heat shield is highly recommended for these as well. Quantum Mechanics offers these as used sets with manifold pulled from old Spits in England. They can be rebuilt/modified by anyone who knows SU's. They look 10X better in the engine bay on a 1300 or a 1500 than a ZS or Weber and have been around forever.

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